A Super-Earth Transiting a Naked-Eye Star

Abstract

We have detected transits of the innermost planet "e" orbiting 55 Cnc (V = 6.0), based on two weeks of nearly continuous photometric monitoring with the MOST space telescope. The transits occur with the period (0.74 d) and phase that had been predicted by Dawson & Fabrycky, and with the expected duration and depth for the crossing of a Sun-like star by a hot super-Earth. Assuming the star's mass and radius to be 0.963 +0.051 −0.029 M ⊙ and 0.943 ± 0.010 R ⊙ , the planet's mass, radius, and mean density are 8.63 ± 0.35 M ⊕ , 2.00 ± 0.14 R ⊕ , and 5.9 +1.5 −1.1 g cm −3 . The mean density is comparable to that of Earth, despite the greater mass and consequently greater compression of the interior of 55 Cnc e. This suggests a rock-iron composition supplemented by a significant mass of water, gas, or other light elements. Outside of transits, we detected a sinusoidal signal resembling the expected signal due to the changing illuminated phase of the planet, but with a full range (168 ± 70 ppm) too large to be reflected light or thermal emission. This signal has no straightforward interpretation and should be checked with further observations. The host star of 55 Cnc e is brighter than that of any other known transiting planet, which will facilitate future investigations.

SECURE MASS MEASUREMENTS FROM TRANSIT TIMING: 10 KEPLER EXOPLANETS BETWEEN 3 AND 8 M ⊕ WITH DIVERSE DENSITIES AND INCIDENT FLUXES

“…Radial velocity (RV)spectroscopy has provided the bulk of mass measurements of transiting exoplanets (Batalha et al 2011;Winn et al 2011;Fressin et al 2012;Gillon et al 2012;Gautier et al 2012;Howard et al 2013;Pepe et al 2013;Weiss et al 2013;Haywood et al 2014;Marcy et al 2014;Dressing et al 2015). However, among subNeptune-mass planets, RV mass detections are limited to planets on short orbital periods, because the RV signal depends on the strength of planet-star interactions and declines with orbital distance.…”

Variability in the super-Earth 55 Cnc e

“…composed of an Iron core (30% by mass) overlaid by a silicate mantle and crust. Previous studies which used a larger radius of the planet required a thick water envelope (Demory et al 2011;Winn et al 2011) or a carbon-rich interior ), neither of which are now required but cannot be ruled out either. However, under the assumption of an Earth-like interior, which is also the case for Io, the outgassed products could comprise of gaseous CO2 and sulfur compounds (Moses et al 2002;Schaefer & Fegley 2005) along with particulate matter composed of silicates (Spencer et al 1997;McEwen et al 1998).…”

On the potentially dramatic history of the super-Earth ρ 55 Cancri e

“…The ability to generate a substantial inclination offset from the orbital plane of the original planetary system offers an explanation for the tension between the astrometric measurement of 55 Canc d's inclination by McArthur et al (2004) and the transitting nature of 55 Canc e (Winn et al 2011;Demory et al 2011). One observation potentially in conflict with this scenario is the statistically marginal detection of an extended H atmosphere of 55 Canc b by Ehrenreich et al (2012).…”

“…The physical parameters of the innermost planet, 55 Cancri e, are estimated to be a mass of 7.99 ± 0.25M⊕ and a radius of 2.00±0.14R⊕ (Nelson et al 2014;Winn et al 2011;Demory et al 2011). The radius of a perovskite planet of this mass is ∼ 1.9R⊕, using the fitting formulae of Seager et al (2007).…”