A US team announced over the weekend it had discovered a 10th planet in the icy Kuiper belt 15 billion kilometres from Earth.
It said the discovery, provisionally named 2003 UB313, is the first object bigger than Pluto ever found in the outer solar system.
But Dr Andrew Prentice, a mathematical astronomer with the Centre for Stellar and Planetary Astrophysics at Australia's Monash University, believes the new object is more likely to be a refugee from a hostile solar system.
"I believe this new planet may more likely be an interloper that's been thrown out of another nursery and is now looking for a new mum," he says.
Interstellar interloper or 10th planet?
Prentice says an object must technically travel around the Sun on a flat plane and have a circular orbit to be considered a planet.
2003 UB313, which is rumoured to soon be named Xena, orbits at a 45° angle to the rest of the solar system.
It was also found in a region of space littered with rubble from the formation of the solar system 4.5 billion years ago.
Prentice says the same criteria rule out Pluto, making Neptune the last of the true planets.
He says 2003 UB313 is probably an irregular chunk of evicted interstellar debris and a companion of Sedna, another large object recently discovered in the Kuiper belt.
A third new object in the belt, Quaoar, holds more claim to being our 9th planet because of its circular orbit and a flat plane, he says.
Dr Quentin Parker, an astronomer with Australia's Macquarie University and the Anglo-Australian Observatory, agrees 2003 UB313's peculiar orbit mitigates against it being a planet, as does its size and distance from the Sun.
Parker says the only way to prove its planetary credentials is to obtain samples to see "if it's made out of the same stuff as the rest of the solar system".
"If you discover the isotopic abundancies in the materials do not match the isotopic abundancies in the rest of our solar system it's come from somewhere else."
"Either we have a 10th planet or we have only eight planets," he says.
Time to rewrite the textbooks?
Professor of planetary astronomy Mike Brown of the California Institute of Technology, who announced the discovery, is confident this is the first planet found since Pluto was controversially identified in 1930.
He says the sheer size of 2003 UB313 qualifies it as a planet.
He says its brightness and distance suggests it is at least as large as Pluto, which measures 2302 kilometres in diameter.
Brown and colleagues at the Gemini Observatory in Mauna Kea, Hawaii and Yale University, realised they had found something new on 8 January 2005, after analysing data collected more than a year earlier.
They first spotted the candidate new planet on 31 October 2003 at Palomar Observatory near San Diego, California.
Astronomy's governing body, the International Astronomical Union, is expected to rule on whether or not 2003 UB313 is a new planet later this year.