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 Earths 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-to-noise 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 co-adding of many transits will be essential.
Deep Dive into Transits of Earth-Like Planets.
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 Earths 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-to-noise ratio for spectral features in the primary eclipse spectrum of an Earth-like exoplanet around a Sun-like star and also M st
The transiting system shows that transiting Super-Earths have already been detected, and recent detections of several Super-Earths (Mayor et al. 2009) show that transiting Earths are expected to be detected in the near future. The current status of exoplanet characterization shows a surprisingly diverse set of planets. For a subset of these, some properties have been measured or inferred using radial velocity, micro-lensing, transits, and astrometry. These observations have yielded measurements of planetary mass, orbital elements, planetary radius (for transits), and some physical characteristics of the upper atmospheres. Specifically, observations of transits, combined with radial velocity (RV) information, have provided estimates of the mass and radius of the planet (see e.g., Torres et al. 2008), planetary brightness temperature (Charbonneau et al. 2005;Deming et al. 2005), planetary day-night temperature difference (Harrington et al. 2006;Knutson et al. 2007), and even absorption features of planetary upper-atmospheric constituents: sodium (Charbonneau et al. 2002), hydrogen (Vidal-Madjar et al. 2003), water (Tinetti et al. 2007,(disputed by Ehrenreich et al. 2007), Beaulieu et al 2008, Swain et al. 2008) and methane (Swain et al. 2008), showing that the transit technique has great value. Several groups are modeling the transmission spectra of extrasolar giant planets in detail (see e.g. the review article by Charbonneau et al. 2006 and references therein). That success has led to speculation that the transit technique might also be useful for characterizing terrestrial planets.
In this paper we use the Earth itself as a proxy to show the potential, and limits, of the transiting technique to detect biomarkers on Earth-analog exoplanets. We calculate the visible and infrared transit spectra of the Earth. With this information we calculate the signal to noise ratio (SNR) for major spectral features, for the case of a 6.5-m telescope in space, like JWST, during the time of a single transit, as well as for co-added transits, for a Sun-like star and for M stars. We note that M stars have been suggested as good targets for characterizing a planet’s atmosphere with transmission spectroscopy due to the improved contrast ratio between star and planet. Ground based transit searches are underway focusing on M stars (see e.g. Irwin et al. 2009).
Theoretical transmission spectra of terrestrial exoplanets have been published by Ehrenreich et al. (2006) in the wavelength range from 0.2-2 µm for simplified atmospheric profiles consisting of water vapor (H 2 O), molecular oxygen (O 2 ), ozone (O 3 ), carbon dioxide (CO 2 ) and molecular nitrogen (N 2 ), and an opaque cloud layer below 10 km for a F2, K2 and G2 dwarf star. The work presented in this paper extends the wavelength range of the calculations to the infrared 0.3-20 µm, uses a realistic atmospheric temperature profile with aerosol, Rayleigh scattering, and three different cloud layers, validates the model with ATMOS 3 infrared transmission spectra of the Earth’s limb, and presents a complete set of SNR calculations of atmospheric species during the transit for the Sun as well as M0 to M9 dwarf stars.
Currently fifteen exoplanets (including three pulsar planets) are known to have a mass (times sin i, where i is the orbital inclination, for RV planets) less than 10 M Earth , a somewhat arbitrary boundary that distinguishes terrestrial from giant planets (Valencia et al. 2006 and references therein). Accordingly, we identify masses in the range 1-10 M Earth as being Super Earths, likely composed of rock, ice, and liquid, and masses greater that 10 M Earth as being giant planets, likely dominated by the mass of a gaseous envelope. The fifteen planets are: COROT-7b ～7 M Earth (Leger et al. in prep), GJ 876 d, ～7.5 M Earth (Rivera et al. 2005); OGLE-05-390L b, ～5.5 M Earth (Beaulieu et al. 2006); Gl 581 c and Gl 581 d, ～ 5.03 M Earth and 8.6 M Earth (Udry et al. 2007); HD40307 b, HD40307 c and HD40307 d～4.2, 6.7, and 9.4 M Earth (Mayor et al. 2009); MOA-2007-BLG-192L b ～3.3 M Earth (Bennett et al. 2008); HD 181433 b ～7.6 M Earth (Bouchy et al. 2009); HD 285968 b ～8.4 M Earth (Forveille et al. 2009); HD 7924 b ～9.2 M Earth (Howard et al. 2009); as well as three planets discovered by pulsar timing (Wolszczan & Frail 1992). None of those planets orbits its star within the habitable zone (HZ), but Gl581 c and especially Gl581 d are close to the HZ edges (see Selsis et al. 2007, Kaltenegger et al. in prep).
In this paper we ask, what are the limits to characterizing an Earth-analog during a transit? We explore potential spectral signatures and biomarkers for an Earth-like planet. This translates into the accuracy needed to measure the planet’s effective radius to detect atmospheric species.
We calculate a model transmission spectrum from the UV to mid-IR, including realistic opacities and clouds, and use this to assess how well we could characterize our own planet i
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