Global Courant
A new discovery from a distant planet, published today in the journal Nature Astronomybrings 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 orbit each other.
To some, it may recall a powerful scene from the original Star Wars movie – Luke Skywalker staring at the horizon, pining for a fate greater than on Tatooine’s dustball, while the aptly named John Williams-themed 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, a postdoctoral researcher at The Open University in Milton Keynes, England, and first author of the study.
“It is only the second multi-planetary, circumbinary system and the first-ever circumbinary planet ever discovered with radial velocity.”
Unpacking those terms is key to why experts think it could lay the groundwork for finding similar hidden exoplanets. But it requires understanding the chaos of such cosmic neighborhoods and the methods used to detect exoplanets.
Abundant binaries
In reality, binary star systems are far from unique.
“Half of the stars we observe in the night sky have a stellar companion,” said Jason Rowe, Canada Research chair in exoplanet astrophysics and assistant professor at Bishop’s University in Sherbrooke, Que.
Finding evidence of multiple planets in such systems is rarer.
“It’s not your typical, run-of-the-mill planetary system,” noted Rowe, who was not involved in the discovery. “It studies the eccentrics who sometimes create breakthroughs in our understanding of how planetary systems form.”
It is not known in great detail exactly how planets form in systems like this. What is known: the gravitational dance of two stars can make for some inhospitable zones.
“There’s an area around binary stars where a planet’s orbit would be unstable,” Standing said, “So that means planets must exist a little further away.”
Featured in today’s paper, BEBOP-1c is one of two planets discovered in that system so far. 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 both stars.
Blink and you won’t miss it
Not only does this neighborhood in space have different rules, it can also make detection tricky.
“You have to be very smart about the techniques you use,” said Chris Mann, an exoplanet researcher at the University of Montreal.
Mann, who was also not involved in the research, describes finding an exoplanet as parsing small signals from a lot of noise – and binary star systems are “noise times two”.
In this screenshot from a NASA visualization video, we can see TOI-1338 b passing in front of the main star, with the brightness fading intermittently. (NASA’s Goddard Space Flight Center/Chris Smith/USRA)
A commonly used technique to find exoplanets is the transit method. Space telescopes (such as NASA’s Transiting Exoplanet Survey Satellite, or TESS) look at a sea of stars and measure their brightness. When a planet passes in front of a certain star, that brightness drops slightly.
“Depending on the size of this dip, you know what the size of the planet is,” said Lalitha Sairam, a postdoctoral researcher at the University of Birmingham and co-author of the paper. “The bigger the planet, the bigger the dip too.”
Thus, the first exoplanet in that system, TOI-1338 b, was discovered.
LOOK | How to find a planet when two stars eclipse each other:
But what if in a binary star system, the gravitational pull of both stars affects a planet’s orbit so that it doesn’t pass for us to see — that is, it orbits the stars without affecting their brightness? That’s where another method, radial velocity, can help.
Like a siren in space
Even though stars are more massive, their planets still gravitate toward 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,” said Mann, referring to the Doppler effect and how we hear a siren differently when it passes us.
“So when the star – the star that wobbles a bit due to (the gravitational pull of) the planet – moves towards us, the light is shifted blue, and when it moves away from us, the light is shifted red. We can detect that with very sensitive instruments.”
Radial velocity data was collected by these ground observatories in Chile. (Rodrigo Garrido/Reuters)
Those tiny shifts, measured from observatories in Chile, allowed researchers to discover BEBOP-1c. They believe its orbit does not currently allow it to pass in front of the stars, making the transit method of detection impossible for now.
The newer hope
Usually radial velocity is used to track and complement the transit method, but in this case it was used for discovery. This raises the idea that there are other planets lurking in these binary star systems.
“It tells us we need to do more radial velocity surveys of other circumbinaries because there will be other non-transiting planets in those systems,” Rowe said.
Sairam says it’s also about stimulating our imaginations to think of planetary systems in this way.
“It’s not just one star and seven or eight planets revolving around it. It’s much more complex,” Sairam said in a joint video interview with Standing. “For us to imagine this system that is stable, that exists out there.”
Standing also sees it as a win for this method of detection.
“Not using multi-hundred-million dollar space telescopes means it will open the doors for a lot of people to propose the idea of … observing more of these targets,” he said.
“And hopefully someone might find a real Tatooine.”
