Resolve the relative roles of high-eccentricity migration and atmospheric escape

Determine whether the Neptunian desert is predominantly shaped by high-eccentricity tidal migration, or by a combination in which high-eccentricity tidal migration establishes the upper boundary while atmospheric escape shapes the lower and Neptunian boundaries.

Background

Two principal explanations are discussed for the desert: high-eccentricity tidal migration followed by circularization near the tidal-disruption limit, and irradiation-driven atmospheric escape. The paper notes that atmospheric escape can reproduce the lower boundary in the sub-Neptune regime, whereas the upper boundary appears comparatively stable against photoevaporation.

The confirmed planets TOI-426 b and TOI-1839 b are highly irradiated yet volatile-rich, while the systems’ outer companions provide evidence relevant to tidal migration. Their properties therefore constrain, but do not settle, the relative contribution of these mechanisms.

References

Therefore, whether the desert is predominantly shaped by HEM or instead arises from a combination of HEM setting the upper boundary and atmospheric escape shaping the lower and Neptunian boundaries remains an open question.

— Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert  (2609.05413 - Castro-González et al., 4 Sep 2026) in Section 1, Introduction, paragraph discussing proposed desert mechanisms