Special Guest:Elizabeth S. Sexton-Kennedy, who works at FermiLab as Compact Muon Solenoid (CMS) Offline Coordinator. CMS (at CERN/LHC) is a particle detector that is designed to see a wide range of particles and phenomena produced in high-energy proton collisions in the LHC.
Why do the other planets, like Venus (shown above) have a different atmosphere than Earth? Credit: ESA
Here on Earth, we tend to take our atmosphere for granted, and not without reason. Our atmosphere has a lovely mix of nitrogen and oxygen (78% and 21% respectively) with trace amounts of water vapor, carbon dioxide and other gaseous molecules. What’s more, we enjoy an atmospheric pressure of 101.325 kPa, which extends to an altitude of about 8.5 km.
In short, our atmosphere is plentiful and life-sustaining. But what about the other planets of the Solar System? How do they stack up in terms of atmospheric composition and pressure? We know for a fact that they are not breathable by humans and cannot support life. But just what is the difference between these balls of rock and gas and our own?
For starters, it should be noted that every planet in the Solar System has an atmosphere of one kind or another. And these range from incredibly thin and tenuous (such as Mercury’s “exosphere”) to the incredibly dense and powerful – which is the case for all of the gas giants. And depending on the composition of the planet, whether it is a terrestrial or a gas/ice giant, the gases that make up its atmosphere range from either the hydrogen and helium to more complex elements like oxygen, carbon dioxide, ammonia and methane.
Mercury’s Atmosphere:
Mercury is too hot and too small to retain an atmosphere. However, it does have a tenuous and variable exosphere that is made up of hydrogen, helium, oxygen, sodium, calcium, potassium and water vapor, with a combined pressure level of about 10-14 bar (one-quadrillionth of Earth’s atmospheric pressure). It is believed this exosphere was formed from particles captured from the Sun, volcanic outgassing and debris kicked into orbit by micrometeorite impacts.
A High-resolution Look over Mercury’s Northern Horizon. Credit: NASA/MESSENGER
Because it lacks a viable atmosphere, Mercury has no way to retain the heat from the Sun. As a result of this and its high eccentricity, the planet experiences considerable variations in temperature. Whereas the side that faces the Sun can reach temperatures of up to 700 K (427° C), while the side in shadow dips down to 100 K (-173° C).
Venus’ Atmosphere:
Surface observations of Venus have been difficult in the past, due to its extremely dense atmosphere, which is composed primarily of carbon dioxide with a small amount of nitrogen. At 92 bar (9.2 MPa), the atmospheric mass is 93 times that of Earth’s atmosphere and the pressure at the planet’s surface is about 92 times that at Earth’s surface.
Venus is also the hottest planet in our Solar System, with a mean surface temperature of 735 K (462 °C/863.6 °F). This is due to the CO²-rich atmosphere which, along with thick clouds of sulfur dioxide, generates the strongest greenhouse effect in the Solar System. Above the dense CO² layer, thick clouds consisting mainly of sulfur dioxide and sulfuric acid droplets scatter about 90% of the sunlight back into space.
Another common phenomena is Venus’ strong winds, which reach speeds of up to 85 m/s (300 km/h; 186.4 mph) at the cloud tops and circle the planet every four to five Earth days. At this speed, these winds move up to 60 times the speed of the planet’s rotation, whereas Earth’s fastest winds are only 10-20% of the planet’s rotational speed.
Venus flybys have also indicated that its dense clouds are capable of producing lightning, much like the clouds on Earth. Their intermittent appearance indicates a pattern associated with weather activity, and the lightning rate is at least half of that on Earth.
Earth’s Atmosphere:
Earth’s atmosphere, which is composed of nitrogen, oxygen, water vapor, carbon dioxide and other trace gases, also consists of five layers. These consists of the Troposphere, the Stratosphere, the Mesosphere, the Thermosphere, and the Exosphere. As a rule, air pressure and density decrease the higher one goes into the atmosphere and the farther one is from the surface.
Closest to the Earth is the Troposphere, which extends from the 0 to between 12 km and 17 km (0 to 7 and 10.56 mi) above the surface. This layer contains roughly 80% of the mass of Earth’s atmosphere, and nearly all atmospheric water vapor or moisture is found in here as well. As a result, it is the layer where most of Earth’s weather takes place.
The Stratosphere extends from the Troposphere to an altitude of 50 km (31 mi). This layer extends from the top of the troposphere to the stratopause, which is at an altitude of about 50 to 55 km (31 to 34 mi). This layer of the atmosphere is home to the ozone layer, which is the part of Earth’s atmosphere that contains relatively high concentrations of ozone gas.
Space Shuttle Endeavour sillouetted against the atmosphere. The orange layer is the troposphere, the white layer is the stratosphere and the blue layer the mesosphere. Credit: NASANext is the Mesosphere, which extends from a distance of 50 to 80 km (31 to 50 mi) above sea level. It is the coldest place on Earth and has an average temperature of around -85 °C (-120 °F; 190 K). The Thermosphere, the second highest layer of the atmosphere, extends from an altitude of about 80 km (50 mi) up to the thermopause, which is at an altitude of 500–1000 km (310–620 mi).
The lower part of the thermosphere, from 80 to 550 kilometers (50 to 342 mi), contains the ionosphere – which is so named because it is here in the atmosphere that particles are ionized by solar radiation. This layer is completely cloudless and free of water vapor. It is also at this altitude that the phenomena known as Aurora Borealis and Aurara Australis are known to take place.
The Exosphere, which is outermost layer of the Earth’s atmosphere, extends from the exobase – located at the top of the thermosphere at an altitude of about 700 km above sea level – to about 10,000 km (6,200 mi). The exosphere merges with the emptiness of outer space, and is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide
The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the Aurora Borealis and Aurora Australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere.
Photo of the aurora taken by astronaut Doug Wheelock from the International Space Station on July 25, 2010. Credit: NASA/Johnson Space Center
The average surface temperature on Earth is approximately 14°C; but as already noted, this varies. For instance, the hottest temperature ever recorded on Earth was 70.7°C (159°F), which was taken in the Lut Desert of Iran. Meanwhile, the coldest temperature ever recorded on Earth was measured at the Soviet Vostok Station on the Antarctic Plateau, reaching an historic low of -89.2°C (-129°F).
Mars’ Atmosphere:
Planet Mars has a very thin atmosphere which is composed of 96% carbon dioxide, 1.93% argon and 1.89% nitrogen along with traces of oxygen and water. The atmosphere is quite dusty, containing particulates that measure 1.5 micrometers in diameter, which is what gives the Martian sky a tawny color when seen from the surface. Mars’ atmospheric pressure ranges from 0.4 – 0.87 kPa, which is equivalent to about 1% of Earth’s at sea level.
Because of its thin atmosphere, and its greater distance from the Sun, the surface temperature of Mars is much colder than what we experience here on Earth. The planet’s average temperature is -46 °C (51 °F), with a low of -143 °C (-225.4 °F) during the winter at the poles, and a high of 35 °C (95 °F) during summer and midday at the equator.
The planet also experiences dust storms, which can turn into what resembles small tornadoes. Larger dust storms occur when the dust is blown into the atmosphere and heats up from the Sun. The warmer dust filled air rises and the winds get stronger, creating storms that can measure up to thousands of kilometers in width and last for months at a time. When they get this large, they can actually block most of the surface from view.
Mars, as it appears today, with a very thin and tenuous atmosphere. Credit: NASA
Trace amounts of methane have also been detected in the Martian atmosphere, with an estimated concentration of about 30 parts per billion (ppb). It occurs in extended plumes, and the profiles imply that the methane was released from specific regions – the first of which is located between Isidis and Utopia Planitia (30°N260°W) and the second in Arabia Terra (0°N310°W).
Ammonia was also tentatively detected on Mars by the Mars Express satellite, but with a relatively short lifetime. It is not clear what produced it, but volcanic activity has been suggested as a possible source.
Jupiter’s Atmosphere:
Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.
Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.
Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. These clouds are located in the tropopause and are arranged into bands of different latitudes, known as “tropical regions”. The cloud layer is only about 50 km (31 mi) deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region.
There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter, which would be caused by the water’s polarity creating the charge separation needed for lightning. Observations of these electrical discharges indicate that they can be up to a thousand times as powerful as those observed here on the Earth.
Saturn’s Atmosphere:
The outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. The gas giant is also known to contain heavier elements, though the proportions of these relative to hydrogen and helium is not known. It is assumed that they would match the primordial abundance from the formation of the Solar System.
Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. The upper clouds are composed of ammonia crystals, while the lower level clouds appear to consist of either ammonium hydrosulfide (NH4SH) or water. Ultraviolet radiation from the Sun causes methane photolysis in the upper atmosphere, leading to a series of hydrocarbon chemical reactions with the resulting products being carried downward by eddies and diffusion.
Saturn’s atmosphere exhibits a banded pattern similar to Jupiter’s, but Saturn’s bands are much fainter and wider near the equator. As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. In the upper cloud layers, with temperatures in range of 100–160 K and pressures between 0.5–2 bar, the clouds consist of ammonia ice.
Water ice clouds begin at a level where the pressure is about 2.5 bar and extend down to 9.5 bar, where temperatures range from 185–270 K. Intermixed in this layer is a band of ammonium hydrosulfide ice, lying in the pressure range 3–6 bar with temperatures of 290–235 K. Finally, the lower layers, where pressures are between 10–20 bar and temperatures are 270–330 K, contains a region of water droplets with ammonia in an aqueous solution.
On occasion, Saturn’s atmosphere exhibits long-lived ovals, similar to what is commonly observed on Jupiter. Whereas Jupiter has the Great Red Spot, Saturn periodically has what’s known as the Great White Spot (aka. Great White Oval). This unique but short-lived phenomenon occurs once every Saturnian year, roughly every 30 Earth years, around the time of the northern hemisphere’s summer solstice.
These spots can be several thousands of kilometers wide, and have been observed in 1876, 1903, 1933, 1960, and 1990. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, which was spotted by the Cassini space probe. If the periodic nature of these storms is maintained, another one will occur in about 2020.
The winds on Saturn are the second fastest among the Solar System’s planets, after Neptune’s. Voyager data indicate peak easterly winds of 500 m/s (1800 km/h). Saturn’s northern and southern poles have also shown evidence of stormy weather. At the north pole, this takes the form of a hexagonal wave pattern, whereas the south shows evidence of a massive jet stream.
The persisting hexagonal wave pattern around the north pole was first noted in the Voyager images. The sides of the hexagon are each about 13,800 km (8,600 mi) long (which is longer than the diameter of the Earth) and the structure rotates with a period of 10h 39m 24s, which is assumed to be equal to the period of rotation of Saturn’s interior.
The south pole vortex, meanwhile, was first observed using the Hubble Space Telescope. These images indicated the presence of a jet stream, but not a hexagonal standing wave. These storms are estimated to be generating winds of 550 km/h, are comparable in size to Earth, and believed to have been going on for billions of years. In 2006, the Cassini space probe observed a hurricane-like storm that had a clearly defined eye. Such storms had not been observed on any planet other than Earth – even on Jupiter.
Uranus’ Atmosphere:
As with Earth, the atmosphere of Uranus is broken into layers, depending upon temperature and pressure. Like the other gas giants, the planet doesn’t have a firm surface, and scientists define the surface as the region where the atmospheric pressure exceeds one bar (the pressure found on Earth at sea level). Anything accessible to remote-sensing capability – which extends down to roughly 300 km below the 1 bar level – is also considered to be the atmosphere.
Diagram of the interior of Uranus. Credit: Public Domain
Using these references points, Uranus’ atmosphere can be divided into three layers. The first is the troposphere, between altitudes of -300 km below the surface and 50 km above it, where pressures range from 100 to 0.1 bar (10 MPa to 10 kPa). The second layer is the stratosphere, which reaches between 50 and 4000 km and experiences pressures between 0.1 and 10-10 bar (10 kPa to 10 µPa).
The troposphere is the densest layer in Uranus’ atmosphere. Here, the temperature ranges from 320 K (46.85 °C/116 °F) at the base (-300 km) to 53 K (-220 °C/-364 °F) at 50 km, with the upper region being the coldest in the solar system. The tropopause region is responsible for the vast majority of Uranus’s thermal infrared emissions, thus determining its effective temperature of 59.1 ± 0.3 K.
Within the troposphere are layers of clouds – water clouds at the lowest pressures, with ammonium hydrosulfide clouds above them. Ammonia and hydrogen sulfide clouds come next. Finally, thin methane clouds lay on the top.
In the stratosphere, temperatures range from 53 K (-220 °C/-364 °F) at the upper level to between 800 and 850 K (527 – 577 °C/980 – 1070 °F) at the base of the thermosphere, thanks largely to heating caused by solar radiation. The stratosphere contains ethane smog, which may contribute to the planet’s dull appearance. Acetylene and methane are also present, and these hazes help warm the stratosphere.
Uranus, as imaged by the Hubble Space Telescope. Image credit: NASA/Hubble
The outermost layer, the thermosphere and corona, extend from 4,000 km to as high as 50,000 km from the surface. This region has a uniform temperature of 800-850 (577 °C/1,070 °F), although scientists are unsure as to the reason. Because the distance to Uranus from the Sun is so great, the amount of sunlight absorbed cannot be the primary cause.
Like Jupiter and Saturn, Uranus’s weather follows a similar pattern where systems are broken up into bands that rotate around the planet, which are driven by internal heat rising to the upper atmosphere. As a result, winds on Uranus can reach up to 900 km/h (560 mph), creating massive storms like the one spotted by the Hubble Space Telescope in 2012. Similar to Jupiter’s Great Red Spot, this “Dark Spot” was a giant cloud vortex that measured 1,700 kilometers by 3,000 kilometers (1,100 miles by 1,900 miles).
Neptune’s Atmosphere:
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown constituent is thought to contribute to Neptune’s more intense coloring.
Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.
A modified color/contrast image emphasizing Neptune’s atmospheric features, including wind speed. Credit Erich Karkoschka)
For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.
This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.
The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter life span than Jupiter’s.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot – a nickname that first arose during the months leading up to the Voyager 2 encounter in 1989. The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.
In sum, the planet’s of our Solar System all have atmospheres of sorts. And compared to Earth’s relatively balmy and thick atmosphere, they run the gamut between very very thin to very very dense. They also range in temperatures from the extremely hot (like on Venus) to the extreme freezing cold.
And when it comes to weather systems, things can equally extreme, with planet’s boasting either weather at all, or intense cyclonic and dust storms that put storms here n Earth to shame. And whereas some are entirely hostile to life as we know it, others we might be able to work with.
Planet Mercury as seen from the MESSENGER spacecraft in 2008. Credit: NASA/JPL
Virtually every planet in the Solar System has moons. Earth has The Moon, Mars has Phobos and Deimos, and Jupiter and Saturn have 67 and 62 officially named moons, respectively. Heck, even the recently-demoted dwarf planet Pluto has five confirmed moons – Charon, Nix, Hydra, Kerberos and Styx. And even asteroids like 243 Ida may have satellites orbiting them (in this case, Dactyl). But what about Mercury?
If moons are such a common feature in the Solar System, why is it that Mercury has none? Yes, if one were to ask how many satellites the planet closest to our Sun has, that would be the short answer. But answering it more thoroughly requires that we examine the process through which other planets acquired their moons, and seeing how these apply (or fail to apply) to Mercury.
Gravity is a fundamental force of physics, one which we Earthlings tend to take for granted. You can’t really blame us. Having evolved over the course of billions of years in Earth’s environment, we are used to living with the pull of a steady 1 g (or 9.8 m/s²). However, for those who have gone into space or set foot on the Moon, gravity is a very tenuous and precious thing.
Basically, gravity is dependent on mass, where all things – from stars, planets, and galaxies to light and sub-atomic particles – are attracted to one another. Depending on the size, mass and density of the object, the gravitational force it exerts varies. And when it comes to the planets of our Solar System, which vary in size and mass, the strength of gravity on their surfaces varies considerably.
Planets and other objects in our Solar System. Credit: NASA.
First the quick facts: Our Solar System has eight “official” planets which orbit the Sun. Here are the planets listed in order of their distance from the Sun:
If you add in the dwarf planets, Ceres is located in the asteroid belt between Mars and Jupiter, while the remaining dwarf planets are in the outer Solar System and in order from the Sun are Pluto, Haumea, Makemake, and Eris. There is, as yet, a bit of indecision about the Trans-Neptunian Objects known as Orcus, Quaoar, 2007 O10, and Sedna and their inclusion in the dwarf planet category.
A mnemonic for this list would be “My Very Educated Mother Could Just Serve Us Noodles, Pie, Ham, Muffins, and Eggs” (and Steak, if Sedna is included.) You can find more tricks for remembering the order of the planets at our detailed article here.
Now, let’s look at a few details including the definition of a planet and a dwarf planet, as well as details about each of the planets in our Solar System.
Artistic impression of the Solar System, with all known terrestrial planets, as giants, and dwarf planets. Credit: NASA
What is a Planet?
In 2006, the International Astronomical Union (IAU) decided on the definition of a planet. The definition states that in our Solar System, a planet is a celestial body which:
is in orbit around the Sun,
has sufficient mass to assume hydrostatic equilibrium (a nearly round shape),
has “cleared the neighborhood” around its orbit.
is not a moon.
This means that Pluto, which was considered to be the farthest planet since its discovery in 1930, now is classified as a dwarf planet. The change in the definition came after the discovery three bodies that were all similar to Pluto in terms of size and orbit, (Quaoar in 2002, Sedna in 2003, and Eris in 2005).
With advances in equipment and techniques, astronomers knew that more objects like Pluto would very likely be discovered, and so the number of planets in our Solar System would start growing quickly. It soon became clear that either they all had to be called planets or Pluto and bodies like it would have to be reclassified.
With much controversy then and since, Pluto was reclassified as a dwarf planet in 2006. This also reclassified the asteroid Ceres as a dwarf planet, too, and so the first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens more dwarf planets awaiting discovery.
Later, in 2008, the IAU announced the subcategory of dwarf planets with trans-Neptunian orbits would be known as “plutoids.” Said the IAU, “Plutoids are celestial bodies in orbit around the Sun at a distance greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighborhood around their orbit.”
This subcategory includes Ceres, Pluto, Haumea, Makemake, and Eris.
The Planets in our Solar System:
Having covered the basics of definition and classification, let’s get talking about those celestial bodies in our Solar System that are still classified as planets (sorry Pluto!). Here is a brief look at the eight planets in our Solar System. Included are quick facts and links so you can find out more about each planet.
Mercury: Mercury is the closest planet to our Sun, at just 58 million km (36 million miles) or 0.39 Astronomical Unit (AU) out. But despite its reputation for being sun-baked and molten, it is not the hottest planet in our Solar System (scroll down to find out who that dubious honor goes go!)
Mercury, as imaged by the MESSENGER spacecraft, revealing parts of the never seen by human eyes. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
Mercury is also the smallest planet in our Solar System, and is also smaller than its largest moon (Ganymede, which orbits Jupiter). And being equivalent in size to 0.38 Earths, it is just slightly larger than the Earth’s own Moon. But this may have something to do with its incredible density, being composed primarily of rock and iron ore. Here are the planetary facts:
Diameter: 4,879 km (3,032 miles)
Mass: 3.3011 x 1023 kg (0.055 Earths)
Length of Year (Orbit): 87.97 Earth days
Length of Day: 59 Earth days.
Mercury is a rocky planet, one of the four “terrestrial planets” in our Solar System. Mercury has a solid, cratered surface, and looks much like Earth’s moon.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mercury.
Mercury does not have any moons.
Temperatures on Mercury range between -173 to 427 degrees Celcius (-279 to 801 degrees Fahrenheit)
Just two spacecraft have visited Mercury: Mariner 10 in 1974-75 and MESSENGER, which flew past Mercury three times before going into orbit around Mercury in 2011 and ended its mission by impacting the surface of Mercury on April 30, 2015. MESSENGER has changed our understanding of this planet, and scientists are still studying the data.
Venus:
Venus is the second closest planet to our Sun, orbiting at an average distance of 108 million km (67 million miles) or 0.72 AU. Venus is often called Earth’s “sister planet,” as it is just a little smaller than Earth. Venus is 81.5% as massive as Earth, and has 90% of its surface area and 86.6% of its volume. The surface gravity, which is 8.87 m/s², is equivalent to 0.904 g – roughly 90% of the Earth standard.
A radar view of Venus taken by the Magellan spacecraft, with some gaps filled in by the Pioneer Venus orbiter. Credit: NASA/JPL
And due to its thick atmosphere and proximity to the Sun, it is the Solar Systems hottest planet, with temperatures reaching up to a scorching 735 K (462 °C). To put that in perspective, that’s over four and a half times the amount of heat needed to evaporate water, and about twice as much needed to turn tin into molten metal (231.9 °C)!
Diameter: 7,521 miles (12,104 km)
Mass: 4.867 x 1024 kg (0.815 Earth mass)
Length of Year (Orbit): 225 days
Length of day: 243 Earth days
Surface temperature: 462 degrees C (864 degrees F)
Venus’ thick and toxic atmosphere is made up mostly of carbon dioxide (CO2) and nitrogen (N2), with clouds of sulfuric acid (H2SO4) droplets.
Venus has no moons.
Venus spins backwards (retrograde rotation), compared to the other planets. This means that the sun rises in the west and sets in the east on Venus.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Venus.
Venus is also known and the “morning star” or “evening star” because it is often brighter than any other object in the sky and is usually seen either at dawn or at dusk. Since it is so bright, it has often been mistaken for a UFO!
More than 40 spacecraft have explored Venus. The Magellan mission in the early 1990s mapped 98 percent of the planet’s surface. Find out more about all the missions here.
Earth: Our home, and the only planet in our Solar System (that we know of) that actively supports life. Our planet is the third from the our Sun, orbiting it at an average distance of 150 million km (93 million miles) from the Sun, or one AU. Given the fact that Earth is where we originated, and has all the necessary prerequisites for supporting life, it should come as no surprise that it is the metric on which all others planets are judged.
Earth, pictured by the crew of the Apollo 17 mission. Credit: NASA
Whether it is gravity (g), distance (measured in AUs), diameter, mass, density or volume, the units are either expressed in terms of Earth’s own values (with Earth having a value of 1) or in terms of equivalencies – i.e. 0.89 times the size of Earth. Here’s a rundown of the kinds of
Diameter: 12,760 km (7,926 miles)
Mass: 5.97 x 1024 kg
Length of Year (Orbit): 365 days
Length of day: 24 hours (more precisely, 23 hours, 56 minutes and 4 seconds.)
Surface temperature: Average is about 14 C, (57 F), with ranges from -88 to 58 (min/max) C (-126 to 136 F).
Earth is another terrestrial planet with an ever-changing surface, and 70 percent of the Earth’s surface is covered in oceans.
Earth has one moon.
Earth’s atmosphere is 78% nitrogen, 21% oxygen, and 1% various other gases.
Mars: Mars is the fourth planet from the sun at a distance of about 228 million km (142 million miles) or 1.52 AU. It is also known as “the Red Planet” because of its reddish hue, which is due to the prevalence of iron oxide on its surface. In many ways, Mars is similar to Earth, which can be seen from its similar rotational period and tilt, which in turn produce seasonal cycles that are comparable to our own.
Global image of the planet Mars. Credit: NASA
The same holds true for surface features. Like Earth, Mars has many familiar surface features, which include volcanoes, valleys, deserts, and polar ice caps. But beyond these, Mars and Earth have little in common. The Martian atmosphere is too thin and the planet too far from our Sun to sustain warm temperatures, which average 210 K (-63 ºC) and fluctuate considerably.
Diameter: 6,787 km, (4,217 miles)
Mass: 6.4171 x 1023 kg (0.107 Earths)
Length of Year (Orbit): 687 Earth days.
Length of day: 24 hours 37 minutes.
Surface temperature: Average is about -55 C (-67 F), with ranges of -153 to +20 °C (-225 to +70 °F)
Mars is the fourth terrestrial planet in our Solar System. Its rocky surface has been altered by volcanoes, impacts, and atmospheric effects such as dust storms.
Mars has a thin atmosphere made up mostly of carbon dioxide (CO2), nitrogen (N2) and argon (Ar).If you weigh 45 kg (100 pounds) on Earth, you would weigh 17 kg (38 pounds) on Mars.
Mars has two small moons, Phobos and Deimos.
Mars is known as the Red Planet because iron minerals in the Martian soil oxidize, or rust, causing the soil to look red.
Jupiter: Jupiter is the fifth planet from the Sun, at a distance of about 778 million km (484 million miles) or 5.2 AU. Jupiter is also the most massive planet in our Solar System, being 317 times the mass of Earth, and two and half times larger than all the other planets combined. It is a gas giant, meaning that it is primarily composed of hydrogen and helium, with swirling clouds and other trace gases.
Io and Jupiter as seen by New Horizons during its 2008 flyby. (Credit: NASA/Johns Hopkins University APL/SWRI).
Jupiter’s atmosphere is the most intense in the Solar System. In fact, the combination of incredibly high pressure and coriolis forces produces the most violent storms ever witnessed. Wind speeds of 100 m/s (360 km/h) are common and can reach as high as 620 km/h (385 mph). In addition, Jupiter experiences auroras that are both more intense than Earth’s, and which never stop.
Diameter: 428,400 km (88,730 miles)
Mass: 1.8986 × 1027 kg (317.8 Earths)
Length of Year (Orbit): 11.9 Earth years
Length of day: 9.8 Earth hours
Temperature: -148 C, (-234 F)
Jupiter has 67 known moons, with an additional 17 moons awaiting confirmation of their discovery – for a total of 67 moons. Jupiter is almost like a mini solar system!
Jupiter has a faint ring system, discovered in 1979 by the Voyager 1 mission.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 115 kg (253) pounds on Jupiter.
Jupiter’s Great Red Spot is a gigantic storm (bigger than Earth) that has been raging for hundreds of years. However, it appears to be shrinking in recent years.
Many missions have visited Jupiter and its system of moons, with the latest being the Juno mission will arrive at Jupiter in 2016. You can find out more about missions to Jupiter here.
Saturn’s relatively thin main rings are about 250,000 km (156,000 miles) in diameter. (Image: NASA/JPL-Caltech/SSI/J. Major)
Saturn: Saturn is the sixth planet from the Sun at a distance of about 1.4 billion km (886 million miles) or 9.5 AU. Like Jupiter, it is a gas giant, with layers of gaseous material surrounding a solid core. Saturn is most famous and most easily recognized for its spectacular ring system, which is made of seven rings with several gaps and divisions between them.
Diameter: 120,500 km (74,900 miles)
Mass: 5.6836 x 1026 kg (95.159 Earths)
Length of Year (Orbit): 29.5 Earth years
Length of day: 10.7 Earth hours
Temperature: -178 C (-288 F)
Saturn’s atmosphere is made up mostly of hydrogen (H2) and helium (He).
If you weigh 45 kg (100 pounds) on Earth, you would weigh about 48 kg (107 pounds) on Saturn
Saturn has 53 known moons with an additional 9 moons awaiting confirmation.
Five missions have gone to Saturn. Since 2004, Cassini has been exploring Saturn, its moons and rings. You can out more about missions to Saturn here.
Uranus: Uranus is the seventh planet from the sun at a distance of about 2.9 billion km (1.8 billion miles) or 19.19 AU. Though it is classified as a “gas giant”, it is often referred to as an “ice giant” as well, owing to the presence of ammonia, methane, water and hydrocarbons in ice form. The presence of methane ice is also what gives it its bluish appearance.
Uranus as seen by NASA’s Voyager 2 space probe. Credit: NASA/JPL
Uranus is also the coldest planet in our Solar System, making the term “ice” seem very appropriate! What’s more, its system of moons experience a very odd seasonal cycle, owing to the fact that they orbit Neptune’s equator, and Neptune orbits with its north pole facing directly towards the Sun. This causes all of its moons to experience 42 year periods of day and night.
Diameter: 51,120 km (31,763 miles)
Mass:
Length of Year (Orbit): 84 Earth years
Length of day: 18 Earth hours
Temperature: -216 C (-357 F)
Most of the planet’s mass is made up of a hot dense fluid of “icy” materials – water (H2O), methane (CH4). and ammonia (NH3) – above a small rocky core.
Uranus has an atmosphere which is mostly made up of hydrogen (H2) and helium (He), with a small amount of methane (CH4). The methane gives Uranus a blue-green tint.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 41 kg (91 pounds) on Uranus.
Uranus has 27 moons.
Uranus has faint rings; the inner rings are narrow and dark and the outer rings are brightly colored.
Voyager 2 is the only spacecraft to have visited Uranus. Find out more about this mission here.
Neptune: Neptune is the eighth and farthest planet from the Sun, at a distance of about 4.5 billion km (2.8 billion miles) or 30.07 AU. Like Jupiter, Saturn and Uranus, it is technically a gas giant, though it is more properly classified as an “ice giant” with Uranus.
Neptune photographed by the Voyager 2 space probe. Credit: NASA/JPL
Due to its extreme distance from our Sun, Neptune cannot be seen with the naked eye, and only one mission has ever flown close enough to get detailed images of it. Nevertheless, what we know about it indicates that it is similar in many respects to Uranus, consisting of gases, ices, methane ice (which gives its color), and has a series of moons and faint rings.
Diameter: 49,530 km (30,775 miles)
Mass: 1.0243 x 1026 kg (17 Earths)
Length of Year (Orbit): 165 Earth years
Length of day: 16 Earth hours
Temperature: -214 C (-353 F)
Neptune is mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core.
Neptune’s atmosphere is made up mostly of hydrogen (H2), helium (He) and methane (CH4).
Neptune has 13 confirmed moons and 1 more awaiting official confirmation.
Neptune has six rings.
If you weigh 45 kg (100 pounds) on Earth, you would weigh 52 kg (114 pounds) on Neptune.
Neptune was the first planet to be predicted to exist by using math.
Voyager 2 is the only spacecraft to have visited Neptune. You can find out more about this mission here.
Find out more about Neptune at this series of articles on Universe Today and this NASA webpage. We have written many articles about the planets for Universe Today. Here are some facts about planets, and here’s an article about the names of the planets.If you’d like more info on the Solar System planets, dwarf planets, asteroids and more, check out NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator.We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.Venus is the second planet from the Sun, and it is the hottest planet in the Solar System due to its thick, toxic atmosphere which has been described as having a “runaway greenhouse effect” on the planet.
Now you know! And if you find yourself unable to remember all the planets in their proper order, just repeat the words, “My Very Educated Mother Just Served Us Noodles.” Of course, the Pie, Ham, Muffins and Eggs are optional, as are any additional courses that might be added in the coming years!
Portrait of Galileo Galilei by Giusto Sustermans (1636). Credit: nmm.ac.uk
When it comes to scientists who revolutionized the way we think of the universe, few names stand out like Galileo Galilei. A noted inventor, physicist, engineer and astronomer, Galileo was one of the greatest contributors to the Scientific Revolution. He build telescopes, designed a compass for surveying and military use, created a revolutionary pumping system, and developed physical laws that were the precursors of Newton’s law of Universal Gravitation and Einstein’s Theory of Relativity.
But it was within the field of astronomy that Galileo made his most enduring impact. Using telescopes of his own design, he discovered Sunspots, the largest moons of Jupiter, surveyed The Moon, and demonstrated the validity of Copernicus’ heliocentric model of the universe. In so doing, he helped to revolutionize our understanding of the cosmos, our place in it, and helped to usher in an age where scientific reasoning trumped religious dogma.
Early Life:
Galileo was born in Pisa, Italy, in 1564, into a noble but poor family. He was the first of six children of Vincenzo Galilei and Giulia Ammannati, who’s father also had three children out of wedlock. Galileo was named after an ancestor, Galileo Bonaiuti (1370 – 1450), a noted physician, university teacher and politician who lived in Florence.
His father, a famous lutenist, composer and music theorist, had a great impact on Galileo; transmitting not only his talent for music, but skepticism of authority, the value of experimentation, and the value of measures of time and rhythm to achieve success.
The Camaldolese Monastery at Vallombrosa, 35 km southeast of Florence, where Galileo was educated until 1581. Credit: nobility.org
In 1572, when Galileo Galilei was eight, his family moved to Florence, leaving Galileo with his uncle Muzio Tedaldi (related to his mother through marriage) for two years.When he reached the age of ten, Galileo left Pisa to join his family in Florence and was tutored by Jacopo Borghini -a mathematician and professor from the university of Pisa.
Once he was old enough to be educated in a monastery, his parents sent him to the Camaldolese Monastery at Vallombrosa, located 35 km southeast of Florence. The Order was independent from the Benedictines, and combined the solitary life of the hermit with the strict life of a monk. Galileo apparently found this life attractive and intending to join the Order, but his father insisted that he study at the University of Pisa to become a doctor.
Education:
While at Pisa, Galileo began studying medicine, but his interest in the sciences quickly became evident. In 1581, he noticed a swinging chandelier, and became fascinated by the timing of its movements. To him, it became clear that the amount of time, regardless of how far it was swinging, was comparable to the beating of his heart.
When he returned home, he set up two pendulums of equal length, swinging one with a large sweep and the other with a small sweep, and found that they kept time together. These observations became the basis of his later work with pendulums to keep time – work which would also be picked up almost a century later when Christiaan Huygens designed the first officially-recognized pendulum clock.
Galileo Demonstrating the New Astronomical Theories at the University of Padua, by Félix Parra (1873). Credit: munal.gob.mx
Shortly thereafter, Galileo accidentally attended a lecture on geometry, and talked his reluctant father into letting his study mathematics and natural philosophy instead of medicine. From that point onward, he began a steady processes of inventing, largely for the sake of appeasing his father’s desire for him to make money to pay off his siblings expenses (particularly those of his younger brother, Michelagnolo).
In 1589, Galileo was appointed to the chair of mathematics at the University of Pisa. In 1591, his father died, and he was entrusted with the care of his younger siblings. Being Professor of Mathematics at Pisa was not well paid, so Galileo lobbied for a more lucrative post. In 1592, this led to his appointment to the position of Professor of Mathematics at the University of Padua, where he taught Euclid’s geometry, mechanics, and astronomy until 1610.
During this period, Galileo made significant discoveries in both pure fundamental science as well as practical applied science. His multiple interests included the study of astrology, which at the time was a discipline tied to the studies of mathematics and astronomy. It was also while teaching the standard (geocentric) model of the universe that his interest in astronomy and the Copernican theory began to take off.
Telescopes:
In 1609, Galileo received a letter telling him about a spyglass that a Dutchman had shown in Venice. Using his own technical skills as a mathematician and as a craftsman, Galileo began to make a series of telescopes whose optical performance was much better than that of the Dutch instrument.
Galileo Galilei’s telescope with his handwritten note specifying the magnifying power of the lens, at an exhibition at The Franklin Institute in Philadelphia. Credit: AP Photo/Matt Rourke
As he would later write in his 1610 tract Sidereus Nuncius (“The Starry Messenger”):
“About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby. Of this truly remarkable effect several experiences were related, to which some persons believed while other denied them. A few days later the report was confirmed by a letter I received from a Frenchman in Paris, Jacques Badovere, which caused me to apply myself wholeheartedly to investigate means by which I might arrive at the invention of a similar instrument. This I did soon afterwards, my basis being the doctrine of refraction.”
His first telescope – which he constructed between June and July of 1609 – was made from available lenses and had a three-powered spyglass. To improve upon this, Galileo learned how to grind and polish his own lenses. By August, he had created an eight-powered telescope, which he presented to the Venetian Senate.
By the following October or November, he managed to improve upon this with the creation a twenty-powered telescope. Galileo saw a great deal of commercial and military applications of his instrument(which he called a perspicillum) for ships at sea. However, in 1610, he began turning his telescope to the heavens and made his most profound discoveries.
Galileo Galilei showing the Doge of Venice how to use the telescope, by Giuseppe Bertini (1858). Credit: gabrielevanin.it
Achievements in Astronomy:
Using his telescope, Galileo began his career in astronomy by gazing at the Moon, where he discerned patterns of uneven and waning light. While not the first astronomer to do this, Galileo artistic’s training and knowledge of chiaroscuro – the use of strong contrasts between light and dark – allowed him to correctly deduce that these light patterns were the result of changes in elevation. Hence, Galileo was the first astronomer to discover lunar mountains and craters.
In The Starry Messenger, he also made topographical charts, estimating the heights of these mountains. In so doing, he challenged centuries of Aristotelian dogma that claimed that Moon, like the other planets, was a perfect, translucent sphere. By identifying that it had imperfections, in the forms of surface features, he began advancing the notion that the planets were similar to Earth.
Galileo also recorded his observations about the Milky Way in the Starry Messenger, which was previously believed to be nebulous. Instead, Galileo found that it was a multitude of stars packed so densely together that it appeared from a distance to look like clouds. He also reported that whereas the telescope resolved the planets into discs, the stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope – thus suggesting that they were much farther away than previously thought.
Using his telescopes, Galileo also became one the first European astronomer to observe and study sunspots. Though there are records of previous instances of naked eye observations – such as in China (ca. 28 BCE), Anaxagoras in 467 BCE, and by Kepler in 1607 – they were not identifies as being imperfections on the surface of the Sun. In many cases, such as Kepler’s, it was thought that the spots were transits of Mercury.
In addition, there is also controversy over who was the first to observe sunspots during the 17th century using a telescope. Whereas Galileo is believed to have observed them in 1610, he did not publish about them and only began speaking to astronomers in Rome about them by the following year. In that time, German astronomer Christoph Scheiner had been reportedly observing them using a helioscope of his own design.
At around the same time, the Frisian astronomers Johannes and David Fabricius published a description of sunspots in June 1611. Johannes book, De Maculis in Sole Observatis (“On the Spots Observed in the Sun”) was published in autumn of 1611, thus securing credit for him and his father.
In any case, it was Galileo who properly identified sunspots as imperfections on the surface of the Sun, rather than being satellites of the Sun – an explanation that Scheiner, a Jesuit missionary, advanced in order to preserve his beliefs in the perfection of the Sun.
Using a technique of projecting the Sun’s image through the telescope onto a piece of paper, Galileo deduced that sunspots were, in fact, on the surface of the Sun or in its atmosphere. This presented another challenge to the Aristotelian and Ptolemaic view of the heavens, since it demonstrated that the Sun itself had imperfections.
On January 7th, 1610, Galileo pointed his telescope towards Jupiter and observed what he described in Nuncius as “three fixed stars, totally invisible by their smallness” that were all close to Jupiter and in line with its equator. Observations on subsequent nights showed that the positions of these “stars” had changed relative to Jupiter, and in a way that was not consistent with them being part of the background stars.
The Galilean moons, shown to scale – Io (top right), Europa (upper left), Ganymede (right) and Callisto (bottom left). Credit: NASA/JPL
By January 10th, he noted that one had disappeared, which he attributed to it being hidden behind Jupiter. From this, he concluded that the stars were in fact orbiting Jupiter, and they were satellites of it. By January 13th, he discovered a fourth, and named them the Medicean stars, in honor of his future patron, Cosimo II de’ Medici, Grand Duke of Tuscany, and his three brothers.
Later astronomers, however, renamed them the Galilean Moons in honour of their discoverer. By the 20th century, these satellites would come to be known by their current names – Io, Europa, Ganymede, and Callisto – which had been suggested by 17th century German astronomer Simon Marius, apparently at the behest of Johannes Kepler.
Galileo’s observations of these satellites proved to be another major controversy. For the first time, a planet other than Earth was shown to have satellites orbiting it, which constituted yet another nail in the coffin of the geocentric model of the universe. His observations were independently confirmed afterwards, and Galileo continued to observe the satellites them and even obtained remarkably accurate estimates for their periods by 1611.
Heliocentrism:
Galileo’s greatest contribution to astronomy came in the form of his advancement of the Copernican model of the universe (i.e. heliocentrism). This began in 1610 with his publication of Sidereus Nuncius, which brought the issue of celestial imperfections before a wider audience. His work on sunspots and his observation of the Galilean Moons furthered this, revealing yet more inconsistencies in the currently accepted view of the heavens.
Galileo’s Sidereus Nuncius (“Starry Messenger”), published in 1610, laid out his observations of the moon’s surface, which included mountains and impact craters. Credit: brunelleschi.imss.fi.it
Other astronomical observations also led Galileo to champion the Copernican model over the traditional Aristotelian-Ptolemaic (aka. geocentric) view. From September 1610 onward, Galileo began observing Venus, noting that it exhibited a full set of phases similar to that of the Moon. The only explanation for this was that Venus was periodically between the Sun and Earth; while at other times, it was on the opposite side of the Sun.
According to the geocentric model of the universe, this should have been impossible, as Venus’ orbit placed it closer to Earth than the Sun – where it could only exhibit crescent and new phases. However, Galileo’s observations of it going through crescent, gibbous, full and new phases was consistent with the Copernican model, which established that Venus orbited the Sun within the Earth’s orbit.
These and other observations made the Ptolemaic model of the universe untenable. Thus, by the early 17th century, the great majority of astronomers began to convert to one of the various geo-heliocentric planetary models – such as the Tychonic, Capellan and Extended Capellan models. These all had the virtue of explaining problems in the geocentric model without engaging in the “heretical” notion that Earth revolved around the Sun.
In 1632, Galileo addressed the “Great Debate” in his treatise Dialogo sopra i due massimi sistemi del mondo (Dialogue Concerning the Two Chief World Systems), in which he advocated the heliocentric model over the geocentric. Using his own telescopic observations, modern physics and rigorous logic, Galileo’s arguments effectively undermined the basis of Aristotle and Ptolemy’s system for a growing and receptive audience.
Frontispiece and title page of the Dialogue, 1632. Credit: moro.imss.fi.it
In the meantime, Johannes Kepler correctly identified the sources of tides on Earth – something which Galileo had become interesting in himself. But whereas Galileo attributed the ebb and flow of tides to the rotation of the Earth, Kepler ascribed this behavior to the influence of the Moon.
Combined with his accurate tables on the elliptical orbits of the planets (something Galileo rejected), the Copernican model was effectively proven. From the middle of the seventeenth century onward, there were few astronomers who were not Copernicans.
The Inquisition and House Arrest:
As a devout Catholic, Galileo often defended the heliocentric model of the universe using Scripture. In 1616, he wrote a letter to the Grand Duchess Christina, in which he argued for a non-literal interpretation of the Bible and espoused his belief in the heliocentric universe as a physical reality:
“I hold that the Sun is located at the center of the revolutions of the heavenly orbs and does not change place, and that the Earth rotates on itself and moves around it. Moreover … I confirm this view not only by refuting Ptolemy’s and Aristotle’s arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.“
More importantly, he argued that the Bible is written in the language of the common person who is not an expert in astronomy. Scripture, he argued, teaches us how to go to heaven, not how the heavens go.
Galileo facing the Roman Inquisition. by Cristiano Banti’s (1857). Credit: law.umkc.edu
Initially, the Copernican model of the universe was not seen as an issue by the Roman Catholic Church or it’s most important interpreter of Scripture at the time – Cardinal Robert Bellarmine. However, in the wake of the Counter-Reformation, which began in 1545 in response to the Reformation, a more stringent attitude began to emerge towards anything seen as a challenge to papal authority.
Eventually, matters came to a head in 1615 when Pope Paul V (1552 – 1621) ordered that the Sacred Congregation of the Index (an Inquisition body charged with banning writings deemed “heretical”) make a ruling on Copernicanism. They condemned the teachings of Copernicus, and Galileo (who had not been personally involved in the trial) was forbidden to hold Copernican views.
However, things changed with the election of Cardinal Maffeo Barberini (Pope Urban VIII) in 1623. As a friend and admirer of Galileo’s, Barberini opposed the condemnation of Galileo, and gave formal authorization and papal permission for the publication of Dialogue Concerning the Two Chief World Systems.
However, Barberini stipulated that Galileo provide arguments for and against heliocentrism in the book, that he be careful not to advocate heliocentrism, and that his own views on the matter be included in Galileo’s book. Unfortunately, Galileo’s book proved to be a solid endorsement of heliocentrism and offended the Pope personally.
Portrait of Galileo gazing at the words “E pur si muove” scratched on the wall of his prison cell, attributed to Bartolomé Esteban Murillo (1618-1682). Credit: Wikipedia Commons
In it, the character of Simplicio, the defender of the Aristotelian geocentric view, is portrayed as an error-prone simpleton. To make matter worse, Galileo had the character Simplicio enunciate the views of Barberini at the close of the book, making it appear as though Pope Urban VIII himself was a simpleton and hence the subject of ridicule.
As a result, Galileo was brought before the Inquisition in February of 1633 and ordered to renounce his views. Whereas Galileo steadfastly defended his position and insisted on his innocence, he was eventually threatened with torture and declared guilty. The sentence of the Inquisition, delivered on June 22nd, contained three parts – that Galileo renounce Copernicanism, that he be placed under house arrest, and that the Dialogue be banned.
According to popular legend, after recanting his theory publicly that the Earth moved around the Sun, Galileo allegedly muttered the rebellious phrase: “E pur si muove” (“And yet it moves” in Latin). After a period of living with his friend, the Archbishop of Siena, Galileo returned to his villa at Arcetri (near Florence in 1634), where he spent the remainder of his life under house arrest.
Other Accomplishments:
In addition to his revolutionary work in astronomy and optics, Galileo is also credited with the invention of many scientific instruments and theories. Much of the devices he created were for the specific purpose of earning money to pay for his sibling’s expenses. However, they would also prove to have a profound impact in the fields of mechanics, engineering, navigation, surveying, and warfare.
Galileo’s La Billancetta, in which he describes a method for hydrostatic balance. Credit: Museo Galileo
In 1586, at the age of 22, Galileo made his first groundbreaking invention. Inspired by the story of Archimedes and his “Eureka” moment, Galileo began looking into how jewelers weighed precious metals in air and then by displacement to determine their specific gravity. Working from this, he eventually theorized of a better method, which he described in a treatise entitled La Bilancetta (“The Little Balance”).
In this tract, he described an accurate balance for weighing things in air and water, in which the part of the arm on which the counter weight was hung was wrapped with metal wire. The amount by which the counterweight had to be moved when weighing in water could then be determined very accurately by counting the number of turns of the wire. In so doing, the proportion of metals like gold to silver in the object could be read off directly.
In 1592, when Galileo was a professor of mathematics at the University of Padua, he made frequent trips to the Arsenal – the inner harbor where Venetian ships were outfitted. The Arsenal had been a place of practical invention and innovation for centuries, and Galileo used the opportunity to study mechanical devices in detail.
In 1593, he was consulted on the placement of oars in galleys and submitted a report in which he treated the oar as a lever and correctly made the water the fulcrum. A year later the Venetian Senate awarded him a patent for a device for raising water that relied on a single horse for the operation. This became the basis of modern pumps.
A replica of the earliest surviving telescope attributed to Galileo Galilei, on display at the Griffith Observatory. Credit: Wikipedia Commons/Mike Dunn
To some, Galileo’s Pump was a merely an improvement on the Archimedes Screw, which was first developed in the third century BCE and patented in the Venetian Republic in 1567. However, there is no apparent evidence connecting Galileo’s invention to Archimedes’ earlier and less sophisticated design.
In ca. 1593, Galileo constructed his own version of a thermoscope, a forerunner of the thermometer, that relied on the expansion and contraction of air in a bulb to move water in an attached tube. Over time, he and his colleagues worked to develop a numerical scale that would measure the heat based on the expansion of the water inside the tube.
The cannon, which was first introduced to Europe in 1325, had become a mainstay of war by Galileo’s time. Having become more sophisticated and mobile, gunners needed instruments to help them coordinate and calculate their fire. As such, between 1595 and 1598, Galileo devised an improved geometric and military compass for use by gunners and surveyors.
During the 16th century, Aristotelian physics was still the predominant way of explaining the behavior of bodies near the Earth. For example, it was believed that heavy bodies sought their natural place of rest – i.e at the center of things. As a result, no means existed to explain the behavior of pendulums, where a heavy body suspended from a rope would swing back and forth and not seek rest in the middle.
The Sector, a military/geometric compass designed by Galileo Galilei. Credit: chsi.harvard.edu
Already, Galileo had conducted experiments that demonstrated that heavier bodies did not fall faster than lighter ones – another belief consistent with Aristotelian theory. In addition, he also demonstrated that objects thrown into the air travel in parabolic arcs. Based on this and his fascination with the back and forth motion of a suspended weight, he began to research pendulums in 1588.
In 1602, he explained his observations in a letter to a friend, in which he described the principle of isochronism. According to Galileo, this principle asserted that the time it takes for the pendulum to swing is not linked to the arc of the pendulum, but rather the pendulum’s length. Comparing two pendulum’s of similar length, Galileo demonstrated that they would swing at the same speed, despite being pulled at different lengths.
According to Vincenzo Vivian, one of Galileo’s contemporaries, it was in 1641 while under house arrest that Galileo created a design for a pendulum clock. Unfortunately, being blind at the time, he was unable to complete it before his death in 1642. As a result, Christiaan Huygens’ publication of Horologrium Oscillatoriumin 1657 is recognized as the first recorded proposal for a pendulum clock.
Death and Legacy:
Galileo died on January 8th, 1642, at the age of 77, due to fever and heart palpitations that had taken a toll on his health. The Grand Duke of Tuscany, Ferdinando II, wished to bury him in the main body of the Basilica of Santa Croce, next to the tombs of his father and other ancestors, and to erect a marble mausoleum in his honor.
Tomb of Galileo Galilei in the Santa Croce Basilica in Florence, Italy. Credit: Wikipedia Commons/stanthejeep
However, Pope Urban VIII objected on the basis that Galileo had been condemned by the Church, and his body was instead buried in a small room next to the novice’s chapel in the Basilica. However, after his death, the controversy surrounding his works and heliocentricm subsided, and the Inquisitions ban on his writing’s was lifted in 1718.
In 1737, his body was exhumed and reburied in the main body of the Basilica after a monument had been erected in his honor. During the exhumation, three fingers and a tooth were removed from his remains. One of these fingers, the middle finger from Galileo’s right hand, is currently on exhibition at the Museo Galileo in Florence, Italy.
In 1741, Pope Benedict XIV authorized the publication of an edition of Galileo’s complete scientific works which included a mildly censored version of the Dialogue. In 1758, the general prohibition against works advocating heliocentrism was removed from the Index of prohibited books, although the specific ban on uncensored versions of the Dialogue and Copernicus’s De Revolutionibusorbium coelestium (“On the Revolutions of the Heavenly Spheres“) remained.
All traces of official opposition to heliocentrism by the church disappeared in 1835 when works that espoused this view were finally dropped from the Index. And in 1939, Pope Pius XII described Galileo as being among the “most audacious heroes of research… not afraid of the stumbling blocks and the risks on the way, nor fearful of the funereal monuments”.
A bust of Galileo at the Museo Galileo in Florence, Italy, where recovered parts of his body and many of his possessions are on display. Credit: NYT/Kathryn Cook
On October 31st, 1992, Pope John Paul II expressed regret for how the Galileo affair was handled, and issued a declaration acknowledging the errors committed by the Catholic Church tribunal. The affair had finally been put to rest and Galileo exonerated, though certain unclear statements issued by Pope Benedict XVI have led to renewed controversy and interest in recent years.
Alas, when it comes to the birth of modern science and those who helped create it, Galileo’s contributions are arguably unmatched. According to Stephen Hawking and Albert Einstein, Galileo was the father of modern science, his discoveries and investigations doing more to dispel the prevailing mood of superstition and dogma than anyone else in his time.
These include the discovery of craters and mountains on the Moon, the discovery of the four largest moons of Jupiter (Io, Europa, Ganymede and Callisto), the existence and nature of Sunspots, and the phases of Venus. These discoveries, combined with his logical and energetic defense of the Copernican model, made a lasting impact on astronomy and forever changed the way people look at the universe.
Galileo’s theoretical and experimental work on the motions of bodies, along with the largely independent work of Kepler and René Descartes, was a precursor of the classical mechanics developed by Sir Isaac Newton. His work with pendulums and time-keeping also previewed the work of Christiaan Huygens and the development of the pendulum clock, the most accurate timepiece of its day.
The €25 coin minted for the 2009 International Year of Astronomy, showing Galileo on the obverse. Credit: coinnews.net
Galileo also put forward the basic principle of relativity, which states that the laws of physics are the same in any system that is moving at a constant speed in a straight line. This remains true, regardless of the system’s particular speed or direction, thus proving that there is no absolute motion or absolute rest. This principle provided the basic framework for Newton’s laws of motion and is central to Einstein’s special theory of relativity.
The United Nations chose 2009 to be the International Year of Astronomy, a global celebration of astronomy and its contributions to society and culture. The year 2009 was selected in part because it was the four-hundredth anniversary of Galileo first viewing the heavens with his a telescope he built himself.
A commemorative €25 coin was minted for the occasion, with the inset on the obverse side showing Galileo’s portrait and telescope, as well as one of his first drawings of the surface of the moon. In the silver circle that surrounds it, pictures of other telescopes – Isaac Newton’s Telescope, the observatory in Kremsmünster Abbey, a modern telescope, a radio telescope and a space telescope – are also shown.
Other scientific endeavors and principles are named after Galileo, including the NASA Galileo spacecraft, which was the first spacecraft to enter orbit around Jupiter. Operating from 1989 to 2003, the mission consisted of an orbiter that observed the Jovian system, and an atmospheric probe that made the first measurements of Jupiter’s atmosphere.
This mission found evidence of subsurface oceans on Europa, Ganymede and Callisto, and revealed the intensity of volcanic activity on Io. In 2003, the spacecraft was crashed into Jupiter’s atmosphere to avoid contamination of any of Jupiter’s moons.
The European Space Agency (ESA) is also developing a global satellite navigation system named Galileo. And in classical mechanics, the transformation between inertial systems is known as “Galilean Transformation“, which is denoted by the non-SI unit of acceleration Gal (sometimes known as the Galileo). Asteroid 697 Galilea is also named in his honor.
Yes, the sciences and humanity as a whole owes a great dept to Galileo. And as time goes on, and space exploration continues, it is likely we will continue to repay that debt by naming future missions – and perhaps even features on the Galilean Moons, should we ever settle there – after him. Seems like a small recompense for ushering in the age of modern science, no?
The conjunction of Venus (brightest), Jupiter (above Venus) and Mars (dimmer below Venus & Jupiter) looking east in the morning twilight on October 25, 2015, as seen from the west shore of Lake Annette, in Jasper National Park, Alberta. The mountain is the Watchtower. Morning mist covers the lake waters. Haze in the sky adds the natural glows around the planets — no filters were used. Credit and copyright: Alan Dyer.
Have you seen the views in the morning skies this week, with three planets huddling together at dawn? Just one degree separated planets Jupiter and Venus, with Mars sneaking in nearby. Astrophotographers were out in full force to capture the scene!
Above, the very talented photographer Alan Dyer from Canada captured a stunning image of the planetary trio over Lake Annette, in Jasper National Park, Alberta, Canada. He took several gorgeous shots, and so we’ve added one more of his below, plus dozens of other wonderful shots from our astrophotographer friends around the world. Each of these images are from Universe Today’s Flickr pool, so you can click on each picture to get a larger view on Flickr.
Enjoy these great views, as there won’t be a more compact arrangement of three planets again until January 10, 2021.
A panorama of roughly 120° showing a star- and planet-filled sky in the dawn twilight over Lake Annette in Jasper National Park, Alberta, on the morning of October 25, 2015. At left, to the east, are the two bright planets, Venus (brightest) and Jupiter in a close conjunction 1° apart (and here almost merging into one glow), plus reddish Mars below them, all in Leo, with the bright star Regulus above them. Right of centre, to the south, is Orion and Canis Major, with the bright star Sirius low in the south. At upper right are the stars of Taurus, including Aldebaran and the Hyades star cluster. Venus was near greatest elongation on this morning. Credit and copyright: Alan Dyer.Taken from Coral Towers Observatory in Queensland, Australia on October 28, 2014. Venus is to the right of and slightly below Jupiter and Mars is to the right of and below Venus. The pre-dawn landscape is illuminated by moonlight. Credit and copyright: Joseph Brimacombe.Jupiter, Venus, and Mars rise behind the 14,155 foot peak of Mount Democrat in Colorado. Credit and copyright: Patrick Cullis.Spooky Selfie, Three Planets and a Dead Satellite. The planetary conjunction of Jupiter, Venus and Mars on October 26, 2015, along with the ADEOS II satellite, which died in orbit in 2003 after the solar panels failed. Credit and copyright: Tom Wildoner.Planetary conjunction of Jupiter, Venus and Mars as seen from Search Results Map of Le Puy Saint-Bonnet, 49300 Cholet, France Le Puy Saint-Bonnet, 49300 Cholet, France Le Puy-Saint-Bonnet in France on October 26, 2015. Credit and copyright: David de Cueves.Venus, Jupiter and Mars grace the morning skies in France on October 26, 2015. Credit and copyright: Frank Tyrlik.
Here’s a timelapse from Damien Weatherley of his planet imaging session from the morning of October 25, 2015:
Venus, Jupiter & Mars create a close triangle in the eastern sky at dawn! John Chumack captured this image above his backyard Observatory in Dayton, Ohio on 10-26-2015. Credit and copyright: John Chumack.A zoomed out view of the planetary trio from John Chumack’s observatory in Dayton, Ohio on October 25, 2015. Credit and copyright: John Chumack.Conjunction of Venus, Jupiter & Mars on the morning of Monday Oct. 26, 2015. Credit and copyright: Holly Roberts.Jupiter and Venus conjunction on October 25, 2015. They were approximately with a degree and a half of each other. Jupiter’s moons are visible. Credit and copyright: Chris Lyons. Venus and the almost invisible Jupiter struggled to shine through the haze on the morning of October 25, 2015, as seen in Malaysia. Credit and copyright:Shahrin Ahmad.Venus, Jupiter and Mars in the hazy, cloudy morning skies over the UK on October 25, 2015. Credit and copyright: Sarah and Simon Fisher.
And here’s just a reminder that this planetary conjunction has been setting up for a while. Here’s a shot from October 10 of the planets as they started moving closer together:
A spooky planetary conjunction of Venus, Jupiter and Mars on October 10, 2015 on the Isle of Mull, Scotland. Credit and copyright: Shaun Reynold.
Doesn’t it feel like the Universe is perfectly tuned for life? Actually, it’s a horrible hostile place, delivering the bare minimum for human survival.
Consider that incomprehensible series of events that brought you to this moment. In a way that we still don’t understand, a complex mix of chemicals came together in just the right combination to kick off the evolution of life.
Generation after generation of bacteria, insects, fish, lizards, mammals and eventually humans somehow successfully found a buddy and passed along their genetic material to another generation. Clever humans invented computers, the internet, YouTube, and somehow you found your way to this exact video, to hear these words. Whoa.
It’s amazing to consider the Universe we live in, and how it’s perfectly tuned for life. If just a single variable was a little bit different, life as we know it probably wouldn’t exist. Gravity might be a repulsive force. Pokemons might catch you.
Doesn’t it feel like the Universe was created especially for us? I mean, didn’t I already tell you that we’re all the center of the Universe?
I’m sad to say, but this couldn’t be further from the truth. The reality is that the Universe is 100% completely inhospitable. Well, apart from a thin layer on the surface of our Earth, but that’s got to be a rounding error. A fraction of a fraction of a fraction of the teeniest percent of the volume of the Universe. The rest of the Universe is bunk.
If I was plucked out of our cozy environment and dropped into the near vacuum of pretty much anywhere else, the only resource would be a handful of hydrogen atoms. And what can you do with a few hydrogen atoms? Nothing. It might even give Bear Grylls a run for his money. He might have a little more trouble on a star’s surface, crisping up in a heartbeat.
Into a black hole? Surface of a neutron star? Near an exploding supernova? Please enjoy the crushing pressures and hellish temperatures of Venus, or the freezing irradiated surface of Mars.
Earth itself is mostly a deathtrap. Travel down a few kilometers and you’d bake and crush from the rising temperatures of the Earth’s interior. Travel up and the air gets thin, cold and killy. In fact, without our technology heating, cooling, or helping us breathe, we wouldn’t last more than a few days on most of the planet.
Panorama of the part of Mars, from Sol 173. Credit: NASA/JPL/Caltech/Malin Space Science Systems. Image editing by
When you think about the landscape of time, we even live in a brief thumbnail of a moment when Earth is hospitable. Over the next few billion years, the Sun is going to heat up to the point that the surface of Earth will resemble the surface of Venus. And then the last hospitable hidey-hole in the entire Universe, that we know of, will wink out. The Universe is as inhospitable as it could possibly be. That is, without being completely inhospitable.
Especially when you consider the timeframes, and the long future when all the stars have died, where there’s nothing but black holes and frozen matter, and the Universe finally ditches that rounding error, and becomes 100% purely inhospitable.
Cosmologists use a term known as the anthropic principle to explain this very special moment we find ourselves in. There’s the greater anthropic principle that says the Universe wouldn’t be here without us to observe it, but that seems nutty and egotistical.
The lesser anthropic principle says that if the Universe turned out any differently, we wouldn’t be here to observe it.
First ever image of Earth Taken by Mars Color Camera aboard India’s Mars Orbiter Mission (MOM) spacecraft while orbiting Earth and before the Trans Mars Insertion firing on Dec. 1, 2013. Image is focused on the Indian subcontinent. Credit: ISRO
Imagine you threw a dart out the window of an airplane and it landed in a tiny spot on the surface of the Earth. What were the chances that it would land there? Almost zero. What a lucky spot.
You can imagine all kinds of other even more inhospitable Universes, where the conditions were never good enough for life to evolve, and so intelligent civilizations could never even ask the question, “Is Our Universe Perfect for Life.”
So when you look out across a meadow in the springtime. The birds are chirping, and there’s new growth everywhere, don’t forget about the boiling rock magma beneath your feet, the frigid air and then vacuum above your head, and the whole Universe of burning, radiating, impacting objects trying their best to kill you.
Of all the extreme environments in the Universe, which ones do you find most fascinating? Tell us in the comments below.
![Space Shuttle Endeavour sillouetted against the atmosphere. The orange layer is the troposphere, the white layer is the stratosphere and the blue layer the mesosphere.[1] (The shuttle is actually orbiting at an altitude of more than 320 km (200 mi), far above all three layers.) Credit: NASA](https://www.universetoday.com/wp-content/uploads/2012/03/Endeavour_silhouette_STS-130-580x387.jpg)
