The Goldilocks problem: detecting water in terrestrial planets
Scientists explored whether a future space mission can detect habitable conditions through the presence of liquid water.
A future space mission called the Large Interferometer for Exoplanets (LIFE) is in development to search for life beyond the Solar System. This mission would use mid-infrared interferometry to study Earth-like exoplanets and search for classic biosignature gases like ozone and methane. Researchers investigated its ability to map out habitable planets, by determining which ones have stable liquid water on the surface.
Water is considered a key ingredient for life, making it a prime target in the search for habitable worlds. While visible-light telescopes may attempt to directly detect oceans through reflected sunlight, LIFE would instead look for the infrared signatures of water vapor in planetary atmospheres.
To test the mission’s capabilities, the team modelled Earth-like planets with water abundances ranging from extremely dry (Mars-like), to water-rich planets. They simulated how LIFE would observe these planets in the mid-infrared and then performed Bayesian atmospheric retrievals to determine how accurately water abundance could be inferred.
A key focus was how the amount of water vapor in an atmosphere varies with altitude, and three profiles were tested: a vertically constant water profile, an Earth-like profile where water decreases with altitude because of condensation and precipitation, and a diffusion and photochemistry profile, where upper-atmosphere water is controlled by transport and chemical reactions.
The team found that the ability for LIFE to detect water largely depends on the vertical profile assumed.
Planets with very little atmospheric water—comparable to Mars—would likely remain undetectable. At the opposite extreme, planets with extremely water-rich atmospheres could also prove challenging. In those cases, water vapor absorbs so much infrared radiation that it masks its own spectral signatures. The planets which produced the clearest atmospheric signatures are those where water levels are similar to those of Earth.
Thus, water detectability follows a ‘Goldilocks’ principle: too little water is invisible, too much water hides itself, and intermediate levels are easiest to characterise.
The researchers also discovered that assumptions about how water is distributed vertically in an atmosphere significantly affect the results. Simplified models often assume water is evenly mixed throughout the atmosphere, but more realistic Earth-like profiles show water concentrations decreasing with altitude because of condensation and precipitation. These physically realistic profiles allowed LIFE to detect water over a much wider range of conditions.
Although LIFE cannot directly image oceans, detecting water vapor in the atmosphere may be strong evidence for surface liquid water, since water is chemically reactive and would otherwise be removed by interactions with rocks and minerals.
The study concludes that LIFE should be capable of identifying atmospheric water, and this could make it one of the most powerful tools yet developed for identifying potentially habitable worlds beyond our Solar System.