I'm guessing that real data about exoplanets will be unhelpful because it's hard to detect low-mass planets, and thus answers are likely to be based on the kind of educated-guesswork that we relied on for decades when we really only knew about planets in the solar system.


  • "No atmosphere"

    I'm looking for things like Earth's Moon, or Mars, about which Wikipedia says:

    The average surface pressure [of Mars] is only about 610 pascals (0.088 psi) which is less than 1% of the Earth's value.

    So, surface pressure no greater than 1% of Earth's atmosphere.

  • 3
    $\begingroup$ This is currently impossible to answer - a Superearth with a Venusian atmosphere would look the same as a bare core in transit spectroscopy. Ground based high resolution spectro might do the trick when the extremely large telescope come online. $\endgroup$ Commented Jan 26, 2023 at 23:49
  • 1
    $\begingroup$ We can only list plausible cthonian planets i.e gas giants having atmosphere more specifically hot jupiters, because it is really tough to find out without much uncertainty. One way to find planets having hydrodynamic escape is that if it is near the host star, we can measure the temperature, then we need to estimate the gravity to calculate whether it is a cthonian, many of these steps require spectroscopy $\endgroup$
    – user47732
    Commented Jan 28, 2023 at 15:16
  • $\begingroup$ My guess would be a mean of 1-3 planets having hydrodynamic escape in a solar system. $\endgroup$
    – user47732
    Commented Jan 29, 2023 at 6:33
  • $\begingroup$ In near future, NASA Roman space telescope is planned to make gratational microlensing studies. There is hope it will constrain the abundances of small (mars-sized and below) exoplanets. $\endgroup$
    – Heopps
    Commented Jan 31, 2023 at 12:43
  • 3
    $\begingroup$ @Tom For a recent look with James Webb, see this paper on the arxive: ui.adsabs.harvard.edu/abs/2023arXiv230104191L/abstract Even for a $1 R_{\oplus}$, 1 $m_{\oplus}$ planet at only 12.5 pc distance, with the best instrument available, we cannot distinguish whether what we see is a thick, cloudy Venus-like atmosphere or bare rock. $\endgroup$ Commented Jan 31, 2023 at 17:01


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