Screaming Penguin - Source Does Matter

Astronomical detective work led to the stunning discovery of a large new world beyond Pluto – and hiding in plain sight. The object could be the biggest in the Kuiper belt of rocky objects that orbit the outer reaches of the solar system.

The first data made public about the object suggested the object could be up to twice the size of Pluto, but newly revealed observations indicate the object is about 70% Pluto's diameter.

The find suggests more such objects are waiting to be discovered and is likely to reignite the fierce debate about what constitutes a planet.

On Thursday, an email with the subject, "Big TNO discovery, urgent" was sent to a popular astronomy mailing list. The message described the discovery of a "very bright" object that was creeping along slowly beyond the orbit of Neptune - making it a Trans-Neptunian Object, or TNO.

If the reflectivity is as dim as most other distant, rocky objects that have been studied, the object "would be larger than Pluto," Jose-Luis Ortiz, an astronomer at the Sierra Nevada Observatory in Spain, wrote in the email. Pluto is about 2300 kilometres across.

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The observations were then verified by the International Astronomical Union's Minor Planet Center (MPC) in Cambridge, Massachusetts, US, which designated the object 2003 EL61.

Estimates of the object's brightness posted by the MPC on Friday at 0027 GMT suggested the object could be as large as twice Pluto's diameter if it was relatively non-reflective object. In the hours since, another team of astronomers revealed independent data on the object taken with some of the world's most powerful telescopes. They give the object's size at about 70% Pluto's diameter, in line with estimates for a relatively reflective object in the first MPC notice. They say also say the object is orbited by a tiny moon.

NewSci

Mass by moon

The moon is not the first discovered around a Kuiper Belt object, but it is the smallest, only about 1% the mass of 2003 EL61. More importantly, observations of the satellite's 49-day orbit allowed Brown to precisely calculate the masses of both 2003 EL61 and its moon.

Brown's results - posted on his website - show the object is about 32% as massive as Pluto. Assuming its composition is similar, that implies its diameter is about 70% of Pluto's, or about 1600 kilometres. That would probably make it larger than Sedna, an object beyond the Kuiper Belt discovered earlier by Brown's group.

Brown had not made his findings public because he was waiting for infrared data from the Spitzer Space Telescope, which he could use to calculate how much visible light 2003 EL61 reflected. That would allow him to calculate its size more precisely.

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