Billions of habitable planets in Milky Way

By Nick Collins, Science Correspondent

By scouring millions of stars in the night sky over six years, researchers found that the majority of the 100 billion stars in the Milky Way have planets similar to Earth or Mercury, Venus or Mars, the other similar planets in our solar system.

 

They estimated that in our galaxy there are about 10 billion stars with planets in the "habitable zone" – the distance from the star where solid planets can be found – many of which could in theory be capable of supporting life.

 

Dr Martin Dominik, a German research fellow at St Andrews University, said: "Even if life existed on only one planet in each galaxy there would still be 100 billion in the universe.

 

"We still don't have the evidence of life on another planet, and we could be unique, but confronted with these numbers it seems highly unlikely.

 

"There are a small number of planets which we think could harbour life, a small number of candidates with what we believe might be the right conditions."

 

More than 1,000 planets have already been detected in our galaxy, but the two different methods used to find them are best suited to those which are large and close to their host star – unlike anything in our solar system.

 

Using a third technique known as gravitational microlensing, the international team of astronomers were able to confirm the existence of planets at a similar distance from their star as Earth without directly seeing them.

 

Uffe Gråe Jørgensen of the University of Copenhagen, another of the researchers, said: "Our results show that planets orbiting around stars are more the rule than the exception.

 

"In a typical solar system approximately four planets have their orbits in the terrestrial zone, which is the distance from the star where you can find solid planets.

 

"On average, there are 1.6 planets in the area around the stars that corresponds to the area between Venus and Saturn."

 

The microlensing technique requires two stars to be positioned in a straight line from Earth, meaning the light from the one behind is amplified by the gravity of the one in front like a magnifying glass.

 

As the stars pass by one another the brightness steadily increases and decreases again, and any unexpected bump in this transition indicates a planet circling the star in the foreground.

 

The bump would only be visible if the planet was between 1.5 billion and 75 billion kilometres from its sun – which in our solar system would range from Venus to Saturn – meaning the method is perfectly suited to finding planets at an "Earthlike distance" from the star.

 

Researchers carried out 500 high-resolution observations of stars between 2002 and 2007. In 10 cases they could directly detect signs of a planet, and in the remainder they used statistical analysis to estimate how many planets were in their orbit.

 

The technique was able to detect planets with masses from five times the size of Earth to ten times that of Jupiter.

 

On average one in six stars had a planet with a Jupiter-like mass, half had one with a mass similar to Neptune and two thirds contained super-Earths. Telegraph