Note: (9/4/2014) Some details in this article have been clarified. Sincere apologies for any confusion the errors may have caused. -LW
Astronomers using NASA's Kepler Space Telescope have identified a transiting exoplanet that completes an orbit in 704 days, making it the longest known year. The research was lead by David Kipping of the Harvard-Smithsonian Center for Astrophysics and has been accepted for publication in The Astrophysical Journal.
Kepler-421b is about 1,000 light-years away in the constellation Lyra. It is about the same size as Uranus and exists over 177 million km away from its orange K-type parent star, taking 704 days to complete a single orbit. Of course, 704 days doesn’t sound like much when compared to our own solar system. Mars circles the Sun every 780 days and Neptune takes a staggering 164.8 years to complete an orbit. However, it is important to keep in mind that spotting exoplanets is a fairly difficult process.
Exoplanets are typically identified when they transit their parent star, blocking out some of the light. For this reason, most exoplanets that have been spotted are very large and very close to the star. This proximity to the star results in a fairly short orbit, such as Kepler-70b which takes only 5.8 hours.
After observing the same portion of sky for 4 long years, Kepler-421b was only spotted twice. Observing exoplanets that are more distant from the star makes it much more difficult and require longer observations than closer planets that transit every few days. While NASA’s TESS is geared up to replace Kepler and identify exoplanet candidates, it won’t be looking at one spot long enough to detect these longer orbits.
"Finding Kepler-421b was a stroke of luck," Kipping explained in a press release. "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.”
Gas giants are assumed to have formed beyond the system’s frost line. Planets formed on the side of the line nearest to the star are rocky, while those that are more distant are gas. As the majority of the candidate exoplanets are gaseous with short orbits, the planets might have migrated inward at some point. Kepler-421b still resides past the frost line, indicating that not all transiting gas giants have to make this journey.
"The snow line is a crucial distance in planet formation theory. We think all gas giants must have formed beyond this distance," explains Kipping. "This is the first example of a potentially non-migrating gas giant in a transiting system that we've found.”
It was announced back in May that astronomers had discovered an exoplanet that takes 80,000 years to orbit its star. However, that planet was imaged directly and transiting identification techniques were not used.