By Helen Briggs
BBC News Online
A little-known star about 42 light-years away is the top target for European astronomers searching for planets that might harbour life.
Darwin: Not one but six telescopes
The star, which has the rather dull designation HD 172051, is much like our own Sun but is just a little cooler.
Scientists believe it is one of the best contenders in nearby space to have planets in orbit that resemble Earth.
It will be a key observational target when Europe launches its Darwin space telescope system in the next decade.
"It's a top star because it is quite similar to our own Sun and thus if there is an Earth-like planet around it, it should have evolved like ours and we can tell a lot about our own Solar System from the information we get," says Lisa Kaltenegger, a trainee at the European Space Agency's research centre, Estec, in The Netherlands.
"We have other top candidates for other stellar types as well, but to find life like ours it seems like this one is a very good target," she told BBC News Online.
The Darwin mission is an ambitious plan by the agency (Esa) to station a network of six telescopes 1.5 million kilometres from Earth.
The flotilla would seek to pinpoint other worlds around nearby stars capable of supporting life.
The launch of Darwin is more than a decade away but astronomers are already putting in the groundwork, by preparing a list of 150 target stars.
The Darwin catalogue includes stars like our own Sun (G stars) as well as stars that are a bit hotter (F stars) and stars that are a little colder (K and M stars).
"Darwin has the amazing capability to search for planets like our own, where life could evolve, around a big number of nearby stars," says Kaltenegger.
"It can collect the light from the planet itself and thus for the first time can give us insight into the conditions on those extra-solar planets, as well as find planets in all stages of evolution around their host stars, so we can puzzle together how planets form, how our own Solar System formed and what great variety of planets exist out there."
Signature of life
The first step is to build a ground-based instrument to investigate the target stars using a new technique.
By 2006, Esa and the European Southern Observatory (Eso) will build Genie (Ground-based European Nulling Interferometer).
Light will be analysed for spectra produced by water or ozone
It will work alongside the Very Large Telescope (VLT), a collection of four 8-metre telescopes in Chile.
Genie will test the principle of what is known as nulling interferometry - combining the signal from a number of different telescopes in such a way that the light from the central star is cancelled out.
The theory is that this makes any extra-solar planets in orbit around the star easier to detect - they are much fainter than stars and are usually drowned out by the light from the parent star.
There is another bonus. By collecting light from the planet itself, Darwin will be able to investigate its atmosphere, looking for the presence of water, oxygen and carbon dioxide.
Finding all three would hit the jackpot, suggesting that some form of biological life had evolved on the planet.
For now, though, further analysis of HD 172051 will have to wait. But the star, which can be found in the southern sky in the constellation Sagittarius, is already arousing the interest of astronomers.
Professor Martin Barstow, of the department of physics and astronomy at the University of Leicester, UK, says the star is interesting because it is similar to the Sun and is relatively nearby.
"To detect an Earth-sized planet, your best chance is to look at nearby stars," he says.
"You're looking for single stars and a good fraction will be like the Sun because we think that's the best bet for planetary systems that might have life."