The tenth planet in the solar system has a moon at least a tenth of its size. The discovery, made on 10 September with an adaptive optics system on the Keck II telescope, will allow astronomers to pin down the mass of both objects.The tenth planet is not officially a planet - for now its only official designation is 2003 UB313. By convention, its moon - announced by the International Astronomical Union on Sunday - is designated S/2005 (2003 UB313) 1.But Mike Brown, the Caltech astronomer who revealed 2003 UB313 in July and nicknamed it Xena after the television warrior princess, is calling the moon Gabrielle, after the princess's companion.Moons of "trans-Neptunian" objects - which orbit farther from the Sun than Neptune - are very hard to see because they are faint and appear very close in the sky to the objects they orbit. But Brown began searching for a moon as soon as he found 2003 UB313. "Having a moon is just inherently cool," he says.Moons can also reveal vital information about how a planet formed.

And astronomers can calculate the masses of both planet and moon by measuring the moon's orbit and period.So far Brown's group has only one night of observations of the moon, in which Gabrielle appeared as a faint dot 4.4 magnitudes fainter than Xena, moving with it across the sky. The moon is ten times closer to its planet that our Moon is to Earth, and the pair are 97 times further from the Sun than the Earth is. Viewed from Earth, they are separated by only half an arc-second.The best estimate of Xena's size so far is about 2700 kilometres in diameter, about one-fifth the size of the Earth. Gabrielle, which is about a tenth the size of Xena, is about one-eighth the size of our Moon.An intriguing consequence of the latest discovery is that three of the four largest objects beyond Neptune are now known to have moons, including Pluto and 2003 EL61.

View: Full Article | Source: New Scientist Space

How many "10th" planets do we have now? I know of this one, Sedna, Quaoar, 2003 EL61, etc...

some of them were later dismissed or confused for other objects i think....

there must be alot of objects resembling planets out there...

^True, but for this very small "planet" to have a moon is very intriguing!

i wonder if the ratio between its moon and the 'planet' is like pluto and charion

There have been a few KBO's found with a companion, (moon). Heck, we've even found an asteroid with one: http://www.space.com/scienceastronomy/sola...moon991006.html

I personally don't consider Pluto a planet myself, I't is just the first binary KBO found.

yeh there have been a few reports, all i knew was sedna and the quaoar, bout time they made up their minds, it will be easy to remember though for education.

lol new way to learn the planets lol not much changed now though

my very easy method just speeds up naming planet x

Is there actually a planet x? I've always heard about it, but where's the proof?

The so called "planet X" is a theory to explain what it is in the Kuiper belt or Oort cloud that disturbs the orbit of comets and asteroids, sending them towards the inner solar system. They say that it'd need to be either a gas giant, Jupiter like, or a red or brown dwarf star, (the so-called Nemesis theory). None of these new planetoids are even close to the size needed for that. Most of the new planetoids are much smaller than Pluto, a couple are the same or slightly larger. It is difficult to find a planetoid, planet or dwarf star in the Kuiper belt or Oort cloud due to both the plethora of other bodies present and the lack of reflecting light, (the sun from Sedna, the furthest planetoid found I believe, can be blocked by a pinhead at arms length). Hopefully with the new generation telescopes and other instruments being made, all kinds of new planetoids will be found, though I doubt whether a planet X will be found.