Exoplanets are around most stars, study suggests

There may be billions of Earth-sized planets out there

Every star twinkling in the night sky plays host to an average of 1.6 planets, a new study suggests.

That implies there are some 10 billion Earth-sized planets in our galaxy.

Using a technique called gravitational microlensing, an international team found a handful of exoplanets that imply the existence of billions more.

The findings were released at the 219th American Astronomical Society (AAS) meeting, alongside reports of the smallest "exoplanets" ever discovered.

Gravitational microlensing is a method that uses the gravity of a far-flung star to amplify the light from even more distant stars that have planets.

Astronomers used a number of relatively small telescopes that make up the Microlensing Network for the Detection of Small Terrestrial Exoplanets, or Mindstep, to look for the rare event of one star passing directly in front of another as seen from Earth.

The team witnessed 40 of these microlensing events, and in three instances spotted the effects of planets circling the more distant stars.

While the number of actual events and detected planets was low, the team was able to estimate how many such exoplanets must exist.

Most news of exoplanets in recent years has come from the Kepler telescope, which spots planets by looking for the slight dimming of their host stars' light as planets pass in front of them.

That method is better at finding large planets close to their host stars.

While a more difficult effect to catch, gravitational microlensing is better at finding planets of all sizes and distances.

It can currently spot a planet as small as Mercury, orbiting at a similar distance to its host star, or as far away as Saturn.

The study, also published in the journal Nature, was a collaboration between researchers from more than 20 international institutes and universities.

"Just the recent 15 years have seen the count of known planets beyond the Solar System rising from none to about 700, but we can expect hundreds of billions to exist in the Milky Way alone," said co-author Dr Martin Dominik, from the University of St Andrews, UK.

Ever smaller

Complementing the microlensing approach, Kepler measurements hold a number of small-planet surprises as well.

The KOI-961 star would, as in this artist's view, be huge in the sky of the new planets

In December, the Kepler team announced the first Earth-sized planet, the smallest yet detected.

At the AAS meeting on Wednesday, the Kepler team announced even smaller planets, all three orbiting a tiny red dwarf star called KOI-961.

The planets are just 0.78, 0.73 and 0.57 times the radius of Earth.

The discovery came from an analysis of Kepler catalogue data released to the public in January 2011.

Among those poring through the data was John Johnson, a California Institute of Technology astronomer, who told the meeting that, as in the case of other red dwarfs, little is known about the size of the KOI-961.

Because of the way Kepler detects exoplanets, star size is crucial to the measurements of planet sizes. But UK amateur astronomer and longtime collaborator with Prof Johnson contacted the team with a clue.

"When he looked at the colours and other properties that we measure for KOI-961, he sent us an email immediately and said, 'Do you know you guys are looking at a twin of a very famous star called Barnard's star?'," Prof Johnson told the meeting.

The team was able to use known data from the well-studied Barnard's star to make guesses about KOI-961's properties.

That, Mr Apps told BBC News, was when "we realised that it was even more remarkable than we thought: the star was fainter, the planets were smaller. The whole thing was like a very compact triple planetary system."

Or, as Prof Johnson told the meeting, "It's like you took your shrink ray gun and set it to seven times smaller... What we have here is a planetary system that's shrunk down because the central star is so tiny."