Has a tenth planet really been found?
Yes, probably, but the answer depends on the exact definition of a planet. This Summer, astronomers discovered in the outer part of the Solar System an object that is very likely larger than Pluto. Since most of us consider Pluto to be a planet, something larger than Pluto would plausibly also be a planet. The new object's temporary name is 2003UB313, although a permanent name will certainly be selected before long.
This short answer is only part of the story, however. The full story involves some uncertainties in measurement, other discoveries, a bit of history, and even astronomical politics.
A little over a decade ago, astronomers began discovering a whole new class of objects in the outer part of the solar system, largely beyond the orbits of Neptune and Pluto. Many thousands of these objects make up a collection that has become known as the Kuiper Belt. Most of these are small pieces of ancient material circling the fringes of the Solar System. Some of the short-period comets entering the inner Solar System are thought to originate from the Kuiper Belt. Although most of these Kuiper Belt Objects (KBO) are too small to be individually interesting, a number of the larger ones have acquired names, including Sedna, Quaoar, Ixion, Varuna, and Chaos. None of these is as large as Pluto.
2003UB313 is the largest KBO yet discovered, and it may well be larger than Pluto. The size cannot be measured directly, because 2003UB313 is too far away (currently 97 times the distance from the Earth to the Sun, compared with Pluto at about 30 times the Earth-Sun distance). Based on its brightness, it has to be almost as large as Pluto, even if it reflects like a mirror. If its reflectivity is less (like fresh snow), then 2003UB313 is larger than Pluto.
Does that make 2003UB313 a tenth planet? That raises the question of the definition of a planet, something that not all astronomers agree on. These days we do not call the asteroids between Mars and Jupiter planets. largely because they are small, even though they too are in orbit around the Sun. Because Pluto is smaller than any of the inner planets (even smaller than Earth's moon), and because it resembles the Kuiper Belt Objects in its properties, some astronomers prefer to call Pluto a KBO instead of a planet. Others prefer the traditional approach of calling Pluto (which does have its own moon, Charon) a planet.
One of the reasons that 2003UB313 does not yet have an official name is that the International Astronomical Union (IAU) is trying to decide how to deal with it. One IAU committee is trying to decide whether it is a planet. If it is, then a second committee of the IAU has responsibility for naming it. If instead 2003UB313 is a KBO, then its name must be approved by yet another IAU committee.
For
more details and pictures showing the discovery of 2003UB313, please visit:
http://www.gps.caltech.edu/~mbrown/planetlila/#name
If
you would like to learn more about Kuiper Belt Objects (and read a different
view of whether 2003UB313 is a planet), please see:
http://www.ifa.hawaii.edu/faculty/jewitt/kb.html
This week's question comes from Dr. Dave Thompson. Dr. Thompson is an astrophysicist who studies gamma rays in the Exploration of the Universe Division. He helped build, test, and analyze data from EGRET on the Compton Gamma Ray Observatory, and he is now helping build part of the Gamma Ray Large Area Space Telescope (GLAST), scheduled for launch in 2007. His particular scientific interest is gamma-ray pulsars.