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Last Updated: Wednesday, 16 August 2006, 13:36 GMT 14:36 UK
Q&A New planets proposal
Image: NASA, ESA, and A Schaller (for STScI)
2003 UB 313 will become the 12th planet, if the plan is approved(Image: Nasa/Esa/A Schaller)
Astronomers have drawn up a new way to define the word planet. This would mean adding three new planets to the Solar System, boosting the current tally of nine to 12.

The draft proposal by the International Astronomical Union (IAU) will be put to a vote at a meeting underway in the Czech capital Prague.

Did we really have no definition for a planet before this?

None that was formally agreed. For centuries, planets were simply objects that moved in the sky with respect to the background of fixed stars. The word "planet" comes from the Greek word for "wanderer". But astronomers now know of hosts of large objects orbiting the Sun in the outer reaches of our Solar System. Finally, the IAU decided it was time to agree on a scientific definition of the word.

Infographic, BBC

So what definition have they decided on?

For a celestial object to be considered a planet, it must satisfy two conditions:

  • The object must be in orbit around a star, but must not itself be a star
  • It must have enough mass for the body's own gravity to pull it into a nearly spherical shape

Any object with a mass greater than 0.6% that of our Moon and a diameter greater than 800km would normally meet the second condition. But borderline cases will have to be resolved by more observation.

Some objects currently considered to be moons and asteroids could be eligible if they meet those basic tests. So the definition leaves the door open for other objects to join the expanding club.

So was Pluto downgraded or not?

In one sense, Pluto has been demoted. This world has always been the odd one out; it has less than one four-hundredth the mass of Earth and has a tilted, elliptical orbit around the Sun.

Trans-Neptunian Objects
2003 EL61
2005 FY9
2002 TX300
2002 AW197

The IAU's draft proposal recognises eight "classical" planets - Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune - three "plutons" - Pluto, Charon and UB313 - and the asteroid Ceres.

Many astronomers think Pluto is part of a vast population of icy objects that orbit the Sun beyond Neptune, in a region called the Kuiper Belt. These objects have a different origin from planets like Earth and Jupiter; they are thought to be leftover debris from the formation of the Solar System.

The new category of plutons distinguishes Pluto and other "icy dwarfs" from classical planets in that they reside in orbits around the Sun that take longer than 200 years to complete, circle the Sun with high "inclination" (are highly tilted with respect to the classical planets) and typically have orbits that are far from being perfectly circular.

But the plutons fit all the criteria of the new definition of a planet, so Pluto does not receive the full demotion some astronomers had hoped for.

Is the Moon now eligible to be called a planet?

No. The Moon is a satellite of the Earth. There is a common centre of gravity between the Earth and Moon, or barycentre, that resides below the surface of the Earth.

So what do we know about the new "planets"?

Ceres is the largest object orbiting in the asteroid belt between Mars and Jupiter. It was recognised as a planet when it was first discovered in 1801. But because 19th century astronomers could not resolve the size and shape of this object, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, it has been referred to as an asteroid or minor planet.

Ceres  Image: Nasa/Esa/J Parker (SwRI)/P Thomas (Cornell)/L McFadden (University of Maryland, College Park)
Ceres: regains the status it lost more than a century ago
Ceres fits the new definition because it has enough mass for its own gravity to pull it into a nearly spherical shape.

Charon was discovered in 1978 by James Christy, an astronomer at the US Naval Observatory in Washington DC. Until now, it has been regarded as a moon of Pluto. But both Pluto and Charon have enough mass to be spherical - both bodies independently satisfy the proposed definition of a planet.

Pluto and Charon are being referred to as a "twin" planet because their common centre of gravity, or barycentre, is located in free space outside the surface of Pluto.

2003 UB313 was discovered by US astronomer Mike Brown and his colleagues at the California Institute of Technology three years ago. This year, measurements by the Hubble Space Telescope confirmed that it is larger than Pluto. Like Pluto, it is an icy body that orbits beyond Neptune in the Kuiper Belt.

It is almost 10 billion miles from the Sun and more than three times more distant than Pluto. 2003 UB 313 Pluto takes more than twice as long to orbit the sun as Pluto.

The name is a temporary one; the IAU must agree on an official one. Mike Brown's group have nicknamed the object Xena, after the TV warrior princess, and its moon Gabrielle, after the fictional character's sidekick.

Does this mean the number of planets in the Solar System could rise even higher?

Yes, it is almost certain to. Since the early 1990s, astronomers have spotted many new objects orbiting the Sun beyond Neptune - a feat made possible by advances in technology.

The IAU has a "watchlist" of at least a dozen other potential candidates that could become planets once more is known about their sizes and orbits.

These include Sedna, which could be the first object to be discovered in a hypothetical region of the Solar System known as the Oort Cloud, as well as several Kuiper Belt objects with comparable sizes to Pluto.

Three asteroids - Vesta, Pallas and Hygiea - are also on the watchlist.

Astronomers recently discovered two new satellites around Pluto. Does this make Pluto a quadruple planet?

No. The two newly discovered smaller bodies in orbit around Pluto are too small and not massive enough for gravity to force them in to a spherical shape. They are satellites, even though the centre of gravity about which they orbit is located outside the surface of Pluto.

Planets plan boosts tally to 12
16 Aug 06 |  Science/Nature
Experts meet to decide Pluto fate
14 Aug 06 |  Science/Nature
Crunch time for Planet Pluto
20 Jun 06 |  Science/Nature
Nasa reinstates asteroid mission
28 Mar 06 |  Science/Nature
Distant world tops Pluto for size
01 Feb 06 |  Science/Nature
Mission guide: New Horizons
19 Jan 06 |  Science/Nature
Pluto probe launches from Florida
20 Jan 06 |  Science/Nature
The girl who named a planet
13 Jan 06 |  Science/Nature
Two new moons found around Pluto
01 Nov 05 |  Science/Nature
Astronomers detect '10th planet'
30 Jul 05 |  Science/Nature

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