Space Forum / Astronomy / August 2006

http://www.nasa.gov/vision/universe/solarsystem/planetsf-20060824.html

Honey, I Shrunk the Solar System
August 24, 2006

Media contact: Jane Platt/JPL
(818) 354-0880

If you woke up Thursday morning and sensed something was different
about
the world around you, you're absolutely right. Pluto is no longer a
planet.

The International Astronomical Union, wrapping up its meeting in
Prague,
Czech Republic, has resolved one of the most hotly-debated topics in
the
cosmos by approving a specific definition that gives our solar system
eight planets, instead of the nine most of us grew up memorizing.

NASA has already visited all eight planets that retain their official
title: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and
Neptune.
In addition, the agency has its New Horizons spacecraft en route to
Pluto, which the astronomical union has designated as the prototype for
a new class of celestial objects, to be called "dwarf planets."

"NASA will, of course, use the new guidelines established by the
International Astronomical Union," said Dr. Paul Hertz, Chief Scientist
for the Science Mission Directorate at NASA Headquarters. "We will
continue pursuing exploration of the most scientifically interesting
objects in the solar system, regardless of how they are categorized."

Ceres, which orbits in a belt between Mars and Jupiter and is the
largest known asteroid, is one of those interesting objects. In 2007,
NASA will launch the Dawn spacecraft on a mission to study Ceres, which
the astronomers have placed in the dwarf planet category, alongside
Pluto. The dwarf planet family also includes 2003 UB313, nicknamed
"Xena." When Dr. Mike Brown of Caltech and his colleagues announced
last
summer that they'd discovered the object, which is bigger and farther
away than Pluto, many astronomers decided it was time to figure out
once
and for all, "What exactly is a planet, anyway?"

Here's how it all shakes out. The International Astronomical Union has
decided that, to be called a planet, an object must have three traits.
It must orbit the sun, be massive enough that its own gravity pulls it
into a nearly round shape, and be dominant enough to clear away objects
in its neighborhood.

To be admitted to the dwarf planet category, an object must have only
two of those traits -- it must orbit the sun and have a nearly round
shape. And no, moons don't count as dwarf planets. In addition to
Pluto,
Ceres and 2003 UB313, the astronomical union has a dozen potential
"dwarf planets" on its watchlist.

What's to become of the other objects in our solar system neighborhood,
the ones that are not planets, not dwarf planets and not moons? The
organization has decided that most asteroids, comets and other small
objects will be called "small solar-system bodies."

Despite the establishment of these three distinct categories, there are
bound to be gray areas. As technologies improve and more objects are
found, the International Astronomical Union will set up a process to
decide which of the three categories are most appropriate for specific
objects.

Even before the discovery of Xena, not all was calm in the planetary
world. There was debate after Clyde Tombaugh discovered Pluto in 1930.
With its small size, distant location and odd orbit, some questioned
whether Pluto was really a planet or just an icy remnant of the
planet-forming process.

That issue has been resolved by the International Astronomical Union.
Among those most keenly following the debate -- Mike Brown, who has
been
awaiting word on Pluto and the object he found, Xena.

"I'm of course disappointed that Xena will not be the tenth planet, but
I definitely support the IAU in this difficult and courageous
decision,"
said Brown. "It is scientifically the right thing to do, and is a great
step forward in astronomy."

Although the revamping of our solar system might seem unsettling, it's
really nothing new. In fact, when Ceres was first discovered in 1801,
it
was called a planet, as were several similar objects found later. But
when the count kept on growing, astronomers decided "enough is enough,"
and they demoted Ceres and its siblings, placing them in a new
category,
called asteroids.

The International Astronomical Union has been naming planets and moons
since its founding in 1919. For more information, visit the
International Astronomical Union home page at www.iau.org
or www.iau2006.org .