Even in the rush of new planets discovered in the last decade, some can surprise. The latest example was actually the Kepler Space Telescope's second detection, which unconfirmed at the time. Finally verified, its curious orbit could teach us something about star system formation

The majority of stars come in pairs, but since it is harder to detect planets in such circumstances we know of relatively few “Tatooine planets” with two suns. Triple star systems are much rarer, so inevitably only a handful are known to have planets. Where we do, it's usually when one is found around a star with a very distant orbit from the other two, such as Proxima Centauri. 

Kepler's recording of dips in the light coming from the KOI-5 in 2009 aroused excitement when such events were very new. However, interference from the system's stars made making KOI-5Ab, as the possible world was named, a major challenge. “There were easier pickings than KOI-5Ab, and we were learning something new from Kepler every day so that KOI-5 was mostly forgotten.” Dr David Ciardi of NASA's Exoplanet Science Institute said in a statement. 

By 2018 Kepler had been replaced by TESS (Transiting Exoplanet Survey Satellite), which also picked up dips in brightness coming from the system. The slight reduction in light occurs every 5 days as if a planet very close to its star keeps passing in front. Ciardi used spectral data collected at the Keck telescope to reveal a wobble in KOI-5A, the brightest star in the system, consistent with a 5-day orbit of a mass about half that of Saturn. The findings were presented at the American Astronomical Society's virtual meeting. 

Putting together observations from the two satellites and the Keck, Gemini North and Palomar observatories Ciardi has revealed a highly unexpected feature of the system – the orbits of the star KOI-5B and the planet KOI-5Ab are 50 degrees out of alignment. Planets form from a common disk of gas and dust, leading to orbits in a common plane

Planets and stars in multi-star systems are expected to be the same, although of course, we have fewer examples to confirm this. KOI-5C's orbit is so far from the other two stars (taking 400 years to go around) a misalignment would be unsurprising. Instead of having formed from the same disk, it could be a captured star that passed nearby.

KOI-5B is a different matter. It and KOI-5A circle each other every 30 years – similar to Saturn's journey. They almost certainly formed together. Ciardi and colleagues think some interaction between KOI-5B and KOI-5Ab must have thrown the unfortunate planet out of its orbit, likely also explaining its close, and very hot, orbit to its star.

“We don’t know of many planets that exist in triple-star systems, and this one is extra special because its orbit is skewed,” Ciardi said. “We still have a lot of questions about how and when planets can form in multiple-star systems and how their properties compare to planets in single-star systems. By studying this system in greater detail, perhaps we can gain insight into how the universe makes planets."