Scientists have discovered a new star with three nearly Earth-sized planets—one of which may have temperatures moderate enough for liquid water—and maybe even life—to exist.
NASA’s Kepler Space Telescope was able to find the star, EPIC 201367065—a cool red M-dwarf about half the size and mass of our own sun, despite being hobbled by the loss of critical guidance systems, .
At a distance of 150 light-years, it ranks among the top 10 nearest stars known to have transiting planets. The star’s proximity means it is bright enough for astronomers to study the planets’ atmospheres, to determine whether they are like Earth’s—and possibly conducive to life.
“A thin atmosphere made of nitrogen and oxygen has allowed life to thrive on Earth. But nature is full of surprises,” says Ian Crossfield, an astronomer at University of Arizona who led the study. “Many exoplanets discovered by the Kepler mission are enveloped by thick, hydrogen-rich atmospheres that are probably incompatible with life as we know it.”
Rocky like Earth
The three planets are 2.1, 1.7, and 1.5 times the size of Earth. The smallest and outermost planet, at 1.5 Earth radii, orbits far enough from its host star that it receives levels of light from its star similar to those received by Earth from the sun, says Erik Petigura, a graduate student at University of California, Berkeley.
Petigura discovered the planets January 6 while conducting a computer analysis of the Kepler data NASA has made available to astronomers. In order from farthest to closest to their star, the three planets receive 10.5, 3.2, and 1.4 times the light intensity of Earth.
“Most planets we have found to date are scorched. This system is the closest star with lukewarm transiting planets,” Petigura says. “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.”
Extrasolar planets are discovered by the hundreds these days, although many astronomers are left wondering if any of the newfound worlds are really like Earth, says Andrew Howard, an astronomer at University of Hawaii. The newly discovered planetary system will help resolve this question.
“We’ve learned in the past year that planets the size and temperature of Earth are common in our Milky Way galaxy,” he says. “We also discovered some Earth-size planets that appear to be made of the same materials as our Earth, mostly rock and iron.”
After Petigura found the planets in the Kepler light curves, researchers quickly employed telescopes in Chile, Hawaii, and California to characterize the star’s mass, radius, temperature, and age.
Two of the telescopes involved—the Automated Planet Finder on Mount Hamilton near San Jose, California, and the Keck Telescope on Mauna Kea, Hawaii—are University of California facilities.
The next step will be observations with other telescopes, including the Hubble Space Telescope, to take the spectroscopic fingerprint of the molecules in the planetary atmospheres. If these warm, nearly Earth-size planets have puffy, hydrogen-rich atmospheres, Hubble will see the telltale signal, Petigura says.
The discovery, submitted to Astrophysical Journal, is all the more remarkable, he says, because the Kepler telescope lost two reaction wheels that kept it pointing at a fixed spot in space.
Kepler was reborn in 2014 as “K2” with a clever strategy of pointing the telescope in the plane of Earth’s orbit, the ecliptic, to stabilize the spacecraft. Kepler is now back to mining the cosmos for planets by searching for eclipses or “transits,” as planets pass in front of their host stars and periodically block some of the starlight.
“This discovery proves that K2, despite being somewhat compromised, can still find exciting and scientifically compelling planets,” Petigura says. “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.”
Kepler sees only a small fraction of the planetary systems in its gaze: only those with orbital planes aligned edge-on to our view from Earth. Planets with large orbital tilts are missed by Kepler. A census of Kepler planets the team conducted in 2013 corrected statistically for these random orbital orientations and concluded that one in five sunlike stars in the Milky Way galaxy has Earth-size planets in the habitable zone. Accounting for other types of stars as well, there may be 40 billion such planets galaxy-wide.
The original Kepler mission found thousands of small planets, but most of them were too faint and far away to assess their density and composition and thus determine whether they were high-density, rocky planets like Earth or puffy, low-density planets like Uranus and Neptune.
Because the star EPIC-201 is nearby, these mass measurements are possible. The host star, an M-dwarf, is less intrinsically bright than the sun, which means that its planets can reside close to the host-star and still enjoy lukewarm temperatures.
The system most like that of EPIC-201 is Kepler-138, an M-dwarf star with three planets of similar size, though none are in the habitable zone, Howard says.
NASA and the National Science Foundation funded the research.
Source: University of Arizona