Imagine an amazing star system in which a large hot Jupiter orbits a pair of stars. This terribly named system, TIC 172900988, is the first system in which a planet has been observed to pass in front of two different stars during a TESS observing run.

The Transiting Exoplanet Survey Satellite (TESS) mission observes different regions of the sky for 27 days at a time to look for planets transiting in front of their host star. During these transits, we receive slightly less light, in a distinctive way that is consistent with a round object slowly moving into and out of the light of a star. To confirm that we’re seeing a planet and not a deceptive star spot or other phenomena, astronomers want to see three transits spaced out in a way consistent with a world circling its star. With just a 27-day window per observing set, TESS can only do this for worlds on exceedingly tiny orbits or in very special cases.

And it turns out, one planet passing in front of two stars is one of the special cases.

The two stars in this system are both about 30% larger than the Sun, while the planet appears to be about the size of Jupiter. While the entire orbit of the planets takes less than a year, it only took the planet five days to go from blocking the slightly larger star to blocking the slightly smaller star. But in itself, these transits are only enough to say “there is probably something here”, but it isn’t enough to say “there is definitely a planet here.”

And this is where data archives are so important. This paper is published in The Astronomical Journal with lead author Veselin Kostov, and the paper’s abstract notes: Archival data from ASAS-SN, Evryscope, KELT, and SuperWASP reveal a prominent [motion of the long axis] of the binary orbit, caused by the dynamical interactions between the binary and the planet. 

The stars orbit in an ellipse, and over time, the planet changes the alignment of the ellipse and reveals itself even when we can’t see it. Luckily, thanks to some artwork [Ed. note: Courtesy of Dr. Pamela Gay], we can at least imagine this system with one Jupiter-sized world orbiting two hot bright stars.

More Information

PSI press release

“TIC 172900988: A Transiting Circumbinary Planet Detected in One Sector of TESS Data,” Veselin B. Kostov et al., 2021 November 10, The Astronomical Journal