Extrasolar planets are being discovered by the hundreds, but are any of these newfound worlds really like Earth? A planetary system recently discovered by the Kepler spacecraft will help resolve this question.
The system of three planets, each just larger than Earth, orbits a nearby star called EPIC 201367065. The three planets are 1.5-2 times the size of Earth, and the outermost planet orbits on the edge of the so-called “habitable zone,” where the temperature may be just right for liquid water, believed necessary to support life, on the planet’s surface.
“We’ve learned in the past year that planets the size and temperature of Earth are common in our Milky Way galaxy,” explains University of Hawaii astronomer Andrew Howard. “We also discovered some Earth-size planets that appear to be made of the same materials as our Earth, mostly rock and iron.”
The compositions of these newfound planets are unknown. “There is a very real possibility that the outer planet is rocky like Earth,” noted Erik Petigura, a University of California, Berkeley graduate student who spent a year visiting the UH Institute for Astronomy. “If so, this planet could have the right temperature to support liquid water oceans.”
In addition to Howard and Petigura, UH graduate students Benjamin Fulton and Kimberly Aller, and UH astronomer Michael Liu are among the two dozen scientists who contributed to the study. The confirmed planets were by the NASA Infrared Telescope Facility (IRTF) and Keck Observatory in Hawaii as well as telescopes in California and Chile.
The new discovery paves the way for studies of the atmosphere of a warm planet nearly the size of Earth. The three new planets are particularly favorable for atmospheric studies because they orbit a nearby, bright star. Next, the team of astronomers that made the discovery hopes to observe the planets with the Hubble Space Telescope and other observatories to determine what elements are in the planets’ atmospheres. If Hubble finds that these warm, nearly Earth-size planets have thick, hydrogen-rich atmospheres, they will learn that there is not much chance for life.
“A thin atmosphere made of nitrogen and oxygen has allowed life to thrive on Earth. But nature is full of surprises. Many extrasolar planets discovered by the Kepler Mission are enveloped by thick, hydrogen-rich atmospheres that are probably incompatible with life as we know it,” says Ian Crossfield, the University of Arizona astronomer who led this study.
The discovery is all the more remarkable because Kepler is now hobbled by the loss of two reaction wheels that kept it pointing at a fixed spot in space. Kepler, launched in 2009, was reborn in 2014 as “K2” with a clever strategy of pointing the telescope in the plane of the Earth’s orbit to stabilize the spacecraft. Kepler is back to mining the cosmos for planets by searching for eclipses, or transits, as planets orbit in front of their host stars and periodically block some of the starlight.
“I was devastated when Kepler was crippled by a hardware failure,” Petigura added. “It’s a testament to the ingenuity of NASA engineers and scientists that Kepler can still do great science.” Kepler sees only a small fraction of the planetary systems in its gaze, those with orbital planes aligned edge-on to our view from Earth. Planets with large orbital tilts are simply missed by Kepler.
“It’s remarkable that the Kepler telescope is now pointed in the ecliptic, the plane that Earth sweeps out as it orbits the Sun,” Fulton explains. “This means that some of the planets discovered by K2 will have orbits lined up with Earth’s, a celestial coincidence that allows Kepler to see the alien planets, and Kepler-like telescopes in those very planetary systems (if there are any) to discover Earth.”
“Here’s looking at you, looking at me,” muses Howard.
The paper presenting this work, “A Nearby M Star with Three Transiting Super-Earths Discovered by K2,” has been submitted to the Astrophysical Journal and is available for free at http://arxiv.org/abs/1501.03798.