PLUTO is warming up as its orbit takes it further from the Sun. The finding is baffling astronomers, who had expected it to cool.
The planet moves in a 248-year elliptical orbit, and has been retreating from the Sun since its closest approach in 1989, when it was 4.4 billion kilometres out, just inside Neptune鈥檚 orbit. It will reach its most distant point, 7.4 billion kilometres from the Sun, in 2114. Astronomers had worried that the atmosphere would freeze before NASA could send a spacecraft to explore the planet. But Pluto may be more complex than they thought. 鈥淲e just don鈥檛 know what is causing these effects,鈥� says Jim Elliot of MIT.
Pluto is so faint and so far away that the only way to probe its atmosphere from Earth is to wait for it to pass in front of a distant star and observe how the starlight is dimmed. Elliot made the first such measurements in 1988, but other opportunities were rare until the planet recently moved in front of the rich star fields of the Milky Way.
Advertisement
Astronomers had little luck on their first attempt on 19 July, when clouds blocked most observations. The available data showed the atmosphere had changed, but it was impossible to tell how (麻豆传媒, 24 August, p 8).
Researchers got a second shot on 20 August, when the weather was better and the eclipse passed over large telescopes in Hawaii, California and Arizona. A team led by Elliot concluded that Pluto鈥檚 atmosphere is now three times as thick as it was in 1988, suggesting the average temperature of nitrogen ice on Pluto鈥檚 surface has increased by nearly 2 掳C.
That sent astronomers on the team scrambling for an explanation. David Tholen of the University of Hawaii says perhaps we are simply seeing a time-lag effect. 鈥淚t takes time for materials to warm up and cool off, which is why the hottest part of the day on Earth is usually around 2 or 3 pm,鈥� he says.
We are already 13 years past Pluto鈥檚 closest approach to the Sun, and Tholen says the planet could go on warming for as long again, but why it鈥檚 continuing for so long is a mystery.
The best way to find out would be to take a closer look at Pluto, the only planet NASA has never visited. The cash-strapped space agency dropped the planned Pluto-Kuiper Belt fly-by from its budget this year, but the US Senate Committee on Appropriations has reinjected $105 million for a 2006 launch. If Congress and the White House agree, the mission would fly past Pluto in 2016.
