Pluto bulging with carbon monoxide

It may have been demoted to dwarf planet status, but researchers have found Pluto's atmosphere is much bigger than previously thought.

The finding is also good news for NASA's New Horizons mission, which is expected to arrive at the faraway outpost in a few years.

Astronomers first detected an atmosphere around Pluto when it briefly passed in front of a star in 1988. Light from the star passed through the dwarf planet's atmosphere, causing it to dim slightly.

Now, Jane Greaves of the University of St Andrews and colleagues have used the James Clerk Maxwell Telescope in Hawaii to study Pluto's atmosphere in more detail.

Their findings, presented at this week's National Astronomy Meeting in Wales, will appear in the journal Monthly Notices of the Royal Astronomical Society.

Previously the atmosphere was known to be over 100 kilometres thick. But the observations of Greaves and her colleagues have pushed that out to more than 3,000 kilometres one quarter of the distance to Pluto's largest moon, Charon.

"[Pluto's atmosphere] has a very low pressure, about a microbar [0.1 pascals], so expands very readily," she says. "In particular, fluctuations in the energy from the sun during the solar cycle can make big changes to its size."

The researchers also detected carbon monoxide, an atmospheric coolant, which they say helps balance the warming effect of methane already known to exist in the dwarf planet's atmosphere.

"The newly discovered carbon monoxide may hold the key to slowing loss of the atmosphere," they write.

Pluto's changing nature

It is thought the dwarf planet's thin atmosphere is the result of ice evaporating from its surface.

Since Pluto made its closest approach to the sun in 1989, the planet's surface has undergone rapid and surprising changes. Observations between 1988 and 2002 show the surface becoming redder and the northern pole brighter.

"We think it is delayed warming after Pluto's closest approach to the sun in 1989," says Greaves. "There is a thermal lag, similar to it being warmer after midday on Earth."

She says the researchers plan to continue monitoring the atmosphere over the next few years.

"We're observing again in two weeks time, hoping to measure the temperature of the carbon monoxide gas, which will give us a lot more idea of the balance of the atmosphere."

Greaves believes similar atmospheres may exist on other dwarf planets.

"A few are of similar size to Pluto, for example, Eris and Sedna. But they are all more distant than Pluto at the moment so really hard to observe," she says.

"[But] we think Charon, being smaller, has probably had all its ices boil away already during the 4.5 billion years since the solar system formed."

The discovery of carbon monoxide raises hopes that NASA's New Horizons mission, expected to pass Pluto in 2015, will be able to observe the atmosphere in detail.

"It was thought that the atmosphere might be starting to snow out as New Horizons arrived," says Greaves. "I think there will still be plenty of gas left, given our discovery, but its state is very unpredictable if it can change over just a few years."