 Pluto by Hubble
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July 9, 2003 — Pluto, a planet so puzzling that some astronomers say it isn't a planet at all, seems to be defying the laws of atmospheric physics, according to studies published on Thursday in the British weekly journal Nature.
The most distant planet of the solar system follows an egg-shaped path with agonizing slowness as it crawls around the sun, each orbit taking the equivalent of 248 Earth years.
In 1989, as Pluto swung by on its closest point to the sun, evidence emerged that it had a thin atmosphere.
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Since then, the planet has been pulling away from the sun, which means that, in theory, its surface temperature should be falling and its atmosphere should be contracting. Atmospheric pressure depends on heat, because gases expand when the temperature rises.
But astronomers in the United States and France say the opposite is happening: not only has the atmosphere failed to collapse as expected, its pressure seems to have doubled over the past 14 years.
Even more puzzling: Pluto's atmosphere, believed to be mostly nitrogen derived from nitrogen ice, has warmed by around one degree Celsius (1.8 Fahrenheit) since perihelion in 1989.
The studies are written by a team led by James Elliot of the Massachusetts Institute of Technology (MIT) and another by Bruno Sicardy of the Paris Observatory.
They took advantage of a stroke of good luck; in 2002, Pluto passed in front of a bright star. Over time, the dimmed starlight, seen through Earth-bound telescopes, allowed them to make the most accurate measurements yet of the planet's atmosphere.
Quite why Pluto's atmosphere is expanding rather than contracting is unclear.
In a guest commentary in Nature, William Hubbard of the University of Arizona's Lunar and Planetary Laboratory suggested it may be something simple, such as a time lag: it may take many years, even by Pluto's tortoise-like calendar, for changes in solar heat to affect the heating and cooling of a planet's surface.
Pluto is the only solar system planet that has not been visited by a spacecraft.
NASA has a mission, New Horizon, that is scheduled for launch in 2006, with a flyby of Pluto a decade later, but the project is in doubt because of costs.
Pluto was discovered in 1930. It is unusual because it is a small rocky planet, whereas the four other outer planets of the solar system are "gas giants" — they are believed to have a small solid core, swathed in roiling, suffocating layers of methane, nitrogen and other gases.
For this and other reasons, some astronomers don't consider Pluto a true planet, but rather an object belonging to the Kuiper Belt, a region on the fringes of the solar system that is named after Dutch-U.S. astronomer Gerard Kuiper, who in 1951 predicted that other bodies would be found beyond the orbit of Neptune.
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