"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.
frank.roylance@baltsun.com
