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Alien World Looms Large in its Neighbor World's Sky
Artist's conception of Kepler-36 system
Artwork entitled Planetrise. An artist's conception shows Kepler-36c as it might look from the surface of neighboring Kepler-36b. Credit: Harvard-Smithsonian Center for Astrophysics/David Aguilar

Today NASA Kepler Mission astronomers are publishing in the journal Science Express the discovery of a star, Kepler-36, about 1200 light-years away, with two planets orbiting very close to each other but with very dissimilar densities. "These two worlds are having close encounters," said Josh Carter, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). Co-author Eric Agol of the University of Washington added, "They are the closest to each other of any planetary system we've found."

The inner world, Kepler-36b, is a rocky planet 1.5 times the size of Earth and weighing 4.5 times as much. It orbits about every 14 days at an average distance of less than 11 million miles, whereas the outer world, Kepler-36c, orbits at a distance of 12 million miles, about every 16 days, and is a gaseous planet 3.7 times the size of Earth. The authors believe the smaller planet is 30 percent iron, less than 1 percent atmospheric hydrogen and helium and probably no more than 15 percent water. The larger planet, by contrast, likely has a rocky core surrounded by a substantial amount of atmospheric hydrogen and helium, a "hot Neptune."

The two planets have closest approach (conjunction) about every 97 days, when they are separated by less than 5 Earth-Moon distances. At those times Kepler-36c would present a spectacular view in the sky of the smaller rocky Kepler-36b and both planets would experience significant tidal forces.

This system presents a puzzle as to how these two very different worlds ended up in such close orbits. Within our solar system, rocky planets reside close to the Sun while the gas giants remain distant.

This result was made possible by the incredibly precise brightness-measuring capability of the Kepler telescope which allows Kepler asteroseismologists to the study of oscillations stars. Stars resonate like musical instruments, with larger stars, having "deeper sounds" are its resonances. Co-author Bill Chaplin noted, "By measuring the oscillations we were able to measure the size, mass and age of Kepler-36 to exquisite precision." Kepler-36 is much like the Sun except several billion years older. He added, "Without asteroseismology, it would not have been possible to place such tight constraints on the properties of the planets."

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