Astronomers have discovered a planet comparable to Earth, but it's much too far away to plan a visit - yet, writes Dick Ahlstrom, Science Editor.
Astronomers may have located a promising home away from home - an Earth-like planet in orbit around a distant star 20 light-years away. The space science world is aflutter with the news, given that this is the first almost Earth-sized planet yet found. And its Swiss, French and Portuguese discoverers also believe the planet may have liquid water and an environment that could harbour life.
The new exoplanet - the term for a planetary body located outside our own solar system - is the 211th discovered so far in an international astronomical effort to find a planet similar to our own.
"Its discovery represents at least 10 years of systematic work by large teams of astronomers to find planets orbiting around other stars," says University College Cork astronomer, Dr Paul Callanan.
"This is one of the first planets that has been found with a size comparable to that of the Earth." More importantly, it seems to be orbiting its parent star at just the right distance where it will not be too hot or too cold.
"This planet is in the region called the habitable zone and so at least in theory, it can have liquid water. This has long been the assumed prerequisite needed to support life," Callanan says.
European astronomers made the discovery using the European Southern Observatory's 3.6 metre telescope based at La Silla, Chile.
They may have got a little ahead of themselves in the rush to make this unnamed planet seem like Earth. If this planet ever becomes a genuine home away from home, it will take some getting used to for visiting earthlings.
It orbits the star Gliese 581, one of the 100 closest stars to us, and just 20.5 light-years away in the constellation Libra. It is known as a "super-Earth", not because it is a terrific place to visit, but because it is bigger and denser than our home planet.
Its radius is half as big again, making for a larger planet with a volume almost three and a half times bigger.
It would be a place where standing upright would be a challenge. It is at least five times bigger than Earth and so would deliver five times the gravitational pull. So a 76kg (12-stone) man on Earth would weigh 381kg (60 stone) on the new planet. It also moves about a lot, completing a full orbit of its sun in just 13 days, compared with the Earth's 365 days.
The really big difference, however, is how the planet's sun would look overhead. Our sun is bright and hot, while Gliese 581 is a "red dwarf", a small, relatively cool star. It is 50 times fainter than our sun and so delivers less heat and light. The new exoplanet is also 14 times closer to its star, so its sun would loom large in the sky.
As its discoverers pointed out, one thing that the planet does have going for it is it orbits in the habitable zone, where average surface temperatures allow water to remain liquid. "We estimate that the mean temperature of this super-Earth lies between zero and 40 degrees Celsius, and water would thus be liquid," states Stephane Udry from the Geneva Observatory and lead author of the report on the discovery, published in Astronomy and Astrophysics.
While those involved were quick to raise the possibility of liquid water, there is no evidence yet for this. The planet is massive, which Udry's models indicate means either it could be a rocky planet like Earth or a planet covered with deep oceans.
It resides in the habitable zone where water does not freeze or boil off, but it could just be very dense and as dry as a board, with absolutely no water.
Its large mass would mean however that if there was any water or atmosphere about, its gravity would keep it there, suggests Armagh Observatory's director, Prof Mark Bailey. And it could have accumulated water during its formation, just as the Earth did.
"I think all these discoveries are very exciting. It is great to speculate," Bailey says. "Here is a planet that could have the building blocks for life."
Certainly, it is not surprising that astronomers are beginning to find near Earth-sized planets around distant stars, but this is the smallest yet found, he says.
"It makes it slightly more likely that there is life on another world and a not particularly distant world. The probability of life on another world is slightly higher because of this discovery."
We may learn in time through further observations whether it has water, but a visit any time soon is definitely out of the question. It lies more than 20 light years away, a distance of about 194,750,000,000,000 km (121, 012, 000, 000, 000 miles).
"We are talking about how hard it is to get to Mars and we see that this is one of the closest stars to our own," says Bailey.
"It would take the current generation of spacecraft aeons to get there."
So much for dropping in and visiting the neighbours - at least for now.
Starry-eyed scientists: how planets are discovered
A wobbling star is enough to make a planet hunter's heart skip a beat. The minute wobble is the tell-tale sign that the star has planets in orbit around it.
Detecting stars that wobble is a huge challenge, however. "They are very, very had to find because the variation is so small," explains University College Cork astronomer, Dr Paul Callanan.
It is impossible to directly see any planets in orbit around a distant star, he says. Planets can only reflect the light of their parent star. The bright starlight simply washes out and makes invisible any light reflecting from even very large planets.
This means planet-hunting astronomers must rely on something else when trying to spot exoplanets orbiting around distant stars.
What the scientists look for is a minute change in the light output from the star. The gravitational pull of the star holds its planets in orbit, but the planets also exert a gravitational pull on the star.
They cause the star to move, to wobble slightly. This small movement causes the light reaching us to change, varying its wavelength, depending on whether the star is being pulled towards us by the planet or away from us.
"It is a very challenging observation," says Callanan. It requires very powerful telescopes and highly sensitive detectors to register these minute changes.
Geoffrey W Marcy and R Paul Butler of the University of California Berkeley liken the challenge to being able to spot something the size of a €2 coin viewed from 10,000 km (6,213 miles) away.
The variation in wavelength is tiny, they point out, in the order of one part in 10 million in a star the size of our own pulled by a planet the size of Jupiter.
Even so, scientists have so far detected 211 exoplanets in orbit around distant stars, with new discoveries coming almost every week as the techniques improve. Still more are certain to be found, including Earth-like planets.
The process is likely to speed up considerably however when the James Webb space telescope, the replacement for US space agency Nasa's Hubble telescope, is launched into orbit in 2013.
University of Colorado astronomer Webster Cash and others are campaigning to use the Webb to find exoplanets directly by observation.
The plan involves putting a "starshade" into orbit some distance from the Webb telescope. The shade would block out the light of a star, allowing the weak light reflected from any orbiting planets to be seen.
Cash likens the technology to an elaborate pinhole camera. If accomplished, it would make it much easier to spot whether a star had orbiting planets.
He believes there could be as many as 10,000 exoplanets within 30 light years of our sun. Not surprisingly he has argued: "Let's find the future home of mankind, let's do it now, let's find life elsewhere in the universe."
The Berkeley astronomy
website is at http://astro.berkeley.edu/ ~gmarcy/sciam.html