Thursday, astronomers announced that they had found the most Earth-like planets they had ever seen in the far reaches of the universe. They are a pair of planets that seem to be able to support life and that orbit a star 1,200 light-years away in the northern constellation Lyra.
They are the two most distant of five planets that orbit a yellowish star that is slightly smaller and dimmer than our Sun. This star was previously unknown, but now it will be called Kepler 62 after the NASA spacecraft that found it. These planets are about twice as big as Earth and are probably made of rocks. They may have oceans and humid, cloudy skies, but that is at best a guess.
A picture made by an artist of a sunrise on Kepler 62f. The two outer planets of the Kepler 62 system may be in the habitable zone, where liquid water could be on the surface. Credit… The American Association for the Advancement of Science
Nobody will probably ever know if anything lives on these planets, and the chances are that humans will only go there in their faster-than-light dreams, but the news has astronomers singing praises to the heavens. William Borucki, who runs the Kepler project at NASA’s Ames Research Center, said that one of the new worlds is the best place to look for life that Kepler has found in its four-year search for other Earths. He bought pizza and beer for his team to celebrate the find on his own dime (this being the age of sequestration). He said, “It’s a big deal.”
The two planets circle their star at 37 million and 65 million miles apart, which is about how far Mercury and Venus are from each other in our solar system. Most importantly, their orbits put them both in the “Goldilocks zone” of just-right temperatures for liquid water, which is essential for life as we know it.
David Charbonneau, an exoplanet hunter from the Harvard-Smithsonian Center for Astrophysics and a member of the Kepler team, said that previous claims of Goldilocks planets with “just so” orbits that were close to red dwarf stars that were much cooler and dimmer than the Sun were uncertain about their size, mass, and even existence.
Dr. Charbonneau said of the farthest planet, Kepler 62f, “This is the first planet that checks both boxes.” “It’s just the right size and temperature.” With a 267-day year, Kepler 62f is 40 percent bigger than Earth and right in the middle of the habitable zone. Mr. Borucki said in an interview that it was the best planet that Kepler had found.
Its partner, Kepler 62e, is about 60 percent bigger than Earth and has an orbit that takes 122 days. This puts it on the edge of the Goldilocks zone. Astronomers said that it is warmer, but probably could be lived on.
The Kepler 62 system is similar to our own solar system, which also has two planets in the habitable zone: Earth and Mars, which once had water and would still be habitable today if it were bigger and had kept its atmosphere.
The Kepler 62 planets are the latest in a series of discoveries made by astronomers in the last 20 years. The first planets found around other stars, called exoplanets, were boiling balls of gas bigger than Jupiter. Since then, astronomers have been able to find smaller and more moderate planets, like iceballs like Neptune and now bodies only a few times the mass of Earth, called super-Earths. Size is important when it comes to planets because we can’t live on a world like Jupiter, where gas clouds are so heavy that they would crush us. For life as we know it to exist, there must be solid ground and liquid water. In other words, there must be a gentle environment on Earth.
The new worlds found by Kepler 62 aren’t quite the same size as Earth, but the results have made astronomers even more sure that there are billions of Earth-sized planets in the galaxy, maybe even one for every star, and that they will soon find Earth 2.0, our lost twin planet bathing in the light of an alien sun.
Geoffrey Marcy, a Kepler team member and longtime exoplanet hunter at the University of California, Berkeley, said that Kepler and other experiments are finding planets that look more and more like Earth. “This is a very exciting time in science. We haven’t found Earth 2.0 yet, but we can taste and smell it right now, thanks to technology.
A group of 60 authors led by Mr. Borucki announced the discovery of the Kepler 62 planets on Thursday at a news conference in Ames and in an online article in the journal Science.
As if that wasn’t enough, a group led by Thomas Barclay of Ames and the Bay Area Environmental Research Institute also found a planet that is 1.7 times the size of Earth. It is in the Goldilocks zone of Kepler 69, a star that is almost the same as the Sun and is 2,700 light-years away. Dr. Barclay said at the news conference that the planet might be a “Super-Venus.” The paper that the group wrote came out in The Astrophysical Journal on Thursday.
In another paper sent to The Astrophysical Journal, a group led by Lisa Kaltenegger of the Harvard-Smithsonian Center for Astrophysics and the Max Planck Institute for Astronomy in Heidelberg, Germany, tried for the first time to model conditions on the Kepler 62 planets. This is hard to do because the system is too far away for astronomers to measure the planets’ masses, which would let them figure out their densities and make-ups, or use telescopes to look at and study their atmospheres.
Dr. Kaltenegger and her colleagues figured out that both of them were probably ocean worlds with humid, cloudy skies by comparing them to Earth. She said that if they had life, it would probably be aquatic, but that “it could be even cooler life than what we have here. When we look at the oceans, we see a lot of interesting life.
Dr. Kaltenegger said she thought of the pair as a kind of Darwinian test tube and wondered in an e-mail if life would evolve on both worlds and, if so, “Would life evolve ‘the same’ way or would there be very different life?”
“This is huge for the overall life-elsewhere question,” said Sara Seager, a planetary expert at the Massachusetts Institute of Technology who did not work on the project.
Alan Boss, an expert on planets at the Carnegie Institution of Washington and a member of the Kepler team, said that the new results are the mission’s crowning achievement. “I would argue,” he wrote in an e-mail, “that the cost of this mission was worth it if this is all we learned from Kepler.”
Kepler, which was launched in March 2009, looks at 150,000 stars in a patch of the Milky Way sky to find planets. It does this by watching how bright the stars are and looking for blips that happen when planets pass in front of their home stars. The spacecraft has found 115 planets so far, and it has a list of 2,740 other possible planets. (Astronomers around the world have found almost 1,000 exoplanets.)
But Kepler is just now getting into its prime because its mission was extended by four years last spring. At least three blips are needed to count a planet, so planets like Earth, which take a year to go around their star, are just now starting to show up in the Kepler data. In fact, each of the new Kepler 62 planets only had three “transits,” which is another name for a move.
But Dr. Seager and others warned that there is a catch. Because the Kepler stars are all hundreds or thousands of light-years away and the planets we want to find are so small, astronomers will never know for sure what each planet is made of or if anything can or does live there.
Natalie Batalha, a Kepler scientist from San Jose State University, said that astronomers figured out what Kepler 62 was made of by comparing it to three other objects that were the same size and were made of rocks.
In an email, Dr. Batalha called it “mass by association.”
Which is fine if all you want to know is how big the universe is. Dr. Seager said, “Kepler wasn’t made to tell us where to live; it was only meant to tell us how common planets like Earth are.”