Suspected circumtriple planet: a planet orbiting 3 stars

GW Orionis, or GW Ori, is a triple star system located at the head of Orion the Hunter. A massive protoplanetary disk surrounds the triple star system. This disc is a dense region of gas and dust that remains from the formation of these young stars and has not yet been blown away by their stellar winds. The reason why scientists call it a protoplanetary disk is because the remaining material could merge and form planets. GW Ori’s protoplanetary disc has a significant gap inside, and scientists announced in September that they believe a planet exists within that gap. If confirmed, it would be the first known circumtriple planet, or a planet that orbits three stars.

Scientists published their study on September 17, 2021 in the peer-reviewed journal Monthly notices from the Royal Astronomical Society.

First circumtriple planet?

The circumtriple planet (s), if any, are about 1,300 light years away from us.

Scientists have already seen protoplanetary disks around stars. And in 2018, they captured the first image of a planet forming into a protoplanetary disc.

GW Ori’s protoplanetary disk is different, because it surrounds not one but Three stars, and has a ringed and deformed structure with a large gap. The researchers used computer models to simulate the unique star system. They also considered the idea that the gap could be a by-product of the three-star couple pulling on each other. The other option was that it could be a planet or planets massive enough to dig space into the dust ring.

In the end, the simulations showed that the couple of the three stars was not sufficient to cause the rupture of the disc. Scientists then concluded that the gap is more likely due to undetected planets.

Senior author Jeremy Smallwood of the University of Nevada, Las Vegas, told New York Times:

This may be the first evidence of a circumtriple planet breaching in real time.

Planets in triple star systems

Suspicious planets are not the first to be found in triple star systems. But the planets found in these other systems orbit only on one of the stars, not on the three stars. As the scientists explain in their article:

Observations show that the majority of stars form in relatively dense regions within star clusters, naturally leading to multi-star systems. It is estimated that over 40% to 50% of stars are in a binary pair, while about 20% are observed in triple or higher order systems. To date, planets have been found in 32 triple star systems. However, no planet in circumtriple orbit has yet been discovered.

The gap in GW Ori’s disk is 100 astronomical units (AU) from its central stars. Compare that to Pluto, which is about 40 AU from the sun. Or the intrepid travelers Voyager 1 and 2, which are 150 and 125 AU from the sun.

Of the three stars in the GW Ori system (called A, B, and C), stars A and B are approximately 1 AU apart, while star C is 8 AU further away. The protoplanetary disk around these stars is immense. The dusty part of the ring extends up to 400 AU, while the gaseous regions reach 1300 AU. The stars in this system are babies themselves, only about a million years old.

The next step will be to search for a planet or planets in the disk to confirm the scientists’ computer simulations.

Animations by GW Ori

The animations below show how GW Ori obtained his strange protoplanetary disc and the three stars that orbit within it.

Conclusion: Scientists believe that one or more planets orbit the GW Orionis triple star system. It would be the first circumtriple planet discovered.

Source: GW Ori: circumtripe circles and planets

Pre-publication: GW Ori: circumtripe rings and planets

Via the New York Times