Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars

Hydrogen escaping from a pair of exoplanets smaller than Neptune

Exoplanet surveys have shown a class of abundant exoplanets smaller than Neptune on close, <100-day orbits1-4. These planets form two populations separated by a natural division at about 1.8 R⊕ termed the radius valley. It is uncertain whether these populations arose from separate dry versus water-rich formation channels, evolved apart because of long-term atmospheric loss or a combination of both5-14. Here we report observations of ongoing hydrogen loss from two sibling planets, TOI-776 b (1.85 ± 0.13 R⊕) and TOI-776 c (2.02 ± 0.14 R⊕), the sizes of which near the radius valley and mature (1-4 Gyr) age make them valuable for investigating the origins of the divided population of which they are a part. During the transits of these planets, absorption appeared against the Lyman-α emission of the host star, compatible with hydrogen escape at rates equivalent to 0.03-0.6% and 0.1-0.9% of the total mass per billion years of each planet, respectively. Observations of the outer planet, TOI-776 c, are incompatible with an outflow of dissociated steam, suggesting both it and its inner sibling formed in a dry environment. These observations support the strong role of hydrogen loss in the evolution of close-orbiting sub-Neptunes5-8,15,16.

A radius valley between migrated steam worlds and evaporated rocky cores

The radius valley (or gap) in the observed distribution of exoplanet radii, which separates smaller super-Earths from larger sub-Neptunes, is a key feature that theoretical models must explain. Conventionally, it is interpreted as the result of the loss of primordial hydrogen and helium (H/He) envelopes atop rocky cores. However, planet formation models predict that water-rich planets migrate from cold regions outside the snowline towards the star. Assuming water to be in the form of solid ice in their interior, many of these planets would be located in the radius gap contradicting observations. Here we use an advanced coupled formation and evolution model that describes the planets' growth and evolution starting from solid, moon-sized bodies in the protoplanetary disk to mature Gyr-old planetary systems. Employing new equations of state and interior structure models to treat water as vapour mixed with H/He, we naturally reproduce the valley at the observed location. The model results demonstrate that the observed radius valley can be interpreted as the separation of less massive, rocky super-Earths formed in situ from more massive, ex situ, water-rich sub-Neptunes. Furthermore, the occurrence drop at larger radii, the so-called radius cliff, is matched by planets with water-dominated envelopes. Our statistical approach shows that the synthetic distribution of radii quantitatively agrees with observations for close-in planets, but only if low-mass planets initially containing H/He lose their atmosphere due to photoevaporation, which populates the super-Earth peak with evaporated rocky cores. Therefore, we provide a hybrid theoretical explanation of the radius gap and cliff caused by both planet formation (orbital migration) as well as evolution (atmospheric escape).

A cool runaway greenhouse without surface magma ocean

Water vapour atmospheres with content equivalent to the Earth's oceans, resulting from impacts1 or a high insolation2,3, were found to yield a surface magma ocean4,5. This was, however, a consequence of assuming a fully convective structure2-11. Here, we report using a consistent climate model that pure steam atmospheres are commonly shaped by radiative layers, making their thermal structure strongly dependent on the stellar spectrum and internal heat flow. The surface is cooler when an adiabatic profile is not imposed; melting Earth's crust requires an insolation several times higher than today, which will not happen during the main sequence of the Sun. Venus's surface can solidify before the steam atmosphere escapes, which is the opposite of previous works4,5. Around the reddest stars (Teff  <  3,000 K), surface magma oceans cannot form by stellar forcing alone, whatever the water content. These findings affect observable signatures of steam atmospheres and exoplanet mass-radius relationships, drastically changing current constraints on the water content of TRAPPIST-1 planets. Unlike adiabatic structures, radiative-convective profiles are sensitive to opacities. New measurements of poorly constrained high-pressure opacities, in particular far from the H2O absorption bands, are thus necessary to refine models of steam atmospheres, which are important stages in terrestrial planet evolution.

Magnesium oxide-water compounds at megabar pressure and implications on planetary interiors

Magnesium Oxide (MgO) and water (H₂O) are abundant in the interior of planets. Their properties, and in particular their interaction, significantly affect the planet interior structure and thermal evolution. Here, using crystal structure predictions and ab initio molecular dynamics simulations, we find that MgO and H₂O can react again at ultrahigh pressure, although Mg(OH)₂ decomposes at low pressure. The reemergent MgO-H₂O compounds are: Mg₂O₃H₂ above 400 GPa, MgO₃H₄ above 600 GPa, and MgO₄H₆ in the pressure range of 270-600 GPa. Importantly, MgO₄H₆ contains 57.3 wt % of water, which is a much higher water content than any reported hydrous mineral. Our results suggest that a substantial amount of water can be stored in MgO rock in the deep interiors of Earth to Neptune mass planets. Based on molecular dynamics simulations we show that these three compounds exhibit superionic behavior at the pressure-temperature conditions as in the interiors of Uranus and Neptune. Moreover, the water-rich compound MgO₄H₆ could be stable inside the early Earth and therefore may serve as a possible early Earth water reservoir. Our findings, in the poorly explored megabar pressure regime, provide constraints for interior and evolution models of wet planets in our solar system and beyond.

Three types of planets orbit red dwarfs

Precise densities of red dwarf exoplanets help distinguish potential "water worlds".