Astronomers used the James Webb Space Telescope (JWST) to study the atmospheres of two similar small planets that orbit the same Sun‑like star in the TOI‑125 system. The planets, called TOI‑125 b and TOI‑125 c, are both sub‑Neptunes—larger than Earth but smaller than Neptune. TOI‑125 b orbits every 4.65 days and has a radius of 2.7 Earth radii, a mass around 10 Earth masses, and an equilibrium temperature near 1100 K. TOI‑125 c orbits every 9.15 days, has a radius of 2.85 Earth radii, a mass near 6.8 Earth masses, and an equilibrium temperature near 825 K.

The team observed starlight that passed through each planet’s atmosphere during transit. This technique is called transmission spectroscopy: different gases and clouds absorb different wavelengths of light and leave telltale patterns. Observations used JWST’s NIRSpec G395H instrument, which records light in the 3–5 micrometer range at a spectral resolution of about 100. The researchers obtained one full transit of TOI‑125 b and about 70% of a transit of TOI‑125 c; the latter was cut short because the planet’s transit time shifted more than expected (transit timing variations, or TTVs). The resulting spectra have median uncertainties of about 50 parts per million for TOI‑125 b and 85 parts per million for TOI‑125 c.

The data were analyzed with atmospheric retrievals, which are statistical fits that test different compositions and cloud settings. For TOI‑125 b the fits are compatible with either a very metal‑rich atmosphere—about 150 times the Sun’s heavy‑element abundance—or an atmosphere with clouds sitting above roughly 3 millibars of pressure (a millibar is a unit of atmospheric pressure). For TOI‑125 c the fits are compatible with roughly 100 times solar metallicity or clouds above about 10 millibars. “Metallicity” here means the fraction of elements heavier than hydrogen and helium.