For the second time, scientists have discovered a 10th planet in the Solar System. Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University) have found that a member of the Kuiper Belt, an object designated 2003 UB313 by the International Astronomical Union (IAU), has a radius comparable to that of Pluto. These researchers had previously found the distant object 2003 VB12, dubbed Sedna, that is no more than 80% of Pluto's radius. This earlier discovery was hailed by some as the discovery of a tenth planet, although the IAU threw cold water on that designation. Undoubtedly the discovered body will reinvigorate the debate of which bodies should be considered planets, and which should be considered planetoids.
The Kuiper Belt is an planetoid belt beyond the orbit of Neptune. The planetoids in this belt are the remnants of a gas disk that encircled the Sun early in the Sun's life. The objects in this belt have a broad range of sizes, with Pluto being the largest. All of the bodies in the Kuiper belt are characterized by very eccentric orbits that generally are out of the plane of the ecliptic.
The new Kuiper Belt object 2003 UB313 is in a highly elliptical orbit with a semimajor axis of the orbit is 67.5 AU. The orbit has a perihelion at 38 AU, which is 8 AU outside of the orbit of Neptune, and an aphelion at 97 AU, giving the orbit an eccentricity of 0.44 (the eccentricity times the semimajor axis give the offset of the Sun from the center of the elliptical orbit). The orbital period is 560 years. The plane of 2003 UB313's orbit is out of the ecliptic by 44°. These characteristics are common for the scattered subclass of Kuiper Belt objects; members of this subclass are thought to have been scattered by Uranus and Neptune from orbits inside of 30 AU to their current orbits far from Neptune.
How is the size of 2003 UB313 determined? The ideal way to size a planetoid is to observing the infrared radiation it emits. Because this radiation is black-body radiation, one can determine the surface area of the object by measuring the temperature of the black-body and the energy flux reaching Earth. But 2003 UB313 is too distant to observe in the infrared with the Spitzer infrared telescope, which is a satellite orbiting Earth and the best instrument available for such observations. One is therefore forced to estimate the size from the amount of sunlight the planetoid reflects. If the planetoid scatters 100% of the sunlight striking it, then its size is 97% of Pluto's, and if it scatters the amount of light from its surface that Pluto scatters, then its size is 125% of Pluto's. The optical observations therefore suggest that 2003 UB313 is slightly larger than Pluto.
As matters stand, 2003 UB313 and Pluto are the largest of the Kuiper Belt objects, with Pluto's moon Charon and Sedna the next largest objects. Research on the Kuiper belt suggest that it could contain up to 10 objects the size of Pluto, all farther away than Pluto.
So is 2003 UB313 a planet? This likely will be a contentious issue, because some, including this author, feel that Pluto itself should never have been classified as a planet. Undoubtedly this issues will be debated for some time. As it stands, Pluto is only 47% the radius of Mercury, and because Mercury is about twice as dense as Pluto, Pluto is only about 5% of Mercury's mass. Given this large difference, it may be better to remove Pluto from the class of planets than to promote 2003 UB313 to this class.