Additional planet discovered in “Tatooine System”.

Illustration of the method that led to the discovery of the circumbinary planet: In the radial velocity method, tiny shifts in the light spectrum reveal the gravitational effect that planets have on their host stars. Amanda Smith / University of Birmingham

As with the fictitious Star Wars planet "Tatooine", they are illuminated by two suns: in addition to an already known exoplanet, astronomers have discovered another celestial body in the binary system TOI-1338. This is the second known double star system with more than one planet. It could now provide new clues about the processes involved in planet formation, say the scientists.

For a long time, planets with double sunsets only seemed to exist in the imagination: In a film scene from the science fiction saga "Star Wars", two suns can be seen in the sky of Luke Skywalker's home planet. Astronomers believed such a system to be almost impossible, because the conditions around a pair of stars were considered too chaotic for planet formation. But as it turned out, the so-called circumbinary systems actually exist: Among the numerous exoplanets that have now been discovered, there are also a few specimens that orbit a binary star system as the center. So far, however, more than just one has only been discovered in the case of the Kepler-47 system. Now follows another example of a more complex planetary system around a stellar duo.

A circumbinary system in sight

This is the TOI-1338 system, which is also alternatively referred to as BEBOP-1. It is about 1300 light-years from us in the constellation Painter. It is a binary system formed by two stars orbiting each other once every 15 days. One partner is about ten percent more massive than our sun, while the second star has only a third of the sun's mass and is comparatively cool and faint. Already in 2020, using data from NASA's TESS space telescope, astronomers discovered that the duo's shared center of gravity is orbited by a planet, dubbed TOI-1338b.

The proof was achieved using the so-called transit method. A brief eclipse provides clues to an exoplanet: as it passes in front of its star, it swallows some of the starlight, creating a characteristic fluctuation in starlight received through telescopes. "The transit method allowed us to determine the size of TOI-1338b, but the mass remained unclear, which is a very important parameter of a planet," says first author Matthew Standing of the University of Birmingham. Therefore, he and his colleagues also investigated the system using the second method for detecting exoplanets, which in turn can provide mass information: With the so-called radial velocity method, astronomers can use tiny shifts in the light spectrum to detect the gravitational effect that a planet exerts on its star. Using data from instruments installed on two telescopes in Chile's Atacama Desert, the team of astronomers attempted to determine the mass of the planet discovered by TESS.

A couple with two children

As they now report, this plan was unsuccessful, but number two in orbit around TOI-1338 got caught: They discovered a second planet using the radial velocity method, which they named BEBOP-1c. It orbits the two stars in a slightly wider orbit than TOI-1338b. "Twelve circumbinary systems are known so far, and this is only the second to host more than one planet," said co-author David Martin of Ohio State University, explaining the significance of the discovery. "BEBOP-1c has an orbital period of 215 days and a mass 65 times that of Earth, which is about five times less than the mass of Jupiter," says Standin.

For the new planet, the information is now available that is missing in the case of the already known planet TOI-1338b - and vice versa. Because the researchers have not yet been able to determine the size of BEBOP-1c, only its mass. However, as they announce, they will now try to use the transit method to determine the dimensions of BEBOP-1c. As for TOI-1338b, the missing results at least provided clues to its rather small mass: in combination with the information about its dimensions, it appears to have an unusually low density.

The TOI-1338 system should now continue to be in the sights of astronomy. Because although they are rare, circumbinary planets could contribute to the understanding of the formation processes of planetary systems, the researchers say. "Planets are born in a disc of matter surrounding a young star, in which the mass gradually condenses into planets," explains co-author Lalitha Sairam from the University of Birmingham. “In the case of circumbinary constellations, the disk surrounds both stars. As they orbit each other, they churn up the disk, disrupting the formation of planets,” says the astronomer. Under certain circumstances, however, they can obviously arise. Information from circumbinary systems could thus shed light on which conditions are important in planet formation.

Source: University of Birmingham

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