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How much water is in that exoplanet?
Thousands of exoplanets have been identified, but little is known about their atmospheres, especially for bodies smaller than Jupiter. The extent and composition of an atmosphere can provide evidence for how an exoplanet formed. Wakeford et al. used the Hubble and Spitzer space telescopes to measure the spectrum of the atmosphere around HAT-P-26b, a Neptune-sized exoplanet discovered in 2011. They detected signatures of water and clouds; this allowed them to constrain the atmospheric composition, which appears not to have been altered substantially since it formed.
Science, this issue p. 628
A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System and has served as a cornerstone of planet-formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 micrometers, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands with a maximum base-to-peak amplitude of 525 parts per million in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b’s atmospheric heavy element content ( times solar). This likely indicates that HAT-P-26b’s atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.