From NASA
January 11, 2021
This illustration shows the planet KOI-5Ab moving across the face of a sun-like star that is part of a triple star system located 1,800 light years away in the Cygnus constellation. Credits: Caltech / R. Injured (Infrared Processing and Analysis Center or IPAC)
Shortly after NASA’s Kepler mission began operations in 2009, the space telescope discovered a planet in a multi-star system that was believed to be about half the size of Saturn. KOI-5Ab was just the second planetary candidate found by the mission, and as exciting as it was back then, it was ultimately put aside as Kepler continued to gather more planetary discoveries.
By the end of the spacecraft’s deployment in 2018, Kepler had discovered a whopping 2,394 exoplanets or planets orbiting stars beyond our sun, and another 2,366 exoplanet candidates that have yet to be confirmed.
“KOI-5Ab was abandoned because it was complicated and we had thousands of candidates,” said David Ciardi, chief scientist at NASA’s Exoplanet Science Institute. “There was an easier choice than KOI-5Ab, and we learned something new from Kepler every day, so KOI-5 was largely forgotten.”
Now, after a long hunt that spanned many years and many telescopes, Ciardi said he “raised KOI-5Ab from the dead”. Thanks to new observations from NASA’s second planet-hunting mission, the Transiting Exoplanet Survey Satellite (TESS) and a series of ground-based telescopes, Ciardi was finally able to unravel all of the evidence surrounding KOI-5Ab and prove its existence. There are some interesting details about it to think about.
KOI-5Ab is most likely a gas giant planet like Jupiter or Saturn in our solar system. Because of its size, it orbits a star in a system with two other companion stars and orbits on a plane that is not aligned with at least one of the stars. The arrangement questions how each member in this system was formed from the same swirling clouds of gas and dust. Ciardi, who is based in Caltech, Pasadena, California, presented the results at a virtual meeting of the American Astronomical Society.
“We don’t know of many planets that exist in triple star systems, and this one is special because its orbit is crooked,” said Ciardi. “We still have many questions about how and when planets can form in multiple star systems and how their properties can be compared with those of planets in single star systems. If we examine this system more closely, we may be able to gain some insight into how the universe makes planets. “
Take up the path
Following its first discovery by Kepler, Ciardi and other researchers tracked KOI-5Ab as part of a cache of planetary candidates they were chasing. Ciardi and other astronomers found, using data from the WM Keck Observatory in Hawaii, the Caltech Palomar Observatory near San Diego, and Gemini North in Hawaii, that KOI-5b appeared to be orbiting a star in a three-star system. However, they still couldn’t figure out whether the planetary signal was actually a buggy error from either of the other two stars, or, if the planet was real, which of the stars it was orbiting from.
Then, in 2018, TESS came along. Like Kepler, TESS searches for the blinking starlight that occurs when a planet crosses or crosses a star. TESS observed part of Kepler’s field of view, including the KOI-5 system. Sure enough, TESS also identified KOI-5Ab as a candidate planet, even though TESS calls it TOI-1241b. As Kepler had previously observed, TESS found that the planet orbited its star approximately every five days.
“I figured I remember that goal,” Ciardi said after seeing the TESS data. “But we still couldn’t definitively determine if the planet was real or if the error in the data was from another star in the system – it could have been a fourth star.”
Notes in the wobble
Then he went back and re-analyzed all the data, then looked for new clues from ground-based telescopes. The Keck Observatory uses an alternative technique to Kepler and TESS and is often used for exoplanet follow-up by measuring the slight wobble in a star as a planet orbits it and wields a gravity tug. Working with fellow scientists through an exoplanet collaboration group called California Planet Search, Ciardi looked for wobbles in Keck’s data on the KOI-5 system. They were able to extract a wobble of the inner companion star orbiting the primary star from the wobbling of the apparent planet as it orbits the primary star. Together, the various data collections from the space- and ground-based telescopes have confirmed that KOI-5Ab is indeed a planet orbiting the primary star.
“Bingo – it was there! If TESS hadn’t looked back at the planet I would never have gone back and done all this detective work, ”he said. “But it really took a lot of time to collect data from many different telescopes to pin this planet down for good.”
KOI-5Ab orbits star A, which has a relatively close companion, star B. Star A and star B orbit each other every 30 years. A third gravitational star, star C, orbits stars A and B every 400 years.
The KOI 5-star system in this diagram consists of three stars with the designations A, B and C. The stars A and B orbit each other every 30 years. Star C orbits stars A and B every 400 years. The system is home to a well-known planet called KOI-5Ab, which was discovered and characterized using data from NASA’s Kepler and Transiting Exoplanet Survey Satellite missions and ground-based telescopes. KOI-5Ab is about half the size of Saturn and orbits star A about every five days. Its orbit is titled 50 degrees relative to the plane of stars A and B. Astronomers suspect that this misaligned orbit was caused by star B, which gravitationally kicked the planet during its evolution, distorted its orbit, and caused it to travel inward.Credit: Caltech / R. Injured (Infrared Processing and Analysis Center, or IPAC )
A distorted orbit
The combined data set also shows that the planet’s orbital plane is not aligned with the orbital plane of star B, the second inner star, as would be expected if the stars and planet were all formed from the same disk of swirling material. Astronomers are not sure what caused the misalignment of KOI-5Ab, but believe that the second star gravitationally entered the planet during its evolution, distorting its orbit, and causing it to migrate inward. Three star systems make up about 10% of all star systems.
This is not the first evidence of planets in double and triple star systems. A conspicuous case concerns the three-star system GW Orionis, in which a planet-forming disk was torn into different, misaligned rings, in which planets possibly form. Despite hundreds of planetary discoveries in the multi-star system, far fewer planets have been observed than in single-star systems. This could be due to an observation distortion (single star planets are easier to spot) or to the fact that planet formation is actually less frequent in multiple star systems.
“This research underscores the importance of NASA’s entire fleet of space telescopes and their synergy with ground-based systems,” said Jessie Dotson, project scientist for the Kepler Space Telescope at NASA’s Ames Research Center in California’s Silicon Valley. “Discoveries like this can go a long way.”
New and future instruments such as the Palomar Radial Velocity Instrument on the 200-inch Hale Telescope in Palomar, the NEID instrument from NASA and the National Science Foundation in southern Arizona, and the Keck Planet Finder open up new opportunities for learning about exoplanets.
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