Our Solar System may have just one star in its sky. Two-star worlds such as Tatooine, which have two stars in the Milky Way galaxy, are not unusual.
However, star systems are limited to two stars. There have been systems with seven or more stars that are linked in complex orbital dances. Scientists have now discovered what they believe to be a first in astronomy: An exoplanet orbiting three stars is known as a stellar trionary.
To be clear, exoplanets were found in three-dimensional systems before they orbited one star in the system. This new discovery will confirm that the exoplanet orbits all three stars. This is a first time it's been done.
Observations of GW Orionis. (ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.)
The Milky Way stars are usually not born in isolation. Their birthplaces are huge molecular clouds where dense clumps degas collapse under gravity.
These clumps create a cloud that spins, and the material within the cloud becomes a disk. This disk accretes onto forming stars. This disk can be broken up into multiple stars or other stars. A little family of stellar siblings may form if it is fragmented. The remnants of the disk that is left can be used to create planets after the star has been formed.
Some 40 to 50% of stars are believed to have a binary companion. Another 20% live in systems with three or more stars.
These systems are very gravitationally complex and may make it hard for smaller objects to stay around. However, approximately 2.5 percent of exoplanets have been estimated to be in multiple systems that consist of at least three stars.
To date, 32 exoplanets were found in trinary systems. Then came GW Orionis.
GW Orionis is located approximately 1,300 light years away. Astronomers noticed it because it is surrounded by a large, misaligned protoplanetary disc that circles 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.
Our models of planet formation suggest that gaps in protoplanetary discs could be caused by the formation of planets. These planets, as they circle the star, sweep up any dust or gas that is in their orbital paths, leaving a gap.
Things are not always so simple in GW Orionis. The complex gravitational field created by three stars could explain why there are some odd features found in the disk.
An analysis of the past suggested otherwise. The gravitational interaction between stars is not enough to have created a hole in the disk. This leaves a likely explanation: a forming exoplanet.
A new analysis now supports this interpretation. A team of researchers led by Jeremy Smallwood, an astronomer at the University of Nevada in Las Vegas, constructed a model of the GW Orionis System, incorporating N-body simulations and three-dimensional hydrodynamic simulations.
Just like previous researchers, they found that the torque generated from the stars was not enough to split the protoplanetary disc.
The culprit is most likely a gas giant like Jupiter in the process or multiple gas giants. Although we haven't seen the exoplanet, there is still some doubt. However, the agreement between the separate research efforts seems to favor the baby exoplanet interpretation.
This could indicate that the planet formation process may be able survive more extreme environments than we anticipated, such as complex environments like those found in the vicinity of triple stars.
Smallwood stated that "it's really exciting because this makes the theory about planet formation really robust." It could indicate that planet formation may be more active than we thought. That's pretty cool.
The team hopes that astronomers can see the exoplanet directly through upcoming observations of GW Orionis.
The research was published in the Monthly Notices of the Royal Astronomical Society.
