Exoplanet observations promise one day to unveil the presence of extraterrestrial life. Atmospheric compounds in strong chemical disequilibrium would point to large-scale biological activity just... Show moreExoplanet observations promise one day to unveil the presence of extraterrestrial life. Atmospheric compounds in strong chemical disequilibrium would point to large-scale biological activity just as oxygen and methane do in the Earth's atmosphere. The cancellation of both the Terrestrial Planet Finder and Darwin missions means that it is unlikely that a dedicated space telescope to search for biomarker gases in exoplanet atmospheres will be launched within the next 25 years. Here we show that ground-based telescopes provide a strong alternative for finding biomarkers in exoplanet atmospheres through transit observations. Recent results on hot Jupiters show the enormous potential of high-dispersion spectroscopy to separate the extraterrestrial and telluric signals, making use of the Doppler shift of the planet. The transmission signal of oxygen from an Earth-twin orbiting a small red dwarf star is only a factor of three smaller than that of carbon monoxide recently detected in the hot Jupiter {$τ$} Boötis b, albeit such a star will be orders of magnitude fainter. We show that if Earth-like planets are common, the planned extremely large telescopes can detect oxygen within a few dozen transits. Ultimately, large arrays of dedicated flux-collector telescopes equipped with high-dispersion spectrographs can provide the large collecting area needed to perform a statistical study of life-bearing planets in the solar neighborhood. Show less
Kok, R.; Brogi, M.; Snellen, I.A.G.; Birkby, J.L.; Albrecht, S.; Mooij, E. de 2013
Context. After many attempts over more than a decade, high-resolution spectroscopy has recently delivered its first detections of molecular absorption in exoplanet atmospheres, both in... Show moreContext. After many attempts over more than a decade, high-resolution spectroscopy has recently delivered its first detections of molecular absorption in exoplanet atmospheres, both in transmission and thermal emission spectra. Targeting the combined signal from individual lines in molecular bands, these measurements use variations in the planet radial velocity to separate the planet signal from telluric and stellar contaminants. Aims: We apply high-resolution spectroscopy to probe molecular absorption in the day-side spectrum of the bright transiting hot Jupiter HD 189733b. Methods: We observed HD 189733b with the CRIRES high-resolution near-infrared spectograph on the Very Large Telescope during three nights, targeting possible absorption from carbon monoxide, water vapour, methane, and carbon dioxide, at 2.0 and 2.3 {$μ$}m. Results: We detect a 5-{$σ$} absorption signal from CO at a contrast level of ~{}4.5 { imes} 10$^{-4}$ with respect to the stellar continuum, revealing the planet orbital radial velocity at 154$^{+4}$$_{-3}$ km s$^{-1}$. This allows us to solve for the planet and stellar mass in a similar way as for stellar eclipsing binaries, resulting in 0.846$^{+0.068}$$_{-0.049}$M{sun} and M$_p$ = 1.162$^{+0.058}$$_{-0.039}$ M$^{Jup}$. No significant absorption is detected from H$_{2}$O, CO$_{2}$, or CH$_{4}$ and we determine upper limits on their line contrasts. Conclusions: The detection of CO in the day-side spectrum of HD 189733b can be made consistent with the haze layer proposed to explain the optical to near-infrared transmission spectrum if the layer is optically thin at the normal incidence angles probed by our observations, or if the CO abundance is high enough for the CO absorption to originate from above the haze. Our non-detection of CO$_{2}$ at 2.0 {$μ$}m is not inconsistent with the deep CO$_{2}$ absorption from low-resolution NICMOS secondary eclipse data in the same wavelength range. If genuine, the absorption would be so strong that it blanks out any planet light completely in this wavelength range, leaving no high-resolution signal to be measured. Based on observations collected at the European Southern Observatory (186.C-0289). Show less
Brogi, M.; Snellen, I.A.G.; Kok, R.; Albrecht, S.; Birkby, J.L.; Mooij, E. de 2013
In this paper, we present ground-based high-resolution spectroscopy of 51 Pegasi using CRIRES at the Very Large Telescope. The system was observed for 3 { imes} 5 hr at 2.3 {$μ$}m at a spectral... Show moreIn this paper, we present ground-based high-resolution spectroscopy of 51 Pegasi using CRIRES at the Very Large Telescope. The system was observed for 3 { imes} 5 hr at 2.3 {$μ$}m at a spectral resolution of R = 100,000, targeting potential signatures from carbon monoxide, water vapor, and methane in the planet's dayside spectrum. In the first 2 { imes} 5 hr of data, we find a combined signal from carbon monoxide and water in absorption at a formal 5.9{$σ$} confidence level, indicating a non-inverted atmosphere. We derive a planet mass of M $_P$ = (0.46 {plusmn} 0.02)M $_{Jup}$ and an orbital inclination i between 79.{deg}6 and 82.{deg}2, with the upper limit set by the non-detection of the planet transit in previous photometric monitoring. However, there is no trace of the signal in the final five hours of data. A statistical analysis indicates that the signal from the first two nights is robust, but we find no compelling explanation for its absence in the final night. The latter suffers from stronger noise residuals and greater instrumental instability than the first two nights, but these cannot fully account for the missing signal. It is possible that the integrated dayside emission from 51 Peg b is instead strongly affected by weather. However, more data are required before we can claim any time variability in the planet's atmosphere. Show less
Brogi, M.; Keller, C.U.; Juan Ovelar, M. de; Kenworthy, M.A.; Kok, R.J. de; Min, M.; Snellen, I.A.G. 2012
