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Orphaned Planet Finds Parent Star
published during a waning crescent moon.
02/04/2016
orphaned planet

Artist’s impression of the planet with its star in the background. Credit: University of Hertfordshire/Neil James Cook.

Astronomers have calculated the age and motion of a giant world and a neighboring star to find the widest planetary system yet observed. Not only does this system make the hypothetical solar system world “Planet Nine” appear close to Earth, it also significantly dwarfs any previously observed systems.

When the world known as 2MASS J2126 was first spotted in 2008, it was identified as a brown dwarf, a massive body not quite large enough to kick off fusion in its heart and thus often called a ‘failed star’. In 2014, observations suggested it could be a member of collection of stars and brown dwarfs with ages around 45 million years, which allowed scientists to narrow down its age and temperature enough to determine that it could be a free-floating planet, a world without a star.

Planetary systems don’t form smoothly; their birth is often accompanied with violence. In our own solar system, a large impact formed Earth’s moon, as well as several others. Recent research suggested that Mercury could be the survivor of a vicious game of interplanetary ping-pong, one of several objects formed in the inner solar system, and the only one to remain, though it likely suffered a violent impact. And the recently-proposed “Planet Nine” may be an icy world relegated to the edges of the solar system by gravitational interactions with other giant planets.

Finding a booted, or free-floating, planet, isn’t a shock, then. Several have been identified in the last few years, and the fact that they are difficult to identify suggests that there are a wealth of worlds drifting through space without a star.

But Niall Deacon, of the University of Hertforshire in the United Kingdom, wasn’t satisfied to classify these worlds as orphans. He has spent the last few years comparing lists of known young stars, brown dwarfs, and free-floating planets to determine if any might be related.

In the case of 2MASS J2126, Deacon and his colleagues found a potential parent star, a young red dwarf laying roughly the same distance from Earth as the rogue planet. By studying the movement of both objects across the sky, the scientists determined that they were extremely likely to be connected.

The newfound relationship doesn’t make a significant dent in the planet’s solitude. 2MASS J2126 lies a whopping 7,000 times the Earth-sun distance (known as an Astronomical Unit, or AU) from its star, making it the largest planetary system found to date.

Very wide orbit planets, those with orbits higher than 1,000 AUs, have been challenging to spot. Four other objects have been identified to date, all of them weighing in at least 5 times as massive as Jupiter. Before 2MASS J2126 was connected to a star, the most distant of these worlds lay only 2500 AU from its sun.

Planets that lie at extreme distances from their star prove difficult to explain under the current understanding of star formation, particularly since many of them tend to be close to the cutoff between planets and brown dwarfs. The bias towards heavier free-floating planets comes about because it’s easier to spot the larger bodies than drifting Earth-sized worlds. Some scientists have proposed that these massive planets could have formed more like a star, condensing from its own cloud of gas and dust rather than from the disk surrounding the parent star like other planets.

A recent research paper of a planet sitting 650 AU from its star revealed a disturbed inner planetary system that suggested the world was kicked out, rather than formed so far away. It might be interesting to see if similar observations around the stellar parent of 2MASS J2126 turned up similar signs of disruption, though it seems likely more time would have passed and allowed things to settle down.

Despite the fact that the enormous world is no longer alone, having a sun wouldn’t do very much for it. At about 1 trillion kilometers, or a tenth of a light-year, away, the dim young sun would appear as bright star from the surface of the planet, which takes 900,000 Earth-years to complete an orbit. Light from the sun would take about a month to reach the planet, and life as we know it would be unlikely.