Planets beyond our solar system

Shortly after NASA’s Kepler mission began operations in 2009, the space telescope spotted what was thought to be a planet about half the size of Saturn in a multi-star system. KOI-5Ab was only the second candidate planet to be found by the mission, and as exciting as it was at the time, it was ultimately shelved as Kepler accumulated more and more discoveries of planets.

By the end of the spacecraft’s operations in 2018, Kepler had discovered 2,394 exoplanets, or planets orbiting stars beyond our sun, and 2,366 more exoplanet candidates that still need confirmation.

“KOI-5Ab was dropped because it was complicated, and we had thousands of applicants,” said David Ciardi, chief scientist at NASA’s Exoplanet Science Institute. “There were easier choices than KOI-5Ab, and we were learning something new from Kepler every day, so KOI-5 was almost forgotten.”

Now, after a long hunt that spanned many years and many telescopes, Ciardi has said he has “raised KOI-5Ab from the dead”. Thanks to new observations from NASA’s second planet-hunting mission, the Transiting Exoplanet Survey Satellite, or TESS, and a number of ground-based telescopes, Ciardi was finally able to unravel all of the evidence surrounding KOI-5Ab and prove his existence. There are some intriguing details about this to ponder.

Most likely a gas giant planet like Jupiter or Saturn in our solar system given its size, KOI-5Ab is unusual in that it orbits a star in a system with two other companion stars, rotating on a plane that is not aligned with at least one of the stars. The arrangement questions how each member of this system has formed from the same swirling clouds of gas and dust. Ciardi, who is located at Caltech in Pasadena, Calif., Presented the results at a virtual meeting of the American Astronomical Society.

Picking up the track

After its initial detection by Kepler, Ciardi and other researchers took up the trail on KOI-5Ab as part of a cache of candidate planets they were tracking on. Using data from the WM Keck Observatory in Hawaii, Caltech’s Palomar Observatory near San Diego, and Gemini North in Hawaii, Ciardi and other astronomers determined that KOI-5b appeared to encircle a star in a system. with three stars. However, they still couldn’t determine if the planet’s signal was in fact an erroneous glitch from one of the other two stars, or, if the planet was real, which star it is orbiting.

Then, in 2018, TESS arrived. Like Kepler, TESS searches for the flicker of starlight that occurs when a planet crosses or passes 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. As Kepler observed previously, TESS discovered that the planet orbits its star about every five days.

“I was like, ‘I remember that goal,’” Ciardi said, after seeing the TESS data. “But we still couldn’t determine for sure 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.

Clues in the wobbles

He then came back and re-analyzed all the data, then looked for new clues from telescopes on the ground. Deploying an alternative technique to Kepler and TESS, the Keck Observatory is often used for exoplanet tracking research by measuring the slight oscillation of a star as a planet revolves around it and exerts a gravitational tug. Ciardi, teaming up with other scientists as part of an exoplanet collaboration group called California Planet Search, looked for oscillations in Keck’s data on the KOI-5 system. They were able to disentangle an oscillation produced by the inner companion star orbiting the primary star from the oscillation of the apparent planet as it orbits the primary star. Together, the different data collections from space and terrestrial telescopes have helped confirm that KOI-5Ab is indeed a planet orbiting the primary star.

“Bingo – it was there! If TESS hadn’t looked at the planet again, I would never have gone back and done all that detective work, “he said.” But it really took a lot of research into the data collected from many different telescopes to ultimately be successful on this planet.

KOI-5Ab orbits star A, which has a relatively close companion, star B. Star A and star B orbit every 30 years. A third gravitational star, star C, orbiting stars A and B every 400 years.

A biased orbit

The combined dataset also reveals that the planet’s orbital plane is not aligned with the orbital plane of star B, the second inner star as one would expect if the stars and planet were all formed. from the same disc of swirling material. Astronomers are unsure of the cause of KOI-5Ab’s misalignment, but believe the second star gravitationally struck the planet during its development, skewing its orbit and causing it to migrate inward. Three-star systems represent about 10% of all star systems.

This is not the first evidence of planets in two and three star systems. One striking case is the GW Orionis three-star system, in which a planet-forming disc has been torn into separate, misaligned rings, where planets can form. Yet despite hundreds of discoveries of planets in a multi-star system, far fewer planets have been observed 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.

“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 the United States. Silicon Valley in California. “Discoveries like this can take a long time.”

New and future instruments, such as the Palomar Radial Velocity Instrument from the 200-inch Hale Telescope in Palomar, NASA and the National Science Foundation’s NEID instrument in southern Arizona, and the Keck Planet Finder will open new avenues to learn more about exoplanets.