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Exoplanets are around every star, study suggests

By Jason Palmer Science and technology reporter, BBC News, Austin, Texas

Every star twinkling in the night sky plays host to at least one planet, 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.

Artist's conception of KOI-961 system 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."