Trio of Super-Earths Circling a Nearby Star

Following the failure of two of its four reaction wheels in May 2013, NASA’s planet-hunting Kepler space telescope was ingeniously repurposed for a new mission plan named K2. On 18 December 2014, it was announced that the K2 mission had detected its first confirmed exoplanet, a super-Earth or mini-Neptune designated HIP 116454 b (Vanderburg et al. 2014). The detection of HIP 116454 b was based on data collected during the testing run to prepare the space telescope for the nominal K2 mission.

Using data from the K2 mission covering 30 May to 21 August 2014, Crosseld et al. (2015) report the discovery of three super-Earths orbiting a nearby M dwarf star slightly larger than half the size of the Sun. This M dwarf star is designated EPIC 201367065 and it lies at a relatively nearby distance of about 150 light years. The three planets are 2.1, 1.7 and 1.5 times the size of Earth, and take 10.1, 24.6 and 44.6 days to circle the host star, respectively. From their sizes, the planets appear to span the range between rock-dominated “Earths/super-Earths” and lower-density “mini-Neptunes” with substantial volatile content.

Figure 1: Artist’s impression of a rocky planet.

Figure 2: Transit light curves of the three planets around EPIC 201367065. Top: Vertical ticks indicate the location of each planet’s transit. Bottom: Phase-folded photometry and best-fit light curves for each planet. Crosseld et al. (2015)

The outermost of the three planets, designated as planet “d”, is 1.5 times the size of Earth and receives somewhat more insolation from its host star than Earth receives from the Sun. This places the planet at the inner edge of the habitable zone where temperatures might be cool enough for the planet to support Earth-like conditions, and possibly life. If temperatures turn out to be too hot, then the planet is more likely a super-Venus with conditions too inhospitable for life.

All three planets around EPIC 201367065 are probably tidally-locked. This means the same side of each planet always faces the host star (i.e. permanent day and night sides). Since M dwarf stars are much cooler and fainter than the Sun, a planet around an M dwarf star needs to be much closer-in to get a similar amount of insolation as Earth gets from the Sun. As a result, the planet experiences stronger tidal forces from the host star and is expected to be tidally-locked.

A study by Yang et al. (2013) suggests that a tidally-locked planet can support thick water clouds on its dayside as the high amount of insolation drives strong dayside convection. The water clouds reflect away incoming radiation from the host star, leading to lower surface temperatures. If such a stabilizing cloud feedback mechanism works on planet “d”, then its surface temperatures can be lower than would otherwise be, allowing the planet to support cool and clement conditions even though its receives somewhat more insolation than Earth.

The trio of planets around EPIC 201367065 can be conveniently studied in further detail as the host star is relatively bright and nearby. Both the Hubble Space Telescope (HST) and the upcoming James Webb Space Telescope (JWST) have the capabilities to reveal more about this planetary system. Such a discovery so early in the mission shows the ubiquity of planetary systems and that the K2 mission will extend the legacy of Kepler for years to come.

References:
- Vanderburg et al. (2014), “Characterizing K2 Planet Discoveries: A super-Earth transiting the bright K-dwarf HIP 116454”, arXiv:1412.5674 [astro-ph.EP]
- Crosseld et al. (2015), “A nearby M star with three transiting super-Earths discovered by K2”, arXiv:1501.03798 [astro-ph.EP]
- Yang et al. (2013), “Stabilizing Cloud Feedback Dramatically Expands the Habitable Zone of Tidally Locked Planets”, arXiv:1307.0515 [astro-ph.EP]