080612 Pluto assigned “plutoid” tag in new IAU classification

Top Stories

		Successful first test of high speed Moon penetrator ...high speed penetrators that could one day be used to breach the surface of planets, moons and asteroids have successfully passed their first test in the UK... read more (includes video) Hubble brings galaxies out of coma ...NASA’s Hubble Space Telescope has captured the magnificent starry population of the Coma Cluster galaxies, one of the densest known galaxy stockpiles in the Universe..... read more Amateurs help radio astronomers glimpse astrophysical jet ...the first astrophysical radio jet has been detected from an outbursting white dwarf nova, suggesting that all compact accreting binary systems are capable of creating radio jets... read more

Spaceflight Now +

		Subscribe to Spaceflight Now Plus for access to our extensive video collections! How do I sign up? Video archive STS-120 day 2 highlights Flight Day 2 of Discovery's mission focused on heat shield inspections. This movie shows the day's highlights. Play STS-120 day 1 highlights The highlights from shuttle Discovery's launch day are packaged into this movie. Play STS-118: Highlights The STS-118 crew, including Barbara Morgan, narrates its mission highlights film and answers questions in this post-flight presentation. Full presentation Mission film STS-120: Rollout to pad Space shuttle Discovery rolls out of the Vehicle Assembly Building and travels to launch pad 39A for its STS-120 mission. Play Dawn leaves Earth NASA's Dawn space probe launches aboard a Delta 2-Heavy rocket from Cape Canaveral to explore two worlds in the asteroid belt. Full coverage Dawn: Launch preview These briefings preview the launch and science objectives of NASA's Dawn asteroid orbiter. Launch \| Science Become a subscriber More video

Pluto assigned 'plutoid' tag in new IAU classification
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 12, 2008

Almost two years after the International Astronomical Union (IAU) caused a worldwide furore by stripping Pluto of its former status as a 'proper' planet to a dwarf planet, the term 'plutoid' has been introduced to describe "Pluto-like transneptunian dwarf planets".

The IAU's new Solar System, as defined in 2006, with Pluto, Eris and Ceres named as dwarf planets. Now, Pluto and Eris are plutoids, while Ceres remains a dwarf planet. Image: IAU.

The original demotion of Pluto to a dwarf planet came about as a result of numerous discoveries of Pluto-like bodies, some even larger in size, in the far reaches of our Solar System. If they were treated the same as Pluto, they too would have to be called planets, taking the Solar System’s planet inventory to more than 50, a prospect that was even less favourable than relegating just one planet to a sub-category, which also included the bodies Ceres and Eris.

Now, the IAU have once again re-written the textbooks to introduce a new term – 'plutoid' – to describe “celestial bodies in orbit around the Sun at a distance greater than that of Neptune, that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a near-spherical shape, and that have not cleared the neighbourhood around their orbit of debris”. The body must also have an absolute magnitude brighter than +1 to be considered as a plutoid and be named by the IAU as one. If, subsequently, the plutoid candidate turns out to not be massive enough to be classified as one, it will still keep its name, but will change category.

The new plutoid category of Solar System bodies includes Pluto and its moons Charon, Hydra and Nix (left) and Eris and its moon Dysnomia (right). Image: IAU.

The new classification systems means that while Pluto and Eris are the first plutoids of the Solar System, Ceres remains a dwarf planet, because it is located in the asteroid belt between Mars and Jupiter. It is expected that more plutoids will be named as science progresses and new discoveries are made.