Score another one for Kepler: the repurposed equipment has observed three planets transiting a nearby star. The most distant of the planets resides in the star’s habitable zone, where liquid water—and potentially life—could exist. Ian Crossfield from the University of Arizona is lead author of the paper, which has been submitted to The Astrophysical Journal, yet can be read on arXiv.
The planets in question orbit EPIC 201367065, a cool M-dwarf star 150 light-years away. This star is near enough for scientists to observe the planets’ transit. This can provide information about their respective atmospheric compositions, which will indicate if the planet is capable of sustaining life.
"A thin atmosphere made of nitrogen and oxygen has allowed life to thrive on Earth. But nature is full of surprises. Many exoplanets discovered by the Kepler mission are enveloped by thick, hydrogen-rich atmospheres that are probably incompatible with life as we know it," Crossfield said in a press release.
The exoplanets are all slightly larger than Earth, and receive more sunlight. The one in the habitable zone is most similar to our planet, as it has a radius 1.5 times larger than Earth and receives 1.4 times the light. The next planet nearest to the star is 1.7 times larger than Earth, with light 3.2 times more intense. The innermost planet has a radius 2.1 times larger than ours, and the intensity of light hitting it is 10.5 times stronger.
"Most planets we have found to date are scorched. This system is the closest star with lukewarm transiting planets,” added co-author Erik Petigura. "There is a very real possibility that the outermost planet is rocky like Earth, which means this planet could have the right temperature to support liquid water oceans.”
To date, there have been over 1,900 confirmed exoplanets, 29 of which are potentially habitable. The holy grail of exoplanet hunting would be to find an Earth twin, which is approximately the same size, rocky, and capable of sustaining liquid water on its surface. While finding the planet is one thing, determining its composition and true chances of habitability are another. Moving forward, the planets will be studied in greater detail using the Hubble Space Telescope.
"We’ve learned in the past year that planets the size and temperature of Earth are common in our Milky Way galaxy," co-author Andrew Howard of the University of Hawaii added. "We also discovered some Earth-size planets that appear to be made of the same materials as our Earth, mostly rock and iron."
The discovery was made using K2, which is the repurposed survey using the Kepler Space Telescope. Kepler has detected over 1,000 exoplanets during its mission. In November 2013, it suffered the loss of a second reaction wheel, compromising its precision. After several months of testing, K2 proved to be functional in searching for supernovae, forming stars, asteroids, and comets, while still detecting exoplanets. However, the K2 survey is only one-fifteenth as precise as Kepler used to be.
"This discovery proves that K2, despite being somewhat compromised, can still find exciting and scientifically compelling planets," Petigura continued. "This ingenious new use of Kepler is a testament to the ingenuity of the scientists and engineers at NASA. This discovery shows that Kepler can still do great science.”