News in Science

Amateurs help nab alien world in weird orbit

Alien world An Earth-like planet locked in a weird orbit has been found in a two-star system by an international collaboration that includes amateur astronomers.

The discovery, reported in the journal Science, expands astronomers ideas of where to look for planets like Earth in our galaxy.

Thousands of planets have now been discovered orbiting other stars, and planets orbiting two stars in binary systems have also become common, says the study's lead author, Professor Andrew Gould of Ohio State University.

"However, there are only four other known examples of binary systems where the planet is not orbiting both stars at once," says Gould.

The newly-discovered planet, called OGLE-2013-BLG-0341LBb is located 3261 light-years away and has about twice the mass of the Earth.

It orbits the smaller of two red dwarf stars at almost the same distance as the Earth orbits the Sun -- red dwarfs are small stars, far cooler and less massive than the Sun.

The two stars in this system are separated by about 15 times the distance between the Earth and the Sun.

Cold desolate world

The newly-discovered planet's host star is only about a tenth the mass of the Sun and therefore at least 400 times less luminous.

The authors calculate the planet has a surface temperature of minus 213°C.

"If you were standing on this planet, you would be very cold, you wouldn't be getting much light from the host star, and the light would be very red," says Gould.

"The host star would only look about a seventh as big as the Sun, so it would be small and faint, and the companion star would just be a point of light about as bright as the Moon."

Lucky discovery

Gould admits the discovery of this planet was down to a good deal of luck, using a process called gravitational microlensing.

First theorised by Albert Einstein, the technique uses the mass of a foreground object such as a galaxy or star, to bend and magnify the light coming from a more distant background object.

In this case, the planet's host star acted as the foreground object, magnifying the light from a more distant star located a further 25,000 light years away.

As the planet orbited its host star, it caused a distortion in the magnified image that caused a dip in the light curve.

"It was a very brief distortion ... the little dip caused by the planet lasted less than a day, and that had to be caught by the telescopes," says Gould.

"We had to look at a huge number of events before that happened."

Gould suggests that a new population of yet unseen Earth-like planets may be orbiting single members of binary stars.

Microlensing searches like this one could likely uncover more such systems, and this could in turn help develop models of planet formation and evolution.

Amateurs essential

According to Gould, microlensing data collected by a network of nine amateur and professional telescopes in Western Australia, New Zealand, Israel, and Chile was absolutely essential to the discovery.

"Microlensing events can take over 24 hours to unfold," he says.

"If you're looking from one spot on Earth, you're only going to see it for a maximum of eight or nine hours. So combining all our telescopes working together we were able to trace out a detailed light curve that would have been impossible to do otherwise.

"It's a challenging method to use, you need to have telescopes all around the world in order to be able to find our planets, and the microlensing event only happens to one in a million stars at any given time."