Water is critical to life as we know it on Earth. So naturally, finding evidence of liquid water on a planet in its star’s habitable zone is extremely relevant to searches for extraterrestrial life. Thus far, we’ve only discovered water vapor in the atmospheres of massive, short-period gas giants — but new observations of sub-Neptune K2-18b have now changed that.
Hubble Has Its Eye on You
The Kepler spacecraft was a planet-finding expert, using the transit method to identify thousands of exoplanets and even more exoplanet candidates. K2-18b is one of Kepler’s more notable finds, a sub-Neptune orbiting within the habitable zone of its star.
Fits to K2-18b’s white-light curve (top) and an example spectral light curve (bottom) with data points from the Hubble observations overplotted. The white-light curve considers points across all available wavelengths while the spectral light curve considers points across a specific wavelength range. The wavelength range for each plot can be seen in the lower right. [Adapted from Benneke et al. 2019]With an orbital period of 33 days and an M dwarf (K2-18) as its host, K2-18b receives about as much radiation as the Earth does. It occupies an odd space in exoplanet demographics, being ...