A new technique developed in part by Planetary Science Institute Senior Scientist Nader Haghighipour has allowed astronomers to quickly detect a transiting circumbinary planet orbiting around two suns, according to a new Astronomical Journal paper on which Haghighipour is an author.
Circumbinary planets are planetary bodies that rotate around two stars. Although for years, they were merely a matter of science fiction, thanks to the successful operation of NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) telescopes, an all-sky survey mission designed to discover thousands of exoplanets around nearby bright stars, a team of astronomers, including Haghighipour, have detected 14 such bodies.

“Detecting circumbinary planets is much more complicated than detecting planets orbiting single stars. The most promising technique for detecting circumbinary planets is transit photometry, which measures drops in starlight caused by those planets whose orbits are oriented in space such that they periodically pass between their stars and the telescope. In this technique, the measurements of the decrease in the intensity of the light of a star is used to infer the existence of a planet,” Haghighipour said. “To determine the orbit of the planet, precisely, at least three transit events are required. This becomes complicated when a planet orbits a double-star system because transits will not happen with the same interval over the same star. The planet may transit one star and then transit the other before transiting the first star again, and so on.”

Another complication is that the orbital period of circumbinary planets is always much longer than the orbital period of the binary star. That means, in order to observe three transits, scientists need to observe the binary for a long time. While that was not a problem with the Kepler space telescope (this telescope observed only one region of the sky for 3.5 years), it makes it almost impossible to use the TESS telescope to detect circumbinary planets. The reason lies in the fact TESS observes one portion of the sky for only 27 days (and will most likely not observe that area again). This means it is actually impossible to observe three transits of a planet during a TESS observing period.

In an article published in the Astronomical Journal in 2020, the same team of astronomers that have detected circumbinary planets using the Kepler telescope, including Haghighipour, announced that they have invented a new technique that would enable them to detect circumbinary planets using the TESS telescope. These authors showed that if during the 27-day observation, the planet shows only two transits (transiting each star once), they will be able to detect the planet.

Today, the same team of astronomers announced the discovery of the first TESS circumbinary planet using their new technique in the paper “TIC 172900988: A Transiting Circumbinary Planet Detected in One Sector of TESS Data” on which Haghighipour is a co-author. The paper appears in Astronomical Journal. The lead author is Veselin B. Kostov, of NASA Goddard Space Flight Center, SETI Institute, and GSFC Sellers Exoplanet Environments Collaboration. The target binary known by its catalog designation, TIC 172900988, was observed in a single sector by TESS where its lightcurve showed signs of two transits, one across each star – during the same conjunction.

“Our group was able to show that despite its short window of observation, it is still possible to use TESS to detect circumbinary planets. The new planet is the proof of the validity, applicability and success of our invented technique,” said Haghighipour, founder of the TESS Circumbinary Planet Working Group. “This discovery demonstrates that our new technique works and will be able to find many more planets.”

A preprint of the journal article can be found at https://arxiv.org/abs/2105.08614.

Haghighpour’s work on the project was funded by grants from NASA’s Exoplanets Research Program and NASA’s TESS Guest Observer program. He worked out the planet’s orbital stability and determined the habitable zone of the binary. He also contributed to the part of the paper that explained the origin of such planets, that they form farther out and migrate to their current orbit, that they get captured between resonances with their host binaries, etc.