Uranus Archives

August 17, 2016August 21, 2016 by Matt Williams

What are the Planets of the Solar System?

An illustration showing the 8 planets of the Solar System to scale Credit: NASA

At one time, humans believed that the Earth was the center of the Universe; that the Sun, Moon, planets and stars all revolved around us. It was only after centuries of ongoing observations and improved instrumentation that astronomers came to understand that we are in fact part a larger system of planets that revolve around the Sun. And it has only been within the last century that we’ve come to understand just how big our Solar System is.

And even now, we are still learning. In the past few decades, the total number of celestial bodies and moons that are known to orbit the Sun has expanded. We have also come to debate the definition of “planet” (a controversial topic indeed!) and introduced additional classifications – like dwarf planet, minor planet, plutoid, etc. – to account for new finds. So just how many planets are there and what is special about them? Let’s run through them one by one, shall we?

Mercury:

As you travel outward from the Sun, Mercury is the closest planet. It orbits the Sun at an average distance of 58 million km (36 million mi). Mercury is airless, and so without any significant atmosphere to hold in the heat, it has dramatic temperature differences. The side that faces the Sun experiences temperatures as high as 420 °C (788 °F), and then the side in shadow goes down to -173 °C (-279.4 °F).

Like Venus, Earth and Mars, Mercury is a terrestrial planet, which means it is composed largely of refractory minerals such as the silicates and metals such as iron and nickel. These elements are also differentiated between a metallic core and a silicate mantle and crust, with Mercury possessing a larger-than-average core. Multiple theories have been proposed to explain this, the most widely accepted being that the impact from a planetesimal in the past blew off much of its mantle material.

Mercury is the smallest planet in the Solar System, measuring just 4879 km across at its equator. However, it is second densest planet in the Solar System, with a density of 5.427 g/cm³ – which is the second only to Earth. Because of this, Mercury experiences a gravitational pull that is roughly 38% that of Earth’s (0.38 g).

Mercury also has the most eccentric orbit of any planet in the Solar System (0.205), which means its distance from the Sun ranges from 46 to 70 million km (29-43 million mi). The planet also takes 87.969 Earth days to complete an orbit. But with an average orbital speed of 47.362 km/s, Mercury also takes 58.646 days to complete a single rotation.

Combined with its eccentric orbit, this means that it takes 176 Earth days for the Sun to return to the same place in the sky (i.e. a solar day) on Mercury, which is twice as long as a single Hermian year. Mercury also has the lowest axial tilt of any planet in the Solar System – approximately 0.027 degrees – compared to Jupiter’s 3.1 degrees, which is the second smallest.

The MESSENGER spacecraft has been in orbit around Mercury since March 2011 – but its days are numbered. Image credit: NASA/JHUAPL/Carnegie Institution of Washington — *The MESSENGER spacecraft has been in orbit around Mercury since March 2011 – but its days are numbered. Credit: NASA/JHUAPL/Carnegie Institution of Washington*

Mercury has only been visited two times by spacecraft, the first being the Mariner 10 probe, which conducted a flyby of the planet back in the mid-1970s. It wasn’t until 2008 that another spacecraft from Earth made a close flyby of Mercury (the MESSENGER probe) which took new images of its surface, shed light on its geological history, and confirmed the presence of water ice and organic molecules in its northern polar region.

In summary, Mercury is made special by the fact it is small, eccentric, and varies between extremes of hot and cold. It’s also very mineral rich, and quite dense!

Venus:

Venus is the second planet in the Solar System, and is Earth’s virtual twin in terms of size and mass. With a mass of 4.8676×10²⁴ kg and a mean radius of about 6,052 km, it is approximately 81.5% as massive as Earth and 95% as large. Like Earth (and Mercury and Mars), it is a terrestrial planet, composed of rocks and minerals that are differentiated.

But apart from these similarities, Venus is very different from Earth. Its atmosphere is composed primarily of carbon dioxide (96%), along with nitrogen and a few other gases. This dense cloud cloaks the planet, making surface observation very difficult, and helps heat it up to 460 °C (860 °F). The atmospheric pressure is also 92 times that of Earth’s atmosphere, and poisonous clouds of carbon dioxide and sulfuric acid rain are commonplace.

At a closest average distance of 41 million km (25,476,219 mi), Venus is the closest planet to Earth. Credit: NASA/JPL/Magellan — *Venus’ similarity in size and mass has led to it being called “Earth’s sister planet’. Credit: NASA/JPL/Magellan*

Venus orbits the Sun at an average distance of about 0.72 AU (108 million km; 67 million mi) with almost no eccentricity. In fact, with its farthest orbit (aphelion) of 0.728 AU (108,939,000 km) and closest orbit (perihelion) of 0.718 AU (107,477,000 km), it has the most circular orbit of any planet in the Solar System. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.

When Venus lies between Earth and the Sun, a position known as inferior conjunction, it makes the closest approach to Earth of any planet, at an average distance of 41 million km. This takes place, on average, once every 584 days, and is the reason why Venus is the closest planet to Earth. The planet completes an orbit around the Sun every 224.65 days, meaning that a year on Venus is 61.5% as long as a year on Earth.

Unlike most other planets in the Solar System, which rotate on their axes in an counter-clockwise direction, Venus rotates clockwise (called “retrograde” rotation). It also rotates very slowly, taking 243 Earth days to complete a single rotation. This is not only the slowest rotation period of any planet, it also means that a single day on Venus lasts longer than a Venusian year.

Venus’ atmosphere is also known to experience lightning storms. Since Venus does not experience rainfall (except in the form of sulfuric acid), it has been theorized that the lightning is being caused by volcanic eruptions. Several spacecraft have visited Venus, and a few landers have even made it to the surface to send back images of its hellish landscape. Even though there were made of metal, these landers only survived a few hours at best.

Venus is made special by the fact that it is very much like Earth, but also radically different. It’s thick atmosphere could crush a living being, its heat could melt lead, and its acid rain could dissolve flesh, bone and metal alike! It also rotates very slowly, and backwards relative to the other plants.

Earth:

Earth is our home, and the third planet from the Sun. With a mean radius of 6371 km and a mass of 5.97×10²⁴ kg, it is the fifth largest and fifth most-massive planet in the Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System. Like Mercury, Venus and Mars, Earth is a terrestrial planet.

But unlike these other planets, Earth’s core is differentiated between a solid inner core and liquid outer core. The outer core also spins in the opposite direction as the planet, which is believed to create a dynamo effect that gives Earth its protective magnetosphere. Combined with a atmosphere that is neither too thin nor too thick, Earth is the only planet in the Solar System known to support life.

The Earth's layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com — *The Earth’s layers, showing the Inner and Outer Core, the Mantle, and Crust. Credit: discovermagazine.com*

In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun between 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU)

The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though a single solar day on Earth is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).

Earth’s axis is also tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.

Earth has only a single moon: the Moon. Thanks to examinations of Moon rocks that were brought back to Earth by the Apollo missions, the predominant theory states that the Moon was created roughly 4.5 billion years ago from a collision between Earth and a Mars-sized object (known as Theia). This collision created a massive cloud of debris that began circling our planet, which eventually coalesced to form the Moon we see today.

*A picture of Earth taken by Apollo 11 astronauts. Credit: NASA*

What makes Earth special, you know, aside from the fact that it is our home and where we originated? It is the only planet in the Solar System where liquid, flowing water exists in abundance on its surface, has a viable atmosphere, and a protective magnetosphere. In other words, it is the only planet (or Solar body) that we know of where life can exist on the surface.

In addition, no planet in the Solar System has been studied as well as Earth, whether it be from the surface or from space. Thousands of spacecraft have been launched to study the planet, measuring its atmosphere, land masses, vegetation, water, and human impact. Our understanding of what makes our planet unique in our Solar System has helped in the search for Earth-like planets in other systems.

Mars:

The fourth planet from the Sun is Mars, which is also the second smallest planet in the Solar System. It has a radius of approximately 3,396 km at its equator, and 3,376 km at its polar regions – which is the equivalent of roughly 0.53 Earths. While it is roughly half the size of Earth, it’s mass – 6.4185 x 10²³ kg – is only 0.151 that of Earth’s. It’s density is also lower than Earths, which leads to it experiencing about 1/3rd Earth’s gravity (0.376 g).

It’s axial tilt is very similar to Earth’s, being inclined 25.19° to its orbital plane (Earth’s axial tilt is just over 23°), which means Mars also experiences seasons. Mars has almost no atmosphere to help trap heat from the Sun, and so temperatures can plunge to a low of -140 °C (-220 °F) in the Martian winter. However, at the height of summer, temperatures can get up to 20 °C (68 °F) during midday at the equator.

However, recent data obtained by the Curiosity rover and numerous orbiters have concluded that Mars once had a denser atmosphere. Its loss, according to data obtained by NASA’s Mars Atmosphere and Volatile Evolution (MAVEN), the atmosphere was stripped away by solar wind over the course of a 500 million year period, beginning 4.2 billion years ago.

At its greatest distance from the Sun (aphelion), Mars orbits at a distance of 1.666 AUs, or 249.2 million km. At perihelion, when it is closest to the Sun, it orbits at a distance of 1.3814 AUs, or 206.7 million km. At this distance, Mars takes 686.971 Earth days, the equivalent of 1.88 Earth years, to complete a rotation of the Sun. In Martian days (aka. Sols, which are equal to one day and 40 Earth minutes), a Martian year is 668.5991 Sols.

Like Mercury, Venus, and Earth, Mars is a terrestrial planet, composed mainly of silicate rock and metals that are differentiated between a core, mantle and crust. The red-orange appearance of the Martian surface is caused by iron oxide, more commonly known as hematite (or rust). The presence of other minerals in the surface dust allow for other common surface colors, including golden, brown, tan, green, and others.

Although liquid water cannot exist on Mars’ surface, owing to its thin atmosphere, large concentrations of ice water exist within the polar ice caps – Planum Boreum and Planum Australe. In addition, a permafrost mantle stretches from the pole to latitudes of about 60°, meaning that water exists beneath much of the Martian surface in the form of ice water. Radar data and soil samples have confirmed the presence of shallow subsurface water at the middle latitudes as well.

*MSL Curiosity selfie on the surface of Mars. Image: NASA/JPL/Cal-Tech*

Mars has two tiny asteroid-sized moons: Phobos and Deimos. Because of their size and shape, the predominant theory is that Mars acquired these two moons after they were kicked out of the Asteroid Belt by Jupiter’s gravity.

Mars has been heavily studied by spacecraft. There are multiple rovers and landers currently on the surface and a small fleet of orbiters flying overhead. Recent missions include the Curiosity Rover, which gathered ample evidence on Mars’ water past, and the groundbreaking discovery of finding organic molecules on the surface. Upcoming missions include NASA’s InSight lander and the Exomars rover.

Hence, Mars’ special nature lies in the fact that it also is terrestrial and lies within the outer edge of the Sun’s habitable zone. And whereas it is a cold, dry place today, it once had an thicker atmosphere and plentiful water on its surface.

Jupiter:

Mighty Jupiter is the fouth planet for our Sun and the biggest planet in our Solar System. Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 10²⁷ kg, 1.43128 x 10¹⁵ km³, 6.1419 x 10¹⁰ km², and 4.39264 x 10⁵ km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 times the mass of all the other planets in the Solar System combined.

But, being a gas giant, it has a relatively low density – 1.326 g/cm³– which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).

Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.

However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact). Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.

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.

The Juno spacecraft isn't the first one to visit Jupiter. Galileo went there in the mid 90's, and Voyager 1 snapped a nice picture of the clouds on its mission. Image: NASA — *The Juno spacecraft isn’t the first one to visit Jupiter. Galileo went there in the mid 90’s, and Voyager 1 snapped a nice picture of the clouds on its mission. Credit: NASA*

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 composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.

The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.

Jupiter has been visited by several spacecraft, including NASA’s Pioneer 10 and Voyager spacecraft in 1973 and 1980, respectively; and by the Cassini and New Horizons spacecraft more recently. Until the recent arrival of Juno, only the Galileo spacecraft has ever gone into orbit around Jupiter, and it was crashed into the planet in 2003 to prevent it from contaminating one of Jupiter’s icy moons.

*Illustration of Jupiter and the Galilean satellites. Credit: NASA*

In short, Jupiter is massive and has massive storms. But compared to the planets of the inner Solar System, is it significantly less dense. Jupiter also has the most moons in the Solar System, with 67 confirmed and named moons orbiting it. But it is estimated that as many as 200 natural satellites may exist around the planet. Little wonder why this planet is named after the king of the gods.

Saturn:

Saturn is the second largest planet in the Solar System. With a mean radius of 58232±6 km, it is approximately 9.13 times the size of Earth. And at 5.6846×10²⁶ kg, it is roughly 95.15 as massive. However, since it is a gas giant, it has significantly greater volume – 8.2713×10¹⁴ km³, which is equivalent to 763.59 Earths.

The sixth most distant planet, Saturn orbits the Sun at an average distance of 9 AU (1.4 billion km; 869.9 million miles). Due to its slight eccentricity, the perihelion and aphelion distances are 9.022 (1,353.6 million km; 841.3 million mi) and 10.053 AU (1,513,325,783 km; 940.13 million mi), on average respectively.

With an average orbital speed of 9.69 km/s, it takes Saturn 10,759 Earth days to complete a single revolution of the Sun. In other words, a single Cronian year is the equivalent of about 29.5 Earth years. However, as with Jupiter, Saturn’s visible features rotate at different rates depending on latitude, and multiple rotation periods have been assigned to various regions.

This portrait looking down on Saturn and its rings was created from images obtained by NASA's Cassini spacecraft on Oct. 10, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic — *This portrait looking down on Saturn and its rings was created from images obtained by NASA’s Cassini spacecraft on Oct. 10th, 2013. Credit: NASA/JPL-Caltech/Space Science Institute/G. Ugarkovic*

As a gas giant, Saturn is predominantly composed of hydrogen and helium gas. With a mean density of 0.687 g/cm³, Saturn is the only planet in the Solar System that is less dense than water; which means that it lacks a definite surface, but is believed to have a solid core. This is due to the fact that Saturn’s temperature, pressure, and density all rise steadily toward the core.

Standard planetary models suggest that the interior of Saturn is similar to that of Jupiter, having a small rocky core surrounded by hydrogen and helium with trace amounts of various volatiles. This core is similar in composition to the Earth, but more dense due to the presence of metallic hydrogen, which as a result of the extreme pressure.

As a gas giant, the outer atmosphere of Saturn contains 96.3% molecular hydrogen and 3.25% helium by volume. Trace amounts of ammonia, acetylene, ethane, propane, phosphine and methane have been also detected in Saturn’s atmosphere. Like Jupiter, it also has a banded appearance, but Saturn’s bands are much fainter and wider near the equator.

On occasion, Saturn’s atmosphere exhibits long-lived ovals that are thousands of km wide, 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.

The huge storm churning through the atmosphere in Saturn's northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI — The huge storm churning through the atmosphere in Saturn’s northern hemisphere overtakes itself as it encircles the planet in this true-color view from NASA’s Cassini spacecraft. Image credit: NASA/JPL-Caltech/SSI

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.

Of course, the most amazing feature of Saturn is its rings. These are made of particles of ice ranging in size from a grains of sand to the size of a car. Some scientists think the rings are only a few hundred million years old, while others think they could be as old as the Solar System itself.

Saturn has been visited by spacecraft 4 times: Pioneer 11, Voyager 1 and 2 were just flybys, but Cassini has actually gone into orbit around Saturn and has captured thousands of images of the planet and its moons. And speaking of moons, Saturn has a total of 62 moons discovered (so far), though estimates indicate that it might have as many as 150.

*A collage of Saturn (bottom left) and some of its moons: Titan, Enceladus, Dione, Rhea and Helene. Credit: NASA/JPL/Space Science Institute*

So like Jupiter, Saturn is a massive gas giant that experiences some very interesting weather patterns. It also has lots of moons and has a very low density. But what really makes Saturn stand out is its impressive ring system. Whereas every gas and ice giant has one, Saturn’s is visible to the naked eye and very beautiful to behold!

Uranus:

Next comes Uranus, the seventh planet from the Sun. With a mean radius of approximately 25,360 km and a mass of 8.68 × 10²⁵ kg, Uranus is approximately 4 times the sizes of Earth and 63 times its volume. However, as a gas giant, its density (1.27 g/cm³) is significantly lower; hence, it is only 14.5 as massive as Earth.

The variation of Uranus’ distance from the Sun is also greater than that any other planet (not including dwarf planets or plutoids). Essentially, the gas giant’s distance from the Sun varies from 18.28 AU (2,735,118,100 km) at perihelion to 20.09 AU (3,006,224,700 km) at aphelion. At an average distance of 3 billion km from the Sun, it takes Uranus roughly 84 years (or 30,687 days) to complete a single orbit of the Sun.

The standard model of Uranus’s structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the center, an icy mantle in the middle and an outer envelope of gaseous hydrogen and helium. Much like Jupiter and Saturn, hydrogen and helium account for the majority of the atmosphere – approximately 83% and 15% – but only a small portion of the planet’s overall mass (0.5 to 1.5 Earth masses).

Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL). — *Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL)*

The third most abundant element is methane ice (CH⁴), which accounts for 2.3% of its composition and which accounts for the planet’s aquamarine or cyan coloring. Trace amounts of various hydrocarbons are also found in the stratosphere of Uranus, which are thought to be produced from methane and ultraviolent radiation-induced photolysis. They include ethane (C₂H₆), acetylene (C₂H₂), methylacetylene (CH₃C₂H), and diacetylene (C₂HC₂H).

In addition, spectroscopy has uncovered carbon monoxide and carbon dioxide in Uranus’ upper atmosphere, as well as the presence icy clouds of water vapor and other volatiles, such as ammonia and hydrogen sulfide. Because of this, Uranus and Neptune are considered a distinct class of giant planet – known as “Ice Giants” – since they are composed mainly of heavier volatile substances.

The rotational period of the interior of Uranus is 17 hours, 14 minutes. As with all giant planets, its upper atmosphere experiences strong winds in the direction of rotation. Hence its weather systems are also 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).

*Huge storms on Uranus were spotted by the Keck Observatory on Aug. 5 and Aug. 6, 2014. Credit: Imke de Pater (UC Berkeley), Pat Fry (University of Wisconsin), Keck Observatory*

One unique feature of Uranus is that it rotates on its side. Whereas all of the Solar System’s planets are tilted on their axes to some degree, Uranus has the most extreme axial tilt of 98°. This leads to the radical seasons that the planet experiences, not to mention an unusual day-night cycle at the poles. At the equator, Uranus experiences normal days and nights; but at the poles, each experience 42 Earth years of day followed by 42 years of night.

Uranus was the first planet to be discovered with a telescope; it was first recognized as a planet in 1781 by William Herschel. Beyond Earth-based observations, only one spacecraft (Voyager 2) has ever studied Uranus up close. It passed by the planet in 1986, and captured the first close images. Uranus has 27 known moons.

Uranus’ special nature comes through in its natural beauty, its intense weather, its ring system and its impressive array of moons. And it’s compositions, being an “ice” giant, is what gives its aquamarine color. But perhaps mist interesting is its sideways rotation, which is unique among the Solar planets.

Neptune:

Neptune is the 8th and final planet in the Solar System, orbiting the Sun at a distance of 29.81 AU (4.459 x 10⁹ km) at perihelion and 30.33 AU (4.537 x 10⁹ km) at aphelion. With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×10²⁶ kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus.

Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).

Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.

Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.

Just like Jupiter and Saturn, Neptune has bands of storms that circle the planet. Astronomers have clocked winds on Neptune traveling at 2,100 km/hour, which is believed to be due to Neptune’s cold temperatures – which may decrease the friction in the system, During its 1989 flyby, NASA’s Voyager 2 spacecraft discovered the Great Dark Spot on Neptune.

Similar to Jupiter’s Great Red Spot, this is an anti-cyclonic storm measuring 13,000 km x 6,600 km across. A few years later, however, the Hubble Space Telescope failed to see the Great Dark Spot, but it did see different storms. This might mean that storms on Neptune don’t last as long as they do on Jupiter or even Saturn.

The more active weather on Neptune might be due, in part, to its higher internal heat. Although Neptune is much more distant than Uranus from the Sun, receiving 40% less sunlight, temperatures on the surface of the two planets are roughly similar. In fact, Neptune radiates 2.61 times as much energy as it receives from the Sun. This is enough heat to help drive the fastest winds in the Solar System.

Neptune is the second planet discovered in modern times. It was discovered at the same time by both Urbain Le Verrier and John Couch Adams. Neptune has only ever been visited by one spacecraft, Voyager 2, which made a fly by in August, 1989. Neptune has 13 known moons. Th largest and most famous of these is Triton, which is believed to be a former KBO that was captured by Neptune’s gravity.

*Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS*

So much like Uranus, Neptune has a ring system, some intense weather patterns, and an impressive array of moons. Also like Uranus, Neptune’s composition allows for its aquamarine color; except that in Neptune’s case, this color is more intense and vivid. In addition, Neptune experiences some temperature anomalies that are yet to be explained. And let’s not forgt the raining diamonds!

And those are the planets in the Solar System thank you for joining the tour! Unfortunately, Pluto isn’t a planet any more, hence why it was not listed. We know, we know, take it up with the IAU!

We have written many interesting articles about the Solar System here at Universe Today. Here’s the Solar System Guide, What is the Solar System?, Interesting Facts About the Solar System, What Was Here Before the Solar System?, How Big is the Solar System?, and Is the Solar System Really a Vortex?

If you’d like more information on the Solar System, visit the Nine Planets, and Solar Views.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast.

Sources:

August 5, 2016February 22, 2017 by Fraser Cain

Why is Uranus on its Side?

It’s impossible to do an article about Uranus without opening up the back door to a spit storm of potty humour. I get it, there’s something just hilarious about talking about your, mine and everyone’s anus. And even if you use the more sanitized and sterile term urine-us, it’s still pretty dirty, in an unwashed New York stairwell kind of way. You’re in us? No.

This is a no-win solution. It’s a Kobayashi Maru scenario here. We’re all doomed.

Can we call a truce? I dare you commentators, to keep the YouTube comments as pure and clean as driven snow, so we can focus on the super interesting science. Think of the children.

Let’s set the stage, I’m going to let planetary astronomer Kevin Grazier give you the proper pronunciation to clear our minds and let us move forward with grace and civility.

Kevin Grazier:
Strictly speaking, it’s pronounced Youranous, is the pronunciation.

As you probably know, Uranus… I mean Ouranus. No, I can’t do it, my brainwashing is too far along. Save yourselves!. Anyway, Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Jupiter and Saturn get all the spacecraft and Hubble space telescopes, but Uranus is an incredibly worthwhile target to visit.

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

It’s almost exactly 4 times larger than Earth and has its own set of strange dusty rings – perhaps left over from a shattered moon. It has at least 27 moons, that we know of, and many more interesting features that would fascinate astronomers, if we had a spacecraft there, which we don’t. Which is ridiculous. We’ve only made one close flyby of Uranus by Voyager II back in 1986.

We’ve seen Pluto up close, but there are no plans to visit Uranus? Madness.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory. — Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Anyway, perhaps one of the strangest aspects of Uranus is its tilt. The planet is flipped over on its side, like a Weeble, that wouldn’t unwobble.

Actually, all the planets in the Solar System have some level of axial tilt. The Earth is tilted 23.5 degrees away from the Sun’s equator. Mars is 25 degrees, and even Mercury is 2.1 degrees tilted. These tilts are everywhere.

But Uranus is 97.8 degrees. That’s just 0.2 degrees shy of a 90s boy band.

You might be wondering, why have it be more than 90 degrees. High school geometry tells me that 97.8 degrees is the same as 82.2 degrees. And that’s true. But astronomers define the angle as greater than 90 degrees when you take its direction of rotation into account. When you describe it as turning in the same direction as the rest of the planets in the Solar System, then you have to measure it this way.

What could have done that to Uranus, how could it have happened?

The fact that Uranus is flipped over on its side tells us that the calm clockwork motion of the Solar System hasn’t always been this way. Shortly after the formation of the Sun and planets, our neighborhood was a violent place.

The early planets smashed into each other, pushed one another into new orbits. Some planets could have been spun out of the Solar System entirely, while others might have been driven into the Sun. Our own Moon was likely formed when a Mars-sized object crashed into the Earth. Other moons might have been captured from three body interactions between worlds. It was mayhem.

The Solar System that you see today contains the survivors. Everything that wasn’t delivered a death blow.

And something really tried to deliver a death blow to Uranus, very early after it formed. We know this because the moons of Uranus orbit at the same tilt as the planet’s axis. This means that something smashed into Uranus while it was still surrounded by the disk of gas and dust that its moons formed from.

When the massive collision happened, the planet flipped over, wrenching this disk with it. The moons formed within this new configuration.

Astronomers think it was more complicated than that, however. If it was a single, massive collision, models suggest the planet would just flip over entirely, and end up rotating backwards from the other planets in the Solar System.

It’s more likely that another collision or even a series of collisions put the brakes on Uranus’ end over end roll, putting it into its current configuration. It boggles the mind to think about what must have happened.

Uranus' tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI) — Uranus’ tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)

Having such a huge axial tilt makes a big different to Uranus. As it travels around the Sun in its 84-year orbit, the planet still has its poles pointed at fixed locations in space. This means that it spends 42 years with its northern hemisphere roughly pointed towards the Sun, and 42 years with its southern hemisphere in sunlight.

If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later. Then you wouldn’t see it for a few decades until it finally reappeared again. It would be very very strange.

Of course, it’s a gas planet, so you can’t stand on it. If you could stand on it, we’d all be marveling at your ability to stand on planets.

Here we are in our calm, ordered Solar System, everything’s business as usual. But if you look around, you realize it’s pretty amazing that our planet is even here. Poor sideways Uranus is a testament to our good luck.

July 6, 2016July 6, 2016 by Matt Williams

What are the Jovian Planets?

The Jovian planets of the Solar System. Credit: bork.hampshire.edu

Beyond our Solar System’s “Frost Line” – the region where volatiles like water, ammonia and methane begin to freeze – four massive planets reside. Though these planets – Jupiter, Saturn, Uranus and Neptune – vary in terms of size, mass, and composition, they all share certain characteristics that cause them to differ greatly from the terrestrial planets located in the inner Solar System.

Officially designated as gas (and/or ice) giants, these worlds also go by the name of “Jovian planets”. Used interchangeably with terms like gas giant and giant planet, the name describes worlds that are essentially “Jupiter-like”. And while the Solar System contains four such planets, extra-solar surveys have discovered hundreds of Jovian planets, and that’s just so far…

Definition:

The term Jovian is derived from Jupiter, the largest of the Outer Planets and the first to be observed using a telescope – by Galileo Galilei in 1610. Taking its name from the Roman king of the gods – Jupiter, or Jove – the adjective Jovian has come to mean anything associated with Jupiter; and by extension, a Jupiter-like planet.

The giant planets of the Solar System (aka. Jovians). Credit: spiff.rit.edu — *The giant planets of the Solar System (aka. the Jovians). Credit: spiff.rit.edu*

Within the Solar System, four Jovian planets exist – Jupiter, Saturn, Uranus and Neptune. A planet designated as Jovian is hence a gas giant, composed primarily of hydrogen and helium gas with varying degrees of heavier elements. In addition to having large systems of moons, these planets each have their own ring systems as well.

Another common feature of gas giants is their lack of a surface, at least when compared to terrestrial planets. In all cases, scientists define the “surface” of a gas giant (for the sake of defining temperatures and air pressure) as being the region where the atmospheric pressure exceeds one bar (the pressure found on Earth at sea level).

Structure and Composition:

In all cases, the gas giants of our Solar System are composed primarily of hydrogen and helium with the remainder being taken up by heavier elements. These elements correspond to a structure that is differentiated between an outer layer of molecular hydrogen and helium that surrounds a layer of liquid (or metallic) hydrogen or volatile elements, and a probable molten core with a rocky composition.

Due to difference in their structure and composition, the four gas giants are often differentiated, with Jupiter and Saturn being classified as “gas giants” while Uranus and Neptune are “ice giants”. This is due to the fact that Neptune and Uranus have higher concentrations of methane and heavier elements – like oxygen, carbon, nitrogen, and sulfur – in their interior.

These cut-aways illustrate interior models of the giant planets. Jupiter is shown with a rocky core overlaid by a deep layer of metallic hydrogen. Credit: NASA/JPL — *Interior models of the giant planets, showing rocky cores overlaid by solid and gaseous envelopes. Credit: NASA/JPL*

In stark contrast to the terrestrial planets, the density of the gas giants is slightly greater than that of water (1 g/cm³). The one exception to this is Saturn, where the mean density is actually lower than water (0.687 g/cm³). In all cases, temperature and pressure increase dramatically the closer one ventures into the core.

Atmospheric Conditions:

Much like their structures and compositions, the atmospheres and weather patterns of the four gas/ice giants are quite similar. The primary difference is that the atmospheres get progressively cooler the farther away they are from Sun. As a result, each Jovian planet has distinct cloud layers who’s altitudes are determined by their temperatures, such that the gases can condense into liquid and solid states.

In short, since Saturn is colder than Jupiter at any particular altitude, its cloud layers occur deeper within it’s atmosphere. Uranus and Neptune, due to their even lower temperatures, are able to hold condensed methane in their very cold tropospheres, whereas Jupiter and Saturn cannot.

The presence of this methane is what gives Uranus and Neptune their hazy blue color, where Jupiter is orange-white in appearance due to the intermingling of hydrogen (which gives off a red appearance), while the upwelling of phosphorus, sulfur, and hydrocarbons yield spotted patches areas and ammonia crystals create white bands.

Shortly after forming, Jupiter was slowly pulled toward the sun. Saturn was also pulled in and eventually, their fates became linked. When Jupiter was about where Mars is now, the pair turned and moved away from the sun. Scientists have referred to this as the "Grand Tack," a reference to the sailing maneuver. Credit: NASA/GSFC — *Jupiter and Saturn have similar appearances, owing to their similar compositions and atmospheres. Credit: NASA/GSFC*

The atmosphere of Jupiter is classified into four layers based on increasing altitude: the troposphere, stratosphere, thermosphere and exosphere. Temperature and pressure increase with depth, which leads to rising convection cells emerging that carry with them the phosphorus, sulfur, and hydrocarbons that interact with UV radiation to give the upper atmosphere its spotted appearance.

Saturn’s atmosphere is similar in composition to Jupiter’s. Hence why it is similarly colored, though its bands are much fainter and are much wider near the equator (resulting in a pale gold color). As with Jupiter’s cloud layers, they are divided into the upper and lower layers, which vary in composition based on depth and pressure. Both planets also have clouds composed of ammonia crystals in their upper atmospheres, with a possible thin layer of water clouds underlying them.

Uranus’ atmosphere can be divided into three sections – the innermost stratosphere, the troposphere, and the outer thermosphere. The troposphere is the densest layer, and also happens to be the coldest in the solar system. Within the troposphere are layers of clouds, with methane clouds on top, ammonium hydrosulfide clouds, ammonia and hydrogen sulfide clouds, and water clouds at the lowest pressures.

Next is the stratosphere, which contains ethane smog, acetylene and methane, and these hazes help warm this layer of the atmosphere. Here, temperatures increase considerably, largely due to solar radiation. The outermost layer (the thermosphere and corona) has a uniform temperature of 800-850 (577 °C/1,070 °F), though scientists are unsure as to the reason.

Uranus and Neptune, the Solar System’s ice giant planets. (Images from Wikipedia.) — *Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons*

This is something that Uranus shares with Neptune, which also experiences unusually high temperatures in its thermosphere (about 750 K (476.85 °C/890 °F). Like Uranus, Neptune is too far from the Sun for this heat to be generated through the absorption of ultraviolet radiation, which means another heating mechanism is involved.

Neptune’s atmosphere is also predominantly hydrogen and helium, with a small amount of methane. The presence of methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Its atmosphere can be subdivided into two main regions: the lower troposphere (where temperatures decrease with altitude), and the stratosphere (where temperatures increase with altitude).

The lower stratosphere is believed to contain hydrocarbons like ethane and ethyne, which are the result of methane interacting with UV radiation, thus producing Neptune’s atmospheric haze. 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.

Weather Patterns:

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. These are the result of Jupiter’s intense radiation, it’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere.

*Reprocessed view by Bjorn Jonsson of the Great Red Spot taken by Voyager 1 in 1979 reveals an incredible wealth of detail. Credit: NASA/JPL*

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 also periodically experiences flashes of lightning in its atmosphere, which can be up to a thousand times as powerful as those observed here on the Earth.

Saturn’s atmosphere is similar, exhibiting long-lived ovals now and then that can be several thousands of kilometers wide. A good example is the Great White Spot (aka. Great White Oval), a unique but short-lived phenomenon that occurs once every 30 Earth years. Since 2010, a large band of white clouds called the Northern Electrostatic Disturbance have been observed enveloping Saturn, and is believed to be followed by another in 2020.

The winds on Saturn are the second fastest among the Solar System’s planets, which have reached a measured high 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 persisting hexagonal wave pattern measuring about 13,800 km (8,600 mi) and rotating with a period of 10h 39m 24s.

Saturn makes a beautifully striped ornament in this natural-color image, showing its north polar hexagon and central vortex (Credit: NASA/JPL-Caltech/Space Science Institute) — *Saturn makes a beautifully striped ornament in this natural-color image, showing its north polar hexagon and central vortex. Credit: NASA/JPL-Caltech/Space Science Institute*

The south pole vortex apparently takes the form 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’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. 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).

Because Neptune is not a solid body, its atmosphere undergoes differential rotation, with its wide equatorial zone rotating slower than the planet’s magnetic field (18 hours vs. 16.1 hours). 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 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.

Exoplanets:

Due to the limitations imposed by our current methods, most of the exoplanets discovered so far by surveys like the Kepler space observatory have been comparable in size to the giant planets of the Solar System. Because these large planets are inferred to share more in common with Jupiter than with the other giant planets, the term “Jovian Planet” has been used by many to describe them.

Many of these planets, being greater in mass than Jupiter, have also been dubbed as “Super-Jupiters” by astronomers. Such planets exist at the borderline between planets and brown dwarf stars, the smallest stars known to exist in our Universe. They can be up to 80 times more massive than Jupiter but are still comparable in size, since their stronger gravity compresses the material into an ever denser, more compact sphere.

Artist's concept of "hot Jupiter" exoplanet HD 149026b (NASA/JPL-Caltech) — *Artist’s concept of the “Hot Jupiter” exoplanet HD 149026b. Credit: NASA/JPL-Caltech*

Those Super-Jupiters that have distant orbits from their parent stars are known as “Cold Jupiters”, whereas those that have close orbits are called “Hot Jupiters”. A surprising number of Hot Jupiters have been observed by exoplanet surveys, due to the fact that they are particularly easy to spot using the Radial Velocity method – which measures the oscillation of parent stars due to the influence of their planets.

In the past, astronomers believed that Jupiter-like planets could only form in the outer reaches of a star system. However, the recent discovery of many Jupiter-sized planets orbiting close to their stars has cast doubt on this. Thanks to the discovery of Jovians beyond our Solar System, astronomers may be forced to rethink our models of planetary formation.

Since Galileo first observed Jupiter through his telescope, Jovian planets have been an endless source of fascination for us. And despite many centuries of research and progress, there are still many things we don’t know about them. Our latest effort to explore Jupiter, the Juno Mission, is expected to produce some rather interesting finds. Here’s hoping they bring us one step closer to understanding those darn Jovians!

We have written many interesting articles about gas giants here at Universe Today. Here’s the Solar System Guide, The Outer Planets, What’s Inside a Gas Giant?, and Which Planets Have Rings?

For more information, check out NASA’s Solar System Exploration page and Science Daily’s the Jovian planets.

Astronomy Cast has a number of episodes on the Jovian planets, including Episode 56: Jupiter.

June 25, 2016July 22, 2016 by Matt Williams

What are the Different Masses of the Planets?

Planets and other objects in our Solar System. Credit: NASA.

It is a well known fact that the planets of the Solar System vary considerably in terms of size. For instance, the planets of the inner Solar System are smaller and denser than the gas/ice giants of the outer Solar System. And in some cases, planets can actually be smaller than the largest moons. But a planet’s size is not necessarily proportional to its mass. In the end, how massive a planet is has more to do with its composition and density.

So while a planet like Mercury may be smaller in size than Jupiter’s moon Ganymede or Saturn’s moon Titan, it is more than twice as massive than they are. And while Jupiter is 318 times as massive as Earth, its composition and density mean that it is only 11.21 times Earth’s size. Let’s go over the planet’s one by one and see just how massive they are, shall we?

Mercury:

Mercury is the Solar System’s smallest planet, with an average diameter of 4879 km (3031.67 mi). It is also one of its densest at 5.427 g/cm³, which is second only to Earth. As a terrestrial planet, it is composed of silicate rock and minerals and is differentiated between an iron core and a silicate mantle and crust. But unlike its peers (Venus, Earth and Mars), it has an abnormally large metallic core relative to its crust and mantle.

All told, Mercury’s mass is approximately 0.330 x 10²⁴kg, which works out to 330,000,000 trillion metric tons (or the equivalent of 0.055 Earths). Combined with its density and size, Mercury has a surface gravity of 3.7 m/s² (or 0.38 g).

*Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius. Credit: MASA/JPL*

Venus:

Venus, otherwise known as “Earth’s Sister Planet”, is so-named because of its similarities in composition, size, and mass to our own. Like Earth, Mercury and Mars, it is a terrestrial planet, and hence quite dense. In fact, with a density of 5.243 g/cm³, it is the third densest planet in the Solar System (behind Earth and Mercury). Its average radius is roughly 6,050 km (3759.3 mi), which is the equivalent of 0.95 Earths.

And when it comes to mass, the planet weighs in at a hefty 4.87 x 10²⁴kg, or 4,870,000,000 trillion metric tons. Not surprisingly, this is the equivalent of 0.815 Earths, making it the second most massive terrestrial planet in the Solar System. Combined with its density and size, this means that Venus also has comparable gravity to Earth – roughly 8.87 m/s², or 0.9 g.

Earth:

Like the other planets of the inner Solar System, Earth is also a terrestrial planet, composed of metals and silicate rocks differentiated between an iron core and a silicate mantle and crust. Of the terrestrial planets, it is the largest and densest, with an average radius of 6,371.0 km (3,958.8 mi) and a mean of density of 5.514 g/cm³.

And at 5.97 x 10²⁴kg (which works out to 5,970,000,000,000 trillion metric tons) Earth is the most massive of all the terrestrial planets. Combined with its size and density, Earth experiences the surface gravity that we are all familiar with – 9.8 m/s², or 1 g.

Mars:

Mars is the third largest terrestrial planet, and the second smallest planet in our Solar System. Like the others, it is composed of metals and silicate rocks that are differentiated between a iron core and a silicate mantle and crust. But while it is roughly half the size of Earth (with a mean diameter of 6792 km, or 4220.35 mi), it is only one-tenth as massive.

In short, Mars has a mass of 0.642 x10²⁴ kg, which works out to 642,000,000 trillion metric tons, or roughly 0.11 the mass of Earth. Combined with its size and density – 3.9335 g/cm³ (which is roughly 0.71 times that of Earth’s) – Mars has a surface gravity of 3.711 m/s² (or 0.376 g).

Jupiter:

Jupiter is the largest planet in the Solar System. With a mean diameter of 142,984 km, it is big enough to fit all the other planets (except Saturn) inside itself, and big enough to fit Earth 11.8 times over. But with a mass of 1898 x 10²⁴kg (or 1,898,000,000,000 trillion metric tons), Jupiter is more massive than all the other planets in the Solar System combined – 2.5 times more massive, to be exact.

upiter's structure and composition. (Image Credit: Kelvinsong CC by S.A. 3.0) — Jupiter’s structure and composition. (Image Credit: Kelvinsong CC by S.A. 3.0)

However, as a gas giant, it has a lower overall density than the terrestrial planets. It’s mean density is 1.326 g/cm, but this increases considerably the further one ventures towards the core. And though Jupiter does not have a true surface, if one were to position themselves within its atmosphere where the pressure is the same as Earth’s at sea level (1 bar), they would experience a gravitational pull of 24.79 m/s²(2.528 g).

Saturn:

Saturn is the second largest of the gas giants; with a mean diameter of 120,536 km, it is just slightly smaller than Jupiter. However, it is significantly less massive than its Jovian cousin, with a mass of 569 x 10²⁴ kg (or 569,000,000,000 trillion metric tons). Still, this makes Saturn the second most-massive planet in the Solar System, with 95 times the mass of Earth.

Much like Jupiter, Saturn has a low mean density due to its composition. In fact, with an average density of 0.687 g/cm³, Saturn is the only planet in the Solar System that is less dense than water (1 g/cm³). But of course, like all gas giants, its density increases considerably the further one ventures towards the core. Combined with its size and mass, Saturn has a “surface” gravity that is just slightly higher than Earth’s – 10.44 m/s², or 1.065 g.

Diagram of Saturn's interior. Credit: Kelvinsong/Wikipedia Commons — *Diagram of Saturn’s interior. Credit: Kelvinsong/Wikipedia Commons*

Uranus:

With a mean diameter of 51,118 km, Uranus is the third largest planet in the Solar System. But with a mass of 86.8 x 10²⁴kg (86,800,000,000 trillion metric tons) it is the fourth most massive – which is 14.5 times the mass of Earth. This is due to its mean density of 1.271 g/cm³, which is about three quarters of what Neptune’s is. Between its size, mass, and density, Uranus’ gravity works out to 8.69 m/s², which is 0.886 g.

Neptune:

Neptune is significantly larger than Earth; at 49,528 km, it is about four times Earth’s size. And with a mass of 102 x 10²⁴kg (or 102,000,000,000 trillion metric tons) it is also more massive – about 17 times more to be exact. This makes Neptune the third most massive planet in the Solar System; while its density is the greatest of any gas giant (1.638 g/cm³). Combined, this works out to a “surface” gravity of 11.15 m/s² (1.14 g).

As you can see, the planets of the Solar System range considerably in terms of mass. But when you factor in their variations in density, you can see how a planets mass is not always proportionate to its size. In short, while some planets may be a few times larger than others, they are can have many, many times more mass.

We have written many interesting articles about the planets here at Universe. For instance, here’s Interesting Facts About the Solar System, What are the Colors of the Planets?, What are the Signs of the Planets?, How Dense are the Planets?, and What are the Diameters of the Planets?.

For more information, check out Nine Planets overview of the Solar System, NASA’s Solar System Exploration, and use this site to find out what you would weigh on other planets.

Astronomy Cast has episodes on all of the planets. Here’s Episode 49: Mercury to start!

June 21, 2016July 1, 2016 by Matt Williams

What Are The Diameters of the Planets?

Planets in the Solar System. Image credit: NASA/JPL/IAU

The planets of our Solar System vary considerably in size and shape. Some planets are small enough that they are comparable in diameter to some of our larger moons – i.e. Mercury is smaller than Jupiter’s moon Ganymede and Saturn’s moon Titan. Meanwhile, others like Jupiter are so big that they are larger in diameter than most of the others combined.

In addition, some planets are wider at the equator than they are at the poles. This is due to a combination of the planets composition and their rotational speed. As a result, some planets are almost perfectly spherical while others are oblate spheroids (i.e. experience some flattening at the poles). Let us examine them one by one, shall we?

Mercury:

With a diameter of 4,879 km (3031.67 mi), Mercury is the smallest planet in our Solar System. In fact, Mercury is not much larger than Earth’s own Moon – which has a diameter of 3,474 km (2158.64 mi). At 5,268 km (3,273 mi) in diameter, Jupiter’s moon of Ganymede is also larger, as is Saturn’s moon Titan – which is 5,152 km (3201.34 mi) in diameter.

As with the other planets in the inner Solar System (Venus, Earth, and Mars), Mercury is a terrestrial planet, which means it is composed primarily of metals and silicate rocks that are differentiated into an iron-rich core and a silicate mantle and crust.

Also, due to the fact that Mercury has a very slow sidereal rotational period, taking 58.646 days to complete a single rotation on its axis, Mercury experiences no flattening at the poles. This means that the planet is almost a perfect sphere and has the same diameter whether it is measured from pole to pole or around its equator.

Venus:

Venus is often referred to as Earth’s “sister planet“, and not without good reason. At 12,104 km (7521 mi) in diameter, it is almost the same size as Earth. But unlike Earth, Venus experiences no flattening at the poles, which means that it almost perfectly circular. As with Mercury, this is due to Venus’ slow sidereal rotation period, taking 243.025 days to rotate once on its axis.

Earth:

With a mean diameter of 12,756 km (7926 mi), Earth is the largest terrestrial planet in the Solar System and the fifth largest planet overall. However, due to flattening at its poles (0.00335), Earth is not a perfect sphere, but an oblate spheroid. As a result, its polar diameter differs from its equatorial diameter, but only by about 41 km (25.5 mi)

In short, Earth measures 12713.6 km (7900 mi) in diameter from pole to pole, and 12756.2 km (7926.3 mi) around its equator. Once again, this is due to Earth’s sidereal rotational period, which takes a relatively short 23 hours, 58 minutes and 4.1 seconds to complete a single rotation on its axis.

Mars:

Mars is often referred to as “Earth’s twin”; and again, for good reason. Like Earth, Mars experiences flattening at its poles (0.00589), which is due to its relatively rapid sidereal rotational period (24 hours, 37 minutes and 22 seconds, or 1.025957 Earth days).

As a result, it experiences a bulge at its equator which leads to a variation of 40 km (25 mi) between its polar radius and equatorial radius. This works out to Mars having a mean diameter of 6779 km (4212.275 mi), varying between 6752.4 km (4195.75 mi) between its poles and 6792.4 km (4220.6 mi) at its equator.

*Mosaic of the Valles Marineris hemisphere of Mars, similar to what one would see from orbital distance of 2500 km. Credit: NASA/JPL-Caltech*

Jupiter:

Jupiter is the largest planet in the Solar System, measuring some 142,984 km (88,846 mi) in diameter. Again, this its mean diameter, since Jupiter experiences some rather significant flattening at the poles (0.06487). This is due to its rapid rotational period, with Jupiter taking just 9 hours 55 minutes and 30 seconds to complete a single rotation on its axis.

Combined with the fact that Jupiter is a gas giant, this means the planet experiences significant bulging at its equator. Basically, it varies in diameter from 133,708 km (83,082.3 mi) when measured from pole to pole, and 142,984 km (88,846 mi) when measured around the equator. This is a difference of 9276 km (5763.8 mi), one of the most pronounced in the Solar System.

Saturn:

With a mean diameter of 120,536 km (74897.6 mi), Saturn is the second largest planet in the Solar System. Like Jupiter, it experiences significant flattening at its poles (0.09796) due to its high rotational velocity (10 hours and 33 minutes) and the fact that it is a gas giant. This means that it varies in diameter from 108,728 km (67560.447 mi) when measured at the poles and 120,536 km (74,897.6 mi) when measured at the equator. This is a difference of almost 12,000 km, the greatest of all planets.

Uranus:

Uranus has a mean diameter of 50,724 km (31,518.43 mi), making it the third largest planet in the Solar System. But due to its rapid rotational velocity – the planet takes 17 hours 14 minutes and 24 seconds to complete a single rotation – and its composition, the planet experiences a significant polar flattening (0.0229). This leads to a variation in diameter of 49,946 km (31,035 mi) at the poles and 51,118 km (31763.25 mi) at the equator – a difference of 1172 km (728.25 mi).

Neptune:

Lastly, there is Neptune, which has a mean diameter of 49,244 km (30598.8 mi). But like all the other gas giants, this varies due to its rapid rotational period (16 hours, 6 minutes and 36 seconds) and composition, and subsequent flattening at the poles (0.0171). As a result, the planet experiences a variation of 846 km (525.68 mi), measuring 48,682 km (30249.59 mi) at the poles and 49,528 km (30775.27 mi) at the equator.

In summary, the planets of our Solar System vary in diameter due to differences in their composition and the speed of their rotation. In short, terrestrial planets tend to be smaller than gas giants, and gas giants tend to spin faster than terrestrial worlds. Between these two factors, the worlds we know range between near-perfect spheres and flattened spheres.

We have written many articles about the Solar System here at Universe Today. Here’s Interesting Facts about the Solar System, How Long Is A Day On The Other Planets Of The Solar System?, What Are the Colors of the Planets?, How Long Is A Year On The Other Planets?, What Is The Atmosphere Like On Other Planets?, and How Strong is Gravity on Other Planets?

For more information of the planets, here is a look at the eight planets and some fact sheets about the planets from NASA.

Astronomy Cast has episodes on all the planets. Here is Mercury to start out with.

May 21, 2016May 4, 2017 by Matt Williams

What is the Surface Temperature of Neptune?

Our Solar System is a fascinating place. Between its eight planets and many dwarf planets, there are some serious differences in terms of orbit, composition, and temperature. Whereas conditions within the inner Solar System, where planets are terrestrial in nature, can get pretty hot, planets that orbit beyond the Frost Line – where it is cold enough that volatiles (i.e. water, ammonia, methane, CO and CO²) condense into solids – can get mighty cold!

It is in this environment that we find Neptune, the Solar System’s most distance (and hence most cold) planet. While this gas/ice giant has no “surface” to speak of, Earth-based research and flybys have been conducted that have managed to obtain accurate measurements of the temperature in the planet’s upper atmosphere. All told, the planet experiences temperatures that range from approximately 55 K (-218 °C; -360 °F) to 72 K (-200 °C; -328 °F), making it the coldest planet in the Solar System.

Orbital Characteristics:

Of all the planets in the Solar System, Neptune orbits the Sun at the greatest average distance. With a very minor eccentricity (0.0086), it orbits the Sun at an semi-major axis of approximately 30.11 AU (4,504,450,000,000 km), ranging from 29.81 AU (4.459 x 10⁹ km) at perihelion and 30.33 AU (4.537 x 10⁹ km) at aphelion.

Pluto and its cohorts in the icy-asteroid-rich Kuiper Belt beyond the orbit of Neptune. Credit: NASA — *Neptune and the icy-asteroid-rich Kuiper Belt that lies beyond its orbit. Credit: NASA*

Neptune takes 16 hours 6 minutes and 36 seconds (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).

Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. In addition, the planets axial tilt also leads to variations in the length of its day, as well as variations in temperature between the northern and southern hemispheres (see below).

“Surface” Temperature:

Due to their composition, determining a surface temperature on gas or ice giants (compared to terrestrial planets or moons) is technically impossible. As a result, astronomers have relied on measurements obtained at altitudes where the atmospheric pressure is equal to 1 bar (or 100 kilo Pascals), the equivalent of air pressure here on Earth at sea level.

In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka) — *Color-contrasted photo showing Neptune’s atmospheric features. Credit: Erich Karkoschka*

It is here on Neptune, just below the upper level clouds, that pressures reach between 1 and 5 bars (100 – 500 kPa). It is also at this level that temperatures reach their recorded high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).

But as with all gas and ice giants, temperatures vary on Neptune due to depth and pressure. In short, the deeper one goes into Neptune, the hotter it becomes. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun. The huge temperature differences between Neptune’s center and its surface create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System.

Temperature Anomalies and Variations:

Whereas Neptune averages the coldest temperatures in the Solar System, a strange anomaly is the planet’s south pole. Here, it is 10 degrees K warmer than the rest of planet. This “hot spot” occurs because Neptune’s south pole is currently exposed to the Sun. As Neptune continues its journey around the Sun, the position of the poles will reverse. Then the northern pole will become the warmer one, and the south pole will cool down.

Neptune’s more varied weather when compared to Uranus is due in part to its higher internal heating, which is particularly perplexing for scientists. Despite the fact that Neptune is located over 50% further from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets’ surface temperatures are roughly equal.

*Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)*

Deeper inside the layers of gas, the temperature rises steadily. This is consistent with Uranus, but oddly enough, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. The mechanism for this remains unknown.

And while temperatures on Pluto have been recorded as reaching lower – down to 33 K (-240 °C; -400 °F) – Pluto’s status as a dwarf planet mean that it is no longer in the same class as the others. As such, Neptune remains the coldest planet of the eight.

We have written many articles about Neptune here at Universe Today. Here’s The Gas (and Ice) Giant Neptune, What is the Surface of Neptune Like?, 10 Interesting Facts About Neptune, and The Rings of Neptune.

If you’d like more information on Neptune, take a look at Hubblesite’s News Releases about Neptune, and here’s a link to NASA’s Solar System Exploration Guide to Neptune.

We have recorded an entire episode of Astronomy Cast just about Neptune. You can listen to it here, Episode 63: Neptune.

May 11, 2016May 11, 2016 by Matt Williams

What Are The Colors of the Planets?

When we look at beautiful images of the planets of our Solar System, it is important to note that we are looking at is not always accurate. Especially where their appearances are concerned, these representations can sometimes be altered or enhanced. This is a common practice, where filters or color enhancement is employed in order to make sure that the planets and their features are clear and discernible.

So what exactly do the planets of the Solar System look like when we take all the added tricks away? If we were to take pictures of them from space, minus the color enhancement, image touch-ups, and other methods designed to bring out their details, what would their true colors and appearances be? We already know that Earth resembles something of a blue marble, but what about the other ones?

Continue reading “What Are The Colors of the Planets?”

February 17, 2016February 18, 2016 by Matt Williams

How Dense Are The Planets?

Our Solar System Montage. Credit: NASA/JPL

The eight planets of our Solar System vary widely, not only in terms of size, but also in terms of mass and density (i.e. its mass per unit of volume). For instance, the 4 inner planets – those that are closest to the Sun – are all terrestrial planets, meaning they are composed primarily of silicate rocks or metals and have a solid surface. On these planets, density varies the farther one ventures from the surface towards the core, but not considerably.

By contrast, the 4 outer planets are designated as gas giants (and/or ice giants) which are composed primarily of of hydrogen, helium, and water existing in various physical states. While these planets are greater in size and mass, their overall density is much lower. In addition, their density varies considerably between the outer and inner layers, ranging from a liquid state to materials so dense that they become rock-solid.

Continue reading “How Dense Are The Planets?”

February 11, 2016February 23, 2017 by Fraser Cain

Why Do Planets Have Rings?

Planet come in a wide variety of sizes, compositions, and colors – and they can sometimes have rings. Where do these rings come from?
Continue reading “Why Do Planets Have Rings?”

January 29, 2016May 30, 2023 by Matt Williams

The Orbit of the Planets. How Long Is A Year On The Other Planets?

Here on Earth, we to end to not give our measurements of time much thought. Unless we’re griping about Time Zones, enjoying the extra day of a Leap Year, or contemplating the rationality of Daylight Savings Time, we tend to take it all for granted. But when you consider the fact that increments like a year are entirely relative, dependent on a specific space and place, you begin to see how time really works.

Here on Earth, we consider a year to be 365 days. Unless of course it’s a Leap Year, which takes place every four years (in which it is 366). But the actual definition of a year is the time it takes our planet to complete a single orbit around the Sun. So if you were to put yourself in another frame of reference – say, another planet – a year would work out to something else. Let’s see just how long a year is on the other planets, shall we?

Continue reading “The Orbit of the Planets. How Long Is A Year On The Other Planets?”