Something unexpected is happening on distant Pluto - the outermost planet.
By Dr David Whitehouse
BBC News Online science editor
Although it is receding from the Sun, its atmosphere is getting thicker, puzzling astronomers who expect it to "freeze-out" and contract in about 10 years.
Artist's impression of Pluto, its moon Charon and the distant Sun
The recent changes were noted as a result of a rare cosmic alignment last year. Pluto passed in front of two faint stars whose light was dimmed by its atmosphere before being cut-off by its disc.
Data from these events indicate that Pluto's tenuous atmosphere has become swelled in the past 14 years since the last time such an occultation was observed.
What is happening on the planet will only be answered by the arrival of the Pluto-Kuiper Express mission, Pluto's first probe. It is due for launch in 2006, to reach Pluto about a decade later.
Pluto was discovered in 1930 as the result of some celestial detective work. It has always been mysterious. Even the mighty Hubble Space Telescope can only make out the haziest details on its icy surface.
Its atmosphere was first detected in 1985 when it made a star passing behind the planet, as viewed from Earth, appear to waver slightly. The atmosphere is very thin by Earth standards - about a millionth of the thickness of our atmosphere.
Observations and calculations suggested Pluto's atmosphere was made of nitrogen, which is in a delicate balance with surface ice.
Orbiting the Sun every 248 years, tiny Pluto - smaller than our own Moon - is usually the most distant planet in our Solar System, except for a few years either side of its closest approach to the Sun (and then Neptune is slightly more distant).
That last happened in 1989, and since then Pluto has been moving away from the Sun, presumably getting colder.
The last occultation of a star by Pluto was in 1988, so astronomers looked forward to two occultations in July and August 2002, setting up camp in North and South America, and Hawaii.
A star designated P126A passed behind Pluto on 20 July 2002, and immediately astronomers realised that Pluto's atmosphere had changed in some way, but the quality of the observations were not good enough to say exactly how.
A probe will get there in about 13 years time
The occultation of 21 August, of star P131.1, yielded better results, suggesting that Pluto's atmosphere had swelled since 1988, even though it seemed that its temperature had stayed roughly constant.
The astronomers report their observations in the Journal Nature.
The researchers think that thermal lag is the explanation. Pluto will inevitably get colder as it moves away from the Sun, but there might be a delay to heat accumulated when closest to the Sun.
Because of the delicate balance between Pluto's atmosphere and its icy surface, researchers expect the atmosphere to completely contract sometime around 2015, with an uncertainty of a few years.
It was the need to reach planet and make measurements before the atmosphere collapsed that has made Pluto-Kuiper Express an urgent mission. It may, or may not, reach the planet in time.
"The observations are timely," says William Hubbard writing in Nature, "Pluto's orbit over the next few years offers an opportunity to learn more about this planet, at a time when technological developments make consideration of a mission to the planet feasible."