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

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. We present our rereduction and analysis of these iconic observations using the open source and modular Spitzer Phase Curve Analysis 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 day = 3771 − 520 + 669 K), nightside temperature ( T night < 1649 K at 2σ), and longitudinal offset of the planet's hot spot, and quantify how they depend on the reduction and detrending. Our reanalysis suggests that 55 Cancri e has a negligible hot spot offset of − 12 − 18 + 21 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 μm brightness temperature is qualitatively consistent with SiO emission from an inverted rock vapor atmosphere.

Observability of silicates in volatile atmospheres of super-Earths and sub-Neptunes

“…Compositions dominated by CO, N 2 , or O 2 are all possible; low-metallicity H 2 -rich atmospheres are less probable (Demory et al 2016;Angelo & Hu 2017;Zilinskas et al 2020Zilinskas et al , 2021, though there have been claims of detected HCN, which would indicate abundant H 2 and a high C/O ratio (Tsiaras et al 2016). A recent reanalysis of Spitzer phase curve data of 55 Cnc e also suggests a high average dayside temperature of T ⋆ = 3771 K, which may be caused by the presence of SiO (Mercier et al 2022). Still, without observations of broad spectral coverage, conclusions for this planet cannot yet be drawn.…”

Rocky Planet or Water World? Observability of Low-density Lava World Atmospheres

“…Spitzer phase curve observations at 4.5 μm initially revealed a significant eastward phase offset (Demory et al 2016;Angelo & Hu 2017), suggesting the presence of a substantial atmosphere (Hammond & Pierrehumbert 2017). However, a recent reanalysis of the Spitzer data by Mercier et al (2022) indicates a negligible phase offset and a stronger day-night temperature contrast, which are consistent with either a local dayside atmosphere or a global atmosphere with poor heat redistribution efficiency. In addition, 55 Cnc e will be observed spectroscopically in JWST's Cycle 1 using NIRCam (total of five eclipses) and MIRI LRS (one eclipse) (Cycle 1, proposals #2084, PI: A.…”

“…Indeed, several studies have investigated the chemistry and spectral appearance of lava world atmospheres, both for purely rocky lava worlds (e.g., Schaefer & Fegley 2009;Miguel et al 2011;Ito et al 2015;Kite et al 2016;Nguyen et al 2020Nguyen et al , 2022Zilinskas et al 2022) and magma oceans in contact with a primary atmosphere (e.g., Kite & Schaefer 2021;Misener & Schlichting 2022Zilinskas et al 2023). Observations of 55 Cnc e have already provided some initial insights into its possible atmospheric conditions (Ehrenreich et al 2012;Demory et al 2016;Angelo & Hu 2017;Esteves et al 2017;Jindal et al 2020;Zhang et al 2021;Keles et al 2022;Mercier et al 2022). Furthermore, the high observed C/O ratio of the host star (1.12 ± 0.19 compared to the solar value of 0.55, Asplund et al 2009;Delgado Mena et al 2010;Madhusudhan et al 2012), combined with the low bulk density of 55 Cnc e, has led to the suggestion of a carbon-rich interior for this planet (Madhusudhan et al 2012).…”

A Nondetection of Iron in the First High-resolution Emission Study of the Lava Planet 55 Cnc e

“…It has a radius of 2.00 R ⊕ , and a period of just 0.736 day (Bourrier et al 2018;Crida et al 2018). Spitzer phase curves suggest that 55 Cnc e is synchronously rotating (Hammond & Pierrehumbert 2018), with two different dayside brightness temperatures reported: reports that this value is 2700 270 270 -+ K at 4.5 μm; a recent reanalysis performed by Mercier et al (2022) In the last several years, many studies have used transmission spectroscopy to search for the presence and nature of 55 Cnc eʼs atmosphere, with the growing consensus pointing toward a higher likelihood for either a high molecular weight atmosphere or no atmosphere. Initial support for 55 Cnc e having a low mean molecular weight atmosphere, came from Tsiaras et al (2016) who used Hubble Space Telescope/WFC3 data to infer a hydrogen-rich atmosphere and a tentative detection of HCN.…”