Author: Tom Siegfried / Source: Science News
If Dr. Seuss had been an astronomer, Horton the Elephant (who heard a Who) would have said “a planet’s a planet, no matter how small.
”Even Pluto.
But don’t quote Dr. Seuss to the International Astronomical Union. In 2006, the IAU declared Pluto a planet not.
IAU Resolution B5 (not to be confused with Le Petit Prince’s asteroid B 612) declared that in order to be considered a planet, a body must clear the neighborhood around its orbit. Pluto, then, doesn’t qualify, because its “neighborhood” (way out beyond the orbit of Neptune) is populated by other bodies referred to as trans-Neptunian or Kuiper Belt objects. Two of them, Haumea and Makemake, have been recognized as “dwarf planets,” the same designation that the IAU now applies to Pluto.
This demotion of Pluto to dwarf status (no offense intended to dwarfs) makes sense, IAU defenders contend, because the asteroids (orbiting the sun mostly between Mars and Jupiter) aren’t planets, either — no one of them has cleared out the orbital neighborhood. After all, nobody would call an asteroid a PLANET. Except actually, nearly everybody called them planets for 150 years after they were discovered. Only half a century or so ago did astronomers stop considering most asteroids to be planets. And that shift had nothing to do with clearing out any neighborhoods, Philip Metzger of the University of Central Florida and colleagues point out in a new paper.
“The planetary science community did not reclassify asteroids on the basis of their sharing of orbits, which had been known … since the mid-19th century,” write Metzger and coauthors (including Alan Stern of the Southwest Research Institute in Boulder, Colo.). “Rather, they were reclassified beginning in the 1950s on the basis of new data showing asteroids’ geophysical differences from large, gravitationally rounded planets.”
When astronomers first discovered asteroids (Ceres and Pallas, in 1801 and 1802), the famous astronomer William Herschel did not consider them planets. He called them asteroids because they were “starlike,” too small to appear through his telescope as bigger than a point. All previously known planets (Mercury, Venus, Mars, Jupiter, Saturn and Uranus) appeared as discernible disks. Those (except Uranus) had been known since ancient times; the Greeks called them…
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