"Twin" exoplanets formed through very different processes

Identical appearance can hide very different origins.
By Laurel Kornfeld | Jul 18, 2018
Two giant exoplanets in separate star systems are practically identical but likely formed through very different processes, according to a team of scientists led by Trent Dupuy of the Gemini Observatory.

Beta Pictoris b and and 2MASS 0249 c, found via direct imaging in 2009 and 2017 respectively, have the same masses--approximately 13 Jupiter masses--as well as the same brightnesses and spectra.

Although the two planets are alike enough for astronomers to refer to the newly-discovered planet as a "doppelganger" of the first, and the two have parent stars that likely formed in the same stellar nursery of gas and dust, scientists believe their origins are very different.

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

Many stars are born in stellar nurseries but subsequently wander away from one another. By observing both planets with the Canada-France-Hawaii Telescope (CFHT), the researchers determined they originated in the same stellar nursery.

However, the stars they orbit as well as their orbital distances are very different from one another. Beta Pictoris b is in a close orbit at approximately nine astronomical units(AU, with one AU equal to the average Earth-Sun distance or 93 million miles) around a star 10 times brighter than the Sun while 2MASS 0249 c circles a pair of brown dwarfs at a distance of 2,000 AU.

Brown dwarfs are the lowest end of the stellar category. Dim and cool, they are not hot enough to fuse hydrogen in their cores although some fuse deuterium, an isotope of hydrogen.

Gas giant planets typically begin their lives as small rocky cores that grow by gathering gas from their parent stars' protoplanetary disks. Beta Pictoris b likely formed in this manner.

However, this formation process would not have been possible for 2MASS 0249 c because its two brown dwarf parents would not have had a large enough protoplanetary disk to provide the levels of gas needed to form a gas giant. This means the planet absorbed its gas directly from the stellar nursery.

"2MASS 0249 c and Beta Pictoris b show us that nature has more than one way to make very similar looking exoplanets. 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," explained Kaitlin Kratter of the University of Arizona, who took part in the study.

A paper on the findings has been accepted for publication in the Astronomical Journal.

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