Astronomers discover a second planet orbiting two stars

Artist’s rendering of Kepler-16b, the first known planet definitely orbiting two stars – a so-called circumbinary planet. The planet seen in the foreground was discovered by NASA’s Kepler mission. Credit: NASA/JPL-Caltech/T. pyle

Planets orbiting binary stars are in a difficult situation: they have to deal with the gravitational pull of two separate stars. Planet formation around a single star like our sun is relatively straightforward compared to what circumbinary planets go through. Until recently, astronomers were unsure if they existed.

Astronomers rarely find binary stars with planets orbiting them. It may be because they are rare or because they are difficult to spot; probably both. Now a team of researchers has found a double star with more than one planet. This is only the second instance of a multi-planet binary system. What does this tell us about these types of solar systems?

The system is called TOI-1338 and is a binary star about 1,300 light-years distant in the constellation Pictoris. TOI-1338 a is a main sequence star with 1.12 solar masses and TOI-1338 b is an M dwarf (red dwarf) with 0.3 solar masses. The star system is about 4.4 billion years old.

A summer intern at NASA’s Goddard Space Flight Center discovered the first planet around the binary star in 2017. TOI-1338 b is a circumbinary planet about 33 Earth masses and intermediate in size between Saturn and Neptune. It is in a 95-day orbit around the binary stars.

Circumbinary planets are difficult to find in the data because the stars can eclipse each other, making planetary transits difficult to see. Their transits can also be irregular and they can only pass in front of one of the double stars. TOI-1338 b’s transits occur irregularly every 93 to 95 days, making them non-periodic. And since both stars are moving, the depth of the transit varies.

Astronomers discover a second planet orbiting two stars

The angle of TOI 1338 b’s orbit around the stars changes over time, so we believe there will be an eight-year transit gap after 2023. This gap leads astronomers to believe there are many other circumbinary planets out there, but we need to observe at the right time to find them. Photo credit: NASA Goddard Space Flight Center

Due to TOI-1338 b’s tilt, we expect it to stop passing in front of its star in November 2023. Then, around 2031, we will see the transits again.

Now astronomers have found a second planet orbiting TOI-1338. It’s called TOI-1338/BEBOP-1c and was found using the radial velocity method, not the transit method. The name BEBOP comes from an observation project. “In order to increase the number of known circumbinary planets and to provide accurate masses for systems discovered using the transit method, we initiated a radial-velocity observational survey dedicated to the detection of circumbinary planets, named Binaries Escorted By Orbiting Planets (BEBOP)” , the authors explain in their paper.

The researchers reported their findings in an article titled “The First Circumbinary Planet Discovered with Radial Velocities.” It has been accepted for publication in natural astronomy and is available on the arXiv preprint server. The lead author is Matthew R. Standing, a Ph.D. Student at the School of Physics and Astronomy, University of Birmingham, UK.

The new planet is a gas giant about 65 Earth masses. It is in a wider orbit than TOI-1338 b and has an orbital period of about 215 days. Astronomers discovered it using radial velocity data collected with the HARPS and ESPRESSO spectrographs. This discovery marks the first time astronomers have found a circumbinary planet using radial velocity, and the system is only the second multi-planet circumbinary system found.

Astronomers discover a second planet orbiting two stars

This research graphic shows the TOI-1338 system in detail. Planet c has a much wider orbit than planet b, and neither is in the habitable zone of the system. Photo credit: Standing et al. 2023

Astronomers are very interested in circumbinary planets. They were common in science fiction but weren’t confirmed until the Kepler mission found the first one. It’s called Kepler-16b, and it’s an oddball in its own way. It is within the radius that astronomers thought was the inner limit for planets in binary star systems. Kepler-16b has no sibling planets.

Now we know 12 circumbinary planets, and two of them are in multi-planet systems. The first circumbinary multiplanet system found by astronomers is called Kepler-47 and is home to three known explanets. The BEBOP observing program aims to discover more circumbinary planets and learn more about them. Its main goal is to find more of them, and it will accomplish this by overcoming some of Kepler’s observational biases.

Binary star systems are much more complicated than single star systems like ours. Binaries disrupt planet formation in ways that more predictable single star systems do not. The binaries create harsh conditions in the protoplanetary environment. Astronomers used to think that planets in these systems suffered catastrophic collisions or were knocked out of their systems by gravitational disturbances. But all of these recent discoveries show that’s not necessarily true. By finding more circumbinary planets and characterizing their similarities and differences to single-star planets, astronomers will learn much about how planets form and migrate.

One of the difficulties in studying circumbinary planets is determining their masses. BEBOP was designed not only to find planets, but also to more accurately measure their masses. This is crucial as knowing their masses will help determine which are bloated, with extended atmospheres suitable for atmospheric spectroscopy. BEBOP not only found the second planet, but also more accurately measured the mass of TOI-1338’s inner planet.

Finding another circumbinary multiplanet system and determining its masses is an important discovery. While these systems upend some parts of the models for planet formation, they will ultimately make our models more accurate.

Astronomers discover a second planet orbiting two stars

This figure from the study illustrates what the astronomers found in TOI-1338. The red inset shows the magnified 14.6-day binary period associated with the stars and the blue inset shows the magnified 215.5-day period of TOI1338/BEBOP-1c. Photo credit: Standing, et al. 2023

The researchers say TOI-1338/BEBOP-1c is guaranteed to eventually pass through the primary star, but they can’t say when. This is despite the misalignment between the planet and the star. “It may seem counterintuitive at first glance that planetary-binary misalignment makes transitability more likely,” they write. This is because, according to the authors, the planet’s sky tilt oscillates around the binary’s sky tilt, and eventually the planet’s tilt approaches 90 degrees. This means “…the vast majority of circumbinary planets orbiting eclipsing binaries will eventually pass.”

The team also studied the problem of other planets around the binary star. So far, none have been discovered, but they could still be. While they can’t say for sure if there are more planets, they calculated and graphed the limitations on possible discoveries.

Astronomers discover a second planet orbiting two stars

This research density plot shows the detection limits of the researchers’ method. It’s quite complicated, but it basically shows that their method “…is sensitive to additional sub-Saturn-mass planets for periods of up to 2000 days, while we are sensitive to Neptune-mass planets near the instability limit.” Photo credit: Standing, et al. 2023

One of the problems with studying circumbinary planets around binary stars is that most of the ones we know are too faint. This means that for most of them, including the new planet TOI-1338/BEBOP-1c, there is no way to study their atmospheres spectroscopically. But its previously discovered brother TOI-1338b might be sufficiently enlightened. “Therefore,” the researchers write, “despite the challenges, TOI-1338/BEBOP-1b is our only opportunity to shed light on the atmospheric composition of circumbinary planets.”

“Of the now 15 known circumbinary exoplanets, TOI-1338/BEBOP-1b is the only one for which transmission spectroscopy by the James Webb Space Telescope can currently be performed. The TOI-1338/BEBOP-1 system offers new hope,” the authors write in her article.

More information:
Matthew R Standing et al, The First Circumbinary Planet Discovered with Radial Velocities, arXiv (2023). DOI: 10.48550/arxiv.2301.10794

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