Pluto, the most distant of the nine planets in our solar system, has piqued the curiosity of astronomers once again. It seems the planet's atmosphere is expanding as it travels away from the sun, rather than contracting as expected.

The only planet not yet visited by spacecraft, Pluto is relatively poorly known. But when it is aligned with the stars just so, observations from Earth are possible. Analyses of data collected last year during one of these rare configurations revealed the surprising atmospheric findings, which were published today in the journal Nature.

Because Pluto is currently journeying swiftly away from the sun, researchers thought that its temperature would fall and its atmosphere would subsequently collapse. Instead, James Elliot of M.I.T. and his colleagues found, the planet's temperature has increased by one degree Celsius since 1989, when it was closest to Sol.

In explanation, the researchers note that temperature variations in planetary surface layers tend to lag behind solar heating fluctuations. Thus although Pluto has been moving away from the sun's heat, the cooling effects of that distancing may not show up for another 10 years.

"In the long run, cooling and atmospheric contraction are inevitable," writes William Hubbard of the University of Arizona in an accompanying commentary. Further elucidation of what is going on, however, will require a spacecraft mission to this most remote planet, he asserts. "Pluto's orbit over the next few years offers an opportunity to learn more about this planet, at a time when technological developments make it feasible to consider a mission to it," Hubbard remarks. "But both time and money are in short supply."