Astronomers have confirmed the existence of a triple-star exoplanet located 1,800 light-years from Earth. Planets parked in multi-star systems are rare, but this object is particularly unusual owing to its inexplicably weird orbital alignment.
The first trace of KOI-5Ab was spotted by NASA’s Kepler space telescope in 2009, but that was very early in the mission, so the exoplanetary candidate was put aside in favor of easier targets. Not a terrible decision, given that Kepler, during the course of its illustrious nine-year career, spotted 4,760 exoplanet candidates, of which roughly half still need to be confirmed.
“KOI-5Ab got abandoned because it was complicated, and we had thousands of candidates,” David Ciardi, chief scientist of NASA’s Exoplanet Science Institute, explained in a NASA statement. “There were easier pickings than KOI-5Ab, and we were learning something new from Kepler every day, so that KOI-5 was mostly forgotten.”
Ciardi, along with his colleagues, have now gazed upon KOI-5Ab with new eyes, namely NASA’s Transiting Exoplanet Survey Satellite and several ground-based telescopes, including the Keck Observatory in Hawai’i. The team was finally able to confirm KOI-5Ab as a bona fide exoplanet, and in the process uncover some fascinating—if not completely perplexing—aspects about its stellar environment. Ciardi, a research astronomer at Caltech, recently presented his team’s findings at a virtual meeting of the American Astronomical Society.
Confirmation of KOI-5Ab was done with the tried-and true transit method, in which an orbiting planet passes in front of its star from our perspective, causing a brief dimming. The confirmation was further validated by another technique, the wobble method, in which the slight gravitational tug of an orbiting planet causes a detectable lurch in its host star. TESS was used for the transit method, while Keck was used to detect the wobble. The combined data allowed the researchers to rule out other possibilities, such as a fourth star.
KOI-5Ab is likely a gas giant, similar to Neptune in terms of its size. It resides within a triple-star system, and while its orbit is a bit strange, it’s overall environment is less chaotic than it may sound.
Despite having three stellar companions, KOI-5Ab orbits a single star, KOI-5A, once every five days. This host star is caught in a mutual orbit with a nearby star called KOI-5B, and the two twirl around each other once every 30 years. A more distant star, KOI-5C orbits this pair once every 400 years.
The issue has to do with KOI-5Ab’s orbital alignment relative to KOI-5B. The two objects don’t share the same orbital plane, which is an unexpected result—one that calls conventional planetary formation theories into question, such as how such objects are believed to form from a single protostellar disk.
“We don’t know of many planets that exist in triple-star systems, and this one is extra special because its orbit is skewed,” said Ciardi. “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.”
Ciardi and his colleagues don’t know the reason for the misalignment, but their working theory is that KOI-5B exerted a gravitational shrug during the system’s development, perturbing the orbit of KOI-5Ba and causing it to migrate inward toward its host star.
Around 10% of all star systems involve three stars, according to NASA. Planets have been spotted in triple-star systems before, and also within binary star systems, but such discoveries remain rare. Multiple star systems, it would seem, don’t tend to host a lot of planets. This could mean that the conditions for the formation of planets are not ideal in these settings, but it could be the result of an observational selection effect, in that it might be tougher for astronomers to spot planets in multiple-star systems compared to single-star systems.
The answer to this question is important, as it carries serious implications in the search for extraterrestrial life. Multiple-star systems account for upwards of 85% of all star systems in the Milky Way galaxy. Should we confirm that multiple-star systems tend to feature far fewer planets, and by consequence fewer life-bearing planets, astrobiologists and SETI scientists should focus their attention on single-star systems.
This list can be whittled down further still. A whopping three-quarters of all stars in the Milky Way are red dwarfs, which, owing to their propensity to blast nearby planets with solar flares, could likewise be poor candidates in the search for alien life.
Considering these factors, it’s easy to get the feeling that life must be exceptionally rare in the galaxy. This could well be the case, but it’s important to remember that the Milky Way has around 100 billion stars. That still leaves us with a lot to choose from, a handful of which could host civilizations asking the exact same questions as these.