The Unexpected Atmosphere

Core accretion theory predicts that giant planets around M-dwarf stars should be metal-rich. The star is metal-poor, the disk is metal-poor, so the planet needs a proportionally larger solid core to initiate runaway gas accretion. The resulting atmosphere should carry that enrichment signature.

JWST transmission spectroscopy of TOI-5293Ab — a half-Jupiter-mass planet orbiting an M-dwarf — reveals sub-solar metallicity. The atmosphere is less metal-rich than the Sun, orbiting a star less metal-rich than the Sun. The enrichment that formation theory demands is absent.

The through-claim: either the formation pathway didn’t require the expected solid enrichment, or the atmospheric metallicity was diluted by processes not yet accounted for. The planet’s radius is also inflated beyond what thermal models predict at its ~700K equilibrium temperature, adding a second anomaly to the first.

This is one data point, not a refutation of core accretion. But it’s a data point in the direction formation theory doesn’t predict. M-dwarf planets were supposed to be the clearest cases of metal enrichment — the low-metallicity environment should amplify the signature. Instead, the signature is missing.

JWST is now producing atmospheric spectra faster than formation theory can explain them. The predictions were made when atmospheres couldn’t be measured. Now they can, and the first surprise is already here.