Rare planet circles just one of a pair of stars

Odd trio may help astronomers test ideas about planet formation

THREE’S A CROWD  A frozen, rocky planet orbits one of a pair of faint red stars (center and right), roughly 3,300 light-years away, as seen in an artist’s illustration. 

Cheongho Han, Chungbuk National University, Republic of Korea

Astronomers have discovered a frozen, rocky planet orbiting one of a pair of faint red stars. Researchers reported the discovery in the July 4 Science.

It’s not the first planet found orbiting one star in a binary, but it is the first to be discovered with microlensing, the temporary brightening of light from a more distant star. This stellar pair is also much more compact than most other binary systems with planets. And it’s the first planet-hosting binary where both stars are M dwarfs, which make up roughly three-quarters of the stars in the galaxy.

Since roughly half of sunlike stars are part of a pair, such duos are a potentially fertile ground for planet hunters. Planets that orbit binary stars can also help astronomers understand how planets form in unusual environments.

The rocky planet is roughly twice as massive as Earth and is about 3,300 light-years away in the constellation Sagittarius. Its orbit is a bit larger than that of Venus, but unlike Venus, which bakes at around 470 degrees Celsius, the new exoplanet reaches only about –210 degrees. That’s because M dwarfs are relatively small and cool. The other M dwarf in the binary is 10 to 14 times as far away from the planet as the sun is from Earth. To an observer on the planet, the companion star would appear as a ruddy point of light about as bright as a full moon.

The new planet was discovered by chance by a project called OGLE, for Optical Gravitational Lensing Experiment. OGLE uses a telescope in Chile to look for stars and galactic debris that pass in front of more distant stars. When that happens, the gravity of the closer star temporarily amplifies the light of the more distant star, a phenomenon known as microlensing. If the nearer star drags a planet along with it, the planet’s gravity may briefly distort the starlight as well.

FROM NEAR TO FAR A newly discovered exoplanet hugs one star (right) while a second star (left) orbits farther away. The planet is 15 times as far from the more-distant star as from the sun it orbits. Cheongho Han, Chungbuk National University, Republic of Korea
When OGLE first detected a flicker of light in June 2012 and alerted other telescopes, the finding appeared no different from the roughly 2,000 other microlensing events that OGLE sees each year. Eventually, eight other observatories around the world watched as the unusual trio drifted between Earth and a third star located roughly 26,000 light-years away.

Planets in binaries help astronomers test their ideas about planet formation, says Roman Rafikov, an astrophysicist at Princeton University who is not a member of the OGLE team. Planets form in disks of gas and dust that swirl around young stars. Dust sticks together to form rocks, and rocks repeatedly collide to form planets. A second nearby star, however, might stir up the material and make it more difficult for the rocky debris to build planets.

The disk may be held together, Rafikov says, by permeating gas. The gas would slow down the debris whirling around the young star, which would help it collide and stick. Also, gravity from the gas might keep the disk orderly in the presence of another star.

Once the planet forms, “survival is not that difficult,” says Andrew Gould, an astrophysicist at Ohio State University in Columbus and a member of the OGLE team. Because the planet is relatively close to its own sun, the gravity from the companion star doesn’t affect it much.

This type of planet, Gould says, could be discovered only via microlensing. And because so many stars live in pairs, he says, a huge population of planets may await discovery. One of the primary goals of a proposed NASA space telescope named WFIRST-AFTA is to find lots of microlensing events. Scientists hope that WFIRST-AFTA will uncover scores of planets that are largely invisible to other techniques.

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