* Astronomy

Title: Towards the Minimum Inner Edge Distance of the Habitable Zone
Authors: Andras Zsom, Sara Seager, Julien de Wit

We explore the minimum distance from a host star for an exoplanet to be potentially habitable, in order to maximize future chances of finding other habitable worlds. We find that the inner edge of the Habitable Zone (HZ) for hot desert worlds is at 0.5 AU around a solar-like star (well within the orbit of Venus). The relative humidity is the key controlling factor in determining the inner edge distance because water vapour has a strong impact on the greenhouse warming of the atmosphere, yet too little water vapour will deactivate precipitation and enable CO2 to accumulate. We estimate that a relative humidity as low as 1% can be sufficient to maintain a liquid water cycle and wash out CO2 from the atmosphere. If the surface pressure is too low (~0.1 bar), the water loss timescale of the planet is too short to support life. If the surface pressure is too high (~100 bars), we show using atmospheric circulation arguments, that the day-night side temperature difference on slow rotators and tidally locked planets is too small to enable an active water cycle. In contrast, the temperature difference on fast rotators with high surface pressure can be large enough to produce rain. Intermediate surface pressures (~1-10 bars) can provide suitable conditions for a water cycle independent of the planetary rotation period. We additionally find that the water loss timescale is influenced by the atmospheric CO2 level, because it indirectly influences the stratospheric water mixing ratio. If the CO2 mixing ratio of dry planets is smaller than 10^{-4}, the water loss timescale is ~1 billion years, which may be too short for complex life to evolve.

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Researchers develop model for identifying habitable zones around stars

Researchers searching the galaxy for planets that could pass the litmus test of sustaining water-based life must find whether those planets fall in whats known as a habitable zone. New work, led by a team of Penn State researchers, will help scientists in that search.
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More planets could harbour life

New computer models suggest there could be many more habitable planets out there than previously thought.

"So traditionally people have said that if a planet is in this Goldilocks zone - not too hot and not too cold - then it can have liquid water on its surface and be a habitable planet" - Sean McMahon, the PhD student from Aberdeen University.

But researchers are starting to think that the Goldilocks theory is far too simple.

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Title: Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler
Authors: Wesley A. Traub

Data from Kepler's first 136 days of operation are analyzed to determine the distribution of exoplanets with respect to radius, period, and host-star spectral type. The analysis is extrapolated to estimate the percentage of terrestrial, habitable-zone exoplanets. The Kepler census is assumed to be complete for bright stars (magnitude <14.0) having transiting planets >0.5 Earth radius and periods <42 days. It is also assumed that the size distribution of planets is independent of orbital period, and that there are no hidden biases in the data. Six significant statistical results are found: there is a paucity of small planet detections around faint target stars, probably an instrumental effect; the frequency of mid-size planet detections is independent of whether the host star is bright or faint; there are significantly fewer planets detected with periods <3 days, compared to longer periods, almost certainly an astrophysical effect; the frequency of all planets in the population with periods <42 days is 29%, broken down as terrestrials 9%, ice giants 18%, and gas giants 3%; the population has a planet frequency with respect to period which follows a power-law relation dN/dP ~ P^{\beta - 1}, with \beta = 0.71 ±0.08; and an extrapolation to longer periods gives the frequency of terrestrial planets in the habitable zones of FGK stars as \eta_\oplus = (34 ±14)%. Thus about one-third of FGK stars are predicted to have at least one terrestrial, habitable-zone planet.

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