Water has been detected in the atmosphere of an alien world for the first time, a new analysis of Hubble Space Telescope data suggests. Some scientists applaud the result – which had been predicted theoretically but not observed in previous studies, while others say the apparent water signal may simply be instrumental noise.
The planet, called HD 209458b, is about 70% as massive as Jupiter and is scorched by the heat of its parent star, which it orbits 9 times as close as Mercury does to the Sun.
Because it is one of a small number of extrasolar planets observed to pass directly in front of and behind their parent stars as seen from Earth, astronomers have been able to glean a lot of information – such as its size and mass – about the distant world.
In February, researchers using the infrared-sensitive Spitzer Space Telescope announced that there was no sign of water vapour in its atmosphere. Since the molecule is expected to be abundant in the atmospheres of extrasolar planets, some speculated that the water signal was obscured by a dusty haze.
Now, Hubble observations seem to have revealed the missing water. Travis Barman of Lowell Observatory in Flagstaff, Arizona, US, did a computer analysis of previously obtained Hubble data taken when the planet partially eclipsed its parent star.
The amount of light blocked during these eclipses was previously used to precisely determine the planet’s radius, which is about 30% greater than that of Jupiter.
Hubble observed light from the host star that had filtered through the outer reaches of the planet’s atmosphere. Because of its specific chemical composition, the atmosphere is more transparent at some wavelengths than others.
Barman found clues to this composition by making different models of the atmosphere, each with a different chemical makeup, and seeing which fit the observations best.
He says the relatively small amount of light filtering through at about 0.9 microns suggests the presence of water, which absorbs light at this wavelength.
“To me, that’s a clear indication that water is there,” Barman told New Scientist. “I think this is the first time we’ve had strong evidence that there’s water in at least one extrasolar planet.”
But despite the presence of water, he points out that the planet’s prevailing temperatures of about 1000° Celsius mean conditions would not be favourable to life. “It’s not a place you or I would want to visit,” he says.
Mark Swain, a member of one of the two Spitzer teams that found no evidence of water and a scientist at NASA’s Jet Propulsion Laboratory, says the new result may shed light on the planet’s temperature.
In order for Spitzer to detect water molecules by the way they absorb light, the planet’s interior has to be hotter than its upper atmosphere.
If the planet has a relatively uniform temperature throughout, however, that – and not the obscuration by dusty clouds – could explain the lack of a water detection by Spitzer.
“It’s certainly an interesting result,” he told New Scientist. “These planets have been surprising us all along.”
Drake Deming of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who is a member of the other Spitzer team, is also impressed with the results. “He’s certainly shown that it’s a better fit with water absorption than without,” he told New Scientist. “I think even the most sceptical person would say that.”
But David Charbonneau of Harvard University in Cambridge, Massachusetts, US, who is a member of the team that originally obtained the Hubble data used by Barman, says the effect seen is small enough that it could simply be the result of noise in Hubble’s instruments.
“Since we can’t rule out an instrumental origin for these variations, it may be premature to interpret them as arising from molecules in the planetary atmosphere,” he says.
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