Discovery of Water on 51 Peg b

A spectrum of light with bright emission lines and dark absorption lines indicating elements and molecules.

This is an excerpt from my post on Astrobites:

Title: Discovery of water at high spectral resolution in the atmosphere of 51 Peg b
Authors: J. L. Birkby, R. J. de Kok, M. Brogi, H. Schwarz, and I. A. G. Snellen
First Author’s Institution: Harvard-Smithsonian Center for Astrophysics
Status: Accepted into the Astronomical Journal, open access

51 Peg b was the first exoplanet ever discovered orbiting another main sequence star (51 Peg). This Jupiter-sized planet, found orbiting in a 4-day orbit, revolutionized astronomy and upended our understanding of planet formation. It was discovered by measuring the star’s spectrum and seeing periodic shifts in the star’s radial velocity. This radial-velocity shift was caused by the planet gravitationally pulling on the star, which indirectly proves the existence of the planet. However, even early on, it was realized that astronomers should be able to see a similar radial-velocity shift in the light reflected by the planet. Critically, this method could used to determine the planet’s inclination, mass, and atmospheric composition, properties that would otherwise be near impossible to measure. First used in 2010, it has now been used on several hot Jupiters.