Molten iron rains from the evening skies of an exoplanet designated WASP-76b, according to a new study. WASP-76b is a gas giant—a large planet with a vast gaseous envelope, like Jupiter—located about 390 light-years from Earth. Astronomers used an instrument called ESPRESSO, which is installed at the European Southern Observatory’s Very Large Telescope in Chile, to infer the exotic weather on WASP-76b. The observational technique used should prove adept at characterizing other exoplanets' climatic conditions. See also: Exoplanet; Iron; Very Large Telescope
The iron rain on WASP-76b occurs because of tidal locking, in which one hemisphere of an orbiting body constantly faces another body. The Moon, for instance, is tidally locked to Earth. Tidal locking arises when the gravitational pull between two spinning celestial bodies slows the bodies' respective rotational periods over time. Eventually, the stronger gravitational pull of the larger body slows the rotational rate of the smaller body to match the smaller body's orbital period. As a result, as the bodies orbit each other, the smaller body always presents the same face to the larger body. In WASP-76b's case, the planet is tidally locked to its host star. This phenomenon often occurs when planets orbit their stars very closely. WASP-76b is no exception, orbiting its host star at a miniscule distance of about one-tenth of the distance between Mercury and our Sun. The upshot: the star-facing hemisphere of WASP-76b broils in permanent daylight, while the other hemisphere is trapped in perpetual night. The day side of WASP-76b maintains a temperature in excess of 2400°C (4350°F)—enough to vaporize most metals (including iron)—while the night side is significantly cooler at about 1500°C (2730°F). See also: Gravity; Rotational motion
ESPRESSO detected the spectroscopic signature of iron vapor where WASP-76b's day side transitions to its night side (the location on the world that experiences "evening"). The signature did not appear, however, where the night side transitions into the day side (the equivalent of the world's "morning"). This discovery—the first time researchers have identified chemical variations in an ultra-hot gas giant's atmosphere—implies that iron that vaporized on the day side condenses into liquid when it reaches the night side. Astronomers suspect that powerful winds generated by stark temperature imbalances between the day and night sides blow hot iron gas from the former into the latter. Further spectroscopic studies of exoplanets should find similar instances of major, global chemical variations. See also: Astronomical spectroscopy; Condensation; Spectroscopy