Inside-Out Exoplanet System Upends Formation Theory

CHEOPS observations reveal four planets orbiting the red dwarf LHS 1903 far closer to their star than Mercury is to the Sun: two rocky super-Earths and two gaseous mini-Neptunes. The outermost planet, surprisingly rocky, challenges standard models that predict rocky worlds close in and gas giants farther out, suggesting an 'inside-out' assembly in which gas was depleted by inner planets or the atmosphere was stripped after formation. With a mass of about 5.8 Earth masses and a surface temperature around 60°C, it could be marginally habitable, and future JWST studies could probe its atmosphere.