Published online 1 August 2005 | Nature | doi:10.1038/news050801-2

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Astronomers to decide what makes a planet

Status of newly discovered world hangs in the balance.

How many planets in our Solar System? Nine? Eight? Twenty?How many planets in our Solar System? Nine? Eight? Twenty?© NASA

The discovery of a new addition to our Solar System has prompted astronomers to fast-track plans to decide what is and is not a planet. The rules, which could be formulated by the end of this week, could more than double the number of local planets - or they could demote Pluto, leaving us with only eight in our neighbourhood.

The number of planets appeared to rise to ten on 29 July, when US astronomers announced the discovery of 2003 UB313, a chunk of rock and ice that orbits near Pluto, around 15 billion kilometres from the Sun.

“Pluto is a planet because culture says it is.”

Mike Brown
California Institute of Technology

Mike Brown of the California Institute of Technology in Pasadena, one of the three-person team that identified the object, says the body is so big it must surely qualify as a planet. He has submitted a name, which he is not disclosing, to the International Astronomical Union (IAU).

But the IAU, which oversees the naming of stars and asteroids, has no criteria for defining planets. An IAU committee has been working on the issue for around a year and had planned to publish its results next summer. Brown's discovery has made the debate more urgent, says Iwan Williams, president of the planetary systems sciences division of the IAU and an astronomer at Queen Mary, University of London. He says a definition should be ready by the end of the week.

Little and large

Most planets in the Solar System are either solid, such as Mercury, Venus, Earth, and Mars, or gas giants, such as Jupiter, Saturn, Uranus and Neptune.

But Pluto and 2003 UB313, both rocky worlds that lie beyond the gas giants, fall into a final and controversial group.

The orbit of the new planetoid isn't circular or in the plane of the other planets.The orbit of the new planetoid isn't circular or in the plane of the other planets.© M Brown

When discovered in 1930, Pluto was thought to exist alone. Astronomers now know it lies in the Kuiper Belt, a jumble of rocky and icy objects that rings the Sun. New objects are continually discovered in the belt. And several Kuiper Belt objects are a similar size to Pluto - 2003 UB313 is thought to be larger. If Pluto is a defined as a planet, then around ten other Kuiper Belt objects should presumably also qualify.

But many astronomers object to this, and argue that Kuiper Belt objects should have a separate status. Williams, for example, points out that gas giants and terrestrial planets are much larger than Kuiper Belt objects, and don't exist in a ring of debris. If the committee follows this reasoning, Pluto could lose its traditional status.

Out of kilter

The orbits of the inner planets also lie in the same plane. But 2003 UB313 and some other Kuiper Belt objects are in a wildly different orbit, at nearly a 45° angle to the rest. Some experts say this wouldn't necessarily discount it as a planet.

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Brown argues that astronomers cannot control what gets called a planet. "Our culture has fully embraced the idea that Pluto is a planet and scientists have for the most part not yet fully realized that the term 'planet' no longer belongs to them," he says.

"Everyone should ignore the distracting debates of the scientists, and planets in our Solar System should be defined not by some attempt at forcing a scientific definition on a thousands-of-years-old cultural term, but by simply embracing culture," says Brown. "Pluto is a planet because culture says it is." And, he adds, that means his new find is a planet too. 

California Institute of Technology