The last few decades have shown that planets outside our Solar System are a lot more common than previously thought. Most orbit one star, like us. Some form in weirder conditions, orbiting two stars. Now astronomers have found evidence for what could be the first known planet orbiting three stars at once.

The system in question, GW Orionis, is located 1,300 light-years away in the constellation of Orion and is made up of two stars that orbit each other every 241 days, with a third one going around the other two in 11.5 years.

The system was already well known to astronomers for having a large protoplanetary disk – the donut of material from which planets form – that is made of three misaligned rings. This is quite unusual, so astronomers wanted to understand better why the rings were misaligned. In a new paper in the Monthly Notices of the Royal Astronomical Society, astronomers used simulations to calculate how the disk became so misaligned and found a surprising result: It can't be the torque of the three stars alone causing it. They put forward the presence of one or more massive undetected planets instead.

There is a huge gap between the innermost ring and the other two, around 15 billion kilometers across. Some have tried to explain this massive gap by suggesting it was caused by the torque from the three stars ripping apart their disk, but the team thinks their suggestion of an unknown planet fits better.

“We show that the torque is not actually strong enough to break the disk,” Dr Jeremy Smallwood, from the University of Nevada, told IFLScience. “Next we explored the scenario where a planet can carve the gap in the disk. We compared our simulation to observations and they match pretty well.”

The team looked at different scenarios for planets forming and/or evolving with a tilt with respect to the stars in the system. A massive planet going through the disk will quickly gain mass, getting bigger in the process – at least if the disk is quite thin. But if the disk is thicker, the gap spotted by astronomers should close over time, unless the planet is actually quite small. So a planet, or several, could work in different scenarios. 

If a planet or multiple planets are there, it won’t be easy to detect them, unfortunately. Misaligned, or titled, planets are quite tricky to spot using the transit method we use to find most exoplanets, where a dip in the light of a star suggests a body passing in front of it. It wouldn’t be impossible, though. The best evidence of the planet or planets would come from high-resolution imaging of the system, although that precision might currently be beyond observatories' capabilities too as the system is still over 1,000 light-years away.