Blogtrek

A new planet? Probably not

Astronomers today announced the discovery of an object in the distant reaches of the Solar System about 1200 miles in diameter. This makes it 80% the diameter of Pluto, which some say is a planet. Since Pluto is a planet, they say, that makes this new object, named 2003VB12 or Sedna after an Inuit goddess, a planet also. Finally the tenth planet has been discovered. Or has it?

I don't think so. In fact, I do not think of Pluto as being a planet. It is too small. What is a planet? That can be tricky. It orbits the Sun. Hipparcos, the solar observatory satellite, orbits the Sun and is only as big as a living room. So it's more than that. It is large. So therefore Mercury is a planet. So Ganymede, a satellite of Jupiter, is a planet because it is larger than Mercury. It has a gravity so strong that someone could not hop off it. But that would make Ceres a planet. So what is a planet?

I think the definition that seems to fit the best is that it have a mass of about 200 yottagrams (or 0.2 xonagrams) . Ganymede may be larger than Mercury but it is less massive. Ganymede is a snowball as large as my hand, while Mercury is a billiard ball or a cannonball. It is more dense, and it is more massive. Ganymede has a mass of 148 yottagrams, and Mercury has a mass of 330 yottagrams. (Earth's mass is 5.974 xonagrams). Under this criterion, Mercury is a planet but Ganymede is not. I also add the requirement that nuclear fusion is not going on in the object (except for nuclear wars) and that the object does not glow. Under these requirements, there are just eight planets in the Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is not a planet, since its mass is only 13 yottagrams. The mass of Sedna is even less yet, so it is not a planet either.

So what are these? Pluto is clearly a Kuiper Belt object: an asteroid that orbits somewhere just outside Neptune's orbit. About a thousand have been discovered, and some of these are huge, compared to Mars-Jupiter asteroids: Quaoar, Ixion, Varuna, and Charon are all bigger than Ceres. But Sedna is way, way out. At its closest, it is still just outside the Kuiper Belt, whereas at aphelion, it is much farther out, stretching into the Öoo (term used by Nigel Calder, meaning Öpik-Oort, and pronounced eu-oh, where eu is the sound of eu in French feu) Cloud. So Sedna is not a Kuiper Belt object. It is something unique. Maybe it is an Öoo Cloud object, even though so far the main residents of the Öoo Cloud are thought to be long-term comets. If an object that large can be out there and is not a Kuiper Belt Object, might there be other, even larger objects out there? If any are found with a mass of 200 yottagrams or more (it would have to be slightly larger than Mercury at least), then we will truly have found a tenth planet.