Hubble Space Telescope detects water vapor on habitable-zone exoplanet

Bryan Dyne

Astronomers using observations taken with the Hubble Space Telescope have for the first time found evidence of water vapor in the atmosphere of an exoplanet that is potentially a rocky world like Earth and orbiting in its parent star’s Òhabitable zone,Ó the region where liquid water could pool on the planet’s surface.

The discovery was made by two independent teams of researchers, one based in Britain and the other in Canada and the United States, using archived data from Hubble, the Kepler Space Observatory and the Spitzer Space Telescope. The work is the culmination of four years of effort by dozens of scientists and hundreds of engineers who have analyzed different parts of the data and helped maintain the spacecraft.

The exoplanet, K2-18b (also known as EPIC 201912552 b), was originally identified in 2015 by the teams that operated the now-decommissioned Kepler, one of more than 1,200 worlds discovered during the second phase of that telescope’s mission. It was found by carefully cataloging periodic drops in the brightness of the red dwarf star K2-18, using eight different observation campaigns over three years made by Hubble’s advanced Wide Field Camera 3 and after ruling out other possible ways the star’s light could be dimmed.

An artist's conception of K2-18b as it orbits its parent star. Credit: ESA/Hubble, M. Kornmesser

Water vapor was found using similar but more precise techniques. As the exoplanet passes in front of its star, not only does the brightness change, some of the wavelengths (colors) of light emitted are absorbed by the planet’s atmosphere, which acts like a filter. This in turn provides information about the chemical makeup of the atmosphere, revealing hydrogen and helium in addition to water vapor. The team from Britain also suspects that there might methane and nitrogen in the atmosphere, but those molecules have yet to be detected.

One of the main reasons this extrasolar system was chosen as a target for such consistent study is that early analyses of K2-18b revealed the possibility that the surface temperature of the world is within five degrees Celsius of that of Earth and thus capable of supporting liquid water and complex organic molecules, necessary ingredients for life as we know it. The exoplanet is also one of many of those discovered that have masses greater than that of Earth, the heaviest of the four rocky planets that orbit our Sun, and smaller than Neptune, the smallest of the four gas giants. These are poorly understood because they have no analog in our Solar System. Since we know they exist, however, they still must be incorporated into models of planetary formation.

The atmosphere of the exoplanet is also relatively easy to observe. The star K2-18 is much dimmer as compared to our Sun, which means there is less light to obscure the planet, but not so dim that there was too little light to pass through the exoplanet’s atmosphere. The exoplanet itself orbits the star every 33 days, which allowed for multiple observations in short span of time. The target star is also much smaller than the Sun, making the signal caused by the planet transiting in front of it more detectable.