Stellar spin clocks suggest small planets keep changing for billions of years

A gap with a history

Small planets on close orbits fall into two broad size groups: super-Earths, generally interpreted as rocky worlds, and sub-Neptunes, thought to retain hydrogen-and-helium envelopes. Between them lies a shortage of planets roughly 1.5–2 times Earth's radius, known as the radius valley.

Atmospheric loss offers an explanation: removing a gaseous envelope can turn a larger-looking planet into a smaller, exposed core. But how long does that transformation continue? A preprint by Nino Greco and colleagues suggests that the relative abundance of these populations changes mainly during their first two billion years. The manuscript was submitted to Astronomy & Astrophysics; its findings remain provisional.

Using stellar rotation as a clock

The researchers used gyrochronology, which estimates stellar ages by comparing rotation periods and colours with models of rotational evolution. Stellar rotation generally slows with age, providing a clock for comparing planets around different hosts.

Their age-selected sample contained 746 planets around 510 F-, G- and K-type stars, with an allowed age range of 0.1–4 billion years. The planets had radii between half and four times Earth's and orbital periods no longer than 100 days.

The main population analysis used a smaller Kepler sample: 581 planets around 404 stars. For these systems, the team could estimate both detection efficiency and the probability that a detected candidate became a confirmed planet.

That distinction matters. Smaller planets produce shallower transits and are harder to detect and confirm. Simply counting discoveries would therefore understate their abundance. The researchers fitted a statistical mixture of two radius populations, incorporating those selection effects, radius measurement errors and stellar-age uncertainties. They also adjusted age comparisons to use the same distribution of orbital periods and confirmed-companion status.

More super-Earths at older ages

The model inferred a standardised super-Earth population weight of about 43% below one billion years and 67% at two to four billion years. The increase was 24 percentage points, with an uncertainty of about 10 percentage points. There was no statistically significant further increase between the one-to-two-billion-year and two-to-four-billion-year groups.

These are modelled population contributions, not direct counts of bare rocky planets. The two fitted populations overlap in radius, and the analysis includes small planets that are difficult to observe.

An exponential model described the rise towards a plateau. Under the assumption that the older population constrains that plateau, the super-Earth weight reached 90% of its eventual level before 1.9 billion years with 90% posterior probability, among model solutions starting below that threshold. This is an upper bound on when the rise finishes—not proof that it must take that long. Allowing the plateau to vary freely weakened the corresponding bound to 3.8 billion years.

A persistent valley, with a possible shift

The radius valley was consistently distinguishable from about 1.4 billion years onwards. After that, its depth, width and occupancy showed no significant age trend through four billion years.

Its position may nevertheless change. The minimum appeared near 1.5 Earth radii before roughly two billion years and near 1.8 Earth radii later. The super-Earth size distribution also showed possible changes in shape, without a significant change in its median radius. These results were suggestive rather than individually decisive, and identifying the younger valley depended partly on how the distribution was smoothed.

One possible interpretation is that planets with more massive cores finish losing their envelopes later, adding relatively large rocky worlds to the super-Earth population. The study does not directly observe those transitions.

What the clock cannot settle

The strongest limitation is the scarcity of young systems: only seven planets in the reference sample were younger than half a billion years. The youngest host was about 0.3 billion years old, leaving earlier evolution largely inaccessible. Some super-Earths may have formed without substantial envelopes, rather than losing them.

The inference also depends on the assumed shapes of the radius populations, especially where detection efficiency is low. Tests using stronger signals and alternative radius measurements preserved the broad age trend, but cannot remove every modelling uncertainty.

If atmospheric stripping drives the population change, both heat escaping from planetary interiors and prolonged stellar ultraviolet irradiation remain viable explanations. The evidence supports continued evolution for some planets over roughly one to two billion years, without establishing which mechanism dominates or reconstructing any individual planet's history.