Planets orbiting binary stars are in a tough spot. They have to contend with the gravitational pull of two separate stars. Planetary formation around a single star like our Sun is relatively straightforward compared to what circumbinary planets go through. Until recently, astronomers were unsure of their existence.
Astronomers rarely find binary stars with planets orbiting them. Maybe it’s because they’re rare, or because they’re hard to detect, probably both. Now a team of researchers has found a binary star with more than one planet. It is only the second instance of a multiplanetary binary star system. What does it tell us about these types of solar systems?
The system is called TOI-1338 and is a binary star about 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 is about 4.4 billion years old.
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A summer intern at NASA’s Goddard Space Flight Center found the first planet around the binary in 2017. TOI 1338 b is a circumbinary planet with about 33 Earth masses and lies between Saturn and Neptune. It’s on a 95-day orbit around binary stars.
Circumbinary planets are difficult to find in the data because stars can eclipse, making planetary transits difficult to discern. Their transits can also be irregular, and they can transit in front of only one of the binary stars. TOI 1338 b transits occur irregularly, between 93 and 95 days, making it non-periodic. And since the two stars move, the depth of the transit varies.
Due to the inclination of TOI 1338 b, from our perspective, it will stop transiting 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 they found it using the radial velocity method rather than the transit method. The name BEBOP comes from an observation project. “To increase the number of known circumbinary planets and provide accurate masses for systems discovered with the transit method, we initiated a radial velocity observational study dedicated to the detection of circumbinary planets called Binaries escorted by orbiting planets (BEBOP)”, explain the authors in their article.
The researchers reported their findings in an article titled “The first discovered circumbinary planet with radial velocities.“It has been accepted for publication in Nature Astronomy and is available at arxiv.org. The lead author is Matthew R. Standing, Ph.D. student at the School of Physics and Astronomy at the 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 approximately 215 days. The astronomers discovered it using radial velocity data collected with the HARPS and ESPRESSO spectrographs. The 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 discovered.
Astronomers are very interested in circumbinary planets. They are common in science fiction but were only confirmed when the Kepler mission found the first one. His name is Kepler-16b, and he’s an oddball in his own way. This is within the radius that astronomers thought was the inner boundary of planets in binary star systems. Kepler-16b has no sister planets.
Now we know of 12 circumbinary planets, and two of them are in multiplanetary systems. The first multiplanetary circumbinary system discovered by astronomers is called Kepler-47 and hosts three known explanets. The BEBOP observing program is designed to discover more circumbinary planets and learn more about them. Its main goal is to find more, and it will do so by overcoming some of Kepler’s observational biases.
Binary star systems are much more complicated than single star systems like ours. Binary stars disrupt planet formation in ways that more predictable single-star systems do not. Double stars create harsh conditions in the protoplanetary environment. Astronomers believed that planets in these systems would be subject to catastrophic collisions or be thrown out of their systems by gravitational disturbances. But all these recent discoveries show that this is not necessarily true. By finding more circumbinary planets and characterizing their similarities and differences to single-star planets, astronomers will learn a lot 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 measure their masses more accurately. This is essential because knowing their masses helps determine which ones are puffy, with extended atmospheres suitable for atmospheric spectroscopy. BEBOP not only found the second planet, but more accurately measured the mass of TOI-1338’s inner planet.
Finding another multiplanetary circumbinary system and determining their masses is an important discovery. Although these systems change some parts of the planet formation models, they will eventually make our models more accurate.
The researchers say that at some point TOI-1338/BEBOP-1c is guaranteed to transit the main star, but they can’t say when. This is despite the misalignment between the planet and the star. “It may seem counterintuitive at first that a planet-binary misalignment makes transitability more likely,” they write. This is because the planet’s sky inclination oscillates around the binary’s sky inclination, according to the authors, and eventually the planet’s inclination will approach 90oh. This means that “…the vast majority of circumbinary planets orbiting eclipsing binaries will eventually transit.”
The team also considered the question of other planets around the binary star. None have been detected yet, but they may still be. Although they cannot say for sure if there are other planets, they have calculated and graphed the limits of any potential detection.
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 possibility of using spectroscopy to probe their atmospheres. But its previously discovered sibling, TOI-1338b, might shed enough light. “Therefore,” the researchers write, “despite the challenges it may present, TOI-1338/BEBOP-1b is our only opportunity to shed light on the atmospheric composition of circumbinary planets.”
“Of the 15 circumbinary exoplanets now known, TOI-1338/BEBOP-1b is the only one for which James Webb Space Telescope transmission spectroscopy can currently be tracked. If we want to unravel the mysteries of Tatooine-like circumbinary exo-atmospheres, the TOI-1338/BEBOP-1 system provides a new hope“, write the authors in their article.