Astronomers have confirmed a remarkable exoplanet in a triple-star system located 1,800 light-years from Earth, distinguished by its unusual orbital alignment.
The discovery traces back to 2009, soon after the Kepler space telescope's launch. Now retired, Kepler detected a candidate planet roughly half the size of Saturn in a multi-star system—dubbed KOI-5Ab. It was only the second such candidate identified by the mission. NASA set it aside amid a flood of exoplanet data; Kepler ultimately flagged 4,760 candidates, about half of which await confirmation.
KOI-5Ab faded into the background until recently, when David Ciardi and his team at NASA's Exoplanet Science Institute revisited it using the TESS satellite and Keck Observatory in Hawaii.
Employing transit photometry from TESS and radial velocity measurements from Keck, the team verified the planet and its three host stars. Transit method detects a star's brief dimming as a planet passes in front; radial velocity spots stellar wobbles via spectral shifts caused by planetary gravity.
KOI-5Ab, a Neptune-sized gas giant, orbits just one star, KOI-5A, every five days. KOI-5A and KOI-5B mutually orbit every 30 years, while distant KOI-5C circles the pair every 400 years.
Triple-star systems hosting planets are scarce, but this one's misaligned orbits—KOI-5Ab and KOI-5B don't share the same plane—defy standard planetary formation models.
"We don't know of many planets in three-star systems, and this one is especially intriguing because its orbit is skewed," notes David Ciardi. "We have many questions about planet formation in multi-star systems and how they compare to single-star worlds."
The team plans deeper observations to unravel how such systems forge planets.
