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Starts With A Bang

These Are The 10 Largest Non-Planets In Our Solar System

Only 8 worlds make the astronomical cut as planets. Here are the 10 fascinating bodies that didn’t make it.

Astronomically, bodies within the Solar System must achieve three criteria in order to gain the much-vaunted status of planet:

  1. Gravitationally pull themselves into a spheroidal shape, where they obtain hydrostatic equilibrium,
  2. Orbit the Sun in an ellipse and no other smaller, parent body,
  3. and clear their orbit of any substantially-massed objects.
The eight planets of our Solar System and our Sun, to scale in size but not in terms of orbital distances. Note that these are the only eight objects that meet all three of the planetary criteria as set forth by the IAU. (WIKIMEDIA COMMONS USER WP)

In our Solar System, only eight worlds make the cut given those criteria. The four rocky planets (Mercury, Venus, Earth, Mars) and the four gas giant worlds (Jupiter, Saturn, Uranus, Neptune) are the only ones that can be called planets under these definitions. Everything else, no matter how large or massive, fails on one of the latter two criteria.

If you judge whether an object is a planet or not by the IAU’s criteria, that satisfies planets in our solar system, but no others. However, by looking at a distant world’s mass, orbital parameters, and the age of the solar system, you can reproduce the IAU’s definition for 99+% of the worlds we know of. (MARGOT (2015), VIA ARXIV.ORG/ABS/1507.06300)

A simple mass-distance relationship could extend this definition to other Solar Systems as well, transforming the IAU’s current definition into a universal one that defines “planets” for exoplanetary systems as well.

Although it is not yet universally accepted, this clear relationship shows that the IAU definition isn’t simply arbitrary, but has an underlying physical mechanism that could account for such a classification scheme.

Densities of various bodies in the Solar System. Note the relationship between density and distance from the Sun, the similarity of Triton to Pluto, and how even the satellites of Jupiter, from Io to Callisto, vary in density so tremendously. (KARIM KHAIDAROV)

Yet being a planet, by definition, isn’t everything. Many of the non-planets, even in our own Solar System, are fascinating in their own right. Here are the 10 largest ones we have, along with what makes them so interesting.

This natural color image of Ganymede’s anti-Jupiter hemisphere comes from the Galileo spacecraft. It has water-ice on its poles down to about 40° latitude, and a thin atmosphere of oxygen and hydrogen atoms, likely made from the vaporized ices. An underground ocean may contain more water than all of Earth combined. (NASA/JPL (EDITED BY WIKIMEDIA COMMONS USER PLANETUSER))

1.) Ganymede: Jupiter’s largest moon is the largest non-planet in the Solar System. With a diameter of 5,268 km (3,271 miles), it’s 8% bigger than the planet Mercury, although it has less than half the mass of our Solar System’s innermost planet, being made of mostly ices and silicate minerals. At just 45% the mass of Mercury, it has an asteroid-like density rather than a density comparable to the terrestrial planets.

Still, it has an iron core that generates its own magnetic field, which dominates very close to the surface even over the enormous magnetic field of nearby parent planet Jupiter. Observations suggest it has an underground ocean beneath the surface, possibly containing even more water than planet Earth possesses. Its atmosphere is almost non-existent: 100 billion times thinner than Earth’s, made almost exclusively of oxygen and hydrogen compounds arising from vaporized ices.

In this assigned-color image of Titan, the methane haze and atmosphere is shown in a near-transparent blue, with surface features beneath the clouds displayed. A composite of ultraviolet, optical, and infrared light was used to construct this view. (NASA/JPL/SPACE SCIENCE INSTITUTE)

2.) Titan: Saturn’s enormous satellite, Titan, also surpasses Mercury in size, but has little else in common with virtually airless Ganymede. Titan’s atmosphere is the richest of any moon in the Solar System, with an atmospheric pressure at its surface greater than that even of Earth. It forms seasonal clouds and weather patterns at its poles, above the methane hazes that dominate its atmosphere.

The surface pressure allows for the presence of liquids there, most prominently methane. The Huygens lander discovered methane lakes and even waterfalls on Titan’s surface, while Cassini’s infrared imager was able to map Titan’s surface through the clouds. In many ways, of all the moons we know of, it’s the one most like the other rocky planets of the Solar System.

Bright scars on a darker surface testify to a long history of impacts on Jupiter’s moon Callisto in this image of Callisto from NASA’s Galileo spacecraft. The picture, taken in May 2001, is the only complete global color image of Callisto obtained by Galileo, which has been orbiting Jupiter since December 1995. Of Jupiter’s four largest moons, Callisto orbits farthest from the giant planet. Callisto’s surface is uniformly cratered but is not uniform in color or brightness. Scientists believe the brighter areas are mainly ice and the darker areas are highly eroded, ice-poor material. (NASA/JPL/DLR(GERMAN AEROSPACE CENTER))

3.) Callisto: The oldest and most heavily-cratered moon in the Solar System, Mercury-sized Callisto is the largest moon to show very few properties of what we’d call “differentiation” between its layers. The most distant of the four Galilean moons around Jupiter, Callisto receives very little tidal heating at this great distance, and isn’t locked into the same resonant orbits as Io, Europa, and Ganymede. It has the lowest density and surface gravity of any of the Galilean satellites.

Although it’s tidally locked to Jupiter, with the same face always facing its jovian parent, its surface appears to be extremely old. It is the most heavily cratered world known in the Solar System, thought to have the oldest surface of all. Of all the large moons we know of, Callisto shows the smallest differences in composition between core, mantle, and crust, likely due to its formation by slow accretion at such a great distance (and with so little tidal heating) from Jupiter.

Jupiter’s innermost Galilean satellite, Io, is multicolored from sulfur, ices, and volcanic activity. Its lack of craters indicates a near-constant resurfacing, giving it the youngest surface of any known object in the Solar System. (NASA/JPL/UNIVERSITY OF ARIZONA)

4.) Io: Jupiter’s volcanic world is constantly torn apart by tides, resurfacing itself via its molten lava interior. In many ways, Io is the counterpoint of Callisto, showcasing what a large Moon can be like with an extraordinary amount of tidal heating from orbiting too close to a gas giant. Io displays:

  • a total of more than 400 active volcanoes, making it the most geologically active object of all,
  • plumes of sulfur and sulfur dioxide that rise as high as 500 km (300 miles) above its surface,
  • and more than 100 mountains, many rising higher than Earth’s Mt. Everest, due to uplifting events inside Io.

Io has virtually no craters, as it is constantly resurfaced, and many regions with molten lava visible at any given time. Io is the most water/ice-poor world in the entire Solar System, primarily composed of silicate rock with a metal-rich core.

The maria — or seas — of the Moon’s surface visible on the near site. The sea of tranquility (Mare Tranquillitas) was the site of Apollo 11’s landing. Our moon likely formed from a giant impact tens of millions of years after the other planets formed, and makes our Moon the only large satellite of a terrestrial planet known to date. (NASA/GSFC/ARIZONA STATE UNIVERSITY, ANNOTATIONS BY STARDATE / THE UNIVERSITY OF TEXAS MCDONALD OBSERVATORY)

5.) Moon: The only satellite of a rocky world on this list, our Moon may well be the youngest large object in the Solar System. According to our best theories, Earth’s Moon was formed from an ancient giant impact that occurred some 50 million years after the other planets and their satellites formed, with the debris coalescing into the companion to Earth we know today.

Like all other moons on this list, our Moon is tidally locked to its parent planet, with the same side always facing our world. It does have its own internal heat source: primarily from the decay of radioactive elements. The Moon’s composition is very similar to the composition of Earth rocks, making it unique among all the large non-planetary objects in the Solar System.

Europa, one of the solar system’s largest moons, orbits Jupiter. Beneath its frozen, icy surface, a liquid water of ocean is heated by tidal forces from Jupiter. (NASA, JPL-CALTECH, SETI INSTITUTE, CYNTHIA PHILLIPS, MARTY VALENTI)

6.) Europa: The smallest and most hospitable of Jupiter’s four large moons, Europa is covered in water-ice with a subsurface, liquid ocean. Similar to Ganymede, Europa has a very thin atmosphere made mostly of oxygen, due to the sublimation of the volatile ices on its surface. Unlike the other moons on this list so far, however, Europa’s icy surface and large volume make it the smoothest object in the Solar System, despite its striated appearance.

The heat from tidal flexing, induced from Jupiter’s gravitational pull, is thought to cause the subsurface ocean to remain liquid, driving the ice to move in a fashion similar to plate tectonics. With surface chemicals being actively transported to the subsurface ocean below, plus the hydrothermal heating from beneath, Europa’s oceans may potentially harbor extraterrestrial life. Cryovolcanic plumes, similar to Saturn’s Enceladus, were first detected in 2013.

Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. Color was synthesized by combining high-resolution images taken through orange, violet, and ultraviolet filters; these images were displayed as red, green, and blue images and combined to create this color version. The reddish color by the pole is thought to be a result of ultraviolet light reacting with methane, similar to what’s been seen more recently on Pluto, pointing towards a similar origin. (NASA / JPL / USGS)

7.) Triton: Neptune’s largest moon was once the Solar System’s largest Kuiper belt object, but was gravitationally captured a long time ago. Orbiting close in at a mean distance of only 355,000 km, both rings and moons are nowhere to be found around Neptune until you reach a distance more than 15 times as great. Triton, during its capture, must have cleared out a huge fraction of the Neptunian system!

Orbiting in a retrograde fashion (counterclockwise, as opposed to clockwise), Triton is the only large moon to exhibit this characteristic, further evidence of its captured nature. It’s an active world that resurfaces itself over time, with erupting geysers, a thin, Pluto-like atmosphere, and covered in a mix of nitrogen, water, and carbon dioxide ices. Its smoke-emitting cryovolcanoes point to a subsurface ocean and ongoing activity.

Triton makes up 99.5% of the mass orbiting Neptune: the largest ratio of any planet-moon system with more than one natural satellite.

Pluto and its moon Charon; image composite stitched together from many New Horizons images. Pluto is the 8th largest non-Planet in our Solar System; Charon ranks at number 17. (NASA / NEW HORIZONS / LORRI)

8.) Pluto: Finally, we get to everyone’s favorite former planet, and the first non-moon on our list. Smaller and less massive by far than Triton, and less than half the diameter of Mercury, the Plutonian system is the first one in the Kuiper belt to be imaged from up close. Its large natural satellite, Charon, was likely formed from a giant impact, along with its four other moons: Styx, Nix, Kerberos and Hydra.

Charon, in particular, is so large that it makes the Plutonian system a binary one, where the center-of-mass of the system lies outside of Pluto itself. Its geological history also points to an active world, as giant ice mountains, snows, valleys, and sublimating plains show a frozen world in motion. Along with many worlds on this list, Pluto likely has a liquid ocean beneath the surface, raising more questions about biochemistry and organics than it answers.

Eris can barely be imaged even with the most powerful of telescopes, as its extreme distance from the Sun, even with its white color and large size, makes it impossible to resolve with current technology. All that we know about it have had to come from very clever measurement techniques, along with a little bit of serendipity. (WIKIMEDIA COMMONS USER LITEFANTASTIC)

9.) Eris: Nearly as large as Pluto but more massive, Eris’ current location, near the aphelion of its orbit, places it at approximately three times the Sun-Pluto distance. Until last month, Eris was, with the exception of some long-period comets, the most distant object known in the Solar System. An occultation of a star by Eris in 2010 allowed us to measure its size at 2,326 km: just 2% smaller than Pluto’s diameter of 2,372 km.

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

Other than its mass, size, and orbital period, there is very little known about Eris due to its tremendous distance. It has at least one natural satellite: Dysnomia, is whiter in color than either Triton or Pluto, contains surface ices and a thin atmosphere similar to both of those worlds, and takes 558 years to complete an orbit around the Sun. If we launched a fly-by mission to Eris in 2032, a gravity assist from Jupiter could get a spacecraft there in merely 24.7 years.

This high-resolution color composite of Titania was made from Voyager 2 images taken Jan. 24, 1986, as the spacecraft neared its closest approach to Uranus. Voyager’s narrow-angle camera acquired this image of Titania, one of the large moons of Uranus, through the violet and clear filters. The spacecraft was about 500,000 kilometers (300,000 miles) away. (NASA / VOYAGER 2)

10.) Titania: Only by going all the way down to the tenth largest non-planet in the Solar System can we finally arrive at one of Uranus’ moons, of which Titania is the largest. Significantly smaller than Eris, Titania is under 1,600 km (1,000 miles) in diameter, and consists of approximately equal amounts of ice and rock. There may be a thin layer of liquid water at the core-mantle boundary of this world, and displays moderate cratering that points towards a resurfacing event relatively early in its history, after most of the impacts affecting the other nearby moons had already occurred.

There is both water ice and carbon dioxide ice on the surface of Titania, which may indicate a very thin, tenuous carbon dioxide atmosphere. Occultations of a star failed to reveal any atmosphere at all, however; if one exists, it would likely take approximately ten trillion of them to equal the pressure at the surface of Earth. It was only ever studied up close once: by Voyager 2 in 1986.

When you rank all the moons, small planets, and dwarf planets in our Solar System, you can see that many of the largest non-planetary objects are moons, with a few being Kuiper belt objects. It isn’t until you get all the way down to Sedna or Ceres that we find a world that doesn’t fall into one of those two categories. (MONTAGE BY EMILY LAKDAWALLA. DATA FROM NASA / JPL, JHUAPL/SWRI, SSI, AND UCLA / MPS / DLR / IDA, PROCESSED BY GORDAN UGARKOVIC, TED STRYK, BJORN JONSSON, ROMAN TKACHENKO, AND EMILY LAKDAWALLA)

The next-largest objects on the list include other moons of Saturn (like Rhea and Iapetus) and Uranus (e.g., Oberon), followed by the other dwarf planets of the Kuiper belt and Pluto’s giant moon, Charon. If the idea that there is a large object some ~200 AU away, provisionally called either “Planet Nine” or “Planet X” turns out to be correct, it may knock everything on this list down a peg, or may even be classified as a planet itself.

Many of the objects we currently think of as having some importance in the Solar System, such as Ceres, the largest asteroid (at #25), or Sedna, a possible Oort cloud object (at #23), don’t come close to cracking the top 10. There is so much to learn from looking at what’s around us and where it is. Rather than argue about classification, we should appreciate our cosmic backyard for exactly what it is, and all the riches contained therein.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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