Pluto, the coldest and most distant planet in the solar system, is getting a dose of global warming. In 1989 it reached its closest point to the sun, causing bits of its icy surface to evaporate into a slight atmosphere. Last summer, Pluto passed directly in front of two stars, allowing two teams of astronomers to study this tenuous shroud of gas. Observations of the distorted starlight showed that Pluto’s nitrogen-rich atmosphere—although still frigid with temperatures between -274°F to -391°F, depending on the altitude—is distinctly warmer than it was when last observed in 1988. The atmospheric pressure has doubled, too. Since the planet has been moving away from the sun for the past 14 years, the results come as a surprise. “The most likely explanation is thermal lag,” says MIT astrophysicist James Elliot, one of the team leaders. “On Earth, the days are longest in the northern hemisphere near the end of June, but the hottest month is July. Similarly, Pluto may not reach maximum surface temperature until a decade or so from now.” Astronomers hope they will be able to get a close view of Pluto’s enigmatic environment before things cool down and the atmosphere begins to collapse. Despite constant threats from budget-slashing administrators, NASA’s New Horizons mission is on schedule for launch in 2006, with a Pluto flyby anticipated for 2015.