[meteorite-list] Astronomers to Decide What Makes a Planet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Aug 1 19:08:50 2005
Message-ID: <200508012307.j71N7t425273_at_zagami.jpl.nasa.gov>

http://www.nature.com/news/2005/050801/full/050801-2.html

Astronomers to decide what makes a planet
Jim Giles
nature.com
August 1, 2005

Status of newly discovered world hangs in the balance.

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.

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.

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.

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.
Received on Mon 01 Aug 2005 07:07:54 PM PDT