The hunt for Planet Nine

An elusive Earth-like planet in our solar system could finally be discovered within the next few years.

Known as Planet Nine or Planet X, it is a hypothetical planet that has long been rumoured to exist in the outer solar system. Although "several claims to have found an extra planet have been made over the years" so far "none has proven to be true", said the BBC.

But now researchers who have spent almost a decade hunting for Planet Nine believe the breakthrough could be close.

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'Doughnut region'

There are eight known planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune – and then a possible Planet Nine. Speculation over its existence began following the discovery of Neptune in 1846 and Percival Lowell, a wealthy American businessman and astronomer, kicked off the search for it in 1906.

For more than 70 years, Pluto was classed as the ninth planet but that changed in 2006, when the International Astronomical Union downgraded its status to that of a dwarf planet.

Last September, astronomers in Japan detected a series of objects in the Kuiper Belt – described by the BBC as a "doughnut shaped region of icy bodies" beyond the orbit of Neptune – that had unusually warped orbits around the Sun. Researchers Michael Brown and Konstantin Batygin, from the California Institute of Technology (Caltech) in Pasadena, speculated that only a massive planet's gravitational pull could explain these "orbital anomalies", said Live Science.

Then in February, scientists narrowed down the "likely hiding" place of the "elusive" planet after they "whittled away" 78% of the "hypothetical world's suspected orbital pathway", said Philip Plait in Scientific American.

But this still leaves "a lot of sky left to search, including spots that are much tougher to sift through", he added.

'Planet Nine sceptics'

Opinion is divided over how big this hypothetical planet might be. Astronomers from Japan think it could be three times as big as Earth but Live Science believes the "enigmatic entity" is significantly bigger: around "seven times more massive than Earth".

It's probably located somewhere between 500 and 600 astronomical units from the Sun. At such a distant location it would not support life as we know it, because the temperatures would be too cold.

There are "Planet Nine sceptics", said BBC Earth. Some experts believe "it isn’t even a planet, but a primordial black hole" and other astronomers are convinced that it is "simply natural bias in sky surveys".

Although "as a scientist" Plait "can't say one way or another" if the planet exists. "But as a human being I'll readily admit I want it to be out there" because "we'd learn so much about this distant region", and "it would advance our understanding of how the solar system formed and evolved over the eons".

It does not have an official name and will not receive one unless its existence is officially confirmed through imaging.