A new planet roughly twice the size of Earth has been discovered located within the “habitable zone”—the range of distances from a star where liquid water may exist on the planet’s surface. A research team that included a UChicago graduate student confirmed the finding after volunteer citizens flagged a crucial piece of evidence in data from NASA’s Kepler spacecraft.
The new world, known as K2-288Bb and located in the Taurus constellation, could either be rocky or a gas-rich planet similar to Neptune in our own solar system. Within the system, there are two stars; K2-288Bb orbits the smaller star.
“It’s a very exciting discovery due to how it was found; its equilibrium temperature, which likely is similar to Earth’s; and because planets of this size seem to be relatively uncommon. We probably only know about a handful of planets this size,” said UChicago graduate student Adina Feinstein, lead author of a paper describing the new planet and published in The Astronomical Journal.
Feinstein is part of a group of UChicago scientists shaping the emerging field of exoplanet research—searching for planets beyond our own solar system. She and colleagues announced the discovery Jan. 7 at the 233rd meeting of the American Astronomical Society in Seattle.
Located 226 light-years away, the new planet lies in a stellar system known as K2-288, which contains a pair of dim, cool M-type stars separated by about 5.1 billion miles, or about six times the distance between Saturn and the sun. The brighter star has around half the mass and size of the sun, while its companion is about one-third the sun’s mass. One year on K2-288Bb is approximately 31.3 days.
In 2017, Feinstein and Makennah Bristow, an undergraduate student at the University of North Carolina ,Asheville, worked with Joshua Schlieder, an astrophysicist at NASA’s Goddard Space Flight Center. They searched Kepler data for evidence of transits—the regular dimming of a star when an orbiting planet moves across the star’s face.
While examining data, the team noticed two likely planetary transits in the system. But scientists require at least three transits before claiming the discovery of a candidate planet, and there wasn’t a third signal in the observations they reviewed.
As it turned out, though, the team wasn’t actually analyzing all of the data.
The data they were working from had been processed to remove the first couple of days of observations as the Kepler telescope repositioned itself to look at a different part of the sky every three months. The missing data in the first few days of the observing run was hiding the third transit.
The full data had, however, been posted directly to Exoplanet Explorers, a project in which the public searches Kepler’s K2 observations to locate new transiting planets. In May 2017, volunteers noticed the third transit and began an excited discussion through the program, which caught the attention of Feinstein and her colleagues.
“That’s how we missed it and it took the keen eyes of citizen scientists to make this extremely valuable find and point us to it,” Feinstein said.
“Vetting transits with the human eye is crucial because noise and other astrophysical events can mimic transits,” Schlieder explained.
The team began follow-up observations using NASA’s Spitzer Space Telescope, the Keck II telescope at the W. M. Keck Observatory and NASA’s Infrared Telescope Facility, both in Hawaii, and also examined data from the European Space Agency’s Gaia mission.
Estimated at about 1.9 times Earth’s size, K2-288Bb is half the size of Neptune. This places it within a recently recognized category called the radius gap. Among planets that orbit close to their stars, there’s a curious dearth of worlds between about 1.5 and two times Earth’s size. Scientists think this is due to intense starlight eroding away the atmospheres of some planets over time. Since K2-288Bb’s radius places it in this gap, it may provide a case study for how planets evolve within this size range.
On Oct. 30, Kepler was shut down after nine years, during which it discovered 2,600 confirmed planets around other stars. And while NASA’s Transiting Exoplanet Survey Satellite is the newest space-based planet hunter, this finding shows that more discoveries await scientists in Kepler data.
Citation: “K2-288Bb: A Small Temperate Planet in a Low-mass Binary System Discovered by Citizen Scientists.” Feinstein et al, The Astronomical Journal, Jan. 7, 2019. DOI: 10.3847/1538-3881/aafa70
Funding: NASA, National Science Foundation, North Carolina Space Grant Consortium.
—Adapted from an article by Francis Reddy