Posted on by Fraser Cain

Planet Jupiter

Jupiter photo. Image credit: NASA/SSI

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Jupiter is the 5th planet in the Solar System, and by far the largest, containing 2.5 times the mass of the rest of the planets in the Solar System. Everything about planet Jupiter is big. It has the most moons in the Solar System, including the largest moon, Ganymede. It has the largest storm: the Great Red Spot; the most extreme gravity, and the biggest temperature extremes.

Because you can see Jupiter with the unaided eye, it’s impossible to say who actually discovered Jupiter. But it was Galileo Galilei who first turned his rudimentary telescope on Jupiter in 1610. Even with its dim optics, Galileo was able to make out the fact that Jupiter had 4 bright moons and bands across the planet. Since astronomers believed that everything orbited around the Earth, finding moons orbiting Jupiter threw the Earth-centered theory of the Universe into doubt. Even the smallest telescope will show you what Galileo saw.

Planet Jupiter orbits the Sun at an average distance of 779 million km (484 million miles), and it takes 4,333 Earth days to complete one orbit around the Sun; that’s almost 12 years. But Jupiter rotates once on its axis every 9 hours and 56 minutes. This high rotation speed has flattened out the planet, so that its equator is much further from the center of Jupiter than the poles. Jupiter’s also the largest plane in the Solar System, with an equatorial diameter of 142,984 kilometers (88,846 miles) – 11 times the diameter of Earth.

Jupiter has 318 times more mass than Earth, but it has a relatively low density; only 1/4 the density of Earth. It has such a low density because Jupiter is made up almost entirely of hydrogen with a little bit of helium. The upper atmosphere has tiny amounts of ammonia and other chemicals, which create the bands and clouds we see in photographs. The most familiar feature in Jupiter’s atmosphere is the planet’s Great Red Spot. This is a long-lived storm large enough to swallow up three planets the size of Earth.

It also has the largest number of moons in the Solar System: 63 at last count. The 4 largest moons are the Galilean moons, named after Galileo who discovered them. Ganymede measures 3,273 km across, and it’s the largest moon in the Solar System. Io orbits the closest of these moons and its undergoing almost constant volcanic activity because of tidal flexing being so close to Jupiter. Europa and Callisto probably have oceans of liquid water underneath thick shells of ice, and could be the home to exotic forms of life. Jupiter also has its own set of rings.

Seven spacecraft from Earth have made the journey to Jupiter: Pioneer 10, Pioneer-Saturn, Voyager 1, Voyager 2, Ulysses, Galileo, and New Horizons. Pioneer 10 was the first to reach the planet, making its flyby in 1972. The Galileo spacecraft actually went into orbit around Jupiter, to study the planet and its moons in great detail.

We’ve written many articles about Jupiter for Universe Today. Here’s an article about how Jupiter might protect us in the Solar System, and here’s an article about how you can see Jupiter in a telescope.

If you’d like more info on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Reference:
NASA

Posted on by Fraser Cain

How Big is Jupiter?

Hubble Jupiter

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I’m sure you’ve heard that Jupiter is the largest planet in the Solar System, but just how big is Jupiter?

In terms of size, Jupiter is 142,984 km (88,846 miles) in diameter across its equator. If you just compare that to Earth, it’s 11.2 times the diameter of Earth. But then, it’s only 10% the diameter of the Sun.

The volume of Jupiter is 1.43128×1015 km3. That’s enough to fit inside 1321 planets the size of Earth, and still have room left over.

The surface area of Jupiter is 6.21796×1010 km2. And just for comparison, that’s 122 times more surface area than Earth.

And finally, the mass of Jupiter is 1.8986×1027 kg. That’s enough mass for 318 Earths. In fact, Jupiter is 2.5 times more than the mass of all the other planets in the Solar System. But then, the Sun accounts for 99.9% of the mass of the Solar System.

Jupiter’s big, no question, but don’t worry about the possibility that Jupiter might become a star. It would need another 80 times its own mass to be able to ignite solar fusion.

We’ve written several articles about Jupiter for Universe Today. Here’s an article about an impact that recently happened on Jupiter, and here’s an article about how Jupiter might protect us in the Solar System.

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded a whole episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Posted on by Fraser Cain

Tenth Planet: The Next World in the Solar System

Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)
Tenth planet? Artists concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. Credit: Robert Hurt (IPAC)

Before 1930, there were 8 planets in the Solar System. And then with the discovery of Pluto in 1930, the total number of planets rose to 9. Although astronomers kept searching for more planets, it wasn’t until 2005 that an object larger than Pluto was found orbiting in the distant Solar System. This new object was known as Eris, and many considered it to be a tenth planet; but it actually created a controversy that ended up with Pluto being kicked out of the planet club and becoming a dwarf planet. There really is no 10th planet, in fact, we don’t even have a ninth planet any more.

Discovery of Eris

Eris, originally named 2003 ub 313 was discovered by Palomar observatory researcher Mike Brown; Mike has been behind many of the trans-Neptunian discoveries in the last decade. Mike and his team discovered Eris by systematically scanning the sky for objects moving at the right speed in the right object to be in the outer Solar System.

Further observations of Eris showed that it was actually larger than Pluto by a significant amount; it measured 2,500 km across, compared to Pluto’s 2,300 km diameter. And it orbited at a distance of 67 astronomical units, compared to Pluto’s 39 AU (1 AU is the average distance from the Earth to the Sun).

Tenth Planet, Dwarf Planet

Because there was now a larger object than Pluto found orbiting the Sun, astronomers needed to decide whether this would be come the tenth planet. At a meeting of the International Astronomical Union in 2006, astronomers decided to redefine their classification of a planet. And these new rules excluded Eris. Instead of becoming the tenth planet, Eris became a dwarf planet; the same fate as Pluto.

We’ve written many articles about Eris for Universe Today. Here’s an article about how Eris is changing, and here’s an article about how Xena was renamed to Eris.

If you’d like more info on Eris, check out NASA’s page on Eris.

We’ve also recorded an episode of Astronomy Cast that explains why Pluto isn’t a planet any more. Listen here, Episode 1: Pluto’s Planetary Identity Crisis.

Posted on by Fraser Cain

What Color is Venus?

Venus. Image Credit: NASA/courtesy of nasaimages.org



Here’s a question: what color is Venus? With the unaided eye, Venus just looks like a very bright star in the sky. But spacecraft have sent back images of the cloud tops of Venus, and some have even returned images from the surface of Venus.

If you could actually fly out to Venus and look at it with your own eyes, you wouldn’t see much more than a bright white-yellowish ball with no features. You wouldn’t actually be able to see any of the cloud features that you can see in photographs of Venus. That’s because those photos are taken using different wavelengths of light, where differences in the cloud layers are visible. For example, the photo that accompanies this story was captured in the ultraviolet spectrum.

Although the atmosphere of Venus is almost entirely made up of carbon dioxide, the clouds that obscure our view to the surface are made of sulfur dioxide. These are opaque to visible light, and so we can’t see through them to the surface of Venus. These clouds actually rain droplets of sulfuric acid.

Surface of Venus by Venera.
Surface of Venus by Venera.

If you could get down beneath the cloud tops of Venus, you wouldn’t be able to see much either. That’s because the clouds are so thick that most of the light from the Sun is blocked before it reaches the surface. You would see a dim landscape, like you might see at twilight. The surface of the planet is littered with brownish-red volcanic rocks. The bright red color you see in the Soviet Venera images of Venus have been brightened to show more surface detail.

So, what color is Venus? Yellowish-white.

We’ve written several articles about the color of the planets for Universe Today. Here’s an article about the color of Mercury, and here’s an article about the color of Pluto.

If you’d like more info on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast all about Venus. Listen here, Episode 50: Venus.

Posted on by Fraser Cain

When Was Pluto Discovered?

Clyde Tombaugh

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Five of the planets are visible with the unaided eye and have been known about for thousands of years. Uranus was discovered in 1781, and Neptune was found in 1846. but when was Pluto discovered?

Pluto was discovered February 18th, 1930 by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. Now that Uranus and Neptune had been discovered, astronomers were certain that there were more planets in the outer Solar System. The director of the Lowell Observatory, Vesto Melvin Slipher, handed the job of finding this next planet, dubbed “Planet X” to the 23-year old Tombaugh.

Tombaugh used a tool called a “blink comparator” to compare two photographs of the night sky. He worked methodically, comparing two photographic plates, looking for any object that jumped from one night to another night. And finally on February 18th, 1930, Tombaugh discovered Pluto; a faint object in the right orbit.

The name for Pluto was chosen by an 11-year old British school girl named Venetia Burney. This continued the tradition of naming planets after Roman gods. Pluto was the Roman god of the underworld, the same as Hades in Greek mythology. It also matched the first initials of Percival Lowell, who the observatory was named after. Lowell died in 1916, and so he never saw the discovery of Pluto.

We have written many articles about the discovery of planets in the Solar System for Universe Today. Here’s an article about when Uranus was discovered, and here’s an article about when Neptune was discovered.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded a couple of episodes of Astronomy Cast about Pluto. Here’s a good one, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Fraser Cain

When Was Jupiter Discovered?

Jupiter's Red Spot

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Were you wondering when was Jupiter discovered? Well, there’s no way to know. Jupiter is one of the 5 planets visible with the unaided eye. If you go outside and Jupiter is up in the sky, it’s probably the brightest object up there, brighter than any star; only Venus is brighter. So the ancient people have known about Jupiter for thousands of years, and there’s no way to know when the first person noticed the planet.

Perhaps a better question to ask is, when did we realize that Jupiter was a planet? In ancient times, astronomers used to think that the Earth was the center of the Universe. This was the geocentric model. The Sun, the Moon, the planets, and even the stars all orbited around the Earth in a series of crystal shells. But one thing that was hard to explain was the strange movements of the planets. They would move in one direction, then stop and go backwards in a retrograde motion. Astronomers created ever more elaborate models to explain these bizarre movements.

But then in the 1500s Nicolaus Copernicus developed his model of a Sun-centered, or heliospheric model of the Solar System. The Sun was center of the Solar System, and the planets, including Earth and Jupiter orbited around it. This nicely explained the strange movements of the planets in the sky. They were really following a circular path around the Sun, but the Earth was also traveling around the Sun, and this created different speeds based on our perspective.

The first person to actually view Jupiter in a telescope was Galileo. Even with his rudimentary telescope, he was able to see bands across the planet, and the 4 large Galilean moons that have been named after him. The moons clearly were orbiting Jupiter, which broke the theory that everything in the Universe was orbiting the Earth.

With bigger telescopes, astronomers were able to see more details in Jupiter’s cloud tops and discover more moons. But it wasn’t until the space age that scientists got to really study Jupiter close up. NASA’s Pioneer 10 was the first spacecraft to fly past Jupiter in 1973. It passed within 34,000 km of the cloud tops.

We’ve written several articles about when the planets of the Solar System were discovered. Here’s an article about the discovery of Uranus, and here’s an article about the discovery of Neptune.

If you’d like more information on Jupiter, check out Hubblesite’s News Releases about Jupiter, and here’s a link to NASA’s Solar System Exploration Guide to Jupiter.

We’ve also recorded an entire episode of Astronomy Cast just about Jupiter. Listen here, Episode 56: Jupiter.

Reference:
NASA

Posted on by Fraser Cain

Who Discovered Saturn?

Saturn. Image credit: Hubble

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Were you wondering who discovered Saturn? Well, nobody knows. Here’s the problem. Saturn is one of the 5 planets that you can see with the unaided eye. In fact, if you’re seeing a bright star in the sky, there’s a good chance it’s Saturn. It takes a telescope to see the rings, but anybody can find Saturn, even in a bright city.

So perhaps a better question might be to ask, when did astronomers realize that Saturn was a planet? The ancient astronomers believed in the geocentric model of the Universe. The Earth was at the center of the Universe, and everything else orbited around it in crystal shells: the Sun, the Moon, the planets and the stars. One problem with this model was the strange movements of the planets. They would sometimes slow down, stop and even travel backwards in the sky. And to explain this, astronomers had to create elaborate models for the planets where the orbited inside spheres within spheres.

Anyway, this model was turned on its ear by Nicolaus Copernicus in the 1500s. He placed the Sun at the center of the Solar System, and had all the planets orbiting around it. This nicely explained the strange movements of the planets. They weren’t going backwards, it was just a change in perspective, since the Earth is also going around the Sun.

The first person to actually look at Saturn in a telescope was Galileo. He saw a strange oval-shaped planet. He thought the planet might have ears, or two small balls on either side. Later observations showed that these were actually Saturn’s grand ring system. Galileo also discovered Saturn’s moon Titan.

Better observations of Saturn by Giovanni Cassini turned up 4 additional moons of Saturn, as well a division in the rings that would later be named after him: the Cassini division.

But it wouldn’t be until 1979 that the first spacecraft flew past Saturn. NASA’s Pioneer 11 spacecraft made the journey, getting within 20,000 km of the planet’s cloud tops. This was followed by the Voyager spacecraft, and eventually NASA’s Cassini spacecraft that’s orbiting the planet today. All of our best images of Saturn were sent back by orbiting spacecraft.

We have written many articles about the discovery of planets for Universe Today. Here’s an article about the discovery of Uranus, and here’s an article about the discovery of Neptune.

If you’d like more info on Saturn, start with the NASA Cassini mission homepage. That’s where you’ll see all the latest news and photos sent back from Saturn. Then check out Hubblesite’s News Releases about Saturn.

We’ve also recorded an entire episode of Astronomy Cast just about Saturn. Check it out here, Episode 59: Saturn.

Reference:
NASA

Posted on by Fraser Cain

When Was Venus Discovered?

Venus captured by Magellan.

Were you wondering when was Venus discovered? Actually, there’s no way to know. Venus is one of the 5 planets visible with the unaided eye. In fact, Venus is the brightest object in the night sky after the Sun and the Moon. When Venus is at its brightest, it even casts shadows. So even ancient people would have been aware of Venus, and so there’s no way to know who that first person was, and when it happened.

However, a better question might be to ask, when did we know that Venus was a planet? This happened about the same time that astronomers first realized the Earth was a planet too. In ancient times, astronomers used to think that the Earth was the center of the Universe, and everything orbited around it: the Sun, the Moon, the planets and the stars. One problem with this model was the strange behavior of the planets. Sometimes they would speed up, and then slow down, stop, and even go backwards in the sky.

But then in the 1500s, Nicolaus Copernicus developed his model of a Sun-centered Solar System. The Earth was just a planet, and all of the planets orbited around the Sun instead. This model explained how the planets could have such strange movements. Since the Earth is moving too, we’re really just seeing them from different perspective in they sky.

The first person to see Venus in a telescope was Galileo. Although he wasn’t able to resolve anything but a bright disk (astronomers can’t do any better today), he saw that Venus went through phases like the Moon. This was further evidence that Venus orbits around the Sun – closer than the Earth, and so we see it in various phases of illumination.

Because Venus is shrouded in clouds, astronomers weren’t able to get a better view of Venus until the first spacecraft arrived from Earth. The first spacecraft to visit Venus was NASA’s Mariner 2, which arrived at Venus in 1962. But even then the planet was still blocked by clouds. The Russian Venera landers were able to pierce through the clouds and landed on the surface to send back a few quick images of the planet’s surface. They showed a hellish world, with thick atmosphere, clogging clouds, and blasting heat, hot enough to melt lead. NASA’s Magellan spacecraft (launched in 1989) was equipped with a radar instrument that allowed it to pierce through the clouds on Venus and show the planet’s landscape, craters and volcanoes.

We’ve written many articles about the discovery of planets for Universe Today. Here’s an article about the discovery of Uranus, and here’s an article about the discovery of Neptune.

If you’d like more information on Venus, check out Hubblesite’s News Releases about Venus, and here’s a link to NASA’s Solar System Exploration Guide on Venus.

We’ve also recorded an entire episode of Astronomy Cast about Venus. Listen here, Episode 50: Venus.

Posted on by Fraser Cain

Who Discovered Pluto?

Clyde Tombaugh

Pluto is incredibly faint. You need a powerful backyard telescope to even see it as a dot, so it’s not surprising that Pluto wasn’t discovered until the modern age. Who discovered Pluto? That was the astronomer Clyde Tombaugh, who found Pluto on February 18, 1930.

Tombaugh worked as an astronomer at the Lowell Observatory in Flagstaff, Arizona. He was given the task of finding a trans-Neptunian object which was predicted by Percival Lowell and William Pickering – the search for Planet X. Tombaugh used a tool called a blink comparator to study two images of the same region of the sky taken several nights apart. He would display one image and then blink to the second image to see if any objects had moved from night to night.

And so on February 18, 1930, Tombaugh turned up just such an object moving at the right speed to be the unknown Planet X. The name “Pluto” was suggested by Venetia Burney, and 11-year old English school girl. Pluto was the name of the Roman god of the underworld, and Percival Lowell liked it because the first two letters started with PL, after his own initials.

Pluto was considered the 9th planet in the Solar System until 2006, when the International Astronomical Union reclassified Pluto as a dwarf planet, joining Eris and Ceres as the Solar System’s 3 dwarf planets. There are now only 8 planets in the Solar System.

We have written many articles about the discovery of planets in the Solar System. Here’s an article about who discovered Uranus, and here’s an article about who discovered Neptune.

If you’d like more info on Pluto, check out Hubblesite’s News Releases about Pluto, and here’s a link to NASA’s Solar System Exploration Guide to Pluto.

We’ve also recorded several episodes of Astronomy Cast about Pluto. Start here, Episode 64: Pluto and the Icy Outer Solar System.

Posted on by Jon Voisey

Comets Posing as Asteroids (or is the the other way around?)

Images of known MBCs from UH 2.2-meter telescope data. Credit: Henry Hsieh

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Asteroids are rocky bodies which belong between Mars and Jupiter. Comets are icy bodies that belong way out beyond Pluto. So what are comet-like objects doing in the asteroid belt?

On the night of August 7, 1996, astronomers Eric Elst and Guido Pizarro were observing what was previously thought to be an ordinary asteroid. To their surprise, the object revealed a faint but distinct tail similar to that of a comet. Initially, this was written off as a minor impact kicking up a debris cloud, but when the tail returned in 2002, when the supposed asteroid again returned to perihelion (the closest approach to the Sun), it once again displayed a tenuous tail. The “asteroid” was then given the designation of 133P/Elst-Pizarro. In 2005, two new asteroids were discovered to sport tails: P/2005 U1 and 118401. In 2008, yet another one of these odd objects was found (P/2008 R1). This new class of objects has been dubbed “Main Belt Comets (MBCs)”.

So where are these objects coming from?

A previous article here on Universe Today explored the possibility that these objects formed like other asteroids in the main belt. After all, each of the objects has an orbit consistent with other apparently normal asteroids. They have a similar distance at with they orbit the Sun, as well as similar eccentricities and inclinations of their orbit. So trying to explain these objects as having origins in the outer solar system that migrated just right into the asteroid belt seemed like little more than special pleading.

Furthermore, a 2008 study by Schorghofer at the University of Hawaii predicted that, if such an icy body were to form, it would be able to avoid sublimation for several billion years if only it were covered with a few meters of dust and dirt thus negating the problems of these objects suffering an early death. (Keep in mind that, much like a melting snowball, the water will evaporate but the dirt won’t, so the dirt will pile up quickly on the surface making this entirely plausible!) However, if the ice were covered by such an amount of dust, it would take a collision to remove the dust and trigger the cometary appearance.

In a recent paper, Nader Haghighipour also at the University of Hawaii explores the viability of collisions to trigger this activation as well as the stability of the orbits of these objects to assess the expectation that they were formed at the same time as other asteroids in the main belt.

For the orbital range in which three of the MBCs lie, it was predicted that “on average, one m[eter]-sized object collides … every 40,000 years.” They stress this is an upper limit since their simulation did not include other, nearby asteroids which would likely deplete the number of available impactors.

When they explored the orbital stability of these objects, the discovered at least two of them were dynamically unstable and would eventually be ejected from their orbits on a timescale of 20 million years. As such, it would be unreasonable to expect such objects to have lasted for the nearly 5 billion year history of the solar system. Thus, an in-situ formation was ruled out. However, due to a similarity in orbital characteristics to a family of asteroids known as the Themis family, suggesting they may have resulted from the same break up of a larger body that created this group. This begs the question of whether or not more of these asteroids are secretly hiding water ice reservoirs and are just waiting for an impact to expose them.

Distinctly separate from this orbital family was P/2008 R1 which exists in an especially unstable orbit near one of the resonances from Jupiter. This suggests that this MBC was likely scattered to its present location, but from where remains to be determined.

So while such Main Belt Comets may not have formed simply as they are now, they are likely to be in orbits not far removed from their original formation. Also, this work supported the earlier notion that minor impacts could reliably expected to expose ice allowing for the cometary tails. Whether or not more asteroids have tails tucked between their legs will be the target of future exploration.

Haghighipour’s Paper