Zooniverse citizen scientists find star system with five planets

First multi-planet system to be discovered by citizen scientists has all planets in unusual resonant orbits.
By Laurel Kornfeld | Jan 16, 2018
Citizen scientists processing data from NASA's K2 extended exoplanet-hunting mission discovered a star system with five orbiting planets and a possible sixth world.

Working with Zooniverse's Exoplanet Explorers program, volunteers sift through data collected by K2 searching for signals that indicate a planet transiting in front of its parent star.

Each potential transit signal is reviewed by a minimum of 10 people and requires at least 90 percent yes votes in order to be confirmed.

Now in operation for three years, K2 has observed 287,309 stars. Every few months the observatory adds tens of thousands more to its inventory.

Volunteers go through K2 data on stars identified by computer programs as showing the periodic dimming characteristic of a transiting planet.

"People anywhere can log on and learn what real signals from exoplanets look like, and then look through actual data collected from the Kepler telescope to vote on whether or not to classify a given signal as a transit, or just noise," said program staff scientist Jessie Christiansen of Caltech.

The K2 dataset, referred to as C12, had not been reviewed by professional astronomers when it was assigned to the Exoplanet Explorers program, which was started in April 2017.

When Christiansen and Ian Crossfield of UC Santa Cruz, who together developed the program, reviewed the two million classifications made by volunteers in just 48 hours after after the project was portrayed on the Australian TV show Stargazing Live, they identified 44 Jupiter-sized planets; 72 Neptune-sized planets; 53 super-Earths, whose sizes range from larger than Earth to smaller than Neptune, and 44 Earth-sized planets.

The project scientists decided to have volunteers focus on potential multi-planet systems, which show multiple transits and are therefore less likely to produce false positives.

That effort almost immediately led to the discovery of a system named K2-138, the first ever multi-planet system found solely by volunteers.

Initially thought to host four planets, the system was found to have a fifth. Significantly, all the planets orbit in a pattern known as a resonance, a mathematical relationship where each planet is in an orbit that takes it 50 percent longer to orbit the star than the one next planet inward.

To date, no other known multi-planet system displays a full chain of unbroken resonances. Such a system likely formed in a calmer, less chaotic process than other planetary systems that do not have such an even configuration.

"The clockwork like orbital architecture of this planetary system is keenly reminiscent of the Galilean satellites of Jupiter," said astronomer and Caltech Van Nuys Page Scholar Konstantin Batygin, who did not take part in the study.

"Orbital commensurabilities among planets are fundamentally fragile, so the present-day configuration of the K2-138 planets clearly points to a rather gentle and laminar formation environment of these distant worlds."

A paper on the findings has been published in The Astronomical Journal.

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