Comparative sizes of four dwarf planets, which may be joined by up to 50 more objects.
Credit: Hubble Space Telescope
SYDNEY: The status of former planet Pluto has taken another blow, with new research suggesting up to 50 known objects may also meet the criteria to be dwarf planets.
To be labelled as a dwarf planet, an object must meet two criteria, as determined by the International Astronomical Union (IAU): they must be 'nearly round' and they must orbit the Sun.
Until now, however, the point at which 'potato-shaped' planets become 'nearly round' has not been formally defined, said astronomer Charley Lineweaver from Australian National University (ANU) in Canberra.
Potato radius
Lineweaver used the phrase 'potato radius' to describe the point at which objects shaped like potatoes are pulled into nearly spherical shapes by their own gravity, in a paper published in the Proceedings of the 9th Australian Space Science Conference.
Informally, 400 km has been used as the potato radius. However, this is not appropriate for most objects.
"The strength of the material is an important feature that determines how big you have to be to be a sphere," said Lineweaver.
Icy objects have smaller potato radii
"If you're ice you're kind of weak and you only have a radius of 200 km to make you spherical. If you're rocky, like an asteroid, you need about 300 km or more."
'Trans-Neptunian Objects', or TNOs, are those that orbit the Sun at a greater average radius than Neptune.
Astronomers only know of five that have radii larger than 400km, but there are many more between 200 and 400km.
Growing number of dwarf planets
"Of all those 50 objects that will be let into the club, all of them are icy objects," said Lineweaver, "so the relevant potato radius will be 200 km."
According to Lineweaver, Pluto's family is set to get even bigger. "The whole project of finding trans-Neptunian objects didn't even start until 1995 or so," he said. "In 10 years there might be another 50 or 100."
To be classified as a proper planet, an object must have cleared the neighbourhood around their orbit of other debris, according to the IAU. This was why Pluto was demoted to dwarf planet status in August 2006, despite its radius of 1,150 km.
But Pluto is not the largest known trans-Neptunian object - Eris has a radius of about 1,300 km.
Simon O'Toole, an astronomer at the Anglo-Australian Observatory in Coonabarabran, New South Wales, questions the criteria used by the IAU to demote Pluto, pointing out that Saturn, with its rings, has not cleared its own orbit but is still classified as a planet.
No changes until at least 2012
O'Toole said that the concept of the potato radius is useful because it will provide a more physical, quantitative definition for dwarf planets.
But whether the idea is adopted by the astronomical community could depend on politics. "This would have to be put forward at an IAU general assembly, which is held every three years," said O'Toole.
The next IAU General Assembly is to be held in Beijing in August 2012.
