Why do Neptune Class Exoplanets Appear to be the Most Common?

An Overabundance of Low-density Neptune-like Planets

Authors:

Cubillos et al

Abstract:

We present a uniform analysis of the atmospheric escape rate of Neptune-like planets with estimated radius and mass (restricted to Mp<30m 167="" 25="" 27="" a="" and="" are="" at="" atmospheres="" atmospheric="" atom="" compare="" compute="" confirm="" consistent="" driven="" each="" energy-limited="" equilibrium="" escape="" estimate="" evaluated="" exhibit="" expected="" extreme="" extremely="" for="" further="" high-energy="" high="" hy="" hydrodynamic="" hydrogen-dominated="" hydrogen="" identify="" jeans="" lhy="" mass-loss="" mass="" maximum-possible="" models.="" of="" out="" parameter="" planet="" planetary="" planets="" radius="" rates.="" rates="" restricted="" sample="" simultaneously="" suggest="" tailored="" temperature.="" that="" the="" these="" to="" values="" we="" with="">0.1M⊕Gyr−1), well in excess of the energy-limited mass-loss rates. This constitutes a contradiction, since the hydrogen envelopes cannot be retained given the high mass-loss rates. We hypothesize that these planets are not truly under such high mass-loss rates. Instead, either hydrodynamic models overestimate the mass-loss rates, transit-timing-variation measurements underestimate the planetary masses, optical transit observations overestimate the planetary radii (due to high-altitude clouds), or Neptunes have consistently higher albedos than Jupiter planets. We conclude that at least one of these established estimations/techniques is consistently producing biased values for Neptune planets. Such an important fraction of exoplanets with misinterpreted parameters can significantly bias our view of populations studies, like the observed mass--radius distribution of exoplanets for example.