Quote:
|
Originally Posted by freddie
Bah. Too bad. I thought nostalgia would win them over.
Actually one might say now that Mercury is a dwarf planet as well - I'm sure there are many planet like objects in the kuiper's belt which are bigger than Mercury & Pluto combined.
|
Aside from the definitional point Haku made about being the dominant body in its region of space, Mercury is a lot bigger than Pluto or Xena. It's over twice the diameter, eight times the volume and 25 times the mass. Mars is only about 40% wider than Mercury and twice the mass. Two moons in the solar system - Ganymede and Titan - are slightly bigger than Mercury, but they have less than half the mass. Mercury is a more substantial object than any of the moons in the solar system. Pluto is smaller and less massive than all seven big satellites in the solar system - Ganymede, Titan, Callisto, Io, the Moon, Europa and Triton. Pluto has one-sixth the mass of our Moon.
It's unlikely that there's anything else much bigger than Xena in the Kuiper Belt - Mike Brown's surveyed most of the sky now and anything bigger would have to be in an even more inclined orbit than Xena.
There may be Earth or Mars sized planets much further out than the Kuiper Belt. There are two theories of planet formation. One, the disc instability theory, says that the giant planets formed very quickly out of instabilities in the disc of gas that surrounded the Sun as the solar system formed. It says there should be no big objects beyond Neptune. The other, the oligarchic theory, says that there were about 60 objects that formed and then collided together to create the planets we know. But in computer simulations several Mars to Earth-sized objects have near-misses with Jupiter and get thrown by its gravity into very distant and wild orbits. Neptune and Pluto are about five billion kilometres from the Sun. Sedna is about fifteen billion kilometres from the Sun (100 times as far as the Earth, or 100 Astronomical Units) and its orbit takes it out to about 150 billion kilometres (1000 AU). It's predicted that these planets, if they exist, will be somewhere between 1000 AU and 10 000 AU. Current telescopes can't find them, but there are plans to build an 8.4 metre telescope to scan the sky for them. If they exist, they'll be real planets, not minnows like Pluto. Maybe there are only 8 planets in the solar system, but maybe there will turn out to be twice as many - real planets. We'll be glad we didn't proclaim planet 9 to planet 90 to be tiny ice balls.