Planets in binary star systems face a difficult situation. The gravitational pulls of two separate stars have been overcome by them. Planet formation around a single star, such as our Sun, is simpler than circumbinary planet formation. Until recently, astronomers were unsure they existed.
Astronomers rarely come across binary stars that have planets orbiting them. It could be because they are rare. After all, they are difficult to detect, or both. A group of scientists has discovered a binary star with multiple planets. This is only the second instance of a binary star system with multiple planets. What does it reveal about these solar systems?
TOI-1338 is a binary star system located approximately 1300 light-years away in the constellation Pictoris. TOI 1338 A is a 1.12 solar mass main sequence star, and TOI 1338 B is a 0.3 solar mass M-dwarf (red dwarf). The star system has been around for about 4.4 billion years.
In 2017, a summer intern at NASA’s Goddard Space Flight Center discovered the first planet orbiting the binary. TOI 1338 b is a circumbinary planet with a mass of about 33 Earth masses that is roughly the size of Saturn or Neptune. It orbits the binary stars every 95 days.
Circumbinary planets are difficult to spot in data because stars can collide, making planetary transits harder to identify. They can also have irregular transits, with only one of the binary stars passing in front of them. Every 93 to 95 days TOI 1338 b rarely transits indicating that it is non-periodic. Because both stars are moving, the depth of the transit varies.
TOI 1338 b will no longer transit around its star in November 2023 due to its inclination. After that, the transits will return around 2031.
Astronomers have now discovered a second planet orbiting TOI 1338. The planet is named TOI-1338/BEBOP-1c, and it was discovered using the radial velocity method rather than the transit method. BEBOP is derived from an observation project. “To increase the number of known circumbinary planets and to provide accurate masses for systems discovered with the transit method, we initiated a radial-velocity observing survey dedicated to circumbinary planet detection called Binaries Escorted By Orbiting Planets (BEBOP),” the authors explain in their paper.
The results were published in the paper “The First Circumbinary Planet Discovered with Radial Velocities.” It is available on arxiv.org and has been accepted for publication in Nature Astronomy. The lead author is Matthew R. Standing, a Ph.D. student at the University of Birmingham’s School of Physics and Astronomy.
The newly discovered planet is a gas giant approximately the size of Earth. It has a slightly longer orbit than TOI 1338 b and an orbital period of about 215 days. Moreover, it was discovered by astronomers who examined radial velocity data from the HARPS and ESPRESSO spectrographs. Furthermore, it is only the second multiple-planet circumbinary system discovered, and it is the first time astronomers have discovered a circumbinary planet using radial velocity.
Astronomers are fascinated by circumbinary planets. They’d been mentioned in science fiction before, but the first one was discovered by the Kepler mission. It’s known as Kepler-16b, and it’s an oddity in and of itself. Astronomers believe it is within the radius of the inner limit for planets in binary star systems. Kepler-16b does not have any known siblings.
There are now 12 circumbinary planets discovered by science, two of which are part of multiplanet systems. Kepler-47 was the first multiplanet circumbinary system discovered by astronomers, with three known exoplanets. The BEBOP observing program seeks to learn more about circumbinary planets. Its main goal is to find more of them, which it will do by overcoming some of Kepler’s observational biases.
Binary star systems are far more complex than single-star systems such as ours. Binary star systems interfere with planet formation in ways that single-star systems do not. The dual stars make the protoplanetary environment harsh. Astronomers previously assumed that planets in these systems would be destroyed by catastrophic collisions or ejected by gravitational perturbations. Recent discoveries, however, show that this is not always the case. Discovering more circumbinary planets and characterizing their similarities and differences with single-star planets will teach astronomers a lot about planet formation and migration.
One of the difficulties in studying circumbinary planets is determining their masses. BEBOP was designed not only to find planets but also to precisely measure their masses. This is significant because knowing their masses allows researchers to determine which are puffy and which have extended atmospheres suitable for atmospheric spectroscopy. BEBOP not only discovered the second planet, but it also determined the mass of TOI-1338’s inner planet with greater precision.
Another multi-planet circumbinary system must be discovered and its masses determined. While these systems will challenge some models of planet formation, they will ultimately improve the accuracy of our models.
TOI-1338/BEBOP-1c will transit the primary star at some point, according to the researchers, but they do not know when. Regardless of how the planet and star are misaligned. “It may seem counterintuitive at first that a planet-binary misalignment makes transability more likely,” they write. According to the authors, this is because the planet’s sky inclination oscillates around the binary’s sky inclination, eventually approaching 90o. That means “… the vast majority of circumbinary planets orbiting eclipsing binaries will eventually transit.”
The researchers also investigated the possibility of other planets orbiting the binary star. None have yet been discovered, but they may be. While they can’t say for certain whether or not there are more planets, they calculated and graphed the limits of any potential detections.
Studying circumbinary planets around binary stars is challenging. Most known circumbinary planets are too faint to be identified. As a result, spectra cannot be used to study the atmospheres of the majority of these planets. This includes the newly discovered planet TOI-1338/BEBOP-1c. TOI-1338b, a previously discovered sibling, may be sufficiently illuminated. “Thus, despite the challenges it may present,” the researchers write, “despite the challenges it may present, TOI-1338/BEBOP-1b is our only possibility to shed light on the atmospheric make-up of circumbinary planets.”
The authors have written in their paper: “Of the now 15 known circumbinary exoplanets, TOI-1338/BEBOP-1b is the only one for which James Webb Space Telescope transmission spectroscopy can currently be pursued. If we are to unveil the mysteries of circumbinary Tatooine-like exo-atmospheres, the TOI-1338/BEBOP-1 system provides a new hope,”