NASA has announced that the Kepler space probe has discovered two planetary systems that include the smallest planets yet found that lie in the "habitable zone." The systems include three super-Earth size planets, with one of them being a habitable-zone exoplanet that is the closest in size to Earth yet discovered.
Launched in 2009, the Kepler spacecraft is tasked with finding exoplanets orbiting other stars. So far, it has detected 2,740 candidates and 122 planets have been confirmed. The planets announced today at a media conference discussing the most recent findings of NASA's ongoing Kepler mission orbit the stars Kepler-62, which is 1,200 light-years from Earth, and Kepler-69, which is 2,700 light-years away. Both are in the constellation of Lyra.
Kepler-62 is a K2 dwarf star about two-thirds the size of the Sun and one-fifth as bright. It’s also seven billion years old as compared to the Sun’s relatively youthful five billion. Five planets have been found orbiting it and two, Kepler-62e and 62f, lie within the star’s habitable zone, where the surface temperature might be suitable for water to exist in liquid form. The other three are so close to the star that they orbit only every five, 12 and 18 days, so they are far too hot to sustain life.
The planets in the habitable zone of Kepler-62 include Kepler-62e, which is 60 percent larger than Earth, and Kepler-62f, which orbits Kepler-62 every 267 days and is only 40 percent larger than Earth, making it the smallest habitable-zone planet yet discovered. The Kepler spacecraft carries no spectrographic equipment and can only determine the size and orbit of a planet, but NASA believes, based on previous studies, that Kepler-62f may have a rocky rather than gaseous composition.
Kepler-69 is a smaller system with only two planets, 69b and 69c, found so far. At first glance, it seems a better candidate for life than Kepler-62 because Kepler-69 is a G-type star like the Sun, though it’s only 90 percent of the size and 80 percent of the brightness. The nearer planet, Kepler-69b, is twice the size of Earth, but is so close that it’s year is only 13 days and too hot. However, Kepler-69c is only 70 percent larger and orbits at a period of 242 days. Unfortunately, while its sun resembles ours and the planet is in the habitable zone, NASA suspects that 69c is, in fact, a super-Venus and may be as uninhabitable.
"We only know of one star that hosts a planet with life, the Sun. Finding a planet in the habitable zone around a star like our sun is a significant milestone toward finding truly Earth-like planets," said Thomas Barclay, Kepler scientist at the Bay Area Environmental Research Institute.
The Kepler spacecraft simultaneously and continuously measures the brightness of more than 150,000 stars every 30 minutes, watching to see if any planets pass in front of them. If one does, it causes the light from the star to dip momentarily. By studying how long and how large the dip is, scientists can determine how large the planet is and how far it orbits from its star. This technique first discovered planets larger than Jupiter, but smaller ones have also been detected as the technology matures.
NASA points out that the search for Earth-like planets will take some time. For Kepler to detect Earth from another system would take three transits of Earth across the Sun, which would take three years.
"The Kepler spacecraft has certainly turned out to be a rock star of science," said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. "The discovery of these rocky planets in the habitable zone brings us a bit closer to finding a place like home. It is only a matter of time before we know if the galaxy is home to a multitude of planets like Earth, or if we are a rarity."