Revisiting the Iconic Spitzer Phase Curve of 55 Cancri e: Hotter Dayside, Cooler Nightside and Smaller Phase Offset

TL;DR

This paper re-analyzes the Spitzer phase curve of 55 Cancri e, revealing a smaller hot spot offset and cooler nightside than previously reported, consistent with poor heat transport and possible atmospheric inversion.

Contribution

The study provides a re-analysis of the iconic phase curve using an open-source pipeline, challenging previous findings of significant eastward heat recirculation.

Findings

Negligible hot spot offset of about -12 degrees

Cooler nightside temperature below 1649 K

High dayside brightness temperature consistent with SiO emission

Abstract

Thermal phase curves of short period exoplanets provide the best constraints on the atmospheric dynamics and heat transport in their atmospheres. The published Spitzer Space Telescope phase curve of 55 Cancri e, an ultra-short period super-Earth, exhibits a large phase offset suggesting significant eastward heat recirculation, unexpected on such a hot planet arXiv:1604.05725. We present our re-reduction and analysis of these iconic observations using the open source and modular Spitzer Phase Curve Analysis (SPCA) pipeline. In particular, we attempt to reproduce the published analysis using the same instrument detrending scheme as the original authors. We retrieve the dayside temperature ($T_{\rm day} = 3771^{+669}_{-520}$ K), nightside temperature ($T_{\rm night} < 1649$ K at $2\sigma$), and longitudinal offset of the planet's hot spot and quantify how they depend on the reduction and detrending. Our re-analysis suggests that 55 Cancri e has a negligible hot spot offset of $-12^{+21}_{-18}$ degrees east. The small phase offset and cool nightside are consistent with the poor heat transport expected on ultra-short period planets. The high dayside 4.5-micrometer brightness temperature is qualitatively consistent with SiO emission from an inverted rock vapour atmosphere.