Dozens of comets are seen every year. Understand that, according to Kepler's second law, the time a comet spends passing through our solar system is but a small fraction of its orbital period: it moves far faster at perihelion than at aphelion. Therefore, if we start with any non-perverse distribution of comets (i.e., one that is consistent with the virial theorem), each one we see implies the presence of a large number of comets that we do not see. But it gets worse than that: comets that pass through the solar system get eliminated eventually by collisions and by sublimation. What we see are the survivors. So that's another big factor by which we must multiply our sample. Put those factors together and you end up with substantial populations of comets, even with extremely conservative assumptions.
Asking how we know the Kuiper belt and Oort cloud are there is like asking how we know that the Earth is full of molten rock. We don't see much molten rock, but we get enough samples of it that we can deduce its presence.
August 24, 2001
By Richard Stenger
CNN
(CNN) -- An icy body beyond Pluto has unseated the asteroid Ceres as the largest minor planet in the solar system, breaking a record that stood for two centuries, European astronomers announced this week.
The frozen rock has a diameter of at least 1,200 kilometers (745 miles), much larger than the old titleholder, a colossal boulder discovered in 1801 by Italian astronomer Giuseppe Piazzi.
Ceres, which resides in the main asteroid belt between Mars and Jupiter, has a diameter of about 950 kilometers (590 miles).
The new object is half the size of Pluto and slightly larger than Pluto's moon Charon. It orbits the sun in a ring of primordial comets beyond the planets known as the Kuiper Belt.
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