This artist’s concept shows the planet KOI-5Ab transiting 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. Hurt (IPAC)
Shortly after NASA’s Kepler mission began operating in 2009, the space telescope observed what was believed to be a planet about half the size of Saturn in a multi-star system. The KOI-5Ab was only the second planetary candidate to be found by the mission, and excitingly, as it was at the time, it was eventually set aside, while Kepler amassed more and more discoveries of the planet.
By the end of the spacecraft’s operations in 2018, Kepler has discovered an enormous 2,394 exoplanets or planets orbiting stars beyond our sun and another 2,366 exoplanet candidates that would need confirmation.
“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 an easier choice than the KOI-5Ab, and I learned something new from Kepler every day, so the KOI-5 was largely forgotten.”
Now, after a long hunt that lasted many years and many telescopes, Ciardi said he “raised the KOI-5Ab from the dead.” Thanks to new observations from NASA’s second hunting mission, the Transiting Exoplanet Survey Satellite, or TESS, and a series of ground-based telescopes, Ciardi has finally managed to unravel all the evidence surrounding KOI-5Ab and -provide their existence. There are some interesting details about this that need to be considered.
Most likely, a giant gas 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 accompanying stars, orbiting a plane that it is not aligned with at least one of the stars. The arrangement calls into question how each member of this system formed the same clouds swirling with gas and dust. Ciardi, who is at Caltech in Pasadena, California, presented the findings at a virtual meeting of the American Astronomical Society.
Raising the path
After initial detection by Kepler, Ciardi and other researchers took the route on KOI-5Ab as part of a cache of candidates on the planet they were tracking. 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 surround a star in a three-star system. However, they still could not figure out if the planet’s signal was actually an erroneous error from one of the other two stars or, if the planet was real, which of the stars was orbiting.
Then, in 2018, came TESS. Like Kepler, TESS seeks the flashing of starlight that occurs when a planet crosses in front of or passes through a star. TESS observed a portion of Kepler’s field of vision, including the KOI-5 system. Sure enough, TESS has also identified KOI-5Ab as a candidate planet, although TESS calls it TOI-1241b. As Kepler had previously observed, TESS found that the planet orbited its star about every five days.
“I thought to myself, ‘I remember this target,'” Ciardi said after seeing the TESS data. “But we still couldn’t determine for sure if the planet was real or if the data blip came from another star in the system – it could have been a fourth star.”
Indices in oscillations
Then he turned and reanalyzed all the data, then looked for new clues from ground-based telescopes. Performing an alternative technique to Kepler and TESS, the Keck Observatory is often used for subsequent searches of exoplanets by measuring the slight oscillation of a star as a planet surrounds it and exerts a gravitational tug. Ciardi, teaming up with other scientists through a collaboration group with the exoplanet called California Planet Search, looked for any oscillations in Keck’s data on the KOI-5 system. They managed to pull out an oscillation produced by the inner accompanying star that orbits the primary star from the oscillation of the apparent planet while orbiting the primary star. Together, the various 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 it weren’t for TESS looking at the planet again, I would never go back and do all this detective work, “he said.” But it really took a lot of detail in data collected from several different telescopes to finally nail on this planet. “
KOI-5Ab orbits star A, which has a relatively close partner, 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 KOI-5 star system consists of three stars, labeled A, B, and C in this diagram. Stars A and B orbit each other every 30 years. Star C orbits stars A and B every 400 years. The system hosts a 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 is named 50 degrees from the plane of stars A and B. Astronomers suspect that this misaligned orbit was caused by star B, which gravitationally hit the planet during its development, tilting its orbit and causing inward migration. Credit: Caltech / R. Hurt (IPAC)
An inclined orbit
The combined data set also reveals that the orbital plane of the planet is not aligned with the orbital plane of star B, the second inner star as would be expected if the stars and the planet all formed from the same disk of material that spins. Astronomers are not sure what caused the KOI-5Ab misalignment, but they believe that the second star gravitationally hit the planet during its development, tilting its orbit and causing it to migrate inward. Three-star systems account for about 10% of all star systems.
This is not the first evidence of planets in double and triple star systems. A striking case involves the three-star GW Orionis system, in which a planet-forming disk has been broken into distinct, misaligned rings where planets can form. However, despite hundreds of planetary discoveries in the multi-star system, far fewer planets have been observed than in single-star systems. This may be due to 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 complete fleet of space telescopes and their synergy with ground-based systems,” said Jessie Dotson, project researcher for the Kepler Space Telescope at NASA’s Ames Research Center in Silicon Valley, California. “Discoveries like this can go a long way. ”
New and future instruments, such as the Palomar Radial Velocity Instrument from the Palomar 200-inch Hale Telescope, NASA’s NEID instrument and the Southern Arizona National Science Foundation, and the Keck Planet Finder will open up new avenues for learning 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]