Using NASA’s powerful James Webb Space Telescope (JWST), Mercedes López-Morales and colleagues measured the wavelengths of light traveling through the atmosphere of WASP-39b, an exoplanet, or planet outside our solar system. Different molecules absorb different wavelengths of light, and the wavelength measurements showed the presence of carbon dioxide (CO₂) in WASP-39b’s atmosphere. This finding not only offers the first decisive evidence of CO₂ in the atmosphere of an exoplanet but also illustrates the potential for future scientific breakthroughs held by the JWST.