Pluto, the smallest and most distant planet in our solar system, has revealed a strange phenomenon to astronomers. Instead of shrinking as the planet moves away from the Sun, Pluto鈥檚 atmosphere has grown bigger.
Two independent teams of astronomers drew the conclusion after watching as stars passed behind the planet.
In the fourteen years since Pluto鈥檚 atmosphere was first discovered, the planet has been steadily receding from the Sun. Therefore it should be getting colder making the atmosphere freeze back onto its surface.
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But, according to Bruno Sicardy, of the Observatoire de Paris in France and colleagues, the pressure of the atmosphere has doubled. Falling temperatures would be expected to cause some gas in the atmosphere to freeze, producing a drop in pressure and a contraction of the atmosphere.
By studying the rate at which two stars dimmed as they passed behind Pluto on 20 July 2002 and 21 August 2002, both teams determined that the planet鈥檚 atmosphere has expanded. Even so, it remains a million times more tenuous than Earth鹿s.
鈥淎ll of the obvious reasons to explain an expanding atmosphere just don鈥檛 work here,鈥� says Michael Person, at the Massachusetts Institute of Technology and co-author of the second study.
Strange phenomenon
Person speculates that ice on the planet鈥檚 surface may be getting darker and thus absorbing more sunlight, which would cause it to release more gas into the atmosphere. Or that Pluto鈥檚 passage through its orbit may have perhaps exposed a previously hidden ice cap to the sun.
Unfortunately, neither team will be able to continue their investigation of this strange phenomenon in the near future. Pluto occultations are few and difficult to predict because the planet鹿s orbit is not well known.
鈥淭here are no further Pluto events this year,鈥� says Person. 鈥淭here are a few candidates coming in the next few years but no certain ones.鈥�
On-off mission
The one plan that might help is NASA鈥檚 Pluto fly-by mission, called New Horizons, which has been pencilled in, then cancelled, on a number of occasions. Currently this is due to launch during 2006, encountering Pluto about a decade later.
Pluto is also the only major solar system body never to have been visited by a space probe.
After so much uncertainty, Person hopes that, this time, the mission will fly. He says, 鈥淭here is nothing better than in-situ measurements. A fly-by mission would help enormously.鈥�
Star occultations have played an important role in the study of Pluto before. The planet鈥檚 atmosphere was first discovered when Israeli astronomer Noah Brosch noticed that an occulted star鹿s light gradually dimmed, instead of turning off like a light bulb, as it passed behind the planet.
A follow-up observation in 1988, one year before Pluto鈥檚 closest approach to the Sun in 248 years, confirmed the existence of the atmosphere and furnished astronomers with a candidate composition: molecular nitrogen. The Sun鹿s rays were assumed to have liberated the gas from the icy surface.
Journal reference Nature (vol 424, p165-168)