Artist impression of 2MASS J2126. Credit: University of Hertfordshire / Neil Cook
2MASS J2126 was believed to be a free-floating planet. A planet without a home star. But a team of astronomers in the UK, USA and Australia found the planet’s home star. It’s just really, really far away. The planet is about 1 trillion kilometers away from its star. Or, about 7000 times the distance from the Earth to the Sun.
The extreme distance also means 2MASS J2126 has the widest orbit of any planet found around another star. It takes 900,000 years to complete one orbit. All of recorded human history wouldn’t even begin to approach one ‘year’ if we lived on that planet.
How did astronomers figure this out?
Astronomers have found several free-floating planets in recent years. They tend to be gas giants, like Jupiter, and lack the mass for nuclear reactions. Without these reactions they cool and fade over time. Classifying them as planets is simple enough. First, astronomers try to figure out how old they are.
U.S.-based astronomers spotted 2MASS J2126 during an infrared sky survey and noted that it appeared to be a young, low-mass object. In 2014, Canadian researchers backed up the U.S. researchers by identifying the planet as a possible member of a 45 million-year-old group of stars and brown dwarfs dubbed the Tucana Horologium Association. This made it young enough to possibly be a free-floating planet.
In the same area of the sky lies TYC 9486-927-1. Astronomers identified it as a young star, but couldn’t tie it to any known group of young stars.
So, we have a possible free-floating planet and a young star that doesn’t appear to be part of any known group. It wasn’t until Dr. Niall Deacon of the University of Hertfordshire began searching for wide orbit systems that the connection between the two was made.
See, 2MASS J2126 and TYC 9486-927-1 are both located about 104 light years from the Sun and are travelling through space in the same direction.
“Nobody had made the link between the objects before,” said Deacon. “The planet is not quite as lonely as we first thought, but it’s certainly in a very long distance relationship.”
Because TYC isn’t tied to any known group of stars, astronomers had to study its spectrum. Specifically, they were looking at lithium. It’s destroyed early in a star’s life, so how much it has can give astronomers a good idea of its age. TYC had stronger signatures of lithium than the stars making up the Tucana Horologium Association, but not as much as a group of 10 million-year-old stars. That puts the star’s age somewhere between 10 million years old and 45 million years old.
As for 2MASS J2126? Astronomers estimate its somewhere between 11.6 – 15 times the mass of Jupiter. Right at the edge separating planets and brown dwarfs.
Sorry ET fans, no life on this planet. But even if there were life, they would think their star was just one among countless dotting the night’s sky.