The Third Option: Color Phase Curves to Characterize the Atmospheres of Temperate Rocky Exoplanets

TL;DR

This paper proposes a new method called color phase curves to detect and analyze atmospheres of temperate rocky exoplanets, especially those that do not transit, by using specific infrared photometry ratios to measure heat transfer.

Contribution

The paper introduces the concept of color phase curves as a novel observational technique for characterizing exoplanet atmospheres, applicable to both transiting and non-transiting planets.

Findings

Color phase curves can isolate planetary thermal emission from stellar light.

The method minimizes degeneracies by avoiding wavelengths with strong molecular absorption.

It enables measurement of longitudinal heat transfer independent of orbital inclination.

Abstract

Detecting and characterizing the atmospheres of rocky exoplanets has proven to be challenging for JWST. Transit spectroscopy of the TRAPPIST-1 planets has been impacted by the effects of spots and faculae on the host star. Secondary eclipses have detected hot rocks, but evidence for atmospheres has been difficult to obtain. However, there is a third option that we call color phase curves. This method will apply to synchronously rotating non-transiting planets as well as transiting planets. A color phase curve uses photometry at a long-IR wavelength near the peak of the planetary thermal emission (e.g., 21 microns) divided by photometry at a shorter wavelength where the star dominates more strongly (e.g., 12 microns). We avoid wavelengths having potentially strong molecular absorption (e.g., 15 microns) to minimize degeneracies in the color phase curve, and we aim to detect and characterize the planetary atmosphere via its longitudinal heat transfer. The ratio of two wavelengths observed nearly simultaneously is designed to isolate thermal emission from the planet, discriminate against the star, and largely cancel instrumental systematic effects. Moreover, we show that invoking mass-radius relations, and using self-consistent physical models, will permit the longitudinal heat transfer to be measured independent of the orbital inclination. Radial velocity surveys are detecting many new exoplanets, including temperate rocky worlds with Earth-like masses. Most of those planets will not transit, but color phase curves have the potential to detect and characterize their atmospheres.