Astronomers discover exoplanet with longest known year

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Artists impression of the Uranus-sized exoplanet Kepler-421b (Image: Daid A. Aguilar)

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With the aid of NASA's Kepler spacecraft a team of astronomers, including members from the Harvard-Smithsonian Center for Astrophysics, has discovered an exoplanet with the longest yearly cycle ever recorded. The planet, imaginatively named Kepler-421b, takes an impressive 704 days to orbit its parent, a dim type K star.

The discovery was made by analyzing data from the Kepler spacecraft, which accrued the information over the course of a four year continuous vigil of the patch of sky containing Kepler-421b. The planet was detected using a technique which measures the dip in light as the exoplanet transits over the face of its parent star. Due to the impressive length of Kepler-421b's orbit, only two transits were detected over the entire four year period in which Kepler stood watch.

"Finding Kepler-421b was a stroke of luck," states David Kipping of the Harvard-Smithsonian Center for Astrophysics, and lead author on the paper detailing the discovery. "The farther a planet is from its star, the less likely it is to transit the star from Earth's point of view. It has to line up just right."

The newly-discovered exoplanet sits around 1,000 light years from Earth. It orbits outside its own star's snow line, a hypothetical point of demarcation at which exoplanets cease to be rocky in composition and instead shift towards a predominantly ice-like make-up. The discovery of Keplar-421b holding an apparently stable orbit beyond the snow line may force astronomers to revise the current theories on the early behavior of gas giants that are thought to form in this area.

It is currently believed under the standard model of behavior for these bodies, that soon after formation a gas giant will drift back towards its parent star as Jupiter has done in our own solar system. The discovery of a planet remaining in orbit beyond the snow line means that this is not necessarily the norm, and that current theories will have to be altered to account for the behavior of planets such as Kepler-421b.

The explanation for Kepler-421b's lengthy orbit relates to its surprising distance from its parent star, which it orbits at a range of around 110 million miles, granting the planet a frigid surface temperature of -135º F (-57º C). Most other exoplanets discovered by the Kepler space telescope were found to be much closer to the focus of their orbit, making the length of their year more similar to that of our own.

A paper detailing the findings is due to be published in The Astrophysical Journal.

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