July 22, 2014

Transiting Exoplanet With Longest Known Year Discovered By Kepler Mission

April Flowers for - Your Universe Online

The time that it takes for a planet, or planetary satellite, to orbit its star is considered the planet's year. For example, Earth's year is approximately 365 days, while Mercury's is 88 days and Mar's year is 687 days long.

A group of astronomers working at the Harvard-Smithsonian Center for Astrophysics (CfA) has discovered a transiting exoplanet, named Kepler 421b, with the longest known year of any of the 1,800 exoplanets discovered so far. Kepler-421b's year is 704 days long; most other exoplanets are much closer to their stars and have much shorter years.

"Finding Kepler-421b was a stroke of luck," said David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA) in a recent statement. "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."

The orange, type K star that Kepler-421b orbits is cooler than our own Sun, as well as being dimmer. Kepler-421b, about the size of Uranus, is approximately 110 million miles from its star, making this planet a frosty -135 degrees Fahrenheit. Kepler-421, the host star, is approximately 1,000 light-years from Earth near the constellation Lyra.

NASA's Kepler telescope, designed to survey our region of the Milky Way galaxy in the hopes of finding Earth-sized planets in the habitable zone, discovered Kepler-421b by observing the same quadrant of space for four years. Kepler, the only mission with such long-term, dedicated focus, searched this region by watching for stars that dim as planets pass between the star and the spacecraft. Kepler-421b only made two transits during this time period because of its very long orbital period.

The planet's extra-long orbit places it beyond the "snow line." The snow line is a dividing line between rocky and gas planets. Planets found outside this line have water that condenses into ice grains. These grains stick together to build gas giant planets.

"The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance," explained Kipping.

Some scientists believe that many exoplanets migrate inward towards their stars because gas giant planets can be found in extremely close orbits that last days or even hours. Kepler-421b is evidence against this migration theory as it could have formed exactly where we see it now.

"This is the first example of a potentially non-migrating gas giant in a transiting system that we've found," adds Kipping.

The Harvard-Smithsonian Center for Astrophysics, the world's largest and most diverse center for the study of the Universe, is a collaboration between the Harvard College Observatory and the Smithsonian Astrophysical Observatory. Over 900 scientists and staff, organized into six research divisions, work to further our understanding of the origin, evolution and ultimate fate of the Universe at the Cambridge, Mass. facility.

The results of this study have been accepted for publication in The Astrophysical Journal.