A map of the large day-night temperature gradient of a super-Earth exoplanet

Abstract: Over the past decade, observations of giant exoplanets (Jupiter-size) have provided key insights into their atmospheres, but the properties of lower-mass exoplanets (sub-Neptune) remain largely unconstrained because of the challenges of observing small planets. Numerous efforts to observe the spectra of super-Earths (exoplanets with masses of one to ten times that of Earth) have so far revealed only featureless spectra. Here we report a longitudinal thermal brightness map of the nearby transiting super-Earth 55 Cancri e revealing highly asymmetric dayside thermal emission and a strong day-night temperature contrast. Dedicated space-based monitoring of the planet in the infrared revealed a modulation of the thermal flux as 55 Cancri e revolves around its star in a tidally locked configuration. These observations reveal a hot spot that is located 41 +- 12 degrees east of the substellar point (the point at which incident light from the star is perpendicular to the surface of the planet). From the orbital phase curve, we also constrain the nightside brightness temperature of the planet to 1380 +- 400 kelvin and the temperature of the warmest hemisphere (centred on the hot spot) to be about 1300 kelvin hotter (2700 +- 270 kelvin) at a wavelength of 4.5 microns, which indicates inefficient heat redistribution from the dayside to the nightside. Our observations are consistent with either an optically thick atmosphere with heat recirculation confined to the planetary dayside, or a planet devoid of atmosphere with low-viscosity magma flows at the surface.

A Map of the Large Day-Night Temperature Gradient of a Super-Earth Exoplanet

The study presented investigates the thermal dynamics of the super-Earth exoplanet 55 Cancri e, providing significant insights into its atmospheric and surface properties. Utilizing data from the Spitzer Space Telescope's Infrared Array Camera, the researchers conducted a detailed analysis of the thermal phase curve of this exoplanet. Observations reveal a notable asymmetry in the thermal emission between its day and night sides, indicative of a substantial temperature gradient.

Central to this research is the identification of a hot spot located 41 ± 12 degrees east of the substellar point. This spatial thermal asymmetry was discerned from continuous monitoring of the planet, which showed variations in thermal flux consistent with the planet’s tidally locked orbit around its host star. The day side of 55 Cancri e exhibits a hemispherically averaged temperature of 2697 K (+268, -275 K), while the night side maintains a much lower brightness temperature, estimated at 1376 K (+344, -451 K). This stark contrast suggests a deficiency in effective heat redistribution across the planet’s surface.

The implications of these findings are manifold, supporting two primary hypotheses regarding the nature of 55 Cancri e. Firstly, the possibility of an optically thick atmosphere with limited heat circulation localized to the day side is proposed. However, given the exoplanet’s high density and lack of hydrogen spectral lines, the presence of a significant gaseous atmosphere is deemed unlikely. The second hypothesis considers the absence of a substantial atmosphere, with the observed temperature gradient arising instead from low-viscosity magma flows on the surface, potentially contributing to the offset hot spot.

Additional analyses attempted to attribute the thermal phase variations to stellar or orbital mechanics, including ellipsoidal effects and magnetic interactions, but these were dismissed based on their periodicity and amplitude constraints. The research further postulates that atmospheric escape driven by stellar irradiation has likely stripped significant atmospheric components, constraining any potential atmospheric pressure to over 31 kbar to have survived over the stellar lifetime.

This study provides a framework for understanding energy redistribution on super-Earth exoplanets, offering tools and methodologies that can be applied to similar bodies. Future research into the material composition and surface dynamics of 55 Cancri e will benefit from these findings, potentially utilizing infrared spectroscopy in conjunction with these thermal dynamics to further decode the planet's atmospheric and geological properties. The methodology also sets a precedent for further exploration of similar exoplanets in tight orbits, which commonly experience extreme star-driven environments.