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Our solar system, with only one star in the sky, is maybe a little weird. Most stars in the Milky Way actually have at least one gravitationally bound stellar companion, which means two-star worlds like Tatooine are probably not uncommon.
Star systems, however, are limited to a maximum of two stars. We have found systems of up to seven stars linked together in a complex orbital dance. And now, scientists have discovered what they believe is a first for astronomy: an exoplanet orbiting a three-star system, also known as the stellar trinarian.
To be clear, exoplanets have already been found in trinary systems – orbiting just one of the stars in the system. If this new discovery is validated, however, the exoplanet will be orbiting all three stars, which has not been seen before.
Observations by 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 clusters of gas collapse under gravity.
As these clusters rotate, the material in the cloud forms a disk that accumulates on the forming star. If this disc fragments, another star or several stars can begin to form in the same place – a small stellar family of siblings. Once the star has finished forming, what is left of the disk can continue to form planets.
It is estimated that about 40 to 50 percent of stars have a binary companion, and 20 percent are found in systems that have three or more stars.
These systems will be quite gravitational complex, which can make it difficult to maintain smaller objects – but nonetheless, it is estimated that about 2.5% of exoplanets are found in these multiple systems made up of three or more stars.
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 has caught the attention of astronomers because it is surrounded by a huge, misaligned protoplanetary disk surrounding the three stars.
Using the powerful Atacama Large Millimeter / submillimeter Array (ALMA), astronomers have confirmed something else about the system: there is a substantial gap in the protoplanetary disk.
According to our models of planetary formation, the gaps in protoplanetary discs are likely caused by the formation of planets. By circling around the star, these planets sweep dust and gas off their orbital path, cleanse it, and leave space.
In GW Orionis, things are not necessarily so clear. Because the three stars would generate a complex gravitational field, it is possible that some strange characteristics of the disk could have been created by the stars themselves.
Previous analysis has suggested that this is probably not the case; the gravitational interaction between the stars alone is not sufficient to have breached the disc, leaving a forming exoplanet as a likely explanation.
Now a new analysis has agreed with this interpretation. Led by astronomer Jeremy Smallwood of the University of Nevada in Las Vegas, a team of researchers reconstructed a model of the GW Orionis system, incorporating N-body and three-dimensional hydrodynamic simulations.
They discovered, like researchers before them, that the torque generated by the stars is not enough to split the protoplanetary disk.
Instead, the culprit is probably a gas giant, like Jupiter, in the process of forming, or maybe several gas giants. We haven’t seen the exoplanet itself, which means there’s still room for doubt, but the agreement between the two separate research efforts seems to favor the interpretation of the baby exoplanet. .
Which could mean that the process of forming the planets could survive more extreme conditions than expected, such as complex environments like the space around triple stars.
“It’s really exciting because it makes the theory of how the planets form really solid,” Smallwood said. “It could mean that the formation of the planets is a lot more active than we thought, which is pretty cool.”
The team hopes that astronomers can see the exoplanet or exoplanets directly in the next observations of the GW Orionis system.
The research was published in the Monthly notices from the Royal Astronomical Society.
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