Curious Stellar Object Confirmed As Rogue Planet

Artist's conception of a rogue planet floating through space. Christine Pulliam/CfA

Three years ago, a mysterious object was observed drifting alone in interstellar space, 80 light-years from Earth. Much bigger than Jupiter but smaller than most stars, it was called a rogue planet, and now astronomers can confirm that the object is indeed a planet.

PSO J318.5338-22.8603, or PSO J318.5-22 for short, is a large planetary object in the Beta Pictoris moving group. It has a mass of 8.3 times that of Jupiter and a temperature of about 1100 Kelvins. A detailed description of the findings has been published on ArXiv.

When it was discovered, it was considered a weird brown dwarf, a type of star that has failed to initiate nuclear fusion. PSO J318.5-22 was alone in space without a companion star and had a high temperature suggesting it was a stellar object, but the light emitted was similar to gas exoplanets seen elsewhere in the galaxy, so scientists thought that it might be a rogue planet.

"We have never before seen an object free-floating in space that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” Dr. Michael Liu, leader of the team that discovered PSO J318.5-22, said in a statement at the time.

Multicolor image from the Pan-STARRS1 telescope of the rogue planet PSO J318.5-22. N. Metcalfe & Pan-STARRS 1 Science Consortium

Confirming PSO J318.5-22's planetary status has not been easy, due to the age-temperature degeneracy of brown dwarfs. That might sound like a complicated concept but it’s very simple: Brown dwarfs begin their lives very hot and over time they cool down. Calculating the age of planets is very difficult, but there are ways to establish the age of a group of stars with a high degree of accuracy.

Luckily, PSO J318.5-22 seemed to be part of the Beta Pictoris moving group, a young and small star cluster relatively near Earth. Using the Hawaiian telescope Gemini North, astronomers confirmed that the potential rogue planet was part of the group and thanks to the fact that the age of the group has been calculated already, they were able to provide the long-sought confirmation. PSO J318.5-22 is about 23 million years old, too young and cool to be a brown dwarf. 

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