A year on the newly discovered Uranus-sized exoplanet Kepler-421b lasts for 704.2 days, making it the longest orbital period exoplanet yet found. Its parent star, Kepler-421, is a G9/K0 dwarf star located in the constellation Lyra, about 1,000 light-years from Earth.
This artist’s conception shows Kepler-421b. Image credit: David A. Aguilar / CfA.
Kepler-421b orbits the star at a distance of about 177 million km. As a result, the exoplanet is chilled to a temperature of minus 93 degees Celsius.
Dr David Kipping of the Harvard-Smithsonian Center for Astrophysics and his colleagues discovered the exoplanet by detecting the decrease in brightness of Kepler-421 as the exoplanet passed in front – a transit event.
They detected only two transits of Kepler-421b due to that world’s extremely long orbital period.
The planet’s long year makes it more than merely a new record-holder, it makes it the first transiting planet discovered near the so-called frost-line – the dividing line between rocky and gas planets.
“When our Solar System was first forming, it was at this special distance that the temperature was cold enough for ice grains to form,” explained Dr Kipping, who is the lead author of a paper accepted for publication in the Astrophysical Journal (full text in .pdf).
“Any closer to the Sun and these grains would be boiled off. These ice grains started to stick together and form planetary embryos which then went on to form the gas giant planets. It is for this very reason that the gas giant is rich in ice and water but the rocky planets started out their existence as very dry worlds.”
Since gas giant planets can be found extremely close to their stars, in orbits lasting days or even hours, theorists believe that many exoplanets migrate inward early in their history.
Kepler-421b shows that such migration isn’t necessary. It could have formed right where the astronomers see it now.
“This is the first example of a potentially non-migrating gas giant in a transiting system that we’ve found,” Dr Kipping concluded.
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David M. Kipping et al. 2014. Discovery of a Transiting Planet Near the Snow-Line. ApJ, accepted for publication; arXiv: 1407.4807
