Bizarre alien planets get cozy with each other

A duo of planets some 1,200 light years from earth have been spotted passing within 1.2 million miles of each other, closer than any other known pair of planets, a new study has found.?

By Mike Wall,?SPACE.com / June 21, 2012

An artist's conception shows the newfound alien planet Kepler-36c as it might look from the surface of Kepler-36b.

David Aguilar, Harvard-Smithsonian Center for Astrophysics

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It is science fiction made fact: Astronomers have discovered two alien planets around the same star whose orbits come so close together that each rises in the night sky of its sister world like an exotic full moon.

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The newfound planets are 1,200 light-years from Earth and an unprecedented find, researchers said. They differ greatly in size and composition but come within just 1.2 million miles (1.9 million kilometers) of each other, closer than any other pair of planets known, according to a new study.

One of the newly discovered?alien planets, called Kepler-36b, appears to be a rocky "super-Earth" 4.5 times as massive as our planet. The other, Kepler-36c, is a gaseous, Neptune-size world about eight times as massive as Earth. The two planets meet up every 97 days in a conjunction that would make each dramatically visible in the other's sky.

?These two worlds are having close encounters,? said co-lead author Josh Carter, of the Harvard-Smithsonian Center for Astrophysics, in a statement.

At their closest approach, the two planets are separated by five times the distance between the Earth and the moon. How such different bodies ended up in such similar orbits may be tough for current theories of?planet formation?and migration to explain, researchers said.

"This is unprecedented," co-lead author Eric Agol, of the University of Washington, told SPACE.com via email. "They are as different in density as Earth and Saturn (the highest and lowest density planets in our solar system), yet they are 30 times closer than any pair of planets in our solar system." [Gallery: The Strangest Alien Planets]

Strange neighbors

The two known planets in the Kepler-36 system ? which is located in the constellation Cygnus (The Swan) ? were detected by NASA's?Kepler Space Telescope.

Kepler is staring continuously at more than 150,000 stars, watching for telltale brightness dips caused when planets cross in front of the stars from the telescope's perspective. Since its March 2009 launch, Kepler has flagged more than 2,300 potential alien planets; while only a small fraction have been confirmed to date, mission scientists think more than 80 percent of them will end up being the real deal.

Kepler-36c, which is about 3.7 times wider than Earth, likely has a rocky core surrounded by a substantial atmosphere filled with lots of hydrogen and helium, researchers said.

Kepler-36b, on the other hand, is a?super-Earth?just 1.5 times wider than our planet. Iron likely constitutes about 30 percent of its mass, water around 15 percent and atmospheric hydrogen and helium less than 1 percent, researchers said.

Though they're very different in size and makeup, the two planets travel on surprisingly similar paths around their host star. Kepler-36c orbits once every 16 days, at an average distance of 12 million miles (19 million km). Kepler-36b orbits each 14 days and sits about 11 million miles (18 million km) from the star.

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