10th Planet? And You Never Heard of It

Actual Photos of Pluto in Rotation
(Click to view animation) 

In 2006, the International Astronomical Union downgraded the celestial body Pluto from a "planet" to a "dwarf planet." Many people felt a certain kind of loss, because Pluto had been considered a planet since its discovery 76 years earlier in 1930. Many people, in fact, feel like it should still be a planet because it is round (not like those asteroid things that look like big misshapen rocks floating around), is large (certainly all planets are bigger than moons; that's why they're called "planets"), has moons orbiting it, and orbits the Sun (otherwise, it would be a comet or something like that). Well, if that's your thinking, then I have some interesting facts to tell you.

First, of all, why are some objects in Space round like a ball, but others are rocky, jagged, and highly irregular shapes? Gravity. Once an asteroid or comet or whatever gets to be a certain size -- perhaps through collisions with other objects and they bond together -- the gravity caused by the mass of the object causes it to collapse into a spherical shape. So, all jagged edges (not counting a mountain here and there) are pulled down until the surface is relatively smooth, all points equidistant from the center. A body in Space becomes spherical when it is approximately 800 kms (500 miles) or more in diameter. That's the magic number.

Since Pluto is spherical, we already know that it's got to be at least 800 kms in diameter. In fact, its diameter is about 2,322 km. That sounds pretty big, doesn't it? It must be a planet. No, it's not. If you think that's large, what if I told you that there is another "dwarf planet" in the Solar System that is even larger? And it has its own moon. And it orbits the Sun. And you have never heard of it.

ERIS

Eris, discovered in 2005, is a dwarf planet that is slightly larger than Pluto. It was the discovery of Eris that motivated astronomers to redefine what constitutes a planet. As distant as Pluto is from the Sun, Eris is three times further away -- in fact, it is the furthest known object in the Solar System. So, if you insist on calling Pluto the ninth planet, then you must refer to Eris as the 10th. Oh, so now the designation of Pluto as a dwarf planet is beginning to sound more acceptable to you, eh?  

LUNA

But wait, there's more. There are non-planets in the Solar System that are even larger than Pluto and Eris -- and they're not even called dwarf planets. For instance, there is our own Moon, Luna. Its diameter is a whopping 3,400 km -- about 46% larger than Pluto. Did you know that the Moon is larger than what had been known as a planet? Is the Moon also larger than the current smallest planet, Mercury? I'll give you a minute look outside and compare. No, it's not. Mercury is actually about 40% larger than the Moon, at 4,880 km in diameter.

But there is a moon that is larger than the planet Mercury.

TITAN

Titan is a moon of Saturn. Discovered all the way back in 1655, it is 5,150 km in diameter, about 5.5% more than Mercury. Titan has other peculiarities as well. For instance, it is the only moon in the Solar System known to have its own thick atmosphere, and the only object besides Earth known to sustain large stable bodies of surface liquid. Granted, its oceans consist of pure liquid hydrocarbons instead of salt water that is inundated with spilled hydrocarbons like we have on Earth, but it is in liquid form nonetheless.

In a previous blog entry ("Image of the Day: Saturn's Eclipse of Sun"), I provided a video of the Huygens-Cassini spacecraft as it traversed the Saturn system of rings and moons. Here is the actual video of its landing on Titan in 2005, as it transmitted images back to Earth. In the beginning, you'll see the Earth and Moon as they pass in front of the Sun, an event viewed from Titan only twice in a thousand years. The "soundtrack" is the actual radio wave noise generated by Saturn.

Landing on Titan

So, Titan certainly sounds like the largest non-planet object in the Solar System, even larger than the dwarf planets, and the actual planet Mercury. Well, there is something even larger.

GANYMEDE

Ganymede, discovered in 1610 by Galileo, is a moon that orbits Jupiter. It is larger than Pluto, larger than our Moon, larger than Mercury, and even larger than Titan. With a 5,270-km diameter, it is the largest moon in the Solar System. It is about 52% larger than our Moon, and even reaches 78% the size of the planet Mars (6,800 km). It's big.

So, the next time you are gazing up at the full Moon (ours, that is), try to imagine that it's about 52% larger in the sky. That's how the other larger moons would look if one of them orbited Earth. Then imagine how the lunar tides would be drowning you a few minutes later.

 Ganymede Compared to Earth, Moon