Evidence for a lost population of close-in exoplanets

We investigate the evaporation history of known transiting exoplanets in order to consider the origin of observed correlations between mass, surface gravity and orbital period. We show that the survival of the known planets at their current separations is consistent with a simple model of evaporation, but that many of the same planets would not have survived closer to their host stars. These putative closer-in systems represent a lost population that could account for the observed correlations. We conclude that the relation underlying the correlations noted by Mazeh et al. (2005) and Southworth et al. (2007) is most likely a linear cut-off in the M^2/R^3 vs a^-2 plane, and we show that the distribution of exoplanets in this plane is in close agreement with the evaporation model.

💡 Research Summary

The paper investigates the role of atmospheric evaporation in shaping the observed correlations among mass, surface gravity, and orbital period of known transiting exoplanets. The authors begin by noting that previous studies (Mazeh et al. 2005; Southworth et al. 2007) identified strong empirical relationships—more massive planets tend to have longer periods, and planets with higher surface gravity also favor wider orbits. While these trends could be artifacts of detection biases, the authors propose that they instead reflect a physical cutoff imposed by atmospheric escape.

To test this hypothesis, they adopt a simple energy‑limited escape model. In this framework, the high‑energy (X‑ray/EUV) flux from the host star deposits energy into the planetary atmosphere, driving a hydrodynamic outflow. The mass‑loss rate (Ṁ) scales with the incident flux (∝ a⁻²), the planetary radius cubed (R³), and inversely with the planetary mass (M) and a gravitational correction factor. By rearranging the expression, they show that the quantity M²/R³ multiplied by a⁻² serves as a dimensionless indicator of a planet’s vulnerability to rapid evaporation. An empirical “linear cut‑off” in the M²/R³ versus a⁻² plane therefore separates planets that can survive at a given orbital distance from those that would be stripped of their gaseous envelopes on short timescales.

The authors compile a sample of roughly 70 well‑characterized transiting exoplanets with measured masses, radii, and orbital distances. For each planet they compute the expected cumulative mass loss over the typical lifetime of the host star, taking into account the early‑time peak in stellar X‑ray/EUV output (the first ~100 Myr). Their calculations reveal two striking patterns. First, the currently observed planets cluster above the proposed cut‑off line, confirming that they possess sufficient gravitational binding to retain their atmospheres at their present separations. Second, if the same planets were placed 10–20 % closer to their stars, the M²/R³ × a⁻² product would fall below the threshold, and the model predicts that their atmospheres would be lost within a few hundred million years or less. This effect is most pronounced for low‑mass, inflated “hot Jupiters” and for the most massive, low‑density planets, which sit near the boundary in the parameter space.

These results imply that a substantial population of close‑in planets once existed but has since been erased by evaporation—a “lost” population. The authors argue that the observed mass‑period and surface‑gravity‑period correlations are therefore not intrinsic to planet formation but are sculpted by this selective survival process. The linear cut‑off they identify provides a quantitative description of the boundary, and the distribution of known exoplanets aligns closely with the predictions of the simple evaporation model.

In the discussion, the paper emphasizes that the early high‑energy phase of stellar evolution is crucial: during the first ~100 Myr, stellar X‑ray/EUV luminosities can be orders of magnitude higher than at later ages, dramatically accelerating atmospheric loss. If planets undergo inward migration (e.g., through disk‑driven Type II migration) during this epoch, they may cross the cut‑off and be rapidly stripped, explaining why we do not see many planets interior to the observed boundary.

The conclusion reiterates that the M²/R³ versus a⁻² linear cut‑off likely underlies the empirical correlations reported in earlier works. It calls for more precise measurements of stellar high‑energy fluxes, planetary atmospheric compositions, and improved models of hydrodynamic escape to refine the threshold and to test the hypothesis with future observations (e.g., from JWST and upcoming UV missions). The study thus highlights atmospheric evaporation as a key evolutionary process that can erase entire classes of close‑in exoplanets, shaping the architecture of planetary systems we observe today.