The first planet Kepler spotted has finally been confirmed 10 years later

Astronomers had dismissed the exoplanet candidate as a false alarm

Kepler 1658b

THE ONCE AND FUTURE PLANET  The Jupiter-sized planet Kepler 1658b, which may resemble this artist’s illustration, had been discarded as a potential exoplanet. But after measuring sound waves bouncing around its host star (orange lines), astronomers say the world is real.

Gabriel Pérez Díaz/Instituto de Astrofísica de Canarias

A decade after being found, the first exoplanet candidate spotted by the Kepler space telescope has been confirmed as a real world.

The planet orbits a star initially dubbed KOI 4, for Kepler Object of Interest 4 (KOIs 1 through 3 were known before Kepler launched in March 2009). When the planet passed in front of the star, it blocked a bit of starlight from reaching Kepler in Earth’s orbit, alerting astronomers to the planet’s existence.

The Kepler team originally thought the star was about 1.1 times the width of the sun, which would make the planet about the size of Neptune. But then astronomers saw a second dip in starlight as the world passed behind the star, called a secondary eclipse. That second dip shouldn’t be visible for such a small planet, so the exoplanet candidate was dismissed as a false alarm.

But “there’s still a treasure trove left to be found in the Kepler data,” says astronomer Ashley Chontos, who started to dig into unconfirmed Kepler candidates in 2016 as a first-year graduate student at the University of Hawaii in Honolulu. She and her colleagues ended up revising KOI 4’s size by measuring the way sound waves ripple through the star’s interior, a technique called asteroseismology.

KOI 4 is actually about three times the width of the sun, meaning its planet would be about three times as large as first estimated — or a bit larger than Jupiter, the team found. That’s big enough for the world to block enough starlight that Kepler would notice that secondary eclipse, Chontos said March 5 in a presentation at NASA’s Kepler & K2 science conference in Glendale, Calif.

After the findings were double checked by astronomers using ground-based telescopes, the star was renamed Kepler 1658, and the planet Kepler 1658b, adding the system to Kepler’s long list of discoveries. “It feels really cool to have that first planet there,” Chontos said.

Still, the addition may be bittersweet: Kepler ran out of gas and ended its mission in November, after detecting thousands of exoplanets (SN Online: 10/30/18). 

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.

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