TESS Finds a Planet That Orbits Two Stars

Researchers working with data from NASA’s TESS (Transiting Exoplanet Survey Satellite) have a found a planet that orbits two stars. Initially, the system was identified by citizen scientists as a pair of eclipsing binary stars without a planet. But an intern taking a closer look at that data found that it was misidentified.

The intern’s name is Wolf Cukier and he was a summer intern at NASA’s Goddard Space Flight Center, the agency that manages TESS. Zooniverse citizen scientists in the Planet Hunters TESS program captured variations in star brightness in TESS data and uploaded them. Cukier was examining those uploads more closely when he came across the data from TOI 1338, a binary star system about 1300 light years away in the constellation Pictor.

“I was looking through the data for everything the volunteers had flagged as an eclipsing binary, a system where two stars circle around each other and from our view eclipse each other every orbit,” Cukier said. “About three days into my internship, I saw a signal from a system called TOI 1338. At first I thought it was a stellar eclipse, but the timing was wrong. It turned out to be a planet.”

A circumbinary planet is one that orbits two stars, and this is TESS’s first. There are over 20 other confirmed circumbinaries and a handful of unconfirmed or doubtful ones. This discovery was presented in a panel discussion at the American Astronomical Society’s annual meeting. Cukier and other researchers submitted their findings in a paper to a scientific journal.

The planet is called TOI 1338 b and it’s the only planet that we know of in the system . It’s about 6.9 times larger than Earth, which places it in between Neptune and Saturn for comparison. One of the stars it orbits is about 10% more massive than our Sun, while the other is cooler and dimmer, and only one third of the Sun’s mass. Since TOI 1338 b is on almost the same plane as the stars, it experiences solar eclipses every 15 days.

TESS works by detecting planets passing between us and their stars. Each time that happens it’s called a transit. Its sensitive cameras can detect these tiny drops in light from the star. TESS is on a two year mission to detect exoplanets by studying the same sector of sky for 27 days at a time. Its focus is to find Earth-like worlds around nearby stars, which are easier for other observatories and telescopes to follow up on.

But most of the exoplanets detected so far, by TESS and other planet hunters, are orbiting a single star. Circumbinaries like TOI 1338 b are trickier to detect. Both stars in the system are orbiting as well, making it challenging to untangle the various transits and dips in light. In this case, the smaller star passes in front of the larger one, creating a similar drop in starlight as the transit of the planet.

“These are the types of signals that algorithms really struggle with.”

Vaselin Kostov, Lead Author, SETI Institute and Goddard Space Flight Center

TOI 1338 b’s transits occur irregularly, between every 93 and 95 days, making it non-periodic. And the depth and the duration of the transits is variable as well, because of the motion of both stars. Compounding the difficulty is the fact that TESS can only see the planet’s transit in front of the larger star, not the smaller one.

“These are the types of signals that algorithms really struggle with,” said lead author Veselin Kostov, a research scientist at the SETI Institute and Goddard, who has worked on other studies of circumbinary planets. “The human eye is extremely good at finding patterns in data, especially non-periodic patterns like those we see in transits from these systems.”

Cukier looked at the transit data and was uncertain at first. Each transit had to be examined individually, because both the planet’s transit and the transit of the smaller star in front of the larger one produced similar dips in starlight. Initially it looked like the smaller star was causing the dip, but the timing of it didn’t match an eclipse. The team used a software package called “Eleanor,” named after the protagonist in Carl Sagan’s “Contact,” to help them understand what they were seeing.

“Throughout all of its images, TESS is monitoring millions of stars,” said co-author Adina Feinstein, a graduate student at the University of Chicago. “That’s why our team created eleanor. It’s an accessible way to download, analyze and visualize transit data. We designed it with planets in mind, but other members of the community use it to study stars, asteroids and even galaxies.”

TESS is not the first to study TOI 1338. It’s been studied from the ground in radial velocity surveys. That archival data helped the team identify TOI 1338 b’s orbit. According to them, the planet’s orbit will be stable for the next 10 million years, although from our perspective it will stop transiting in front of its star in November 2023. Then in about 2031 we’ll see the transits again, thanks to the angle of the orbit.

The angle of TOI 1338 b’s orbit around the stars changes over time, so after 2023, there will be an eight year gap in transits from our point of view. This gap leads astronomers to believe that there are many other circumbinary planets out there, but we have to be observing at the right time to find them. Image Credit: NASA Goddard Space Flight Center.

Though we’ve only identified a few, circumbinaries may be relatively common. They’re just hard to find, especially small ones, even though circumbinaries are more likely to transit than planets orbiting a single star.

Circumbinary planets can have some strange characteristics compared to planets orbiting a single star. The orbital characteristics of these systems can be complex and dynamic. Some astronomers call them “violent.”

One circumbinary discovered in 2014, Kepler 413-b, has an axial tilt that can vary by as much as 30% in 11 years. It’s near the inner edge of the habitable zone, and its seasons can fluctuate wildly. Astronomers studying the planet observed three transits occurring 66 days apart. Then there were no transits for 800 days, followed by five more transits also separated by 66 days each. Those gaps in transits lead scientists to believe that there are many more circumbinaries out there, we’re just not observing at the right time to see them transit.

Binary star systems are common, and astronomers expect TESS to find hundreds of thousands more eclipsing binary star systems during the two-year mission. Based on that, the spacecraft is likely to find many more circumbinary planets. For reference, Kepler found 12 circumbinary planets in 10 eclipsing binary star systems.


Evan Gough

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