This artist’s concept shows the planet KOI-5Ab moving across the surface of a sun-like star, which is part of a triple star system located 1,800 light-years away in the constellation Cygnus. Credit: Caltech / R. Pain (IPAC)
Shortly after NASA’s Kepler mission began in 2009, the space telescope discovered a planet about half the size of Saturn in a multiple star system. KOI-5Ab was only the second planet candidate found by the mission, and exciting as it was then, it was eventually put aside as Kepler continued to make more planet discoveries.
By the end of the spacecraft’s operations in 2018, Kepler had discovered a whopping 2,394 exoplanets, or planets orbiting stars beyond our sun, and a further 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. “It was easier to pick than KOI-5Ab, and we learned something new every day from Kepler, so KOI-5 was largely forgotten.”
Now, after a long hunt of many years and many telescopes, Ciardi said he has “raised KOI-5Ab from the dead.” Thanks to new observations from NASA’s second planetary hunting mission, the Transiting Exoplanet Survey Satellite, or TESS, and a number of ground-based telescopes, Ciardi was finally able to unravel all the evidence surrounding KOI-5Ab and prove its existence. There are some intriguing details to consider.
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, orbiting in a plane that is not aligned with at least one of the stars. The arrangement casts doubt on how each member in this system is formed from the same swirling clouds of gas and dust. Ciardi, based in Caltech in Pasadena, California, presented the findings at a virtual meeting of the American Astronomical Society.
Taking up the path
After initial detection by Kepler, Ciardi and other researchers recorded the trail on KOI-5Ab as part of a cache of planet candidates they tracked. 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 orbit one star in a triple star system. However, they still couldn’t figure out whether the planetary signal was actually a false interference from one of the other two stars, or, if the planet was real, which of the stars orbited it.
Then, in 2018, TESS came by. Like Kepler, TESS looks for the blinking of the starlight that comes when a planet crosses or passes through 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, although TESS calls it TOI-1241b. As Kepler noted earlier, TESS found that the planet orbited its star about every five days.
“I thought to myself, I remember this target,” said Ciardi after seeing the TESS data. “But we still couldn’t definitively determine if the planet was real or if the error in the data came from another star in the system – it could be a fourth star.”
Clues in the wobble
He then went back and re-analyzed all the data, then looked for new clues from ground-based telescopes. Applying an alternative technique to Kepler and TESS, the Keck Observatory is often used for follow-up exoplanet studies by measuring the light oscillation in a star as a planet orbits around it and exerts a gravitational drag. Working with other scientists through an exoplanet collaboration group called California Planet Search, Ciardi looked for fluctuations in Keck’s data on the KOI-5 system. They were able to tease a wobble caused by the inner companion star orbiting the primary star from the apparent planet’s wobble as it orbits the primary star. Together, the various data sets from the space and ground 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 never would have gone back to do all this research, ”he said. But it really took a lot of research into the data collected from many different telescopes on this planet. “
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 gravity-bound star, Star C, orbits stars A and B every 400 years.
The KOI-5 star system consists of three stars, indicated in this diagram by A, B and C. 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 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 ground-based telescopes. KOI-5Ab is about half the mass of Saturn and orbits star A about every five days. Its orbit has a title of 50 degrees relative to the plane of stars A and B. Astronomers suspect it became misaligned orbit caused by star B, which kicked the planet by gravity during its development, skewed its orbit and migrated inward. Credit: Caltech / R. Pain (IPAC)
A crooked track
The combined dataset also reveals that the planet’s orbital plane is not aligned with the orbital plane of Star B, the second innermost star as you would expect if the stars and planet are all formed from the same disk of swirling material. Astronomers aren’t sure what caused the KOI-5Ab alignment, but believe the second star kicked the planet by gravity during its development, skewing its orbit and pulling it inward. Triple star systems make up about 10% of all star systems.
This is not the first evidence of planets in two and three star systems. A striking case concerns the triple star system GW Orionis, in which a planet-forming disk has ruptured into several misaligned rings, where planets may form. Yet despite hundreds of planet discoveries in multi-star systems, far fewer planets have been observed than in single-star systems. This could be due to an observational bias (single-star planets are easier to detect), or because planet formation is actually less common in multi-star systems.
“This research highlights the importance of NASA’s entire fleet of space telescopes and their synergy with ground-based systems,” said Jessie Dotson, the Kepler Space Telescope project scientist at NASA’s Ames Research Center in Silicon Valley, California. “can be a long breath.”
New and future instruments, such as the Palomar Radial Velocity Instrument at 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 will open new avenues for more find out about exoplanets.
Media contact:
Elizabeth Landau
NASA Headquarters, Washington
202-923-0167
[email protected]
Alison Hawkes
NASA Ames Research Center, Silicon Valley, California.
650-604-4789
[email protected]