Transits of Earth-Like Planets

Abstract

Transmission spectroscopy of Earth-like exoplanets is a potential tool for habitability screening. Transiting planets are present-day "Rosetta Stones" for understanding extrasolar planets because they offer the possibility to characterize giant planet atmospheres and should provide an access to biomarkers in the atmospheres of Earth-like exoplanets, once they are detected. Using the Earth itself as a proxy we show the potential and limits of the transiting technique to detect biomarkers on an Earth-analog exoplanet in transit. We quantify the Earth's cross section as a function of wavelength, and show the effect of each atmospheric species, aerosol, and Rayleigh scattering. Clouds do not significantly affect this picture because the opacity of the lower atmosphere from aerosol and Rayleigh losses dominates over cloud losses. We calculate the optimum signal-tonoise ratio for spectral features in the primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and also M stars, for a 6.5 m telescope in space. We find that the signal-to-noise values for all important spectral features are on the order of unity or less per transit-except for the closest stars-making it difficult to detect such features in one single transit, and implying that coadding of many transits will be essential.

Effects of refraction on transmission spectra of gas giants: decrease of the Rayleigh scattering slope and breaking of retrieval degeneracies

“…To save on computation time, Bétrémieux & Kaltenegger (2013) modified the code so that one can choose not to include thermal emission. Although the line opacity database is identical to that in Kaltenegger & Traub (2009), Bétrémieux & Kaltenegger (2013) replaced the original Rayleigh scattering parametrisation, appropriate only for Earth, with a database of Rayleigh scattering cross sections for N2, O2, CO2, and Ar (see their Table 2). They also included new routines to compute optical depths from continuous absorption cross sections, and a new database of ultraviolet and visible absorbers (see their Table 1).…”

A High-Precision Near-Infrared Survey for Radial Velocity Variable Low-Mass Stars Using Cshell and a Methane Gas Cell

“…Using the projected separation of GJ725AB (≈47 au) and assuming typical masses of ≈0.36 M e and ≈0.3 M e that correspond to their respective spectral types of M3 and M3.5 (Reid & Hawley 2005;Kaltenegger & Traub 2009), their orbital period should be P≈400 years. This corresponds to a tangential velocity of v≈1.75 km s −1 as measured from the center of mass in the case of a circular orbit.…”

“…Assuming a mass of ≈0.58 M e for GJ458A (Gaidos et al 2014) and a mass of ≈0.36 M e for GJ458B that is typical of a field M3 star (Reid & Hawley 2005;Kaltenegger & Traub 2009) and using the projected separation of ≈228 au, we would expect a period of ≈3550 years for the orbit of the GJ458AB system in a case with zero eccentricity. This would be consistent with a maximal RV slope of only ≈1.2 m s −1 yr −1 .…”

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

“…As more potentially habitable planets are discovered around low-mass stars, a similar analysis as in Section 5 should be undertaken in order to assess the possibility that the planet could in fact be dehydrated. As we may only be able to spectroscopically characterize a few planets with the James Webb Space Telescope (Kaltenegger and Traub, 2009;Seager et al, 2009), prioritization of targets is crucial, and past and present tidal heating will help determine the best planet to observe.…”