Search for an exosphere in sodium and calcium in the transmission spectrum of exoplanet 55 Cancri e

Abstract

Context. The atmospheric and surface characterization of rocky planets is a key goal of exoplanet science. Unfortunately, the measurements required for this are generally out of reach of present-day instrumentation. However, the planet Mercury in our own solar system exhibits a large exosphere composed of atomic species that have been ejected from the planetary surface by the process of sputtering. Since the hottest rocky exoplanets known so far are more than an order of magnitude closer to their parent star than Mercury is to the Sun, the sputtering process and the resulting exospheres could be orders of magnitude larger and potentially detectable using transmission spectroscopy, indirectly probing their surface compositions. Aims. The aim of this work is to search for an absorption signal from exospheric sodium (Na) and singly ionized calcium (Ca + ) in the optical transmission spectrum of the hot rocky super-Earth 55 Cancri e. Although the current best-fitting models to the planet mass and radius require a possible atmospheric component, uncertainties in the radius exist, making it possible that 55 Cancri e could be a hot rocky planet without an atmosphere. Methods. High resolution (R∼110000) time-series spectra of five transits of 55 Cancri e, obtained with three different telescopes (UVES/VLT, HARPS/ESO 3.6m & HARPS-N/TNG) were analysed. Targeting the sodium D lines and the calcium H and K lines, the potential planet exospheric signal was filtered out from the much stronger stellar and telluric signals, making use of the change of the radial component of the orbital velocity of the planet over the transit from −57 to +57 km sec −1 . Results. Combining all five transit data sets, we detect a signal potentially associated with sodium in the planet exosphere at a statistical significance level of 3σ. Combining the four HARPS transits that cover the calcium H and K lines, we also find a potential signal from ionized calcium (4.1 σ). Interestingly, this latter signal originates from just one of the transit measurements -with a 4.9σ detection at this epoch. Unfortunately, due to the low significance of the measured sodium signal and the potentially variable Ca + signal, we estimate the p-values of these signals to be too high (corresponding to <4σ) to claim unambiguous exospheric detections. By comparing the observed signals with artificial signals injected early in the analysis, the absorption by Na and Ca + are estimated to be at a level of ∼ 2.3 × 10 −3 and ∼ 7.0 × 10 −2 respectively, relative to the stellar spectrum.Conclusions. If confirmed, the 3σ signal would correspond to an optically thick sodium exosphere with a radius of 5 R ⊕ , which is comparable to the Roche lobe radius of the planet. The 4.9σ detection of Ca + in a single HARPS data set would correspond to an optically thick Ca + exosphere approximately five times larger than the Roche lobe radius. If this were a real detection, it would imply that the exosphere exhibits extreme variability. Although no formal detection has been made, we advocate th...

A Search for Water in a Super-Earth Atmosphere: High-resolution Optical Spectroscopy of 55Cancri e

“…Evidence for the existence of an extended atmosphere on the planet includes detections reported by Tsiaras et al (2016) of features at 1.42 and 1.54 μm, which they attribute to trace amounts of HCN retained within the planet's extended atmosphere. Additional evidence includes the tentative detection of sodium and singly ionized calcium reported by Ridden-Harper et al (2016), as well as findings from Lammer et al (2013) that 55Cnce's theoretical mass-loss rate is ∼10 times lower than that of a typical hot-Jupiter, and therefore 55Cnce can retain a hydrogen envelope. Furthermore, a water-depleted atmosphere was predicted by Ito et al (2015), who modeled an atmosphere that is in gas/melt equilibrium with an underlying magma ocean; a possible scenario for 55Cnce, given its high temperature.…”

“…However, follow-up secondary eclipse, phase-curve, and transit measurements (Demory et al , 2016a(Demory et al , 2016bRidden-Harper et al 2016;Tsiaras et al 2016), in conjunction with theoretical models (Lammer et al 2013;Ito et al 2015;Kite et al 2016), point to the existence of an envelope.…”

“…Our study, similar to Ridden-Harper et al (2016), uses ground-based high-resolution transmission spectra to search for absorption signatures of 55Cnce's atmosphere. However, we take advantage of the large wavelength coverage and high spectral resolution of our observations to search for the signature of thousands of optical absorption lines produced by water.…”

Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

“…From a subsequent analysis of the same data set, Louden & Wheatley (2015) measured an eastward wind blowing from the trailing limb (−5.3 +1.0 −1.4 km s −1 ) to the leading limb (+2.3 +1.3 −1.5 km s −1 ), after decorrelating the signal from the stellar rotation effect. A recent reanalysis of these same data (Barnes et al 2016) emphasized the role stellar activity might play in producing spurious transit signals and mimicking wind signatures in chromospherically sensitive lines, such as Ca ii H and K lines (e.g., Ridden-Harper et al 2016), and in other strong features usually considered less sensitive to activity, such as Na i lines. Finally, the same HARPS data allowed Cegla et al (2016) to pioneer a method for retrieving the stellar surface properties behind the transiting planet.…”

Atmospheric compositions and observability of nitrogen-dominated ultra-short-period super-Earths

“…Planets in USP orbits are tidally locked and extremely hot on their day side, which makes them particularly good targets for thermal emission observations. 55 Cnc e itself has been observed a multitude of times (Demory et al 2011(Demory et al , 2012Ehrenreich et al 2012;de Mooij et al 2014;Demory et al 2016a,b;Ridden-Harper et al 2016;Tsiaras et al 2016a;Esteves et al 2017). Despite the efforts, its atmospheric structure and composition still remain unknown.…”