An important discovery orbiting 2003 UB313 (the “10th Planet”)

Thursday, October 6th, 2005 at 12:07 PM by Juan

Many amateur astronomers were excited by the discovery (announced earlier this year) of the Kuiper Belt Object 2003 UB₃₁₃, a world in the outer solar system which is (very likely) larger than Pluto. Since there has been no accepted standard for distinguishing “minor planets” (a.k.a. asteroids) from “major planets” (what most of us just call “planets”), it had become generally accepted that Pluto was the smallest planet. And Pluto is indeed, very small, with a diameter of approximately two thirds that of Earth’s moon. What surprised astronomers is that 2003 UB₃₁₃ appears relatively bright given its current distance of 97 Astronomical Units^[1] from the Sun (over twice Pluto’s distance from the Sun). This brightness indicates that 2003 UB₃₁₃ is either 100% reflective of sunlight and about the size of Pluto, or if similar to Pluto in its reflectivity, then it has to be bigger than Pluto … and thus, it has the right to be called a “major planet” In fact, we have been tracking its position as the 10^th planet on our local astronomical info page since its discovery.

(Click on image for full-size version.) And image of 2003 UB313 (center) and moon (right of center) taken September 10, 2005 by the Keck 10-meter telescope.

This week, the folks who discovered 2003 UB₃₁₃, Drs. Mike Brown (Caltech) and Marcos van Dam (Keck Observatory), announced the discovery of a moon orbiting 2003 UB₃₁₃. This discovery has been touted in the mainstream media as more justification for the idea that 2003 UB₃₁₃ is the “Tenth Planet” (indeed, some people are quire upset at NASA for promoting names for these worlds without the approval of the International Astronomical Union). There is a much more important aspect to this discovery: it will allow the determination of the mass of 2003 UB₃₁₃. As I teach my Astronomy 106 students, astronomers can determine the mass of an object as long as something else is orbiting it. It is a simple application of (Newton’s version of) Kepler’s Third Law which relates the masses of the worlds to the time it takes for one world to orbit another and the separation between the two worlds. I expect as Dr. Brown’s group takes more images of 2003 UB₃₁₃, they will be able to determine the separation between these two worlds more accurately and their orbital period, and from that, their masses. From there, they will be able to make additional estimates of their diameters based on their likely composition. All in all, this discovery is very exciting.

Postscript: My opinion on the whole ‘is Pluto a planet or not’ debate can be summarized as follows: There is no obvious physical ‘cutoff’ between “minor planets” and “(major) planets”. The definition of a “(major) planet” has been mostly the result of tradition and not due to some consensus by astronomers. The only obvious distinction I see between asteroids and planets is that most asteroids are not massive enough to make them spherical (due to their own self-gravity). Astronomers could choose the physical cutoff that “(major) planets” need to be massive enough to be spherical. However, if you use that criteria, several known asteroids (including the first one discovered, Ceres) would have to be redefined as “major planets.” I believe most astronomers think that if Pluto were discovered today, it would obviously be classified as a “minor planet.” or asteroid. However astronomers also understand that tradition can be a very powerful influence in any field and so Pluto has remained a “planet.” We will hopefully see the International Astronomical Union (IAU) make a decision on this issue during the next year, although whatever they decide will likely not end the arguments.

Posted in Astronomy News

One Response to “An important discovery orbiting 2003 UB313 (the “10th Planet”)”

1. SCSU Astronomy » Blog Archive » The 10th Planet has been downsized… wrote on 01/31/06 at 12:46 pm :

   [...] Its also interesting to note how much has been learned about this distant world in just a few months[1] … we now have a good idea as to its size, its mass and some idea of its surface composition. [...]