TOWARD THE MINIMUM INNER EDGE DISTANCE OF THE HABITABLE ZONE

Abstract

We explore the minimum distance from a host star where an exoplanet could\npotentially be habitable in order not to discard close-in rocky exoplanets for\nfollow-up observations. We find that the inner edge of the Habitable Zone for\nhot desert worlds can be as close as 0.38 AU around a solar-like star, if the\ngreenhouse effect is reduced ($\\sim$ 1% relative humidity) and the surface\nalbedo is increased. We consider a wide range of atmospheric and planetary\nparameters such as the mixing ratios of greenhouse gases (water vapor and\nCO$_2$), surface albedo, pressure and gravity. Intermediate surface pressure\n($\\sim$1-10 bars) is necessary to limit water loss and to simultaneously\nsustain an active water cycle. We additionally find that the water loss\ntimescale is influenced by the atmospheric CO$_2$ level, because it indirectly\ninfluences the stratospheric water mixing ratio. If the CO$_2$ mixing ratio of\ndry planets at the inner edge is smaller than 10$^{-4}$, the water loss\ntimescale is $\\sim$1 billion years, which is considered here too short for life\nto evolve. We also show that the expected transmission spectra of hot desert\nworlds are similar to an Earth-like planet. Therefore, an instrument designed\nto identify biosignature gases in an Earth-like atmosphere can also identify\nsimilarly abundant gases in the atmospheres of dry planets. Our inner edge\nlimit is closer to the host star than previous estimates. As a consequence, the\noccurrence rate of potentially habitable planets is larger than previously\nthought.\n