Wednesday, October 27, 2021

Mysterious world appears to be the first exoplanet found orbiting 3 stars

Our solar system, with just one star in the sky, can be a bit strange. Most stars in the Milky Way galaxy actually have at least one gravitationally bound stellar companion, meaning that two-star worlds like Tatooine are probably not uncommon.

However, star systems are limited to a maximum of two stars. We’ve got a system of up to seven stars bound together in an intricate 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 found in trinary systems – orbiting just one star in the system. If this new finding is validated, however, the exoplanets will be orbiting all three stars, which is not something that has been observed before.

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 birthplace are massive molecular clouds, where dense clumps of gas collapse under gravity.

As these clusters spin, material in the cloud forms a disk that is deposited on the forming star. If this disk fragments, another star, or several stars, may begin to form in the same location – a small stellar family of siblings. After a star is formed, what is left in the disk can form planets.

It is estimated that about 40 to 50 percent of stars have a binary companion, and another 20 percent are in systems that contain three or more stars.

These systems would be quite gravitationally complex, making it difficult for smaller objects to stick around – but even so, about 2.5 percent of exoplanets are estimated to be in these multiple systems that contain three stars or more.

To date, about 32 exoplanets have been found in the trinary system. 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 orbiting all three stars.

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Using the powerful Atacama Large Millimeter/submillimeter Array (ALMA), astronomers confirmed something else about the system: The protoplanetary disk has a substantial gap.

According to our model of planet formation, gaps in the protoplanetary disk are likely due to the formation of planets. As they go around the star, these planets sweep dust and gas into their orbital path, cleaning it up and leaving a gap.

In GW Orionis, things are not necessarily so clear. Because the three stars would generate a complex gravitational field, there is a possibility that any strange features in the disk were created by the stars themselves.

Previous analysis suggested that this is probably not the case; Gravitational interaction between stars alone is not enough to carve a gap in the disk, making an exoplanet as a possible explanation.

Now, a new analysis agrees 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, as did the researchers before them, that the torque generated by the stars is not enough to split the protoplanetary disk.

Instead, the culprit is likely a gas giant, like Jupiter, in the process of becoming, or perhaps like, several gas giants. We haven’t seen the exoplanet itself, which means there’s still room for doubt, but agreement between the two separate research efforts appears to be in favor of the baby exoplanet interpretation.

Which could mean that the planet formation process may be able to survive in more extreme conditions than we expected, with complex environments such as the space around triple stars.

“It’s really exciting because it makes the theory of planet formation really strong,” Smallwood said. “This could mean that planet formation is much more active than we thought, which is great.”

The team hopes that astronomers will be able to see the exoplanets, or exoplanets, directly in upcoming observations of the GW Orionis system.

research has been published in Monthly Notice of the Royal Astronomical Society.


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