Nasa has announced that there is almost certainly not a giant, dark 'Nemesis' star lurking at the edge of our Solar System.

"Probably".

It has long been theorised - though only by a fringe of the astronomy community - that a massive, unseen celestial body exists beyond the orbit of Pluto.

Known as 'Nemesis', or Planet X, the theories attempt to explain geological studies which suggest oddly regular mass extinctions on Earth.

Above: A nearby star stands out in red in this image from the Second Generation Digitized Sky Survey.

The idea is that this large star or planet might occasionally rush through bands of comets at the edge of the solar system, and send them hurtling towards Earth.

It's not a widely-held belief, though. And now a full-sky sweep by the space agency's Wide Field Infrared Survey Explorer (WISE) has found "no evidence" that such a thing exists.

Nasa said it is now sure there is no object larger than Jupiter beyond Pluto to a distance of 26,000 'AU' (astronomical units) - where the distance to the Sun from Earth, or 93 million miles, is one AU.

"The outer solar system probably does not contain a large gas giant planet, or a small, companion star," said Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University.

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NASA: There Is 'Probably' No 'Nemesis' Death Star Beyond Pluto