Person:

Kreidberg, Laura

The newly detected Earth-mass planet in the habitable zone of Proxima Centauri could potentially host life - if it has an atmosphere that supports surface liquid water. We show that thermal phase curve observations with the James Webb Space Telescope (JWST) from 5-12 microns can be used to test the existence of such an atmosphere. We predict the thermal variation for a bare rock versus a planet with 35% heat redistribution to the nightside and show that a JWST phase curve measurement can distinguish between these cases at 4σ confidence, assuming photon-limited precision. We also consider the case of an Earth-like atmosphere, and find that the ozone 9.8 micron band could be detected with longer integration times (a few months). We conclude that JWST observations have the potential to put the first constraints on the possibility of life around the nearest star to the Solar System.

The James Webb Space Telescope (JWST) will likely revolutionize transiting exoplanet atmospheric science due to a combination of its capability for continuous, long duration observations and its larger collecting area, spectral coverage, and spectral resolution compared to existing space-based facilities. However, it is unclear precisely how well JWST will perform and which of its myriad instruments and observing modes will be best suited for transiting exoplanet studies. In this article, we describe a prefatory JWST Early Release Science (ERS) Cycle 1 program that focuses on testing specific observing modes to quickly give the community the data and experience it needs to plan more efficient and successful transiting exoplanet characterization programs in later cycles. We propose a multi-pronged approach wherein one aspect of the program focuses on observing transits of a single target with all of the recommended observing modes to identify and understand potential systematics, compare transmission spectra at overlapping and neighboring wavelength regions, confirm throughputs, and determine overall performances. In our search for transiting exoplanets that are well suited to achieving these goals, we identify 12 objects (dubbed “community targets”) that meet our defined criteria. Currently, the most favorable target is WASP-62b because of its large predicted signal size, relatively bright host star, and location in JWST’s continuous viewing zone. Since most of the community targets do not have well-characterized atmospheres, we recommend initiating preparatory observing programs to determine the presence of obscuring clouds/hazes within their atmospheres. Measurable spectroscopic features are needed to establish the optimal resolution and wavelength regions for exoplanet characterization. Other initiatives from our proposed ERS program include testing the instrument brightness limits and performing phase-curve observations. The latter are a unique challenge compared to transit observations because of their significantly longer durations. Using only a single mode, we propose to observe a full-orbit phase curve of one of the previously characterized, short-orbital-period planets to evaluate the facility-level aspects of long, uninterrupted time-series observations.

The water abundance in a planetary atmosphere provides a key constraint on the planet's primordial origins because water ice is expected to play an important role in the core accretion model of planet formation. However, the water content of the solar system giant planets is not well known because water is sequestered in clouds deep in their atmospheres. By contrast, short-period exoplanets have such high temperatures that their atmospheres have water in the gas phase, making it possible to measure the water abundance for these objects. We present a precise determination of the water abundance in the atmosphere of the 2 M Jup short-period exoplanet WASP-43b based on thermal emission and transmission spectroscopy measurements obtained with the Hubble Space Telescope. We find the water content is consistent with the value expected in a solar composition gas at planetary temperatures (0.4-3.5 × solar at 1σ confidence). The metallicity of WASP-43b's atmosphere suggested by this result extends the trend observed in the solar system of lower metal enrichment for higher planet masses.

The majority of terrestrial planets in the Galaxy orbit small stars with radii less than 60% that of the Sun1,2. Theoretical models predict that these planets are more vulnerable to at- mospheric escape and collapse than their counterparts orbiting Sun-like stars3–5. To deter- mine whether a thick atmosphere has survived, one approach is to search for signatures of atmospheric heat redistribution in a planet’s thermal phase curve6–9. This technique was previously applied to the super-Earth 55 Cancri e, which showed an offset hot spot indicative of atmosphere heat circulation10. Here we report a phase curve measurement for the exo- planet LHS 3844b, a 1.3 R⊕ world in an 11-hour orbit around a small, nearby star. This is the first such measurement for a planet smaller than 1.6 Re, the size marking the transition from rocky to gaseous worlds11. The phase variation is symmetric and has a large amplitude, implying a dayside brightness temperature of 1040±40 K and a nightside temperature con- sistent with zero K (1σ confidence). The data are best fit by a bare rock model with a low Bond albedo (< 0.2 at 2σ confidence), or a tenuous atmosphere with surface pressure below 0.1 bar. These results support theoretical predictions that hot terrestrial planets orbiting small stars may not retain substantial atmospheres.