Following its launch in March 2009, the unblinking eye of NASA’s powerful Kepler telescope spent more than four years patiently staring at the same corner of the Milky Way in search of exoplanets—planets that orbit stars outside our solar system. Relying on the “transit method,” by which the presence of alien worlds can be detected as the brightness of parent stars dip slightly as orbiting bodies pass in front, astronomers have so far used the Kepler telescope to discover nearly 1,000 exoplanets with more than 3,000 planetary candidates requiring further study.
None of the exoplanets identified by Kepler, however, have had such a lengthy orbit as the newly identified Kepler-421b, located 1,000 light-years from Earth in the direction of the constellation Lyra. While most exoplanets discovered by the transit method have short orbiting periods of weeks or even days, Kepler-421b circles its parent star every 704 Earth days, comparable to the 687-day orbit of Mars around the Sun. (Using non-transiting methods, astronomers have identified exoplanets with far longer orbits, such as GU Piscium b, which has a transit period lasting 160,000 years.) Based on the quantity of light blocked by the exoplanet as it passes in front of its parent star, astronomers estimate that Kepler-421b is nearly equivalent in size as Uranus, which would make it four times the size of Earth.
“Finding Kepler-421b was a stroke of luck,” says astronomer David Kipping of the Harvard-Smithsonian Center for Astrophysics, lead author of an article announcing the exoplanet’s discovery that will be published in an upcoming edition of the Astrophysical Journal. “The farther a planet is from its star, the less likely it is to transit the star from Earth’s point of view. It has to line up just right.”
Kepler-421b circles an orange star that is cooler and dimmer than our Sun at a distance of 110 million miles, which is between the average distance of Earth (93 million miles) and Mars (142 million miles) from the Sun. Since the newly discovered exoplanet revolves around a star, called Kepler-421, that is less powerful and a farther distance away, it receives only about one-quarter the light as Earth does, which makes it a frigid place with an estimated temperature of 135 degrees below zero Fahrenheit.
The discovery is particularly noteworthy for astronomers because it makes Kepler-421b the first transiting exoplanet to be found beyond what astronomers call the “snow line”—the distance from a star where orbiting worlds are divided into rocky and gaseous planets. (In our solar system, the “snow line” divides planets into the rocky, interior ones—Mercury, Venus, Earth and Mars—and the outer gaseous giants—Jupiter, Saturn, Uranus and Neptune.)
“The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance,” Kipping explains. Beyond the snow line, water could condense into ice grains that bind together to form embryonic bodies around which enormous gaseous planets could form. On the other side of the snow line, a star’s heat would have boiled off ice grains.
Since astronomers have discovered giant gas planets extremely close to their host stars, they theorize that many exoplanets migrated inward early in their histories. In the case of Kepler-421b, though, astronomers believe it could have formed at the orbit where it has now been located. “This is the first example of a potentially non-migrating gas giant in a transiting system that we’ve found,” Kipping says.
Kepler’s original mission of searching solely for exoplanets ended in May 2013 when its precision pointing system broke. Although the hobbled telescope can no longer fixate on a single patch of the heavens for years on end, NASA plans to repurpose it for its new K2 mission to hunt for supernovae, galaxy clusters and other cosmic bodies in addition to planetary bodies outside our solar system.