An international team of astronomers have identified the second multi-planet circumbinary system.
Unlike planets in our Solar System, those in circumbinary systems revolve around two stars instead of one. The discovery, led by University of Birmingham researchers, is detailed in a study published today in Nature Astronomy.
Dr Rosemary Mardling, from the Monash University School of Physics and Astronomy, is a co-author of the study.
"We are aware of 12 circumbinary systems so far, and this is only the second one that has more than one planet," said Dr David Martin, a co-author of the paper who is a former student of Dr Mardling’s and currently a Sagan Fellow at the Ohio State University.
The new planet is called BEBOP-1c, which comes from the name of the project that discovered it. BEBOP is an acronym for Binaries Escorted By Orbiting Planets. The system BEBOP-1 is also known as TOI-1338.
Using data from NASA’s TESS space telescope which searches for planets using the transit method, a circumbinary planet named TOI-1338b was found in the same system in 2020 as it passed in front of the larger of the two stars several times.
“The transit method permitted us to measure the size of TOI-1338b, but not its mass which
is the planet’s most fundamental parameter,” said lead author Dr Matthew Standing, formerly from the University of Birmingham but now a researcher at The Open
The BEBOP team was already keeping an eye on this system with something called the Doppler method. Also called the wobble or the radial-velocity method, this technique depends on measuring how fast the stars are moving.
It is the same method that led to the discovery of the first exoplanet, for which Mayor and Queloz won the Nobel Prize in 2019.
The team tried to measure the mass of the planet found by TESS by installing cutting-edge equipment on two telescopes in the Atacama Desert in Chile. Even though the team worked hard for years, they were not successful. Instead, they found a second planet, BEBOP-1c, and were able to measure its mass.
The orbital period of BEBOP-1c is 215 days, and its mass is 65 times that of Earth, which is about five times less than Jupiter's mass.
This was a challenging system to confirm because telescopes in Chile were closed for six months because of the COVID pandemic during a crucial part of the planet’s journey around its two parent stars. This particular section of the orbit could only be observed again last year, enabling the research team to confirm their discovery.
While only two planets are currently known to orbit the TOI-1338/BEBOP-1 binary system, future observations by the team may reveal more.
Even though they are rare, circumbinary planets are important for learning about how planets form.
“With an orbital period around 6.5 times that of the binary, BEBOP-1c is almost as close as it can be to the two stars – any closer and it would be kicked out of the system by the binary’s strong gravitational field,” said Dr Mardling.
“And with a period only twice that of BEBOP-1c, TOI-1338b is also perilously close to being thrown out of the nest. This tells us much about the conditions in which such planets form.”
Planets are born in a disc of matter that circles a young star or binary pair, with gas, dust and pebbles gradually gathering together to form planets.
Two stars act like a giant paddle, stirring up and stopping planets from forming anywhere but in quiet places far from the binary.
But the TOI-1338/BEBOP-1 planets are close to their binary, as are all known circumbinary planets. In fact the disc pushes them inwards as they form, stopping them just before they are evicted from the system.
While the radial velocity method has allowed the research team to measure the mass of BEBOP-1c, it will now try to measure its size using the transit method.
The international team of researchers consists of astronomers from the University of Birmingham, the Open University, Ohio State University, the University of Coimbra, the Observatory of Paris, Queen Mary University London, the Lowell Observatory, Northern Arizona University, the Aix-Marseille University, the University of St. Andrews, the University of Porto, the University of Liege, the Observatory of Geneva, Keele University, Monash University, and San Diego State University.
Funding for the research came from the European Research Council, the Science and Technology Facilities Council, the Leverhulme Trust, the National Air and Space Administration, the US National Science Foundation, the French National Research Agency, the French National Centre for Scientific Research, the Portuguese foundation for Science and Technology, the Belgian National Fund for Scientific Research, and access to telescopes run by the European Research Council. Observations were made at the European Southern Observatory, Chile.