Kepler-421b: A Uranus-Sized Planet near the Snow-Line

“In future, children won’t perceive the stars as mere twinkling points of light: they’ll learn that each is a ‘Sun’, orbited by planets fully as interesting as those in our Solar System.”

- Martin Rees

A protoplanetary disk is a circumstellar disk of material around a young star in which the formation of planets occurs. The snow-line marks the distance from the central star where the protoplanetary disk becomes cool enough for volatiles such as water to condense into solid ice grains. By analysing publicly available data from NASA’s Kepler space telescope, Kipping et al. (2014) present the discovery of a cold transiting planet near the snow-line. This planet, identified as Kepler-421b, is the first of its kind to be discovered. It is similar in size to Uranus and it circles a star that is slightly cooler than the Sun in a nearly-circular orbit with an orbital period of 704.2 days. Kepler-421b is the longest period transiting planet discovered to date.

Figure 1: Artist’s impression of a Uranus-like planet with a large moon in orbit around it.

Figure 2: Transit light curve of Kepler-421b. Based on how much light it blocks when it passes in front its parent star, Kepler-421b is estimated to be ~4 times the Earth’s diameter, roughly the size of Uranus. Kipping et al. (2014).

“Finding Kepler-421b was a stroke of luck,” says lead author David Kipping of the Harvard-Smithsonian Center for Astrophysics (CfA). “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 is ~1.2 AU from its parent star. At that distance, the planet is closer to its parent star than Mars is from the Sun. Since its parents star is only ~40 percent as luminous as the Sun, Kepler-421b receives only ~64 percent of the insolation Mars gets from the Sun, or ~28 percent of the insolation Earth gets from the Sun. Kepler-421b receives the same amount of insolation as an object at ~2 AU from the Sun. If Kepler421b has a Uranus-like albedo, the planet’s effective temperature would be ~180 K. For comparison, Earth has a mean surface temperature of 288 K, or 15°C.

Assuming Kepler-421b has a Uranus-like composition (i.e. an ice giant), the planet probably formed at its current distance from its parent star (i.e. in situ formation). At that distance, it is cool enough for icy planetesimals to form in the protoplanetary disk, eventually leading to the creation of an ice giant. The large orbital period of 704.2 days means that transits of Kepler-421b are relatively infrequent. In fact, only two transits have been observed so far and those were sufficient to result in its initial detection. Unfortunately, the 3rd transit occurred in March 2014, after Kepler’s primary mission. Nevertheless, the 4th transit opportunity is in February 2016. Kepler-412b is the first known transiting Uranus-sized planet in a long-period orbit. Determining its mass and finding more of its kind would be the next logical steps. Finally, the large distance of Kepler421b from its parent star makes it an appealing target in the search for exomoons.

Reference:

Kipping et al. (2014), “Discovery of a Transiting Planet Near the Snow-Line”, arXiv:1407.4807 [astro-ph.EP]