Astronomers have found strong evidence of a massive planet (or planets) — the first planet(s) in a circumtriple orbit — in a hierarchical triple star system called GW Orionis (GW Ori).
This ALMA image shows the shadow of the innermost ring on the rest of the circumtriple protoplanetary disk around GW Orionis. Image credit: ALMA / ESO / NAOJ / NRAO / University of Exeter / Kraus et al.
GW Ori, also known as HD 244138 and HIC 25689, is located some 1,312 light-years away in the constellation of Orion.
It is a hierarchical triple star system, with a binary star (GW Ori A and B) at a separation of about 1 AU (astronomical units) and a tertiary stellar companion (GW Ori C) at a projected distance of about 8 AU.
It has an age of 1 million years and hosts a misaligned circumtriple protoplanetary disk.
“Unlike our Solar System, which consists of a solitary star, it is believed that half of all star systems consist of two or more stars that are gravitationally bound to each other,” said Dr. Jeremy Smallwood from the Department of Physics and Astronomy at the University of Nevada and his colleagues.
“But no planet orbiting three stars — in a circumtriple orbit — has ever been discovered. Perhaps until now.”
This composite image shows both ALMA and SPHERE/VLT observations of the disk around GW Orionis. The ALMA image (blue) shows the disk’s ringed structure, with the innermost ring (part of which is visible as an oblong dot at the very center of the image) separated from the rest of the disk. The SPHERE observations (orange-red) allowed the astronomers to see for the first time the shadow of this innermost ring on the rest of the disk, which made it possible for them to reconstruct its warped shape. Image credit: ALMA / ESO / NAOJ / NRAO / University of Exeter / Kraus et al.
Using observations from the Atacama Large Millimeter/submillimeter Array (ALMA), the astronomers analyzed three dust rings previously observed around the GW Ori system.
They identified a substantial, yet puzzling, gap at 100 AU and misalignments between each of the rings.
They investigated different origins, including the possibility that the gap was created by gravitational torque from the three stars.
But after constructing a comprehensive model of GW Ori, they found that the more likely, and fascinating, explanation for the space in the disk is the presence of one or more massive planets.
“Gas giants are usually the first planets to form within a star system. Terrestrial planets like Earth and Mars follow,” Dr. Smallwood said.
“The planet itself cannot be seen, but the finding suggests that this is the first circumtriple planet ever discovered.”
Further ALMA observations are expected in the coming months, which could provide direct evidence of the circumtriple planet(s).
“It’s really exciting because it makes the theory of planet formation really robust,” Dr. Smallwood said.
“It could mean that planet formation is much more active than we thought, which is pretty cool.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
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Jeremy L. Smallwood et al. 2021. GW Ori: circumtriple rings and planets. MNRAS 508 (1): 392-407; doi: 10.1093/mnras/stab2624
