Researchers have reached a key new milestone in the continuing search for worlds like Earth that could support life elsewhere in the universe. For the first time, water vapor has been detected in the atmosphere of an exoplanet that resides within its star's so-called habitable zone—the temperate band of orbits where water can remain liquid on a planetary surface. Water in its liquid form is deemed all but essential for biochemistry. While highly unlikely to be habitable itself, the exoplanet in question—designated K2-18b—will be a prime target for further research, given its relative proximity (only 110 light-years distant) and intriguing characteristics. See also: Astrobiology; Atmosphere; Biochemistry; Earth; Exoplanet; Light-year; Star; Universe
First discovered in 2015 by the now-retired Kepler spacecraft during the "K2" phase of its mission, the thusly named K2-18b has about eight times the mass of Earth. Its surface is probably blanketed under a thick, unbreathable atmosphere composed mostly of hydrogen and helium. Given this set of properties, K2-18b is at the boundary of what astronomers call a super-Earth (a giant, rocky planet) and a mini-Neptune, where much of a planet's bulk is dominated by gases. Neither of these kinds of worlds exist in our solar system, therefore complicating a firm classification of K2-18b and its attendant properties. See also: Gas; Helium; Hydrogen; Kepler mission; Neptune; Solar system
The presence of water in the exoplanet's atmosphere became known through observations by the Hubble Space Telescope. The orbiting observatory captured light that filtered through K2-18b's atmosphere as the planet transited (crossed the face of) its star. The light bore absorption signatures of water, revealed through spectroscopic analysis. This technique will be used on an ever-expanding number of exoplanets to tease out the compositions of their atmospheres, looking for certain "biosignature" combinations of gases—for instance, water vapor, oxygen, and methane—that could not plausibly exist in their respective concentrations through geological processes alone. Missions such as the Transiting Exoplanet Survey Satellite (TESS), which launched in 2018, are poised to provide hundreds of nearby and newfound worlds of interest for in-depth study in the years ahead. Bit by bit, the scientific community is moving closer to the much-anticipated discovery of an Earth twin. See also: Hubble Space Telescope; Spectroscopy; The Transiting Exoplanet Survey Satellite (TESS) mission begins; Transit (astronomy)
