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Post Info TOPIC: Habitable Exoplanets
Blobrana


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Title: Direct Imaging in the Habitable Zone and the Problem of Orbital Motion
Authors: Jared R. Males, Andrew J. Skemer, Laird M. Close

High contrast imaging searches for exoplanets have been conducted on 2.4-10 m telescopes, typically at H band (1.6 microns) and used exposure times of ~1 hr to search for planets with semi-major axes of > ~10 AU. We are beginning to plan for surveys using extreme-AO systems on the next generation of 30-meter class telescopes, where we hope to begin probing the habitable zones (HZs) of nearby stars. Here we highlight a heretofore ignorable problem in direct imaging: planets orbit their stars. Under the parameters of current surveys, orbital motion is negligible over the duration of a typical observation. However, this motion is not negligible when using large diameter telescopes to observe at relatively close stellar distances (1-10pc), over the long exposure times (10-20 hrs) necessary for direct detection of older planets in the HZ. We show that this motion will limit our achievable signal-to-noise ratio and degrade observational completeness. Even on current 8m class telescopes, orbital motion will need to be accounted for in an attempt to detect HZ planets around the nearest sun-like stars alpha Cen A & B, a binary system now known to harbour at least one planet. Here we derive some basic tools for analysing this problem, and ultimately show that the prospects are good for de-orbiting a series of shorter exposures to correct for orbital motion.

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Blobrana


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Title: Detecting bio-markers in habitable-zone earths transiting white dwarfs
Authors: Abraham Loeb, Dan Maoz

The characterisation of the atmospheres of habitable-zone Earth-mass exoplanets that transit across main-sequence stars, let alone the detection of bio-markers in their atmospheres, will be challenging even with future facilities. It has been noted that white dwarfs (WDs) have long-lived habitable zones and that a large fraction of WDs may host planets. We point out that during a transit of an Earth-mass planet across a WD, the planet's atmospheric transmission spectrum obtains a much higher contrast over the stellar background compared to a main-sequence host, because of the small surface area of the WD. The most prominent bio-marker in the present-day terrestrial atmosphere, molecular oxygen, is readily detectable in a WD transit via its A-band absorption at ~0.76 micron. A potentially life-sustaining Earth-like planet transiting a WD can be found by assembling a suitable sample of ~500 WDs and then surveying them for transits using small telescopes. Once a transit is found, the O_2 absorption in the planetary atmospheric transmission spectrum would be detectable with the James Webb Space Telescope (JWST) in about 5 hours of total exposure time, integrated over 160 2-minute transits. Characterisation of the planet atmosphere using other tracers such as water vapour and CO_2 will be considerably easier. We demonstrate this future discovery space by simulating a possible transmission spectrum that would be detectable with JWST.

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Blobrana


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Title: Characterisation of potentially habitable planets: Retrieval of atmospheric and planetary properties from emission spectra
Authors: P. von Paris, P. Hedelt, F. Selsis, F. Schreier, T. Trautmann

An increasing number of potentially habitable terrestrial planets and planet candidates are found by ongoing planet search programs. The search for atmospheric signatures to establish planetary habitability and the presence of life might be possible in the future. We want to quantify the accuracy of retrieved atmospheric parameters which might be obtained from infrared emission spectroscopy. We use synthetic observations of hypothetical habitable planets, constructed with a parameterised atmosphere model, a high-resolution radiative transfer model and a simplified noise model. Classic statistical tools such as chi2 statistics and least-square fits were used to analyse the simulated observations. When adopting the design of currently planned or proposed exoplanet characterisation missions, we find that emission spectroscopy could provide weak limits on surface conditions of terrestrial planets, hence their potential habitability. However, these mission designs are unlikely to allow to characterise the composition of the atmosphere of a habitable planet, even though CO2 is detected. Upon increasing the signal-to-noise ratios by about a factor of 2-5 (depending on spectral resolution) compared to current mission designs, the CO2 content could be characterised to within two orders of magnitude. The detection of the O3 biosignature remains marginal. The atmospheric temperature structure could not be constrained. Therefore, a full atmospheric characterisation seems to be beyond the capabilities of such missions when using only emission spectroscopy during secondary eclipse or target visits. Other methods such as transmission spectroscopy or orbital photometry are probably needed in order to give additional constraints and break degeneracies.

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