Astroblog

Xena, as UB₃₁₃ is affectionately known, is causing no end of trouble.

On January 25, a study from the Hubble Space Telescope had reported that they measured the disk of UB₃₁₃ and it was just a smidgen over the size of Pluto, with an albedo (reflectivity) of 0.9, roughly the same as fresh snow.

Last week, data from the 30-meter IRAM telescope was reported in Nature (subscription required for the full article). This study, using 1.2 mm infrared light, concluded that UB₃₁₃ had a diameter of 3,000 ± 300 Km, (substantially larger than Pluto at 2,300 km) and an albedo of 0.6 (pretty much like Pluto). They used 1.2 mm IR radiation as at thiswavelengtht object brightness only depends on the surface temperature (calculable for its distance from the Sun) and the object size.

Why the heck are the estimates so different, true the low end of the IRAM study 2,700 may overlap with the Hubble estimate (an accurate figure hasn't been given yet), but the albedo figure's way off. It may be a while before we know. All Mike Brown says on his web site is that the Hubble results will be available late February.

However, both estimates make UB₃₁₃ at least Pluto size or larger. Will it be called a planet now?

Not very likely. There is currently no accepted definition of what is a planet. The International Astronomical Union (IAU) set up a 19 person committee in 2004 to resolve the issue, which reported to the executive committee in November. While there was a reasonable proposall that anything spherical that orbits the Sun and is more than 2,000 kilometers across should be called a planet, there were strong opposing views (including the perennial one to downgrade Pluto), so no decision has been made. As it stands, the IAU looks like it won't make a final decision until August.

Why can't the IAU put together a definition? If astronomers can't, who can? Well,definingg a planet is hard, things like Earth and Jupiter are planets, by tradition, but when does a gas giant stop being a planet and start being a star? The start ofselff sustainedfusionn is an obvious definition, but what about the many large objects that just miss out on starting stellar fusion but are massive isolated objects like stars. Do we call them failed stars or overgrown planets?

Within the solar system we have a continuumm of orbitingg material that proceeds from tiny dust grains, through asteroids to Pluto to the terrestrial planets, with no clear size cut off. Asteroids like Ida and Dactyl, rough hewn chunks of rock of no great size, aren'’t planets. If we say that anything that is large enough to be spherical under its own gravity is a planet, this will make at least two asteroids planets. Vesta is round (large enough for gravity to modify it) differentiated (ie it has a recognizable crust mantle and core, like Earth) and Ceres is also round (but probably not differentiated). If we don't take a recogniseable physical property lie self-gravitation, when does an object become large enough to become a planet?

This isn'’t an easy question, as I said. Hence the The International Astronomical Union's current quandary. Shortly after it was found, it was obvious that Pluto was the odd one out. A ball of ice smaller than all the other planets and many moons (Our Moon, The Galilean Moons of Jupiter, Titan and Triton), it'’s highly elliptical orbit and the large angle of this orbit to the plane of the rest of the planets orbits were all unusual.

Then the Kupier Belt objects were discovered, icy plantessimals left over from the formation of the solar system with highly elliptical orbits at weird angles to the rest of the planets. It was rapidly obvious that Pluto was just a large Kupier belt object, and there have been many calls for Pluto to be demoted, but Pluto has been called a planet for so long that it'’s demotion would be difficult. If we retain Pluto as a planet, then it makes sense to call all things larger than Pluto a planet (and this is the best guess for the result of the IAU committee). Since it is likely that there are several other as yet discovered icy objects out there, be prepared for the 11th and 12th etc planets to be turning up in the next few years.

But if Pluto is a planet, what about 2003 EL61 and 2005 FY9 (or even Vesta or Ceres?). At 3/4 of the mass of Pluto, can we really justify not calling them planets? Making Pluto the cut oarbitrarybitary and not based on any science of planet formation (whereas demoting Pluto is). Here’s the thoughts of Mike Brown, discoverer of UB313, on the subject (with amusing insight into the IAU process.

As I said, a difficult question, and we will have to wait for an answer. But without Xena surprising us on, no one would have got round to making a decision on it.