Last week was a good one for exoplanet enthusiasts, with yet more news relating to how other worlds are now being found by the thousands, and that there may be many habitable planets out there. Now there’s already another discovery being announced of three more planets almost the same size as Earth, all orbiting a nearby star.
The star, EPIC 201367065, is a red M-dwarf star cooler than our Sun and about half the size and mass. Only 150 light-years away, it is one of the 10 closest stars known to have transiting exoplanets.
Being so relatively close to our own Solar System is a bonus for astronomers, as it makes it easier to study the atmospheres of the newly found planets. The ability to analyze exoplanet atmospheres is still in the early stages; most of the ones seen so far are thicker atmospheres around larger worlds similar to Uranus, Neptune, or bigger gas giants. Those are certainly interesting enough, but the real goal is to be able to examine the atmospheres of smaller, rocky planets like Earth. It is ones like those where astronomers are hoping to someday find signs of biological activity, especially planets orbiting in the habitable zone of their stars.
“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,” said Ian Crossfield of the University of Arizona. Cornfield is the astronomer who led the new study.
The three planets are 2.1, 1.7, and 1.5 times the size of Earth, meaning they are classified as “super-Earths,” many of which have already been discovered elsewhere. The smallest of these, and closest in size to the Earth, EPIC 201367065 d, orbits in the star’s habitable zone. It is thought to receive a similar amount of heat and light that Earth does, even though it orbits closer to the star, since the star is cooler than our Sun. It has also been added to the Habitable Exoplanets Catalog, which now currently lists 29 known potentially habitable exoplanets.
“Most planets we have found to date are scorched. This system is the closest star with lukewarm transiting planets,” said UC Berkeley graduate student 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.”
Petigura discovered the planets on Jan. 6 while analyzing data from the Kepler Space Telescope. While this third planet receives about 1.4 times the amount of light that Earth does, the other two receive about 10.5 and 3.2 times the amount. Most of the other smaller exoplanets found so far orbit close to their stars, with temperatures making them unlikely to be habitable. There are exceptions, however, and these three new planets can be added to that growing list.
While a planet actually similar to Earth has yet to be found, astronomers are getting closer all the time. The first smaller planets, close to their stars, were the easiest to detect due to having small orbits that could be observed more easily. But now, astronomers are starting to find others that orbit a bit farther from their stars, in orbits more like Earth’s.
According to University of Hawaii astronomer Andrew Howard: “We’ve learned in the past year that planets the size and temperature of Earth are common in our Milky Way galaxy. We also discovered some Earth-size planets that appear to be made of the same materials as our Earth, mostly rock and iron.”
Ground-based telescopes in Chile, Hawaii, and California were later used to characterize the star’s mass, radius, temperature, and age, including the Automated Planet Finder on Mount Hamilton near San Jose, Calif., and the Keck Telescope on Mauna Kea, Hawaii.
So what’s next? Other telescopes, including the Hubble Space Telescope, will look at the “spectroscopic fingerprint” of the molecules in the planets’ atmospheres. Hubble can determine if any of these nearly Earth-size planets have puffy, hydrogen-rich atmospheres like Earth does.
The discovery is also remarkable since Kepler is in its K2 extended mission phase, a fix for the hobbled telescope after two of its reaction wheels, needed to keep it stable, failed. So far Kepler has been performing very well and continues to return large amounts of data to scientists back on Earth. Plus the fact that there is still a lot of the original Kepler data for scientists to go through, promising even more exciting discoveries yet to come. And Kepler’s findings are only the tip of the iceberg; only a small fraction of planetary systems can be detected by Kepler, the ones in which the orbital planes of the planets are edge-on as seen from Earth. Extrapolations from the Kepler data and discoveries from other telescopes indicate that our galaxy alone is brimming with planets, likely numbering in the billions (Carl Sagan would be proud).
As Petigura noted: “I was devastated when Kepler was crippled by a hardware failure. It’s a testament to the ingenuity of NASA engineers and scientists that Kepler can still do great science.”
The new paper, “A Nearby M Star with Three Transiting Super-Earths Discovered by K2,” has been submitted to The Astrophysical Journal; the abstract and paper (free) are available here. See also this related press release from the University of Hawaii.