The large number of exoplanets and planet candidates detected thus far provides significant observational constraints for theoretical studies on planet formation and evolution. Most of the exoplanets and candidates are orbiting their host star at close-in orbits. For them, the atmospheric escape driven by strong stellar X-Ray and EUV irradiation plays an important role in planetary evolution. A theoretical study of the statistical impact of evaporation and a comparison between the exoplanets could give us a better understanding of atmospheric escape.

   Astronomers at PMO, MPIA and OCA investigated the statistical impact of atmospheric escape. They simulate the thermal evolution of theoretical planet populations using several evaporation models, which are distinguished by the driving force of the escape flow (X-ray or EUV), the heating efficiency in energy-limited evaporation regimes, or both. They found that the planetary radius distributions clearly show a break at approximately 2 Earth Radii. As a result, an “evaporation valley” running diagonally downward in the orbital distance versus planetary radius plane appears (Figure 1). This evaporation valley is not sensitive to the heating efficiency of the description of evaporation. Corresponding to this valley, the one-dimensional radius distribution of close-in low-mass planets is bimodal, with a local maximum at about 1 Earth radius, a local minimum at about 1.5 Earth radii and another maximum at 2-3 Earth radii. The lower maximum in the bimodal distribution corresponds to the bare cores of planets that have lost their entire initial H/He envelope. The minimum corresponds to the “evaporation valley”. The second maximum corresponds to low-mass planets that have kept some primordial H/He (Figure 2).

   The specific shape and location of the second maximum at 2-3 Earth radii in the bimodal distribution is related to envelope evaporation. Stronger evaporation produces a lower outer maximum and moves the peak to smaller radii. Most of our evaporation models lead to a similar outer peak, which is approximately consistent with the size distribution of the Kepler candidates in the radius range of about 2-8 Earth radii. In two extreme cases, the simulation without any evaporation and the simulation with very strong evaporation (100% heating efficiency), the final planet size distributions show clear differences compared with the Kepler data in this range. This indicates that evaporation is indeed important in shaping the characteristics of close-in, low-mass planets.

  This wor k is supported of the joint doctor training program between the Chinese Academy of Sciences and the Max-Planck-Gesellschaft, the National Natural Science Foundation of China, the innovative and interdisciplinary program by CAS, and the Strategic Priority Research Program - The Emergence of Cosmological Structures of the Chinese Academy of Sciences, etc.

This work has been published by ApJ, please see ApJ, 795, 65 for more details. http://iopscience.iop.org/0004-637X/795/1/65/pdf/apj_795_1_65.pdf