Real-life Tatooine: Astronomers discover planet orbiting two stars at once

With the help of the old technique that was used to discover the very first exoplanet in 1995, astronomers have discovered a planet orbiting two stars at the same time.

This rarely observed phenomenon, called circumbinary planets, has never before been detected using this technique.

“Whereas people were previously able to find planets around single stars using radial velocities pretty easily, this technique was not being successfully used to search for binaries,” David Martin, co-author of the study and NASA Sagan Fellow in astronomy at The Ohio State University, said in a press release.

The newly discovered planet is a gas-giant that is 65 times more massive than Earth. It orbits two stars along with another, smaller exoplanet — a term which refers to planets orbiting stars outside of our own — which had previously been detected in 2020. Prior to this discovery of a second planet in this system, only one other multi-planet circumbinary system has ever been found: Keplar-47, which is believed to have three orbiting exoplanets.

The image of Luke Skywalker watching two suns setting on his home planet of Tatooine in “Star Wars: Episode IV – A New Hope” is one of the most iconic images of science fiction, but for a long time, the idea of a planet orbiting two stars was believed to be nothing more than that — science fiction.

However, NASA’s Keplar project, which was a spacecraft launched in 2009 with the goal of detecting planets, helped scientists understand that binary star systems and circumbinary planets were much more common than previously thought.

Half of the stars in the sky are single stars, meaning that if they have planets, those planets only orbit around them. But the other half of the stars are in systems with each other, with some consisting of two stars moving in an established pattern together, others consisting of three or four. Not all of these multi-star systems have a planet or planets orbiting them, but how common they are means that there are likely many more planets orbiting two or more stars than we are currently aware of.

“When a planet orbits two stars, it can be a bit more complicated to find because both of its stars are also moving through space,” Martin said. “So how we can detect these stars’ exoplanets, and the way in which they are formed, are all quite different.”

Currently, there are only 12 binary systems known to host circumbinary planets. Astronomers behind this new discovery, described Monday in the peer-reviewed journal Nature Astronomy, are hoping that this marks progress in how we search for a detect exoplanets in these unique systems.

The vast majority of the exoplanets that astronomers have discovered in the universe thus far have been discovered using the transit method.

This method involves tracking the light from stars over a large period of time to measure if the star’s brightness dims at any point. A dip in brightness that follows a specific pattern suggests that a planet is passing in front of the star on a regular basis as it orbits it.

When astronomers first started looking for exoplanets, however, they found the first exoplanet, called Dimidium, by using the radial velocity method.

This method relies on watching the tiny shifts in a star’s position in the sky as it is pulled by the gravity of an orbiting planet. This gravitational interaction makes a star wobble ever so slightly, a movement that can be detected by looking at the colour signature of the star’s light. A star that is red-shifted is farther away, while a blue shift indicates that it is closer, as longer light wavelengths appear redder.

If this wobble is happening on a regular basis, it almost certainly means there is a planet pulling on the star as it orbits it.

In the past, this method has been best at detecting large exoplanets that are quite close to their stars, as well as establishing the mass of already detected exoplanets.

What it hasn’t been good at is detecting planets in binary or multi-star systems, since in these systems, the stars are not only being affected by a planet’s gravity, but the gravity of the other star or stars in the system.

This new discovery of a binary system with two circumbinary planets is a step forward for the radial-velocity survey called BEBOP — which stands for “binaries escorted by orbiting planets.”

The system is officially called TOI-1338/BEBOP-1, showing that it is the first entry in the BEBOP catalogue of circumbinary planets that have been found using the radial velocity method.

Although the transit method is very reliable in detecting planets, it is also limited in its scope, as it requires a planet to pass between its star on a plane that lines up with Earth’s view of that star, meaning that we could be missing planets that are orbiting on a tilted orbit. The transit method also favours planets with a shorter orbital period, as it’s easier to establish a pattern of orbit if a planet doesn’t take years to complete one orbit.

BEBOP-1c, which refers specifically to the larger of the two planets orbiting in this system, hasn’t transited in front of the star yet, and it’s unknown when it will. The smaller planet hasn’t been picked up by radial velocity data, but researchers were able to infer its presence using existing data on it. 

The BEBOP program is believed to be particularly helpful in detecting circumbinary planets in binary systems where one star is much brighter than the other.

“If we are to unravel the mysteries of circumbinary Tatooine-like exo-atmospheres, the BEBOP-1 system may provide a new hope,” authors wrote in the study.

The inner planet is also sparking interest because it is eligible for atmospheric study by the James Webb Space Telescope, a development which could help look for whether life is possible on this planet. If NASA studies the inner planet further, it would be the first planet orbiting two stars to have its atmosphere studied.