Exoplanet Science Sodium Na 2.05% 2.10% 2.20% 2.15% 2.25% 2.30% 0.5 1.00 1.50 2.00 2.50 3.00 Wavelength (microns) 3.50 4.00 4.50 Data 5.00 Best-fit Model Starlight interacts with water molecules in the exoplanet's atmosphere. Water The ultraviolet light splits the water into hydroxide (OH) and hydrogen atoms. UNEXPECTED WIGGLE This first transmission spectrum from WASP-39b revealed the presence of several compounds in the gas giant's atmosphere, including sulfur dioxide - a detection that surprised theorists. (Note that researchers usually display these spectra mirror-flipped vertically, so that the absorption features are " bumps " instead of valleys; we have flipped all transmission spectra in this article to avoid confusing absorption features with emission ones.) Hydrogen Sulfide Hydrogen sulfide in the atmosphere reacts with hydrogen and hydroxide in a series of steps. HOW WASP-39B MAKES SULFUR DIOXIDE A series of steps transforms water and hydrogen sulfide in the planet's atmosphere into the molecule sulfur dioxide. Water O H2 Water O H2 Carbon Monoxide CO Carbon Dioxide CO2 Water O H2 Sulfur Dioxide SO2 Carbon Dioxide CO2 Carbon Monoxide CO Water O H2 This process strips hydrogen away and adds oxygen, eventually producing sulfur dioxide. Sulfur Dioxide 38 DECEMBER 2024 * SKY & TELESCOPE Photon Hydrogen Oxygen Sulfur Fraction of light blocked by planet WASP-39B'S SPECTRUM: NASA / ESA / CSA / JOSEPH OLMSTED (STSCI), ADAPTED BY BEATRIZ INGLESSIS / S&T; CREATION OF SULFUR DIOXIDE: BEATRIZ INGLESSIS / S&T, SOURCE: NASA / JPL-CALTECH / ROBERT HURT