Billions of planets may well be suitable for life
Habitable planets might be in orbit around billions of stars in the Milky Way, a new study suggests.
Astronomers came to the conclusion after a six-year star survey that suggests planets are commonplace in our galaxy.
Scientists estimate as many as 10 billion stars in the Milky Way might host planets in the "habitable", or "Goldilocks", zone.
This is the orbital band within which conditions are not too cold and not too hot but "just right" to allow surface liquid water and, potentially, life.
The discovery raises the possibility of a universe teeming with life, as depicted in popular sci-fi movies and TV series such as 'Star Wars' and 'Star Trek'.
However, scientists stress that just because a planet has conditions suitable for life, it does not follow that life has evolved there.
Over the past 16 years, astronomers have made more than 700 confirmed detections of "exoplanets" orbiting distant stars.
The vast majority have been Jupiter-like gas giants or scalding hot planets hugging close to their stars. Neither type offers much hope of finding life.
In those cases, astronomers relied on spotting tiny "wobbles" in the host star caused by a planet's gravitational pull, or the minute dimming of starlight as a planet crossed in front of its star.
But the new survey employed a radically different method called "gravitational microlensing".
This involves a foreground star's gravity acting like a "magnifying glass" to bend and amplify light from a background star.
If there is a planet orbiting the foreground star, a small extra "bump" might be seen in the light signal.
The technique just happens to be most sensitive to planets a mid-distance away from the star -- in other words, those in the "habitable zone". However, very special conditions are needed to detect planets by gravitational microlensing. The background and foreground stars have to be lined up, and an additional chance alignment of the planet's orbit is also needed.
Despite these obstacles, analysis of six years' worth of microlensing data from telescopes around the world uncovered an unexpected number of exoplanets.
"In a six-year period from 2002 to 2007 we observed 500 stars at high resolution," said Danish astronomer Dr Uffe Grae Jorgensen, head of Astrophysics and Planetary Science at the University of Copenhagen.
"In 10 of the stars, we directly see the lens effect of a planet, and for the others we could use statistical arguments to determine how many planets the stars had on average."
The results were published yesterday in the journal 'Nature'.
Dr Martin Dominik, who led a British team from the University of St Andrews involved in the research, said: "Our latest results tell us that while we may not see all the planets, wherever in the sky we look, they are there."