Astronomers have discovered a giant gaseous planet orbiting a pair of brown dwarfs called 2MASS J02495639-0557352AB (2MASS 0249 for short). The new exoplanet, 2MASS 0249c, has the same mass, brightness, and spectrum as Beta Pictoris b, a 20-million-year-old gas giant in orbit around the second brightest star in the southern constellation of Pictor.

Gemini Observatory astronomer Trent Dupuy and co-authors identified 2MASS 0249c using images from Canada-France-Hawaii Telescope (CFHT), and their repeated observations revealed the 11.6-Jupiter-mass planet is orbiting at a large distance from its host.

The system belongs to the Beta Pictoris moving group, a widely dispersed set of stars named for its famous planet-hosting star.

The team’s observations at W. M. Keck Observatory determined that the host is actually a closely separated pair of 48- and 44-Jupiter-mass brown dwarfs.

Follow-up spectroscopy of 2MASS 0249c with NASA’s Infrared Telescope Facility and the 3.5-m telescope at Apache Point Observatory demonstrated that it shares a remarkable resemblance to Beta Pictoris b.

“We have found a gas-giant planet that is a virtual twin of a previously known planet, but it looks like the two objects formed in different ways,” Dr. Dupuy said.

The team also determined that 2MASS 0249 c and Beta Pictoris b were born in the same stellar nursery.

This makes the two objects not just look-alikes but genuine siblings. However, the planets have vastly different living situations, namely the types of stars they orbit.

Beta Pictoris is a star 10 times brighter than the Sun, while 2MASS 0249 c orbits a pair of brown dwarfs that are 2,000 times fainter than the Sun.

Furthermore, Beta Pictoris b is relatively close to its star, about 9 AU (astronomical units), while 2MASS 0249 c is 1,950 AU from its binary host.

These drastically different arrangements suggest that the planets’ upbringings were not at all alike.

“The traditional picture of gas-giant formation, where planets start as small rocky cores around their host star and grow by accumulating gas from the star’s disk, likely created Beta Pictoris b,” the researchers said.

“In contrast, the host of 2MASS 0249c did not have enough of a disk to make a gas giant, so the planet likely formed by directly accumulating gas from the original stellar nursery.”

“2MASS 0249c and Beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets,” said co-author Dr. Kaitlin Kratter, an astronomer at the University of Arizona.

“Beta Pictoris b probably formed like we think most gas giants do, starting from tiny dust grains. In contrast, 2MASS 0249c looks like an underweight brown dwarf that formed from the collapse of a gas cloud.”

“They’re both considered exoplanets, but 2MASS 0249c illustrates that such a simple classification can obscure a complicated reality.”

The study will be published in the Astronomical Journal.

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Trent J. Dupuy et al. 2018. The Hawaii Infrared Parallax Program. III. 2MASS J0249-0557 c: A Wide Planetary-mass Companion to a Low-mass Binary in the beta Pic Moving Group. AJ, in press; arXiv: 1807.05235