A warmer, fluffier Pluto.

Because Pluto has been receding from the sun for more than a decade, planetary scientists presumed that its temperature had dropped and its nitrogen atmosphere had shrunk. But between 1988 and 2002, Pluto's atmosphere nearly doubled in size and its temperature increased by about 1[degrees]C, report James L. Elliot of the Massachusetts Institute of Technology and his colleagues in the July 10 Nature. Another team, led by Bruno Sicardy of the Observatory of Paris, reports in the same journal that Pluto's atmospheric pressure doubled during that 14-year interval.

The researchers observed Pluto on Aug. 20, 2002, when the planet passed in front of a bright star (SN: 9/7/02, p. 148). Several large telescopes recorded the starlight filtering through Pluto's tenuous atmosphere, revealing the atmosphere's pressure, temperature, and height. The scientists then compared their measurements with those made in 1988, when Pluto eclipsed another star.

Elliot attributes the paradoxical results to two possible causes. One of them is that Pluto's surface has darkened in recent years, which has led to an increase in the amount of heat it absorbs from the sun. The other is a lag between the time when the planet made the closest approach to the sun, which was in 1989, and when the planet grew warmer.

Elliot estimates that the atmosphere could continue expanding for another 10 to 20 years. That's good news for a proposed mission to Pluto, which would arrive in 2015, when there will still be an active atmosphere to examine. Inevitably, however, as Pluto continues moving away from the sun, the planet's atmosphere will collapse onto its surface, leaving a naked ball of ice.--R. C.