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Pluto's family set to grow tenfold
New research by Australian scientists suggests the number of objects in the solar system classed as dwarf planets should grow by a factor of ten.

The work, which will appear in the Proceedings of the 9th Australian Space Science Conference and published on the prepublication website, would see a number of objects join the former planet Pluto in this newly defined family.

According to the International Astronomical Union (IAU) definition, the boundary between whether an object is defined as a dwarf planet or a smaller solar system body is based on whether the object is round and has a diameter of 400 kilometres or more.

Dr Charley Lineweaver of the Australian National University's Planetary Science Institute believes that definition is fairly arbitrary.

"If a body has enough mass, its own gravity forces it into a spherical shape, without enough mass it remains potato shaped," he says. "If it's large enough to be round then it should classified as a dwarf planet."

Lineweaver and colleague Dr Marc Norman, calculated the minimum size objects such as asteroids, icy moons and Kuiper belt objects must be to become round due to their own gravity.

"The surprise was that icy objects down to roughly 200 kilometres and rocky objects down to 300 kilometres were round," he says. "If hydrostatic equilibrium or roundness is one of the guidelines for classifying an object as a dwarf planet, rather than just a small solar system body, then there are ten times more Pluto like objects out there".

Lineweaver says the correlation between shape and mass is fairly conclusive.
Emotive topic

The definition of planets and dwarf planets has been a controversial and highly emotive topic since the IAU demoted Pluto in 2006.

Previously, some scientists had argued that its highly elongated and tilted orbit made it a large Kuiper Belt object rather than a true planet.

The final nail in Pluto's coffin came soon after the discovery of a number of large objects beyond Neptune - such as Eris, Makemake and Sedna - which bear a striking resemblance to Pluto.

Currently the solar system is known to have eight planets, five dwarf planets and thousands of small solar system bodies orbiting the Sun.

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