A few years ago the solar system got a bit more crowded when astronomers at the Palomar Observatory spotted a tenth ‘planet,’ romantically named 2003 UB313, circling the sun out beyond Pluto’s orbit. Questions of whether this thing constituted a real planet or just an especially reflective asteroid should have been put to bed last year when some folks at the Keck Lab in Hawaii discovered that the rock, codenamed ‘Xena,’ has a moon.
Who’s an asteroid now, tough guy
Some people weren’t satisfied. Maybe it’s a big asteroid with a really little asteroid spinning around it, they said. Nope (Nature, subscription wall. you know the drill):
The recently discovered ‘tenth planet’ of our Solar System is substantially larger than Pluto, astronomers have found.
For many, the discovery that object 2003 UB313 is about 3,000 kilometres across will remove any doubt that it deserves to be called a planet.
…When astronomer Mike Brown of the California Institute of Technology in Pasadena unveiled 2003 UB313 to the world in July 2005, his team was already confident that the new object was at least as large as Pluto, and deserved the status of ‘planet’.
But UB313’s elongated orbit takes it almost twice as far away from the Sun as Pluto ever gets, making it very difficult to measure its diameter precisely. One clue to its larger size came from the fact that it is slightly brighter than Pluto; a larger mirror would reflect more of the Sun’s light. But an alternative explanation could have been that UB313 is simply made of a more reflective material than Pluto.
Using the Institute for Millimetre Radio Astronomy (IRAM) 30-metre telescope in Spain, Bertoldi’s team has now studied the radiowaves coming from UB313, which reveal how much of the Sun’s rays are absorbed and re-radiated as heat. Because very little reflected sunlight is emitted at these wavelengths, the object’s brightness in radiowaves depends only on its size and surface temperature.
Based on its enormous distance from the Sun, UB313 is calculated to be tremendously cold: a staggering -248 °C. Bertoldi and his colleagues combined this value with their measurements of UB313’s radiation to determine its reflectivity and size.
Assuming they’re about the same density (they’re not, but bear with me), that puts Xena at about 2.3 times as massive as Pluto and one-75th as massive as our own blue rock. The International Astronomical Union (IAU) hasn’t thought up a proper name yet, so if you have any ideas you can either leave them here or drop them a line yourself.