With only one star in the sky, our Solar System appears to be an outlier. Most stars in the Milky Way galaxy have at least one gravitationally bound stellar companion, implying that two-star worlds such as Tatooine are not uncommon.
However, star systems are not limited to a maximum of two stars. We discovered systems with up to seven stars linked together in a complex orbital dance. And now, scientists have found what they believe may be a first for astronomy: an exoplanet orbiting a system of three stars, also known as a stellar trinary.
To be clear, exoplanets have previously been discovered in trinary systems – orbiting only one of the system’s stars. If this new discovery is validated, the exoplanet will be in orbit around all three stars, which has never been observed before.
Stars in the Milky Way are not typically born alone. Their birthplaces are massive molecular clouds, where dense clumps of gas collapse under gravity.
As these clumps spin, the cloud’s material condenses into a disk, which accretes onto the forming star. If this disk fragments, another star, or multiple stars, may form in the same location – forming a small stellar family of siblings. What remains of the disk after the star has formed can go on to form planets.
It is estimated that 40 to 50 percent of stars have a binary companion, with another 20 percent in systems with three or more stars.
These systems will be quite gravitationally complex, making it difficult for smaller objects to stick around – but, despite this, it is estimated that around 2.5 percent of exoplanets are in multiple systems consisting of three or more stars.
To date, 32 exoplanets have been discovered in trinary systems. And then a system called GW Orionis came along.
Located about 1,300 light-years away, GW Orionis caught the attention of astronomers because it is surrounded by a massive, misaligned protoplanetary disk that circles all three stars.
Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), astronomers confirmed another feature of the system: a significant gap in the protoplanetary disk.
Gaps in protoplanetary disks are most likely caused by planets forming, according to our models of planet formation. These planets sweep up the dust and gas in their orbital path as they orbit the star, clearing it and leaving a gap.
Things aren’t always so clear-cut in GW Orionis. Because the three stars would generate a complex gravitational field, any strange features in the disk could have been created by the stars themselves.
Previous research suggested that this is unlikely; the gravitational interaction between the stars alone is insufficient to have carved a gap in the disk, leaving a forming exoplanet as the most likely explanation.
A new analysis has now confirmed this interpretation. A team of researchers led by astronomer Jeremy Smallwood of the University of Nevada, Las Vegas rebuilt a model of the GW Orionis system using N-body and three-dimensional hydrodynamic simulations.
They found, just as researchers before them had, that the torque generated by the stars is not sufficient to have split the protoplanetary disk.
Instead, the culprit is most likely a gas giant in the process of formation, such as Jupiter, or a group of gas giants. We haven’t seen the exoplanet, so there’s still room for speculation, but the agreement between the two separate research efforts appears to favor the baby exoplanet interpretation.
This could imply that the planet formation process can survive more extreme conditions than previously thought, such as complex environments like the space around triple stars.
“It’s really exciting because it makes the theory of planet formation really robust,” Smallwood said. “It could mean that planet formation is much more active than we thought, which is pretty cool.”
The team hopes that astronomers will be able to see the exoplanet or exoplanets directly in upcoming observations of the GW Orionis system.
The research has been published in the Monthly Notices of the Royal Astronomical Society.