The Exoplanet With the Longest Year

Astronomers have recently discovered a transient exoplanet with the longest year ever seen. Circling its star once every 704 days, Kepler-421b easily wins the title of longest orbit among known exoplanets.

According to experts from the Harvard-Smithsonain Center for Astrophysics (CfA), most of the more-than 1,800 exoplanets discovered to-date are much closer to their respective stars and have exceptionally shorter years.

David Kipping of CfA led the team that discovered this long-yeared planet - a discovery he says took some luck.

"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," he explained in a recent release.

Exoplanets are generally discovered when they pass in front of their star so that they interfere with otherwise consistent readings from the fiery celestial surface. This process is called a transit.

Named after the spacecraft that discovered it, Kepler-421b has only been seen making two transits across the face of its star due to its incredibly long orbital period. Still, Kipping and his team are confident that it is a real planet and not just solar phenomena masquerading as a planet, as seen with a pair of potential exoplanets once thought habitable.

Interestingly, the planet's exceptionally wide orbit places it beyond a point that astronomers call the "frost line" in a solar system. Past this point, water condenses into ice grains that tend to stick together building gas giants.

This allowed Kipping's team to more conclusively say that Kepler-421b is a gas giant. However, most gas giants have been found to move closer to their star as time passes.

Kepler241b has done no such thing.

"This is the first example of a potentially non-migrating gas giant in a transiting system that we've found," said Kipping, adding that the planet likely formed in a sweet spot along the frost line - making it a rare find.

More details on this long-yeared exoplanet is due to be published in The Astrophysical Journal. All findings should be viewed a preliminary until official publication.