Discovery of TESS Mission’s First Circumbinary Planet

TOI 1338b is a great catch, a circumbinary world that turned up in TESS data and was announced at the ongoing meeting of the American Astronomical Society in Hawai’i. Ravi Kopparapu (NASA GSFC) describes the discovery process in the essay below. The system lies 1,300 light years out in the constellation Pictor, with the planet transiting the larger star. Dr. Kopparapu’s work on exoplanet habitability is well known to Centauri Dreams readers. See, for example, his How Common Are Potential Habitable Worlds in Our Galaxy?, which ran in 2014. He followed this up with a look at an unusual multi-planet system (Ravi Kopparapu: Looking at K2-72). Analyzing habitable zone possibilities around different kinds of stars, as well as modeling and characterizing exoplanet atmospheres, plays a major role in his research interests. Here Dr. Kopparapu tells us about the new world and the significant role of an intern in its discovery, reminding us that the opportunities for young scientists to make a difference are abundant in this burgeoning field.

by Ravi Kumar Kopparapu

Back in December 2015 a couple of my colleagues, including Dr. Veselin Kostov from SETI Institute (who is now based at NASA Goddard) and Prof. Bill Welsh from San Diego State University (SDSU), met at a conference in Hawai’i. Veselin and Bill are astronomers well-known for discoveries of planets around binary star systems in the Kepler mission data. The topic of the discussion was submitting a NASA proposal to study the habitability of planets around binary stars. Planets in multiple star systems experience different kinds of illumination from their Suns (plural), varying periodically as the stars orbit each other, so the seasons or climate may be completely different than ours. It was decided that my theory colleague, Dr. Jacob Haqq-Misra from Blue Marble Space Institute of Science (BMSIS), will lead the proposal. A year or so after we submitted the proposal, we received a notification from NASA that our proposal was accepted. We were thrilled about this outcome and looked forward to start working soon.

About 2 years later, in January 2018, I received an email from a high school student named Wolf Cukier, asking if I could be his summer (2018) mentor for a high school project. His resume looked great, and he did his homework related to our group’s research. All was set and Wolf arrived in the summer of 2018.

Image: TOI 1338 b is silhouetted by its host stars. TESS only detects transits from the larger star. Credit: NASA’s Goddard Space Flight Center/Chris Smith.

Wolf worked with me on the project that we got funded through NASA. The project was to identify the habitable zones (HZs) of an Earth-like planet orbiting around two stars, which themselves are orbiting each other. These are called circumbinary planets (CBPs). We used a climate model to estimate the HZs around a variety of CBPs. I found that Wolf was a quick study and is capable of handling far more complex assignments. However, it was the end of his summer internship, so he had to leave. To tie up Wolf’s project with me, he and I started drafting a paper for publication, while Wolf was attending his school.

Image: NASA intern Wolf Cukier. Credit: Ravi Kopparapu.

Meanwhile at NASA, I mentioned to Veselin that Wolf would be very well suited to look for planets in the TESS mission data, particularly to search for circumbinary planets. At that time, only the Kepler mission had discovered transiting CBPs (about 10 of them). As exciting as these are, the small number leaves a vast gap in our understanding of this new class of worlds, not unlike the state of exoplanet science 20 years ago, when only a handful of hot-Jupiter exoplanets were known. Among the unknowns are the formation and migration efficiency of CBPs, their orbital architectures and occurrence rates. Therefore, discovering more CBPs in TESS mission data will open opportunities to answer these questions.

Veselin agreed that it would be a good project, particularly considering that no CBP was discovered by the TESS mission yet. Consequently, I offered Wolf another summer internship opportunity for 2019, with paid work through the NASA intern program. However, by summer, my calendar was booked with conference travel. It became so busy that I was not even going to be in town when Wolf would arrive to sign him in. I requested Veselin to take Wolf under his wing, which was going to happen anyway because of the work, but much earlier than Veselin expected. Being a great gentleman, and one of the nicest people I know, Veselin agreed.

I came back to my office from one of my conference travels, two days after Wolf joined, and was getting ready for my next conference travel. The next day, I got a cryptic message from Veselin asking if we all three could meet. I was concerned. What could it be? It couldn’t possibly be a discovery because Wolf just started 2 days ago. It would take weeks, sometimes months to even find a candidate planet, and that too for a high school student who is just learning data analysis. Was it not working out between Wolf and Veselin?

They came to my office the next day. I could see both were trying to hide something because both of them were containing, or trying to contain, their smiles. A small suspicion deep inside my mind started taking root. Veselin started, “We got something,” and quickly added “but we have to make sure”. Apparently within three days since he arrived at NASA, Wolf noticed that in one of the light curves there was both a prominent primary eclipse and an additional unknown feature.

This was initially flagged as an eclipsing binary on the Planet Hunters TESS platform. Planet Hunters is a citizen science project where in addition to primarily tagging transit-like features, volunteers may tag targets as various phenomena, including eclipsing binaries, variable stars, etc., thus effectively creating informal catalogs. These catalogs will be later followed-up by professional astronomers, like Veselin, to see if they are indeed candidate planets. Planet Hunters has already successfully contributed to the field of circumbinary planets through the independent discovery of Kepler-64 (also known as Planet Hunters-1).

Wolf immediately notified Veselin about the unknown feature flagged in the Planet Hunters catalog, and Veselin dutifully followed up to verify the authenticity with the help of his fellow astronomers Jeremy Orosz, Adina Feinstein and Bill Welsh. Veselin has a reputation of being extremely thorough and incredibly careful in analyzing candidate planets, so his standards for confirmation of a planet are pretty high, and if he says there is a planet in the data, you can take it to the bank. When Veselin came to my office to tell me that they may have discovered TESS’s first circumbinary planet, with the help of a high school student, I paid full attention.

They found a Saturn-sized planet in a 95-day orbit around both the stars, which means the planet is the longest period circumbinary planet found by TESS. The stars themselves orbit each other in 15 days, with one star being Sun-like and the other one a smaller, cooler star (effective temperatures of 5976 K and 3657 K, respectively. For comparison, the Sun is 5780 K). The planet is not in the habitable zone of its host stars (which Wolf verified based on his work from earlier summer). The system itself is estimated to be 4.4 Gyr old.

Video: Researchers working with data from NASA’s Transiting Exoplanet Survey Satellite (TESS) have discovered the mission’s first circumbinary planet, a world orbiting two stars. The planet, called TOI 1338 b, is around 6.9 times larger than Earth, or between the sizes of Neptune and Saturn. It lies in a system 1,300 light-years away in the constellation Pictor. The stars in the system make an eclipsing binary, which occurs when the stellar companions circle each other in our plane of view. One is about 10% more massive than our Sun, while the other is cooler, dimmer and only one-third the Sun’s mass. TOI 1338 b’s transits are irregular, between every 93 and 95 days, and vary in depth and duration thanks to the orbital motion of its stars. TESS only sees the transits crossing the larger star — the transits of the smaller star are too faint to detect. Its orbit is stable for at least the next 10 million years. The orbit’s angle to us, however, changes enough that the planet transit will cease after November 2023 and resume eight years later. Credit: NASA’s Goddard Space Flight Center.

The chain of events started in Hawai’i with a dinner chat, which led to a NASA grant. It in turn helped to recruit a diligent high school student, who helped meticulous astronomers confirm the first circumbinary planet discovered by the TESS mission. This is an exemplary example of how teamwork and valuable contributions from the most junior scientist to the senior, can produce high impact science.