The new planet, Kepler-150 f, was overlooked for several ears, according to researchers at the Yale University in the US.
Computer algorithms identify most such "exoplanets", which are planets located outside our solar system.
The algorithms search through data from space mission surveys, looking for the telltale transits of planets orbiting in front of distant stars.
However, sometimes the computers miss something. In this case, it was a planet in the Kepler-150 system with a long orbit around its sun.
Kepler-150 f takes 637 days to circle its sun, one of the longest orbits for any known system with five or more planets.
The Kepler Mission found four other planets in the Kepler-150 msyste - Kepler-150 b, c, d, and e - several years ago. All of them have orbits much closer to their sun than the new planet does.
"Only by using our new technique of modeling and subtracting out the transit signals of known planets could we then actually see it for what it really was," said Joseph Schmitt, graduate student at Yale.
"Essentially, it was hiding in plain sight in a forest of other planetary transits," said Schmitt.
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