Astronomers studying a lonely planet drifting through space have found its mum; a star a trillion kilometers away.

The planet, known as 2MASS J2126-8140, has an orbit around its host star that takes nearly a million Earth years and is more than 140 times wider than Pluto's. This makes it easily the largest solar system ever found.

"We were very surprised to find such a low-mass object so far from its parent star," said Dr. Simon Murphy of The Australian National University (ANU) Research School of Astronomy and Astrophysics.

"There is no way it formed in the same way as our solar system did, from a large disc of dust and gas."

Only a handful of extremely wide pairs of this kind have been found in recent years. The distance between the new pair is 6,900 astronomical units (AU) - 1,000,000,000,000 kilometers or 0.1 light-year - nearly three times the previous widest pair, which is 2,500 AU (370,000,000,000 km).

2MASS J2126-8140's parent is a red dwarf star called TYC 9486-927-1. At that distance, it would appear as only a moderately bright star in the sky, and light would take about a month to reach the planet.

Dr. Murphy is part of an international team of scientists that studied 2MASS J2126-8140, a gas giant planet around 12 to 15 times the mass of Jupiter, as part of a survey of several thousand young stars and brown dwarfs close to our solar system.

Once they realized 2MASS J2126-8140 and TYC 9486-927-1 were a similar distance from the Earth - about 100 light-years - they compared the motion of the two through space and realized they were moving together.

"We can speculate they formed 10 million to 45 million years ago from a filament of gas that pushed them together in the same direction," Dr. Murphy said.

"They must not have lived their lives in a very dense environment. They are so tenuously bound together that any nearby star would have disrupted their orbit completely."