by Robert Roy Britt
Senior Science Writer
posted: 29 July 2005,
07:59 pm ET
Updated at 11:17 p.m. ET
have discovered an object in our solar system that is larger than
Pluto. They are calling it the 10th planet,
but already that claim is contested. The new world’s size is not at
issue. But the very definition of planethood is. It is the first
time an object so big has been found in our solar system since the
discovery of Pluto 75 years ago.
The announcement, made today by Mike Brown of Caltech,
came just hours after another newfound object, one slightly smaller
than Pluto , was revealed in a very confusing day for astronomers
and the media.
The new object, temporarily named 2003 UB313, is about three
times as far from the Sun as is Pluto.
"It’s definitely bigger than Pluto," said Brown, a professor
of planetary astronomy. The object is round and could be up to twice
as large as Pluto, Brown told reporters in a hastily called NASA-run
teleconference Friday evening. His best estimate is that it is 2,100
miles wide, about 1-1/2 times the diameter of Pluto.
The object is inclined by a whopping 45 degrees to the main
plane of the solar system, where most of the other planets orbit.
That’s why it eluded discovery: nobody was looking there until now,
Some astronomers view it as a
Kuiper Belt object and not a
planet. The Kuiper Belt is a region of frozen objects beyond
Pluto is called a Kuiper Belt object by many
astronomers. Brown himself has argued in the past for
Pluto’s demotion from planet status, because of its diminutive
eccentric and inclined orbit.
But today he struck a different note.
circled here, is seen moving across a field of stars. The
three images were taken about 90 minutes apart.
The planet, circled here, is seen moving across a field of stars.
The three images were taken about 90 minutes apart.
"Pluto has been a planet for
so long that the world is comfortable with that," Brown
said in the teleconference. "It seems to me a logical extension
that anything bigger than Pluto and farther out is a
Offering additional justification,
Brown said 2003 UB313 appears to be surfaced with methane
ice, as is Pluto. That’s not the case with other large Kuiper
Belt objects, however.
"This object is in a class very much
like Pluto," he said.
NASA effectively endorsed the
idea in an official statement that referred to 2003 UB313 as the
10th planet. Yet in recent years, a bevy of objects roughly half
to three-fourths the size of Pluto have been found.
definition for ’planet’
Brian Marsden, who runs the Minor Planet Center where data on
objects like this are collected, says that if Pluto is a
planet, then other round objects nearly as large as Pluto ought to
be called planets. On that logic, 2003 UB313 would perhaps be
a planet, but it would have to get in line behind a handful of
others that were discovered previously.
"I would not call it the 10th
planet," Marsden told SPACE.com
Alan Boss, a planet-formation
theorist at the Carnegie Institution of Washington, called the
discovery "a major step." But Boss would not call it a planet
at all. Instead, he said Pluto and other small objects
beyond Neptune should be called, at best, "Kuiper Belt
"To just call them planets does an
injustice to the big guys in the solar system," Boss said
in a telephone interview.
The very definition of what constitutes
a planet is currently being debated by Boss and others in a
working group of the International Astronomical Union. Boss said the
group has not reached consensus after six months of discussion.
The debate actually stretches back more than
five years and is rooted in the fact that astronomers
have never had a definition for the word "planet," because the nine
we knew seemed obvious.
"This discovery will likely
re-ignite a healthy debate about what is and what is not a
planet," Boss said.
Alan Stern, of the Southwest Research Institute and leader of
NASA’s New Horizons mission to Pluto, predicted in the
early 1990s that there would be 1,000 Plutos out there. He
has also contended, based on computer modeling, that there should be
Mars-sized worlds hidden in the far corners of our solar system and
even possibly other worlds as large as Earth.
In a telephone interview after Friday’s announcement, Stern,
who was not involved in the discovery, said he stands by those
predictions and expects Mars-sized objects to be found within
"I find this to be very satisfying,"
Stern said of 2003 UB313. "It’s something we’ve
been looking for for a long time."
Stern stopped short of calling it
one of the greatest discoveries in astronomy, however, because he
sees it as just one more of many findings of objects in this size
range. Last year, for example, Brown’s team found
Sedna , which is about
three-fourths as large as Pluto. Others include
2004 DW and
Quaoar. Stern sees the outer
solar system as an attic full of undiscovered objects.
"Now we have the technology to see
them," he said. "We’re just barely scratching the surface."
Way out there
The new world is about 97 astronomical units from the Sun. An
astronomical unit is the distance between the Sun and Earth.
It becomes the farthest-known object
in the solar system, and the third brightest of the Kuiper belt
It is colder than Pluto and
"not a very pleasant place to be."
It was found using the Samuel
Oschin Telescope at Palomar Observatory.
Backyard astronomers with large
telescopes may be able to spot
2003 UB313 - planet No.
A map, made
using Starry Night software, shows the position of 2003 UB313
in the eastern
sky as of about 1:30 a.m. from mid-northern latitudes for the next
amateur astronomers with large backyard telescopes should be able to
"It will be visible over the next
six months and is currently almost directly overhead in the
early-morning eastern sky, in the constellation Cetus,"
says Brown, who made the discovery with colleagues
Chad Trujillo, of the Gemini Observatory, and David
Rabinowitz, of Yale University, on Jan. 8.
The team had hoped to analyze the data
further before announcing the planet but were forced to do so Friday
evening because word had leaked out, Brown said.
"Somebody hacked our website," he
said, and "they were planning to make [the data] public."
Brown and Trujillo first
photographed the new planet with the 48-inch Samuel Oschin Telescope
on Oct. 31, 2003. However, the object was so far away that
its motion was not detected until they reanalyzed the data in
January of this year. In the last seven months, the scientists have
been studying the planet to better estimate its size and its
Scientists infer the size of a solar-system object by its brightness
and distance. The reflectiveness of the new planet is not known,
however, which is why the estimate of its diameter ranges from one
to two times the size of Pluto. But those constraints are
well supported by the data, Brown said.
"Even if it reflected 100 percent of
the light reaching it, it would still be as big as Pluto,"
says Brown. "I’d say it’s probably one and a half times
the size of Pluto, but we’re not sure yet of the final size. But
we are 100 percent confident that this is the first object
bigger than Pluto ever found in the outer solar system."
The upper size limit is constrained by
results from the Spitzer Space Telescope, which records heat
in the form of infrared light. Because the Spitzer can’t detect the
new planet, the overall diameter must be less twice Pluto’s size,
Brown has had a running bet for five years with a friend that
an object larger than Pluto would be found by Jan. 1 this
year. 2003 UB313 was spotted on Jan. 8.
"My first reaction was, ’aw, I lost
the bet by seven days,’" he said.
Brown’s team has submitted a name
proposal to the International Astronomical Union and has chosen not
to divulge it until that body makes a decision.