planet may hold the key to life beyond the solar system

Found: planet that may hold the key to life beyond the solar system
By Steve Connor, Science Editor
Published: 12 July 2007
Independent UK

It is a world where one side of the planet is bathed in perpetual daylight while the other is kept in infinite night. Here, an entire year passes in just over 48 hours and surface temperatures are hot enough to melt lead.

It is also the place where scientists have for the first time found evidence of water on a planet beyond our own solar system - a discovery that marks a milestone in the search for the vital signs of extraterrestrial life.

The planet is known only by its code name, HD 189733b, and it orbits a star similar in size to the Sun in the constellation Vulpecula, the Fox, some 63 light years - or about 380 million million miles - from Earth.

Yesterday a team of astronomers led by Giovanna Tinetti of the European Space Agency announced that it had found the strongest evidence yet for the presence of water on an "exoplanet", a planet in a solar system other than our own.

"We're thrilled to have identified clear signs of water on a planet that is trillions of miles away," said Dr Tinetti, who holds a research post at University College London.

Water is widely assumed to be the essential ingredient for life - or at least life as we know it - but there is little chance of anything surviving the intense heat of HD 189733b, a "gas giant" planet similar to Jupiter.

Yet finding evidence of water on a planet so far away marks an important breakthrough in the search for life on other planets, explained Professor Yuk Yung, of the California Institute of Technology.

"Water is the quintessence of life as we know it. It is exciting to find that it is as abundant in another solar system as it is in ours," said Professor Yung, a member of the research team.

Since 1995, astronomers have discovered about 200 exoplanets by detecting their influence on the stars that they orbit. HD 189733b was detected by the subtle differences in starlight coming from its sun as the planet passed in front of it once every 2.2 Earth days.

Detailed analysis of the wavelengths of starlight absorbed by the planet indicated that its atmosphere must be rich in water vapour, which is kept too hot to ever condense into clouds or liquid water, said Dr Tinetti, whose study is published in the journal Nature.

"Water is the only molecule that can explain that behaviour. Although HD 189733b is far from being habitable, and actually provides rather a hostile environment, our discovery shows that water might be more common out there than previously thought, and our method can be used in the future to study more life-friendly environments," Dr Tinetti said.

It is believed that the best planets for life are relatively smaller ones made of rock, similar to Earth, rather than Jupiter-like gas giants. Life is also assumed to require liquid water, so the planet must be in the "Goldilock's zone" - not too close or too far away from its sun for all its water to freeze or boil away.

"The holy grail for today's planet hunters is to find an Earth-like planet that also has water in its atmosphere... that discovery [would] provide real evidence that planets outside our solar system might harbour life," Dr Tinetti said.

The planet HD 189733b is estimated to be about 1.15 the mass of Jupiter and it orbits its sun at a distance of about 2.8 million miles, close enough for it to be gripped so strongly in the star's gravity that one hemisphere is constantly facing the star. This means one hemisphere is in perpetual light with temperatures of 1,000C or more.

This probably generates intense winds that sweep from the day-side to the night-side of the planet, Dr Tinetti said. "There are a thousand things to learn about these planets," she said. The discovery of water on HD 189733b was made with the help of Nasa's Spitzer space telescope which is viewed as the prototype of future space instruments.

"Finding water on this planet implies that other planets in the universe... could also have water," said Sean Carey of the Spitzer Science Centre at Caltech.

Mao-Chang Liang, also of Caltech, agreed: "The discovery of water is the key to the discovery of alien life."