Kepler-421b, a newly discovered planet that's outside of our solar system, takes the cake for exoplanet with the longest year. It takes 704 Earth days, or about 2 Earth years, for the exoplanet to make one revolution around its star. Though that may sound like a lot, Mars completes an orbit around the sun in 780 days, Phys.org reports. Most known exoplanets are simply much closer to their stars and have shorter orbits.
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