Have astronomers observed the first signs of storms on a distant 'hot Jupiter'? Credit: NASA/JPL-Caltech/T. Pyle

Astronomers have detected water, and possibly even the first signs of weather, on a planet outside our Solar System1. The gas giant, which orbits a star 63 light years from Earth, also has carbon dioxide and methane in its atmosphere, according to recent reports.

Water is one of the most abundant molecules in the Universe and, according to theoretical models, should form easily in planetary atmospheres. So scientists were surprised when their first observations2 of the planet did not show a clear signal for water in its atmospheric spectra. Last year astronomers did find the first traces3,4 of water in the atmospheres of this and another exoplanet, but the data were ambiguous.

The new evidence1, published in Nature, provides "the best spectrum of an extrasolar planet ever taken", says Drake Deming, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the research. He says the spectrum "clearly and unequivocally" shows the presence of water in the planet's atmosphere.

The planet, HD 189733b, is a 'hot Jupiter' — a gas giant that is in a closer orbit to its star than Jupiter is to the Sun, and so is hotter. To study the atmosphere of HD189733b, Carl Grillmair of the Spitzer Science Center in Pasadena, California, and his colleagues used the 'secondary eclipse' method to exclude light from the planet's star and record light from the planet alone.

This technique requires astronomers to record the light from both the planet and its star together, and then subtract the spectrum of just the star by recording its light when the planet is behind it.

Grillmair used the infrared spectrograph aboard the Spitzer Space Telescope to observe light from HD 189733b in the mid-infrared wavelengths, a region of the spectrum where water's signature should be clearly visible. The result, he says, "distinctly showed the presence of water vapour".

Cloudy skies

But why water was not clearly visible in the earlier spectra from HD 189733b or other hot Jupiters is unclear.

Grillmair attributes the disparity between the spectra to changes in cloud cover in the upper atmosphere of HD 189733b. There could be "huge storms and nasty weather with winds [of] thousands of miles an hour" that form clouds high in the atmosphere, he says, and hide the signatures of water in lower clouds.

The team cannot verify this idea with the data they have. But the explanation is plausible and is the most exciting, comments Alan Boss of the Department of Terrestrial Magnetism at the Carnegie Institution of Washington. He says it could hint at the "beginnings of weather reports on worlds" that lie far outside our Solar System.

Adam Burrows, an astrophysicist at Princeton University in New Jersey and a co-author of the new study, says that data to come in the next month are likely to answer this question. They should confirm whether the astronomers are actually seeing the first patterns of weather on an exoplanet or whether the difference between the spectra simply reflects how the data were collected.

Life signs

Water is also one of four chemical markers — along with carbon dioxide, methane and oxygen — that indicate whether a planet might be habitable. Astronomers have now detected three of these components in the atmosphere of HD 189733b (see 'Carbon dioxide discovered on distant planet'). Oxygen has not yet been found in the spectrum of any exoplanet. And even if it had been for HD 189733b, the planet is too hot to harbour life resembling that on Earth.

But getting the spectrum is still a triumph, as Spitzer's infrared spectrograph and other instruments were not designed to measure the spectra of exoplanetary atmospheres.

By pushing the telescope to its limits, planetary scientists have "nailed down" the spectrum of a distant world about a decade before they thought they would, says Burrows. The success "whets astronomers' appetites", he says, as it suggests they could use the secondary eclipse method to study the atmospheres of rocky Earth-like planets, which may harbour life.

But to get the spectrum of a smaller planet like Earth, astronomers need bigger and better tools, Burrows says. Astronomers can now use what they've learned to design more powerful instruments that could record the spectrum of atmospheres of Earth-like exoplanets.