Shortly after NASA’s Kepler mission began operating in 2009, it identified what was believed to be a planet the size of Neptune. Named KOI-5Ab, the planet, which was the second new candidate planet to be found by the mission, was finally forgotten when Kepler amassed more and more discoveries from the planet. By the end of his 2018 mission, Kepler had discovered a whopping 2,394 exoplanets, or planets orbiting stars beyond our sun, and an additional 2,366 exoplanet candidates, including KOI-5Ab.
Now, David Ciardi, chief scientist at NASA’s Exoplanet Science Institute (NExScI), located at Caltech’s IPAC, says he has “resurrected KOI-5Ab from the dead,” thanks to new observations from the mission TESS (Transiting Exoplanet Survey Satellite) from NASA.
“KOI-5Ab fell off the table and was forgotten,” says Ciardi, who presented the findings at a virtual meeting of the American Astronomical Society (AAS). In 2014, Ciardi and other researchers had used the WM Keck Observatory in Hawaii, the Caltech Palomar Observatory near San Diego, and the Gemini North in Hawaii to prove that the star encircled 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 housed a planet or if they saw a wrong signal from one of the other two stars.
Then, in 2018, TESS appeared. Like Kepler, TESS looks for the flicker of stellar light that occurs when a planet crosses in front of or passes a star. TESS observed a portion of Kepler’s field of view, including the KOI-5 system. Indeed, TESS also identified KOI-5Ab as a candidate planet (although TESS calls it TOI-1241b). TESS, like Kepler, found that the planet orbited its star about every five days. But at the time, it was still unclear if the planet was real.
“I thought,‘ I remember that goal, ’” Ciardi says, after seeing the TESS data. He then went back and reanalyzed all the data, including that of the California Planet Search, led by Caltech astronomy professor Andrew Howard. California Planet Search uses ground-based telescopes, including the Keck Observatory, to search for the revealing hesitation of a star that occurs when a planet orbits and exerts a gravitational pull.
“If it weren’t for TESS looking at the planet again, I would never have gone back and done all this detective work,” Ciardi says.
Jessie Dotson, a scientist in the Kepler / K2 project at NASA’s Ames Research Center, says: “This research highlights the importance of NASA’s entire fleet of space telescopes and their synergy with terrestrial systems. Discoveries like this can be long. ”
Together, data from terrestrial and space telescopes helped 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 orbit each other every 30 years. A third gravitationally bound star (star C) orbits stars A and B every 400 years.
The combined data set also reveals that the planet’s orbital plane is not aligned with the orbital plane of the second inner star (star B), as might be expected if the stars and the planet formed from the same rotating disk. . Three-star systems, which account for about 10 percent of all stellar systems, are believed to form when three stars are born together from the same gas and dust disk.
Astronomers are not sure what caused the misalignment of KOI-5Ab, but speculate that the second star gravitationally hit the planet during its development, skewing its orbit and causing it to migrate inward.
This is not the first evidence of planets in double- and triple-star systems. A striking case is the three-star GW Orionis system, in which a planet-forming disk had been broken into different misaligned rings, where planets can form. However, despite hundreds of planetary discoveries from multi-star systems, the frequency of planet formation in these systems is lower than that of single-star systems. This could be due to an observational bias (single-star planets are easier to detect) or because planet formation is in fact less common in multi-star systems.
Future instruments, such as the Palomar Radial Speed Instrument (PARVI) on the 200-inch Hale Telescope in Palomar and the Keck Planet Finder in Keck, will open up new avenues to better answer these questions.
“Stellar comrades can partially attenuate the process of planet 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 are compared to planets in single-star systems. By studying the KOI-5 system in more detail, we can perhaps obtain an idea of how the universe makes planets ”.