Kepler-421b has been found to have the longest-known year for a transiting expoplanet; it has a 704 day orbit. The findings have important implications in the behaviour of exoplanets. 

Mars orbits the Sun once every 780 days, but most of the over 1,800 exoplanets are much closer to their host stars and have shorter orbital periods. Kepler-421b orbits its orange K-type star at a distance of about 110 miles, leaving it at a frigid temperature of negative 135 degrees Fahrenheit. The planet is located about 1,000 light-years from Earth near the constellation Lyra.

"Finding Kepler-421b was a stroke of luck," lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) said. "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 planet was discovered using NASA's Kepler spacecraft, which looks out for the dimming of stars as planets cross in front of them. Over a four year period the craft only detected the presence of Kepler-421b twice. The planet's orbit is beyond the "snow line," which divides rocky and gas planets. When outside of the snow line, water condenses into sticky ice grains that can make up planets.

"The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance," Kipping said.

Gas planets are often found extremely close to their host stars, leaving them with orbits that can be as short as hours. This has led scientists to suggest exoplanets tend to migrate inward sometime in their early lives. Kepler-42b challenges that theory and shows migration isn't always necessary and these planets could have formed in the same position they are in today.

"This is the first example of a potentially non-migrating gas giant in a transiting system that we've found," Kipping said.

The findings have been accepted into The Astrophysical Journal.