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This artist’s concept shows the planet KOI-5Ab transiting through the face of a sun-like star, which is part of a three-star system located 1,800 light years away in the constellation Cygnus. Credit: Caltech / R. Injured (IPAC)
Years after its detection, astronomers learned that a planet called KOI-5Ab orbits in a three-star system with an asymmetric configuration.
Shortly after NASA’s Kepler mission began operations in 2009, she identified what was thought to be a planet the size of Neptune. Called KOI-5Ab, the planet, which was the second new candidate planet to be found by the mission, was ultimately forgotten as Kepler accumulated more and more planet finds. By the end of his mission in 2018, Kepler had discovered 2,394 exoplanets, or planets orbiting stars beyond our sun, and 2,366 more. exoplanet candidates, including KOI-5Ab.
Now David Ciardi, Chief Scientist of NASA‘s Exoplanet Science Institute (NExScI), located at Caltech’s IPAC, says it has “raised KOI-5Ab from the dead”, thanks to new observations from the Transiting Exoplanet Survey Satellite (TESS) mission. NASA.
“KOI-5Ab fell off the table and was forgotten,” said Ciardi, who presented the results at a virtual meeting of the American Astronomical Society (AAS). In 2014, Ciardi and other researchers 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 a member of a three-star system called KOI-5. But they weren’t sure if the KOI-5 system actually hosted a planet or if they saw an erroneous signal from one of the other two stars.
Then, in 2018, TESS came up with. Like Kepler, TESS searches for the flickering starlight that occurs when a planet crosses or transits in front of a star. TESS observed part of Kepler’s field of view, including the KOI-5 system. Indeed, TESS has also identified KOI-5Ab as a candidate planet (although TESS calls it TOI-1241b). TESS, like Kepler, discovered that the planet orbits its star every five days or so. But at this point, it still wasn’t clear if the planet was real.
“I was like, ‘I remember that goal,’” Ciardi says, after seeing the TESS data. He then went back and reanalyzed all the data, including that from California Planet Search, led by Caltech astronomy professor Andrew Howard. The California Planet Search uses ground-based telescopes, including the Keck Observatory, to search for the telltale oscillation of a star that occurs when a planet revolves around it and exerts a gravitational tug.
“If TESS hadn’t looked at the planet again, I would never have gone back and done all that detective work,” Ciardi says.
The KOI-5 star system consists of three stars, labeled A, B, and C in this diagram. The A and B stars are in orbit every 30 years. Star C revolves around stars A and B every 400 years. The system is home to a known planet, called KOI-5Ab, which was discovered and characterized using data from NASA’s Kepler and Transiting Exoplanet Survey Satellite (TESS) missions, as well as ground-based telescopes. KOI-5Ab is roughly half the mass of Saturn and orbits the star A every five days or so. Its orbit is titled 50 degrees from the plane of stars A and B. Astronomers suspect that this misaligned orbit was caused by star B, which gravitationally struck the planet during its development, skewing its orbit and making it migrate inward. Credit: Caltech / R. Injured (IPAC)
Jessie Dotson, Kepler / K2 Project Scientist at the NASA Ames Research Center, says: “This research emphasizes the importance of NASA’s entire fleet of space telescopes and their synergy with ground systems. Discoveries like this can take a long time. “
Together, data from space and ground telescopes have confirmed that KOI-5Ab is a planet. KOI-5Ab is about half the mass of Saturn and revolves around a star (star A) with a relatively close companion (star B). Star A and star B are in orbit every 30 years. A third gravitational star (C star) orbits A and B stars every 400 years.
The combined dataset also reveals that the orbital plane of the planet is not aligned with the orbital plane of the second inner star (star B) as one would expect if the stars and the planet were all formed at from the same disc of swirling material. Three-star systems, which make up about 10% of all star systems, are thought to form when three stars are born together from the same disk of gas and dust.
Astronomers aren’t sure what caused the KOI-5Ab misalignment, but assume the second star gravitated to the planet during its development, skewing its orbit and causing it to migrate inward.
This is not the first evidence of the presence of planets in two and three star systems. A striking case involves the GW Orionis three-star system, in which a planet-forming disk had been torn into separate, misaligned rings, where planets could form. Yet despite hundreds of discoveries of multi-star system planets, the frequency of planetary formation in these systems is lower than in single-star systems. This could be due to an observation bias (single star planets are easier to detect) or the fact that planet formation is actually less common in multi-star systems.
Future instruments, such as the Palomar Radial Velocity Instrument (PARVI) from the 200-inch Hale Telescope at Palomar and the Keck Planet Finder at Keck, will open up new avenues to better answer these questions.
“The stellar companions can partially extinguish the process of planetary formation,” Ciardi says. “We still have a lot of questions about how and when planets can form in multi-star systems and how their properties compare to planets in single-star systems. By studying the KOI-5 system in more detail, we may be able to gain insight into how the universe makes planets.
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