This artist’s concept shows how the planet KOI-5Ab moves across the face of a solar star, which is part of a triple star system located 1800 light-years away in the constellation Cygnus. Credit: Caltech / R. Seer (IPAC)
Years after being detected, astronomers have learned that a planet called KOI-5Ab orbits a three-star system with a skewed configuration.
Shortly after NASA’s Kepler mission began work in 2009, it identified what as a planet is about the size of Neptune. Called the KOI-5Ab, the planet, which was the second new planetary candidate found by the mission, was eventually forgotten as Kepler gained more and more planetary discoveries. At the end of its mission in 2018, Kepler discovered a whopping 2,394 exoplanets, or planets orbiting our stars, and an additional 2,366 exoplanet candidates, including KOI-5Ab.
Now, David Ciardi, chief scientist of NASA‘s Exoplanet Science Institute (NExScI), located in Caltech’s IPAC, says it ‘raised KOI-5Ab from the dead’, thanks to new observations from NASA’s TESS (Transiting Exoplanet Survey Satellite) mission.
“KOI-5Ab fell off the table and was forgotten,” said Ciardi, who presented the findings at a virtual meeting of the American Astronomical Society (AAS). By 2014, Ciardi and other researchers had used the WM Keck Observatory in Hawaii, Caltech’s Palomar Observatory near San Diego and Gemini North in Hawaii to show that the star surrounded by KOI-5Ab is one member of ‘ a three-star system called KOI- 5. But they were not sure if the KOI-5 system really presented a planet and if they saw an erroneous signal from one of the other two stars.
Then, in 2018, TESS came together. Like Kepler, TESS looks at the twinkling starlight that comes when a planet crosses in front of a star, or passes through. TESS observed a portion of Kepler’s field of view, including the KOI-5 system. Sure enough, TESS also identified KOI-5Ab as a candidate planet (although TESS calls it TOI-1241b). TESS, like Kepler, found that the planet orbits its star about every five days. But at that point, it was not yet clear if the planet was real.
“I thought to myself, ‘I remember this target,'” Ciardi said after seeing the TESS data. He then went back and re-analyzed all the data, including the data from the California Planet Search, led by Andrew Howard, professor of astronomy at Caltech. The California Planet Search uses telescopes on the ground, including the Keck Observatory, to search for the wobble in a star that appears as a planet orbits it and a gravitational pull.
“If TESS had not looked at the planet again, I would never have done all this detective work,” says Ciardi.
The KOI-5 galaxy consists of three stars, marked A, B and C in this diagram. Stars A and B rotate each other every 30 years. Star C orbits stars A and B every 400 years. The system houses one known planet, called KOI-5Ab, which was discovered and characterized using data from NASA’s Kepler and TESS (Transiting Exoplanet Survey Satellite) missions, as well as as telescopes on the ground. KOI-5Ab is about half the mass of Saturn and orbit star A approximately every five days. Its orbit has the title 50 degrees relative to the level of stars A and B. Astronomers suspect that this wrong orbit was caused by star B, which kicked the planet during its evolution gravity, skewed its orbit and caused it to migrate inward. . Credit: Caltech / R. Seer (IPAC)
Jessie Dotson, the Kepler / K2 project scientist at NASA’s Ames Research Center, says: “This research highlights the importance of NASA’s complete fleet of space telescopes and their synergy with terrestrial systems. Discoveries like these can be a long way off.”
The data from the space and ground-based telescopes help together to confirm that KOI-5Ab is a planet. KOI-5Ab is about half the mass of Saturn and orbits a star (star A) with a relatively close companion (star B). Star A and star B rotate each other every 30 years. A third gravitational star (star C) orbits stars A and B every 400 years.
The combined data set also reveals that the orbital plane of the planet is not in line with the orbital plane of the second inner star (star B) as might be expected if the stars and planet are all formed from the same disk of swirling material. Three-star systems, which make up about 10 percent of all galaxies, are thought to form when three stars are born together from the same disk of gas and dust.
Astronomers are not sure what caused the misinterpretation of KOI-5Ab, but they do speculate that the second star kicked the planet gravely during its evolution, skewed its orbit, and caused it to migrate inward.
This is not the first evidence for planets in double and triple systems. One notable case is the triple galaxy GW Orionis, in which a planet-forming disk is torn in various misinterpreted rings, where planets can form. Despite hundreds of discoveries of multiple galaxies, the frequency of planetary formation in these systems is lower than that of single galaxies. This may be due to an observational bias (single star planets are easier to detect) or because planet formation is actually less common in multiple galaxies.
Future instruments, such as the Palomar Radial Velocity Instrument (PARVI) at the 200-inch Hale telescope at Palomar and the Keck Planet Finder at Keck, will offer new ways to better answer these questions.
“Star companions can partially extinguish the process of planet formation,” says Ciardi. ‘We still have many questions about how and when planets can form in multiple galaxies and how their properties compare to planets in single galaxies. By studying the KOI-5 system in more detail, we might gain insight into how the universe makes planets. ”