"Tenth Planet" Larger Than Pluto, Study Confirms

Stefan Lovgren
for National Geographic News
February 1, 2006
Update: On April 11, 2006, astronomers announced that the so-called tenth planet is not as large as previously speculated, though they maintained that it is bigger than Pluto. UB313, nicknamed Xena, is about 1,490 miles (2,400 kilometers) wide, according to new measurements made using the Hubble Space Telescope, the scientists say.

Scientists have measured the size of a solar system object discovered last year and confirmed that it is larger than Pluto.

The icy object, called 2003 UB313, is located in the far reaches of the solar system. It measures 1,900 miles (3,000 kilometers) in diameter. Pluto, by contrast, measures 1,400 miles (2,300 kilometers).

The finding has fueled the debate over what constitutes a planet. Pluto is traditionally considered the ninth planet in our solar system (see interactive map).

Some scientists argue that 2003 UB313 should now be considered the tenth planet of our solar system. Others say Pluto's status as a planet should be reconsidered.

"Since UB313 is decidedly larger than Pluto, it is now increasingly hard to justify calling Pluto a planet if UB313 is not also given this status," said Frank Bertoldi, an astronomer at the University of Bonn and Max Planck Institute for Radioastronomy in Germany.

Bertoldi led the study, which will be reported in tomorrow's issue of the journal Nature.

Largest Discovery

Like Pluto, 2003 UB313 is one of the icy bodies found in the Kuiper belt, a ring of some 100,000 objects on the fringes of our solar system beyond Neptune.

These objects are remnants of the clouds of gas and dust from which the sun and the planets are believed to have formed 4.5 billion years ago.

2003 UB313 is the most distant object ever seen in our solar system. It orbits up to 97 times farther away from the sun than Earth does—almost twice as far as the most distant point of Pluto's orbit.

Mike Brown, an astronomer at the California Institute of Technology in Pasadena, discovered 2003 UB313 a year ago.

Brown and his colleagues could not determine the size of the object. But based on its optical brightness, the body was believed to be at least as big as Pluto.

Researchers behind the new study were able to determine the object's size using a powerful radio telescope in the mountains of southern Spain. Bertoldi and his colleagues from the Max Planck Institute for Radioastronomy measured the amount of heat 2003 UB313 radiates, a value known as thermal emission.

The team found that UB313's diameter is about 440 miles (700 kilometers) wider than Pluto's, making UB313 the largest solar system object found since the discovery of Neptune in 1846.

"We expected it to be larger than Pluto, but how large was really up in the air," Bertoldi said. "Our measurement of the thermal emission produces the first good size estimate."

Dropping Pluto?

The discovery of UB313 has reignited the debate over the definition of the term "planet" and precisely how many objects in our solar system deserve the name.

If Pluto were discovered today, it may not have been called a planet, because it is just one among a myriad of objects in the Kuiper belt.

Some scientists argue that anything orbiting the sun over a certain size should count as a planet. If the benchmark is Pluto, 2003 UB313 would now be the tenth recognized planet in our solar system.

"I would prefer to respect the cultural, historical fact [that Pluto is a planet] and to make an arbitrary cut at Pluto's size and call all objects larger than that planets too," Bertoldi said.

Other astronomers prefer a different approach.

"I like the scientific basis more, which is that an object that has a unique orbit and dominates its environment gravitationally should be called a planet," said Scott Sheppard, who wrote an accompanying Nature article on the subject.

Another option would split planets into subcategories.

Mercury, Venus, Mars, and Earth could be known as terrestrial planets, while Jupiter and Saturn could be formally termed gas giant planets. Pluto and UB313, along with thousands of other Kuiper objects, could be grouped as ice dwarfs.

"These objects all have evolved in a very similar manner, and they're best thought of as an ensemble of objects," said Sheppard, an astronomer with the Carnegie Institution in Washington, D.C.

On the Edge

While they say the labeling of planets is not that important to scientists, Bertoldi and Sheppard believe it is an important topic for the public.

The International Astronomical Union, which rules on such matters, has formed a committee to mull over the competing definitions of the term "planet."

"People basically want to know how many planets there are, and that depends on how you define a planet," Sheppard said. "Nature hasn't been nice to us. Objects don't fall into nice little bins."

"The largest [Kuiper belt] objects out there fall on the edge of the word planet," he added. "You can stretch the word planet to fit them, or you can shrink it to not fit them."

Sheppard says that as technology improves and time goes on, astronomers will find many more objects that are much larger than Pluto.

"There could even be Mars-sized objects way out in the distance of the solar system that we haven't been able to find," he said.

"We have searched just beyond Neptune pretty well. But when you go way out in the distant solar system, objects get extremely faint," the astronomer added.

"It's a large space out there."

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