Our Solar System, with just one star in the sky, may be a bit of an oddball. Most of the stars in the Milky Way galaxy actually have at least one gravitationally bound stellar companion, meaning that two-starred worlds like Tatooine are probably not uncommon.
Star systems, however, are not confined to a maximum of two stars. We've found systems of up to seven stars bound 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 been found in trinary systems before – orbiting just one of the stars in the system. If this new discovery is validated, though, the exoplanet will be in orbit around all three of the stars, which isn't something that's been seen previously.
Observations of GW Orionis. (ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.)
Stars in the Milky Way are not usually born in isolation. Their birthplaces are massive molecular clouds, where dense clumps of gas collapse under gravity.
As these clumps spin, material in the cloud forms a disk that accretes onto the forming star. If this disk fragments, another star, or multiple stars, can start forming in the same place – a little stellar family of siblings. After the star is done forming, what's left of the disk can go on to form planets.
It's estimated that some 40 to 50 percent of stars have a binary companion, and another 20 percent are in systems that have three or more stars.
These systems will be quite gravitationally complex, which may make it difficult for smaller objects to stick around – but nevertheless, around 2.5 percent of exoplanets are estimated to be in these multiple systems consisting of three stars or more.
To date, around 32 exoplanets have been found in trinary systems. And then a system called GW Orionis came along.
Located about 1,300 light-years away, GW Orionis caught astronomers' attention because it's surrounded by a huge, misaligned protoplanetary disk circling all three stars.
Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), astronomers confirmed something else about the system: There's a substantial gap in the protoplanetary disk.
According to our models of planet formation, gaps in protoplanetary disks are likely to be caused by planets forming. As they go around the star, these planets sweep up the dust and gas in their orbital path, clearing it and leaving a gap.
In GW Orionis, things aren't necessarily so clear-cut. Because the three stars would generate a complex gravitational field, there's a possibility that any strange features in the disk could have been created by the stars themselves.
Previous analysis suggested that this is probably not the case; the gravitational interaction between the stars alone is not sufficient to have carved a gap in the disk, leaving a forming exoplanet as the likely explanation.
Now, a new analysis has agreed with this interpretation. Led by astronomer Jeremy Smallwood of the University of Nevada, Las Vegas, a team of researchers reconstructed a model of the GW Orionis system, integrating 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 likely a gas giant, like Jupiter, in the process of forming, or perhaps multiple gas giants. We've not seen the exoplanet itself, which means there's still room for doubt, but the agreement between the two separate research efforts does seem to favor the baby exoplanet interpretation.
Which could mean that the planet formation process might be able to survive more extreme conditions than we expected, such as complicated 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.