A new discovery of a faraway planet, published today in the journal Nature Astronomy, is bringing more science to what was once the realm of science fiction.
The new exoplanet, TOI-1338/BEBOP-1c, is a gas giant 65 times the size of Earth and more than 1,300 light years away in a binary star system –– where two suns revolve around each other.
For some, it might recall a powerful scene from the original Star Wars movie — Luke Skywalker staring at the horizon, pining for a greater destiny than on the dustball of Tatooine, as the aptly named John Williams theme Binary Sunset plays in the background.
While the fictional Tatooine was alone, BEBOP-1c is the second planet discovered in the real TOI-1338 system.
“It’s quite an exciting discovery,” said Matthew Standing, post-doctoral researcher at The Open University in Milton Keynes, England, and first author of the study.
“It’s only the second multi-planetary, circumbinary system and the first-ever circumbinary planet discovered with radial velocity.”
Unpacking those terms is key to why experts think this could lay the groundwork for finding similar hidden exoplanets. But it requires understanding the chaos of such cosmic neighbourhoods and the methods used to detect exoplanets.
Bountiful binaries
In reality, binary star systems are far from unique.
“Half of the stars that we look at in the night-time sky have a stellar companion,” said Jason Rowe, Canada Research chair in exoplanet astrophysics and an assistant professor at Bishop’s University in Sherbrooke, Que.
Finding evidence of multiple planets inside such systems is rarer.
“It’s not your typical, run-of-the-mill planetary system,” remarked Rowe, who was not involved in the discovery. “It’s studying the oddballs that sometimes create breakthroughs in our knowledge of how planetary systems form.”
It’s not known in great detail how exactly planets form in systems like this. What is known: the gravitational dance of two stars can make for some inhospitable zones.
“Around binary stars there’s a region where any planet’s orbit would be unstable,” said Standing, “So that means that planets have to exist slightly further out.”
BEBOP-1c, highlighted in today’s paper, is one of two planets discovered so far in that system. The other, TOI-1338 b, made headlines in 2020 after a summer intern at NASA helped discover it. Both are in stable orbits that are circumbinary, meaning they orbit around both stars.
Blink and you won’t miss it
Not only does this neighbourhood in space have different rules, it can make detection tricky.
“You have to be really clever in the techniques you use,” said Chris Mann, an exoplanet researcher at the University of Montreal.
Mann, who also wasn’t involved in the study, describes finding an exoplanet as parsing tiny signals from a lot of noise –– and binary star systems are “noise times two.”
A common technique used to find exoplanets is the transit method. Space telescopes (such as NASA’s Transiting Exoplanet Survey Satellite, or TESS) look on a sea of stars, measuring their brightness. When a planet passes in front of any given star, that brightness dips slightly.
“Depending on the size of this dip, then you know what is the size of the planet,” said Lalitha Sairam, post-doctoral researcher at the University of Birmingham and co-author of the paper. “The larger the planet, then the dip is larger as well.”
This is how the first exoplanet in that system, TOI-1338 b, was discovered.
WATCH | How to find a planet when two stars eclipse each other:
But in a binary star system, what if the pull of both stars affects a planet’s orbit so that it doesn’t pass in front for us to see — that is, it orbits the stars without affecting their brightness? That’s where another method, radial velocitycan help.
Like a siren in space
Though stars are more massive, their planets still pull on them. And in that tug of war, Mann explains, there’s an effect on the wavelength of light the star gives off.
“This is your classic ambulance whizzing by you,” said Mann, referring to the Doppler effect and how we hear a siren differently as it moves past us.
“So when the star –– the star that’s wiggling a little bit due to [the pull of] the planet –– moves towards us, the light is blue-shifted and when it moves away from us the light is red-shifted. We can detect that with very sensitive instruments.”
Those tiny shifts, measured from observatories in Chile, are what allowed researchers to discover BEBOP-1c. They believe its orbit does not currently let it transit in front of the stars, thus making the transit method of detection impossible for now.
The newer hope
Typically, radial velocity is used to follow up and complement the transit method, but in this case it was used for discovery. This raises the idea that other planets might be lurking in these binary star systems.
“It tells us we should do more radial velocity surveys of other circumbinaries, because there’s going to be other non-transiting planets in those systems,” said Rowe.
Sairam says it’s also about pushing our imagination to think of planetary systems this way.
“It’s not simply one star and seven or eight planets that orbits around it. It’s a lot more complex,” Sairam said in a joint video interview with Standing. “For us to even imagine this system which is stable, which exists out there.”
Standing also sees it as a win for this method of detection.
“Not using multi-hundreds-of-million-dollar space telescopes means that it will open the doors for lots of people to have the opportunity to propose the idea to … observe more of these targets,” he said.
“And hopefully, maybe, someone will find a real-life Tatooine.”
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