Astronomers Discover Twin to Famous Exoplanet

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The infrared spectra of 2MASS 0249 c (top) and beta Pictoris b (bottom) are similar. Both objects are of comparable mass and formed in the same stellar nursery. Unlike 2MASS 0249 c, beta Pictoris b orbits much closer to its massive host star and is embedded in a bright circumstellar disk. Credit: T. Dupuy, ESO/A.-M. Lagrange et al.

Hawai‘i Island astronomers have found a new planet that appears to be nearly identical to one of the best studied gas-giant extrasolar planets. However, the newly discovered planet differs in an important way: its origin.

“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,” said Trent Dupuy, astronomer at the Gemini Observatory and leader of the study.

Stars form in bunches, forming varying in energy and size within stellar nurseries filled with gas and dust. Eventually, they drift from their birthplace. Because of this exodus, astronomers believe planets exist that are born at the same time in the same stellar nursery, but are orbiting stars that have moved far away from each other.


“To date, exoplanets found by direct imaging have basically been individuals, each distinct from the other in their appearance and age,” said Michael Liu, astronomer at the University of Hawai‘i Institute for Astronomy, and a collaborator on this work. “Finding two exoplanets with almost identical appearances and yet having formed so differently opens a new window for understanding these objects.”

Dupuy, Liu and their collaborators have identified the first case of such a planetary doppelgänger. One object has long been known—one of the first planets discovered by direct imaging back in 2009 called the 13-Jupiter-mass planet beta Pictoris b. The new object, dubbed 2MASS 0249 c, has the same mass, brightness and spectrum as beta Pictoris b.

After discovering this object with the Canada-France-Hawaii Telescope (CFHT), Dupuy and collaborators then determined that 2MASS 0249 c and beta Pictoris b were born in the same stellar nursery. This makes the two objects not only look-alikes, but genuine siblings.


However, the planets orbit vastly different stars. The host for beta Pictoris b 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 host—about nine astronomical units (AU, the distance from the Earth to the Sun), while 2MASS 0249 c is 2,000 AU from its binary host.

These drastically different conditions suggest the planets’ upbringings were not at all alike. The traditional picture of gas-giant formation dictates that planets start as small rocky cores around their host star and grow by accumulating gas from their star’s disk—which is likely how beta Pictoris b was created. In contrast, the host of 2MASS 0249 c did not have enough of a disk to make a gas giant, so the planet was likely formed by accumulating gas from its original stellar nursery.

“2MASS 0249 c and beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets,” says Kaitlin Kratter, astronomer at the University of Arizona and a collaborator on this work. “Beta Pictoris b probably formed like we think most gas giants do, starting from tiny dust grains. In contrast, 2MASS 0249 c looks like an underweight brown dwarf that formed from the collapse of a gas cloud. They’re both considered exoplanets, but 2MASS 0249 c illustrates that such a simple classification can obscure a complicated reality.”


The team first identified 2MASS 0249 c using images from CFHT, and their repeated observations revealed this object 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 brown dwarfs. So, altogether, the 2MASS 0249 system comprises two brown dwarfs and one gas-giant planet. Follow-up spectroscopy of 2MASS 0249 c with the NASA Infrared Telescope Facility and the Astrophysical Research Consortium 3.5-meter Telescope at Apache Point Observatory demonstrated that it shares a remarkable resemblance to beta Pictoris b.

The 2MASS 0249 system is an appealing target for future studies. Most directly imaged planets are very close to their host stars, hindering detailed studies of the planets due to the bright light generated by their stars. In contrast, the very wide separation of 2MASS 0249 c from its host binary will make measurements of properties like its surface weather and composition much easier, leading to a better understanding of the characteristics and origins of gas-giant planets.


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