Astronomers May Have Found First-Ever Exoplanet Orbiting Three Stars

(ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.) Since last year, astronomers have been peeking at a fascinating triple star system about 1,300 light-years away. The system, known as GW Orionis, is enshrouded in murky dust, indicating it’s still very young. Scientists have struggled to explain the twisting and contortion of the dust rings. According to Bad Astronomy, a new analysis may have hit on a cause: a first-of-its-kind exoplanet that orbits all three stars in this solar system. 

GW Orionis is what’s known as a hierarchical trinary. That’s the same grouping as the Centauri solar system just next door with two closely orbiting stars, plus a companion that orbits both at a greater distance. In GW Orionis, the two closer stars are 180 million kilometers apart, a little more than the distance between Earth and the sun. Meanwhile, the outer member is 1.2 billion kilometers away, which is roughly the distance between the sun and Saturn. 

These stars are surrounded by a disk of dust and gas, which is common for young solar systems. Eventually, some of this material could coalesce into planets, but most of it will be blown away by increasingly powerful solar winds. Around GW Orionis, the dust has fallen into a series of severely warped rings. You can see a simulation of the rings below. Above, the left image is an artist’s impression, and the right is observational data from the ALMA radio telescope array. 

The new research, led by astronomer Jeremy Smallwood, used orbital and particle simulations to search for some way the outer star in the trinary could be disrupting the rings. Try as they might, the researchers couldn’t come up with a way for the star itself to break up the rings. SO, they tried something new and added a planet to the system. There are known exoplanets in trinary systems — Centauri has a few, in fact. However, they all orbit one or two stars in the group. We’ve never found a planet orbiting three stars because such a system is inherently unstable. And yet, the addition of a planet in that configuration seems to work. 

According to the study, if the protoplanetary disk is on the thick side (10 times wider than it is thick) the exoplanet can’t affect the rings as seen from Earth. However, if it were 20 times wider than it is thick, the planet can carve a gap very similar to what we see. There are some features of the GW Orionis system that are still unexplained, like the mirrored arch structures toward the middle. It’s going to take much more analysis to see what impact, if any, the proposed planet could have on those. 

This exoplanet is not yet confirmed, but it could be a first for astronomy. About half of all stars are in binary systems, and maybe twenty percent are in trinaries. Finding out that exoplanets can orbit a trinary could mean there are a lot more worlds out there. GW Orionis is still young, though. It’s possible planets like this form often but then get ejected to become rogue planets, and we do suspect there are a lot of those. 

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