The solar system may have a 10th planet whether astronomers want it or not.
A planet-like object first discovered last summer in orbit 15 billion kilometres (nine billion miles) away from the sun has finally been measured and is less than half as big again as the current "last" planet, Pluto.
The International Astronomical Union now has a problem on its hands. Should the newcomer with the ungainly name, 2003 UB313, join the rest to become the 10th planet given it is bigger than Pluto? Or should it decide that Pluto was too small to be a planet, leaving us with only eight members of the solar system club?
This is no small decision given we have got used to the notion of nine main planets since the discovery of Pluto in 1930. The IAU set up a committee to agree a new definition of what constitutes a planet and it is considering all the options, including the possibility that Pluto might get the chop.
The discovery of 2003 UB313 may have forced the IAU to make a decision on the issue, but finding an agreed view has been made more difficult now that we know how big the new planetoid is. German and French scientists collaborated to get the measure of 2003 UB313 and published their findings this morning in the journal, Nature.
They found its diameter is about 3,100km, compared with Pluto's diameter of 2,300km.
It is too far away to get a handle on its size using optical methods, even when examined by the powerful Hubble space telescope. Instead, to guess its size, the research team used radio telescopes based in the Sierra Nevada mountain range of southern Spain to measure the amount of heat the body radiates.
This data was added to measurements of how much sunlight the body reflects to calculate its assumed diameter. It would take the visit of a satellite to confirm the calculations but scientists believe they are accurate to within 300km.
The new planet is bigger than Pluto even if this error tends towards a smaller body, so any decision on whether to make it the 10th planet in the solar system has been made much more difficult.
If 2003 UB313 goes, however, how can we retain Pluto as a full member of the solar system?
Scientists knew they had to come up with a credible way to decide a body's planetary status given the discovery over the past 14 years of about 1,000 so-called "trans-Neptunian objects". They are in orbit beyond Neptune, the eighth planet from the sun, and more and more are being discovered using very sensitive digital detectors on large telescopes, according to Scott S Sheppard of the Carnegie Institution of Washington, who wrote an accompanying article in Nature.