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Astronomers have discovered an exoplanet with the longest known year - 704 days.

The planet, Kepler-421b, takes almost two Earth years to circle its star. In comparison, Mars orbits our sun once every 780 days.

Most of the 1,800-plus exoplanets discovered to date are much closer to their stars and have much shorter orbital periods, astronomers claim.

It takes Kepler-421b (illustration pictured) circles almost two Earth years to circle its star - or 704 days. Kepler-421b orbits an orange type K star that is cooler and dimmer than our sun

'Finding Kepler-421b was a stroke of luck,' said lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) in Massachusetts.

'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.'

Kepler-421b orbits an orange, type K star that is cooler and dimmer than our sun. It circles the star at a distance of about 110 million miles.

As a result, this Uranus-sized planet is chilled to a temperature of -93°C (-135° F).

As the name implies, Kepler-421b was discovered using data from Nasa Kepler spacecraft (illustration pictured). The spacecraft stared at the same patch of sky for four years, watching for stars that dim as planets cross in front of them

As the name implies, Kepler-421b was discovered using data from the Nasa Kepler spacecraft.

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

Despite its patience, Kepler only detected two transits of Kepler-421b due to that world's extremely long orbital period.

The planet's orbit places it beyond the 'snow line' - the dividing line between rocky and gas planets.

Outside of the snow line, water condenses into ice grains that stick together to build gas giant planets.

'The snow line is a crucial distance in planet formation theory.

'We think all gas giants must have formed beyond this distance,' explained Professor Kipping.

Since gas giant planets can be found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exoplanets migrate inward early in their history.

Kepler-421b shows that such migration isn't necessary. It could have formed right where it can now be seen.

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

The host star, Kepler-421, is located about 1,000 light-years from Earth in the direction of the constellation Lyra.

The Kepler probe was launched in 2009. To find the latest planet, spacecraft stared at the same patch of sky for four years, watching for stars that dim as planets cross in front of them