Q: Find the ratio of the mass of Charon to the mass of Pluto.
Q: Compare the distance between Pluto and Charon to the distance between the Earth and the Moon. Some people call Pluto / Charon a "double planet". Can you see why?
A: They are so close in size, and close together in distance.
Scientific context: How do planets get moons? It depends on the planet.
As discussed above, scientists think that Jupiter's four largest satellites
formed in place around it out of leftover material, like the planets formed
around the sun. Not all moons were formed this way, however. Mars has two
tiny satellites that look a lot like members of the asteroid belt (an area full
of small bodies (house-sized to city-sized) that orbits the sun between Mars
and Jupiter). Since Mars is located right next to the asteroid belt, it seems
likely that it captured these two satellites from there when their orbits got
too close to Mars. Jupiter, on the other side of the asteroid belt, has two
groups of moons that orbit outside the four major ones. Once group orbits
prograde (forwards), the other retrograde (backwards). Scientists think these
bodies might have been captured, like Mars' satellites. They might even have
been captured as two larger bodies, each of which then broke apart into a
number of pieces.
The Earth's Moon is harder to explain. It's so large in comparison to the Earth (as you showed above) that it's very unlikely that the Earth could have captured it unchanged, or formed it in place from leftover material. We also know from rocks brought back from the Moon by the Apollo astronauts that the Moon's composition is quite similar to Earth's in most respects, but that it has much less metal than the Earth does. This makes the capture theory even more unlikely, since a body that formed somewhere else in the solar system is unlikely to have a similar composition at all. The currently favored theory is that a giant impact soon after the Earth formed splattered a large amount of molten and rocky material into orbit. Most of the material would have fallen back to the Earth, but if the size and direction of the impact were within a certain range, enough material could have remained in orbit to clump together and form the Moon. Since metals are heavier than plain rocks, the metal would be more likely to fall back to Earth, while the less dense rock could stay in orbit long enough to form the Moon. So this could explain why the Moon's composition is so similar to the Earth's in most respects, but depleted in metals. Scientists are still working on this theory, and all the details have yet to be fully understood.
Pluto, and its moon Charon, are another question. As described above, the planets can roughly be divided into the four inner terrestrial planets, which are small and rocky, and the four outer gas giants, which are huge and gaseous. Pluto doesn't fit either category: it's far from the sun, near the gas giants, but it seems to be a small body made of rock and ice, rather than a huge ball of gas. Some scientists have thought that Pluto might originally have been a satellite of Uranus or Neptune, which escaped somehow (maybe due to a collision) and began orbiting the sun on its own. More recently, scientists have begun discovering icy bodies out beyond the orbit of Pluto. These bodies make up something called the Kuiper Belt. Not much at all is known about them, since they're so small, dark, and far from the sun, but some scientists think that these objects might be leftover remnants of rock and ice from the formation of the solar system, which were too far out to become part of a planet. Pluto might be one of the largest and closest of this class of objects (it's been called the "King of the Kuiper Belt"), and would therefore be a very interesting sample of what the material which formed the solar system was like over 4 billion years ago. We've never sent a space probe to Pluto, so we know very little about it. There have been some recent proposals for such a mission, however, and if one of these is selected for funding, we may soon know much more about this tiny, cold world so far from the sun.
This module was written by Cynthia Phillips, Dept. of Planetary Sciences, University of Arizona, Tucson AZ, and funded in part by the NASA Spacegrant program.
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