Just a few years ago the search for alien life was the stuff of science
fiction – the domain of writers, film makers and conspiracy theorists.
This weekend, however, in a house in Berkhamsted, Hertfordshire, two of
Europe’s leading astronomers are finalising plans to turn that search into
reality.
Glenn White and Malcolm Fridlund are the lead scientists for the Darwin
mission, a space telescope so powerful that it would be able to spot distant
planets, take photographs of them and even analyse their atmospheres for the
telltale gases that indicate life.
For the past 10 years they and a team of astronomers around Europe have been
perfecting the designs for Darwin. They are now collating that work into a
formal proposal for the European Space Agency (ESA), which has already
indicated it wants to adopt it as the centrepiece of Europe’s space research
programme for the next decade.
“What we will be looking for is no less than life itself,” said White,
professor of astronomy at the Open University. “The Darwin mission will be
able to look at stars up to 100 light years away and see if the planets
orbiting around them have evolved life.”
ESA’s plans for Darwin come at one of the most exciting times for planet
hunters. In the past 12 years this specialised group of astronomers has
found more than 220 planets orbiting other stars in the Milky Way – and its
ability to detect ever-smaller extra-solar bodies is constantly improving.
Last week saw astronomers claim they had found potentially the smallest and
most Earth-like planet yet, orbiting one of our closest neighbouring stars.
Gliese 581c was, it appeared, not just small and rocky, like the Earth. It
also orbited its star (Gliese 581) in the so-called habitable zone, where
the surface temperature could be just right to allow water to remain a
liquid.
This week is likely to see at least one more such announcement, this time from
the team overseeing Europe’s Corot satellite. It was launched just a few
months ago but has already detected its first planet.
Another mission, Kepler, that is due to be launched by Nasa, the American
space agency, next year, will accelerate that task, examining hundreds of
stars for the minuscule fluctuations in light caused by planets passing
across their faces.
“We are finding new planets faster than we ever expected,” said White. “What
it suggests is that planets are very common and that many of them are likely
to be like ours. It suggests that the chances of life having evolved on
other planets are very high – but we won’t find it till we build something
like Darwin.”
Such confidence shows the huge progress made in planet-hunting. Until the
1990s the only planets we knew of were those that orbited the sun in our
solar system.
Then, in 1995, two astronomers, Michel Mayor and Didier Queloz, announced they
had detected a new planet orbiting a star called 51 Pegasi.
Their discovery sent shock-waves through the scientific community. We were no
longer alone. There were new worlds waiting to be discovered – and the
methods used by Mayor and Queloz were simple and cheap enough for many
astronomers to join the hunt.
Mayor’s detection method did not, however, study planets directly. Stars are
so bright that relatively tiny objects such as planets are swamped by their
light. Instead he measured the way stars “wobbled” under the gravitational
pull of the planets orbiting around them.
By measuring such wobbles, astronomers can work out a great deal about a
planet, including its probable size, distance from its star and even how
warm it might be. This is how the Earth-like planet and its two companions
orbiting the star Gliese 581 were detected.
Since 1995, however, astronomers have been working on a new method for
detecting planets. The theory is that if a planet passes between you and a
star, then the star dims slightly as it passes by.
So far this method has been used to detect only around a dozen planets, but it
holds great potential. The Corot probe, for example, is already sending back
data suggesting it has found dozens of new planets.
“The data is still being analysed but there are strong hints that there will
be a lot more planets than we expected,” said Fridlund, a senior ESA
scientist, who is involved in that mission as well as Darwin.
Another breakthrough in the hunt for planets came in 2004, when astronomers
announced the first direct photograph of a planet orbiting another star.
There were all kinds of caveats: the star was a so-called brown dwarf, meaning
it was too small to give off much light, and the planet was huge – around
five times bigger than Jupiter.
What it showed, however, was that a new age of discovery was dawning. In
particular, the astronomers had been able to pick out the telltale
signatures of water and other chemicals in the light reflected from the
planet’s atmosphere.
It meant we could not just find planets but we could also learn about the
chemistry of their atmospheres. What’s more, we could also spot gases such
as ozone, methane and oxygen that are the signatures of life.
The Darwin mission, planned for launch in 2015, will build on all these
discoveries. It will comprise three space telescopes flying in close
formation with a fourth spacecraft acting as a communications hub.
Each telescope will be small compared with those on Earth, but by acting
together they will have the power of a far larger instrument. That power
will be amplified still further by being in space – away from the
distortions caused by the Earth’s atmosphere.
This means Darwin will be able not only to detect planets as small as the
Earth but also to take photographs of them and then analyse their
atmospheres for signs of life.
“This instrument will be more powerful than anything we have today,” said
Fridlund. “This week we heard a lot about Gliese 581. It was a great
discovery, but it took those researchers months to do that work. Darwin
could have done the same detection work in eight minutes. If there was life
there it would take 1.5 days to find it.”
Nasa is planning a mission very similar to Darwin, the Terrestrial Planet
Finder. It, too, has been years in preparation but is now being threatened
by Nasa’s new focus on returning to the moon and sending humans to Mars,
missions decreed by President George Bush.
This means Nasa’s planet hunters are likely to drop their own project and join
forces with Europe in building the Darwin probes, making it a truly global
mission.
Once started, however, the search for alien life may prove never-ending. Sir
Martin Rees, the astronomer royal, has pointed out that the universe
contains at least 100 billion galaxies, each with 100 billion stars. It
means there are more stars than all the grains of sand on every beach and in
every desert on Earth.
For White and Fridlund, however, that is a source of optimism, not despair.
They point out that if humans have been able to find so many planets so
quickly around the relatively few stars close enough to measure, then how
many more must there be waiting to be found?
“Statistically there could be billions of Earth-like planets out there,” said
White. “I have ultimate faith that in the next few years we will find life
elsewhere in the universe.”
