Space: many exoplanets may lack Earth-like cores

Space reported on May 24 that a growing body of exoplanet research is pushing back on a familiar assumption: that most planets are built like Earth, with a relatively small, dense metallic core wrapped in rock. The challenge is aimed at sub-Neptunes — planets larger than Earth and smaller than Neptune — which are among the most common worlds found in the galaxy. Space said recent spectroscopy and interior-structure work suggests many of those planets may be water-rich or otherwise internally unlike the terrestrial planets in our solar system. ### Which planets are at the center of this debate? Kepler-era surveys showed that sub-Neptunes and super-Earths are common around other stars, even though the solar system has no close equivalent. A Nature paper published in January 2026 described compact systems of Earth-to-Neptune-size planets as the galaxy’s most common known planetary systems, underscoring why their interiors matter for planet formation models. (space.com) Aritra Chakrabarty, Gijs D. Mulders, Artyom Aguichine and Natalie Batalha focused one recent study on close-in planets with orbital periods under 100 days. Their paper, accepted for publication in *The Astrophysical Journal*, tested whether many sub-Neptunes could be explained as “water worlds” rather than scaled-up rocky planets with Earth-like interiors. ### What is the new claim about their interiors? The February 2026 paper said updated interior models of water worlds with steam atmospheres reproduce the observed “radius valley” near about 2 Earth radii more accurately than a simple Earth-like picture. (nature.com) In that framework, the split between smaller super-Earths and larger sub-Neptunes can arise from differences in bulk composition, with some planets forming as rocky bodies and others as water-rich ones. (arxiv.org) The same study inferred that rocky planets in its sample peak near 2.6 Earth masses, while water-rich planets peak near about 7 Earth masses. It also estimated that the water mass fraction of the modeled water worlds peaks near 41%, a result that points to interiors very different from Earth’s iron-core-and-silicate-mantle structure. ### Does that mean metallic cores are gone entirely? The model does not say every common exoplanet lacks a dense core. (arxiv.org) Instead, it argues that Earth’s internal layout may not be the default template for many sub-Neptunes. The paper found water worlds fit much of the observed population below roughly 3 Earth radii, while larger planets in the sample still appear to require a hydrogen-helium-rich component. Space framed that result as a broader warning against assuming that a “small, dense metallic heart” is universal. (arxiv.org) That is an inference from population-level modeling, not a direct drill-down measurement of any one planet’s center. Astronomers usually infer interiors indirectly from mass, radius and atmospheric data, and a 2025 review noted that interpreting exoplanet interiors remains difficult because different internal compositions can produce similar observed properties. ### Where does spectroscopy fit into this? Atmospheric spectroscopy gives researchers a way to narrow the list of possible interiors because gases above a planet can carry clues about bulk composition. NASA’s Exoplanet Modeling and Analysis Center describes that broader effort as combining atmosphere, interior and population tools to study exoplanet characteristics and environments. A separate 2026 study on sub-Neptune spectra said multi-molecule observations are needed to distinguish between competing atmospheric models and warned that single-parameter assumptions can misclassify interiors. (space.com) That is why researchers are pairing population statistics from Kepler-like samples with newer atmospheric measurements from instruments such as the James Webb Space Telescope. ### What should readers watch next? The next test is whether atmospheric measurements and mass-radius data keep supporting water-rich or otherwise non-Earth-like interiors for sub-Neptunes. (emac.gsfc.nasa.gov) Papers such as Chakrabarty and colleagues’ 2026 study already point researchers toward that comparison, especially for planets smaller than about 3 Earth radii and for the minority of larger sub-Neptunes that appear to need hydrogen-helium envelopes. (arxiv.org 1) (arxiv.org 2)