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That's No Exomoon, It's an Evaporating Planet

ByAllison EckNOVA NextNOVA Next
While hunting for exomoons, astronomers found an extremely light, Neptune-like exoplanet.

Failed experiments sometimes render unexpected results , and new ideas are born from seemingly banal data.

Even major undertakings, like NASA’s Kepler mission, propel scientists in unintended directions. In December, the Hunt for Exomoons with Kepler (HEK) project —which inspects Kepler’s data in search of distant moons—inadvertently discovered the lightest exoplanet yet.

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The Kepler spacecraft pinpoints exoplanets by looking for transits—the period during which an exoplanet appears to move across the face of its host star, dimming the star’s light. When an exoplanet’s moons are aligned in front of or behind the planet (relative to the host star), the transit looks the same. But when these exomoons are either ahead of or behind the planet’s orbit, they block more light from the sun and exert more gravitational pull on the planet, altering the transit’s duration.

In a new paper, HEK astronomers explain that they found abnormal gravitational tugging in two of the eight systems tested, leading them to believe that there were exomoons in those systems. Unfortunately, their data wasn’t convincing enough, since each of the two systems had two planets in them, raising the possibility that other gravitational interactions caused this effect.

In doing all this work, though, the team wound up calculating the weight of these four exoplanets. One, KOI 314c, was startlingly light.

John Timmer, writing for Ars Technica:

[…] KOI 314c was only slightly more dense than liquid water: 1.31 grams per cubic centimeter. For context, the Earth’s density is 5.52 grams per cubic centimeter. Combined with its relatively small size, that makes KOI 314c the lightest planet we’ve ever discovered.

Although it’s technically possible that the planet is little more than a giant ball of water, the HEK team favors an alternate explanation: it’s an evaporating Neptune. The planet’s density suggests that it has an atmosphere that’s mostly hydrogen and helium, but a very thick one: approximately 17 percent of the planet’s radius. However, the object’s gravity would probably have trouble hanging on to that gas given how close it orbits to its host star every 23 days. The researchers argue that it started its existence as something closer to Neptune, but it has bled off some of its atmosphere since.

Even though the finding calls into question the basic definition of a planet, which is itself nebulous, it underscores the importance of the Kepler mission in uncovering new insights about our universe.

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