Icy Pluto is part of hot debate

It was billed as a debate over the 2006 decision by the International Astronomical Union that kicked Pluto out of the family of planets, leaving just eight.

But in the end, after a jocular and noisy tussle before scientists and educators gathered at the Johns Hopkins University's Applied Physics Laboratory, both debaters agreed that the IAU's definition only muddied the waters, and that more time is needed for science to sort out the increasingly complex range of objects circling our sun and other stars.


"Get the notion of counting things out of your system," said Neil deGrasse Tyson, director of New York's Hayden Planetarium. "The more we learn about anything, the more we have to tune the vocabulary we use to describe it."

The two debaters also expressed delight that a scientific debate has captured so much public attention.


"How many scientists get to have their business in the op-ed pages, in comic strips?" said Tyson.

Mark Sykes, director of the Planetary Science Institute in Tucson, Ariz., agreed that it's not how many planets we count that's important.

"It's that debate, that exposure of issues to people that's more important," he said. "This is helping to expose a little bit more of the messy side of science, this clash of ideas ... and it's a good, positive thing."

Tyson and Sykes faced off before scientists and educators gathered to discuss the IAU's definition of planets to the exclusion of Pluto.

Tyson, an astrophysicist, is OK with Pluto's reclassification as a "dwarf planet" or "plutoid." In his view, the word planet desperately needed a new definition that reflects all that's been learned about the solar system since the ancients first noticed that planets behaved differently than stars.

Pluto has been revealed to be different from the eight other planets. "The time has come to discard the useless words and reinvent an entire system to respect the level of science we have achieved," he argued.

Sykes found the IAU's new definition confusing and disruptive. He argues that anything orbiting the sun that's large enough that gravity has pulled it into a near-spherical shape - including Pluto - should be classed as a planet.

"We're interested in which objects exhibit similar characteristics, so that when we send spacecraft out into the solar system ... we can understand these things better," he said. The IAU's definition won't even work on the planets found circling other stars.


"So why not use a word that's easily understood?" he asked

Tyson argued that such a definition would lump tiny, icy Pluto with giant gaseous Jupiter. "The word you choose gives you tunnel vision."

"That's why God made subcategories," Sykes snapped back. It's why "animal" includes everything from an amoeba to a blue whale.

The debate over Pluto's status began almost immediately after its discovery in 1930, by a 24-year-old Lowell Observatory astronomer named Clyde Tombaugh. The news media trumpeted the young American's find as the "ninth planet." But as astronomers gradually revealed how truly odd Pluto was, doubts began to creep in.

Pluto turned out to be tiny - two-thirds the size of Earth's moon and barely 17 percent the diameter of Earth itself. Its orbit is tilted 17 degrees out of the plane of the rest of the planets. And, for 20 years out of its 249-year journey around the sun, Pluto actually passes inside the orbit of the eighth planet - Neptune. That last occurred between January 1979 and February 1999.

The more they learned, the more some astronomers concluded that Pluto looked less like the other planets and more like the icy objects they had begun to discover beyond Neptune.


Since 1992 more than 1,000 "trans-Neptunian objects", or TNOs, have been cataloged. Like Pluto, they're believed to be made of ices, dust and rock.

The largest found so far is Eris, named for the Greek goddess of strife and discord. First spotted in 2003, it is believed to be spherical and 27 percent larger than Pluto. But there are others: Sedna, Quaoar, Orcus, Ixion, Varuna and Makemake.

NASA initially proclaimed Eris the 10th planet. That spurred the IAU to try to nail down some firm criteria for the designation, which had never been precisely defined.

To qualify as a planet in our solar system, they concluded, an object had to circle the sun and be large enough so that gravity has pulled it into a round shape. And, they said, it must have "cleared the neighborhood around its orbit."

That last proviso baffled some astronomers. As APL astronomer Hal Weaver pointed out, Neptune still hasn't cleared its neighborhood either, as pesky Pluto demonstrates.

The 2006 IAU decision dumped Pluto, Ceres and Eris into a new category, called "dwarf planets."


Sykes and hundreds more scientists reacted with a protest petitions to the IAU.

Everyone, on both sides of the debate, hopes to learn more about Pluto and its trans-Neptunian neighbors when the $700 million APL-managed New Horizons spacecraft flies by Pluto in July 2015. After passing Pluto, New Horizons is programmed to fly on into the Kuiper Belt to study the icy worlds beyond.