[meteorite-list] 2003 UB313 Reignites a Planet-Sized Debate
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Feb 6 14:27:29 2006
Xena reignites a planet-sized debate
06 February 2006
The heated debate over what constitutes a planet has reignited following
last week's confirmation that the most distant planet-like object object
ever seen in the solar system is larger than Pluto. But astronomers
tasked with settling the issue say the argument could drag on for years.
The International Astronomical Union (IAU), responsible for resolving
such issues, assembled a special working group to decide on the
definition two years ago, when a large new body called Sedna was found
in the outer solar system.
But since then, several other large worlds have been discovered,
including 2003 UB313, unofficially dubbed Xena. This body became widely
known as the "tenth planet" as it appeared to be larger than Pluto,
which is about 2300 kilometres across. Now, new results from an
independent team appear to confirm this, finding Xena is about 30% wider
However, astronomers are bitterly divided over what constitutes a
planet. And when the IAU's working group was forced to issue its verdict
in October 2005, it failed to find a definition all 19 members could
agree on. So it simply reported on the relative popularity of three
different proposals - each group member was allowed to vote for more
than one proposal.
A narrow majority of 11 members favoured deeming anything larger than
2000 kilometres a planet. Under this scheme, Pluto would remain a planet
and it would be joined by several newly discovered worlds, including
Xena. But some group members argued such a size cut-off was arbitrary,
set only so Pluto could retain the title of ninth planet.
Another option attempted to come up with a scientific justification for
a size cut-off. In this plan, planets would have to be massive enough
for their gravity to hold them in a stable shape - a requirement that
could be met by objects as small as 600 kilometres across.
"It complicates matters because we get some dozens of new planets, but
on the other hand, there's some scientific justification" for the size
cut-off, says group member Brian Marsden of the Harvard-Smithsonian
Center for Astrophysics in Cambridge, Massachusetts, US, who had the idea.
In this scheme, which eight group members voted for, all planets would
be sorted into four sub-categories based on their location and
composition, those being:
o Terrestrial planets, such as Earth and Mars
o Jovian planets - gas giants such as Jupiter
o Cisjovian planets - large asteroids such as Ceres
o Trans-Neptunian planets, such as Pluto
Another proposal argued that a planet is the dominant body in its
immediate neighbourhood. This would demote Pluto, as it is one of
several bodies of similar size in the Kuiper Belt - a ring of icy
objects beyond Neptune. Six group members voted for this option, which
would leave the solar system with eight official planets.
Division and discord
Because the group was so divided it simply issued a report on its
discord, and not a resolution for the IAU's executive committee to put
up for a wider vote. That vote would most likely occur at an IAU general
assembly meeting, which occur every three years. The next will take
place in Prague, Czech Republic, in August 2006.
That meeting could see a vote if the executive committee, a
representative from an IAU member nation, or one of the IAU's divisions
puts forward a resolution on the definition of a planet. "But I don't
think that is likely given the difficulty the experts had in coming up
with a consensus," says Robert Williams, one of 10 members of the IAU's
executive committee and an astronomer at the Space Telescope Science
Institute in Baltimore, Maryland, US.
Williams adds that waiting may also have scientific merit because
astronomers have been focusing on defining planets in our solar system,
while little is known about the formation and evolution of the 170 or so
known planets around other stars.
"We're trying to define things we don't fully understand," he says. "It
may be a bit frustrating that we're not quite at the point where we can
agree on what a planet is. But the more important thing is a fundamental
understanding of what's going on - I would put that as a higher priority
than the naming convention we adopt."
Received on Mon 06 Feb 2006 02:25:25 PM PST