Kepler-47 star system
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Kepler-47, a two-star galaxy found by Kepler while the space telescope was still alive. Credit: NASA

Sorry Tatooine, but this world in a galaxy far, far away has three suns

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Jan 19, 2021, 10:50 PM EST

Luke Skywalker might have had an otherworldly view of a double sunrise and double sunset on Tatooine, but even he probably didn’t envision what it would be like to live on a planet where he could watch the rising and setting of three stars.

The strange orb known as KOI-5Ab is that planet. Almost thought to be no more than a glitch in Kepler’s observations, its existence has finally been proven by new observations from TESS, which have also made it out to probably be a gas giant like Jupiter or Saturn. KOI-5Ab might as well exist in the Star Wars universe. It has an unusual orbit, and the three stars it orbits are unlikely to have formed from the same primordial cloud of gas and dust. Illuminating more about this sci-fi planet and its system could reveal more about how the universe births stars and planets. (Kepler also found the two-star system above, Kepler-47, which is located in the constellation Cygnus. So just imagine that with one more star.)

So what makes the orbit of KOI-5Ab so unusual? David Ciardi, chief scientist at NASA’s Exoplanet Institute, has an idea.“For KOI-5, there are three stars, and the two inner stars are of nearly equal mass and not that far apart,” Ciardi, who recently led a presentation on the findings at a virtual meeting of the American Astronomical Society, told SYFY WIRE. “"The stars must have interacted with the planet gravitationally such that the planet is no longer in the orbit in which it formed, but rather was likely forced into its current orbit through gravitational interaction with the two inner stars." 

Star systems that have the three suns make up a tenth of all the star systems (that we know of) out there. KOI-5Ab is not the first planet found to be orbiting a triple star system, but it does stand out. There was something off about the arrangement. When Kepler was still alive and scouring the cosmos for exoplanets in 2009, there were so many candidates for alien worlds that the planet, which is believed to be about half the size of Saturn, ended up taking a backseat to more exciting finds. There is going to be some tough competition when you’re up against 2,394 confirmed exoplanets and an additional 2,366 candidates, of which KOI-5Ab was one.

Some of the exoplanet candidates spotted transiting their stars by Kepler still need confirmation. The thought of KOI-5Ab stagnated for years. Enter NASA’s shiny new planet hunter TESS and several observatories, where Ciardi and his team further investigated the planet that almost wasn’t.

"We know of one planet, but it may be  that all the stars have planets," Ciardi said. "That is one of the big questions. Then the gravitational interactions between the outer star and the inner stars could have forced the inner stars closer together, which in turn forced the planet to migrate in and maybe skew its orbit to a different inclination."

TESS seeks out exoplanets by their transits, just as Kepler did. The momentary flicker that happens when starlight is blocked by another object floating by is usually an indication of an exoplanet passing in front of the star. However, some of these signals can be faint. TESS found KOI-5Ab again in 2018, and though its data identified the planet as TOI-1241b, astronomers are still going by the name Kepler tagged it with. Turned out the new planet hunter was re-observing parts of the sky Kepler had previously looked at when KOI-5Ab showed up as a planetary candidate again. What exactly it was remained unsure. It might have been a planet, but there was a chance it also could have been a fourth star.

KOI-5Ab was finally exposed by gravity. Ciardi looked back over everything he had found so far, making sure there was nothing he had missed, and then decided to head to the Keck observatory. Keck is often used as a backup when it comes to making sure whether that really was an exoplanet Kepler or TESS saw. This observatory doesn’t use the transit method like the two space telescopes, but instead gauges the slight wobble of a star that results from the gravity exerted on it by a passing planet. In collaboration with the California Planet Search (CPS) group of astronomers who use Keck and its HIRES spectrometer to reveal new exoplanets, he kept an eye out for any telltale wobbles. There was just one problem: the signals were tangled.

Is KOI-5Ab the only planet that has ever existed in the system? At least for now, it's hard to tell."

"It is likely that each star formed from its own disk and each disk formed the star plus one or more planets," said Ciardi. "It is also possible that the interactions of the three stars rearranged the system, and that planets formed in the system that may have been ejected as the system evolved."

Eventually, Ciardi and the CPS team made it through that labyrinth and determined that one of the wobbles was being caused by the gravity of one of the companion stars (Star B) pulling on the main star (Star A) as it orbited. Star A and B complete an orbit around each other every 30 years while Star C takes 400 years to orbit them both. Got that? Star B and KOI-5Ab both orbit Star A. KOI-5Ab was responsible for the other wobble.

"The planet orbiting the primary star is not in the same orbital plane as the two stars; the outer star is too far away to really know what its orbit is doing," he said. "Furthermore, the planet is a gas giant in a five day orbit, and it certainly did not form there. It had to form further out and then migrate in. What we see today is certainly not how the system formed, so KOI-5 may provide clues on how systems may form and evolve."

Too bad Luke couldn't live on KOI-5Ab anyway, because there is supposedly no land — not even a desert.