Using NASA’s Kepler spacecraft, researchers have discovered an exoplanet with the longest known year.

The newly discovered Kepler-421b circles its star in 704 days. It is the longest period of revolution known yet for a planet outside of our solar system.

Most of the exoplanets discovered till date are much closer to their stars and have much shorter orbital periods.

“Finding Kepler-421b was a stroke of luck,” said lead researcher David Kipping from the Harvard-Smithsonian Centre for Astrophysics.

“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,” Kipping added.

The spacecraft stared at the same patch of sky for four years, watching for stars that dim as planets cross in front of them.

It detected Kepler-421b pass by twice during these four years, said the study that will be published in the Astrophysical Journal.