Chemicals

Water has been detected for the first time in the atmosphere of a planet outside our Solar System.
The planet, known as HD 209458b, is a Jupiter-like gas giant located 150 light-years from Earth in the constellation Pegasus.

Scientists will publish the findings in the Astrophysical Journal.

However, another research team reported in February that it was unable to find evidence of water in this planet's atmosphere.

Water vapour (or steam) was expected to be present in atmospheres of most known extrasolar planets, even those that orbit more closely to their parent star than Mercury is to our Sun.

For the majority of exoplanets, their close proximity to their parent star has made detecting water and other compounds difficult.

The identification reported here takes advantage of the fact that HD209458b, as seen from Earth, passes directly in front of its star every three-and-half days.

As a planet passes in front of a star, its atmosphere blocks a different amount of starlight at different wavelengths.

In particular, absorption by water in the atmosphere of a giant planet makes the planet appear larger across a specific part of the infrared spectrum compared with wavelengths in the visible spectrum.

Cosmic puzzle

Astronomer Travis Barman, from the Lowell Observatory in Flagstaff, US, found what he says is strong evidence for water absorption in the atmosphere of the transiting planet.

The conclusions stemmed from an analysis of Hubble Space Telescope measurements by Harvard University astronomer Heather Knutson and new theoretical models developed by Dr Barman.

He said his findings provided good reason to believe other planets beyond our Solar System also had water vapour in their atmospheres, despite the failure by another group to find water on the same world.

"I'm very confident," said Dr Barman. "It's definitely good news because water has been predicted to be present in the atmosphere of this planet and many of the other ones for some time."

He added that a Jupiter-like gaseous planet such as this one, as opposed to a rocky one like Earth, is highly unlikely to harbour any kind of life.

"Certainly this is part of that puzzle - understanding the distribution of water in other solar systems is important for understanding whether or not conditions for life are possible," he explained.

Planet shine

Telescope technologies are being developed that will probe the very faint light from these objects for tell-tale signs of biology.

These are the same "life markers" known to be present in light reflected off the Earth - so-called "earthshine".

They include signatures for water, and gases such as oxygen and methane and perhaps more complex molecules such as chlorophyll - the pigment which makes the process of photosynthesis possible.

HD209458b belongs to a type of extrasolar planet known as "hot Jupiters". These planets orbit precariously close to their stars.

The planet's outer atmosphere is expanded and heated so much by the nearby star that it is escaping the planet's gravity. Hydrogen "boils off" in the upper atmosphere under the searing heat from the star.

Astronomers have found water vapour in the atmosphere of a giant planet outside our Solar System.
The detection in the extrasolar planet HD 189733b was made using Nasa's powerful Spitzer Space Telescope and is reported in the journal Nature.

The team looked for the signal of water absorption in starlight poking through the edges of the atmosphere when the planet passed in front of its star.

It is only the second time water has been detected on an exoplanet.

Some researchers suggest that the presence of water could be a feature that is common to all gas giants - the type of planet represented by Jupiter, Saturn, Uranus and Neptune in our own Solar System.

HD 189733b orbits a star in the constellation of Vulpecula (the Fox), which is 64 light-years from our Sun.

Although water is a key ingredient for biology, the planet is far too hot to harbour life.

It orbits extremely close to its parent star - more than 30 times closer to its star than the Earth is to the Sun.

As such, temperatures range from a scorching 1,200 Kelvin (930C) on the dayside of the planet, to a relatively balmy 700 Kelvin (427C) on the nightside. This type of planet is known as a "hot Jupiter".

Letting off steam

Giovanna Tinetti, from University College London and colleagues, measured the radius of the hot Jupiter HD 189733b at different wavelengths by tracking how much starlight is blocked by the planet as it crosses in front of its parent star as viewed from Earth.

The planet looked bigger at the wavelength bands that corresponded to water, suggesting water vapour was present in its atmosphere.

"Although HD 189733b is far from being habitable - and actually provides a rather hostile environment - our discovery shows that water might be more common out there than previously thought," said Dr Tinetti.

She added: "Our method can be used in the future to study more 'life-friendly' environments."

Another team of astronomers previously detected water vapour in the atmosphere of a "hot Jupiter" called HD 209458b. The study, by astronomers in the US, was published in the Astrophysical Journal earlier this year.

But some critics have argued that instrument effects in this data could have created the same signal as water vapour.

Life search

Dr Tinetti said: "The 'holy grail' for today's planet hunters is to find an Earth-like planet that also has water in its atmosphere.

"When it happens, that discovery will provide real evidence that planets outside our Solar System might harbour life."

Co-author Sean Carey of the Spitzer Science Center in California commented: "Finding water on this planet implies that other planets in the Universe, possibly even rocky ones, could also have water."

Earlier this year, the Spitzer Space Telescope became the first telescope to analyse, or break apart, the light from two transiting "hot Jupiters", HD 189733b and HD 209458b.

This led to the first-ever "fingerprint", or spectrum, of an exoplanet's light.

A carbon-containing molecule has been detected for the first time on a planet outside our Solar System.
The organic compound methane was found in the atmosphere of a planet orbiting a star some 63 light years away.

Water has also been found in its atmosphere, but scientists say the planet is far too hot to support life.

The discovery, unveiled in the journal Nature, is an important step towards exploring new worlds that might be more hospitable to life, they say.

Methane, made up of carbon and hydrogen, is the simplest possible organic compound.

Under certain circumstances, methane can play a key role in prebiotic chemistry - the chemical reactions considered necessary to form life.

Scientists detected the gas in the atmosphere of a Jupiter-sized planet known as HD 189733b.

HD 189733b
Located 63 light years from Earth, in the constellation Vulpecula, the little fox
About the size of Jupiter but orbits closer to the parent star in its Solar System than Mercury does in our own
Temperatures reach 900 degrees C, about the melting point of silver

Co-author Giovanna Tinetti from University College, London, told BBC News: "This planet is a gas giant very similar to our own Jupiter, but orbiting very close to its star.

"The methane here, although we can call it an organic constituent, is not produced by life - it is way too hot there for life."

Stepping stone

Dr Tinetti, and co-authors Mark Swain and Gautam Vasisht, from Nasa's Jet Propulsion Laboratory in Pasadena, California, found the tell-tale signature of methane in the planet's atmosphere using the Hubble Space Telescope.

The observations were made as the planet passed in front of its parent star, as viewed from Earth. As the star's light passed briefly through the planet's atmosphere, the gases imprinted their chemical signatures on the transmitted light.

A method known as spectroscopy, which splits light into its components, revealed the chemical "fingerprint" of methane.

The researchers also confirmed a previous discovery - made by Nasa's Spitzer Space Telescope - that the atmosphere of HD 189733b also contains water vapour.

It shows that Hubble, Spitzer and a new generation of space telescopes yet to be launched can detect organic molecules on other extrasolar planets using spectroscopy, they say.

Dr Swain said: "This is a crucial stepping stone to eventually characterising prebiotic molecules on planets where life could exist."

Dr Tinetti said the technique could eventually be applied to extrasolar planets that appear more suitable for life than HD 189733b.

She said: "I definitely think that life is out there. My personal view is it is way too arrogant to think that we are the only ones living in the Universe."

Real worlds

The number of known planets orbiting stars other than our own now stands at about 270.

For most of them, scientists know little more than the planet's mass and orbital properties.

Adam Showman of the Department of Planetary Sciences at the University of Arizona, US, said scientists were finally starting to move beyond simply discovering extrasolar planets to truly characterising them as worlds.

Dr Showman, who was not part of the study, said: "The discovery does not by itself have any direct implications for life except that it proves a technique which might potentially be useful for characterising the atmosphere of rocky planets when we finally start discovering them."

Excitement about finding other Earth-like planets is driven by the idea that some might contain life; or that perhaps, centuries from now, humans might be able to set up colonies on them.

The key to this search is the so-called "Goldilocks zone", an area of space in which a planet is "just the right distance" from its parent star so that its surface is not-too-hot or not-too-cold to support liquid water.