Lost planet size of Neptune discovered 'hiding in plain sight'

A fifth planetin the Kepler-150 system has been discovered, several years after the discovery of the first four, Kepler-150 b, c, d and e. It is about the size of Neptune and lies about 3,000 light years from Earth.

The exoplanet Kepler-150 f is the furthest from its star of the five planets in the system discovered so far. It takes 637 days to orbit, giving it one of the longest orbits in a solar system with five or more planets. The other four planets have orbital periods of between 3 and 31 days.

The Kepler Mission, run by NASA, scans the skies to pick up signs of planets in transit – when they pass in front of the star that they are orbiting. When the planet passes in front of the star, the light reaching Earth from the star dims ever so slightly. This can be a dimming of about 1/10,000 of the star's brightness, and can last for between 1 and 16 hours.

The mission has so far detected and confirmed about 4,700 planets in this way. When a planet is detected in transit, all the data points associated with its transit are deleted from the model so as not to interfere with the search for more planetary transits.

However, this can result in another planet around the same star being masked from detection. Researchers led by Joseph Schmitt of Yale University in the US looked into the probability of planets being lost by this method. They found the probability was small but significant, reaching up to 3.3%.

A visual search through the data revealed the existence of Kepler-150 f that had slipped through the net.

"Only by using our new technique of modelling and subtracting out the transit signals of known planets could we then actually see it for what it really was," said Schmitt in a statement.

"Essentially, it was hiding in plain sight in a forest of other planetary transits."

The research is published in a paper in The Astronomical Journal.