On Earth, water is so intertwined with life that our search for life on other worlds is essentially a search for water. When scientists find exoplanets around distant stars, a primary consideration is if they’re in the stars’ habitable zones where liquid water could persist on the planet’s surface. The search for atmospheric biosignatures takes a backseat to the search for water.
Even though scientists have found many planets in their stars’ purported habitable zones, they still can’t be certain if there’s liquid water. That could change if NASA’s proposed Habitable Worlds Observatory (HWO) ever comes to fruition. The HWO is a proposed infrared/optical/ultraviolet space telescope designed to search for and characterize potentially habitable exoplanets. The mission’s primary goas as it stands is to directly image at least 25 potentially habitable worlds.
The HWO would also have the capability to detect surface water, according to the telescope’s Characterizing Exoplanets sub working group. They outline how in a new paper titled “Detecting Surface Liquid Water on Exoplanets.” It will be presented at HWO2025 and submitted to Astronomical Society of the Pacific. The lead author is Nicolas Cowan from the Department of Physics at McGill University in Montreal, Canada.
“Planets with large bodies of water on their surface will have more temperate and stable climates, and such planets are the ideal places for life-as-we-know-it to arise and evolve,” the authors write. “A key science case for the Habitable Worlds Observatory (HWO) is to determine which planets host surface liquid water.” Detecting water directly would tell us a lot about exoplanets and astrobiology, and would also help scientists refine their theories of planet formation and how water makes its way to planets.
Liquids reflect light differently than solids, and that’s at the heart of the HWO’s water-detecting capability. Liquids have smoother surfaces and that means they have distinct light signatures through specular reflection. Specular reflection, also called glint, is when parallel light waves reflect off of a very smooth surface and remain parallel after reflecting. The smoother the surface, the greater their parallelism. “Liquids have smoother surfaces than most solids, and hence exhibit specular reflection instead of diffuse reflection,” the authors write.
Due to specular reflection, oceans and large bodies of water appear dark from most angles. But when viewed from the angle where specular reflection sends light, they appear bright and even mirror-like. On exoplanets that rotate, the HWO should be able to detect large bodies of water.
“Rotational variability in the reflectance of an exoplanet may reveal surface features rotating in and out of view, including oceans,” the authors explain. “Orbital
changes in reflectance and polarization, meanwhile, are sensitive to the scattering phase function of the planetary surface, including specular reflection from large bodies of water.”
An exoplanet’s orbit isn’t the only way the the HWO could detect water through specular reflection. Many exoplanets in the habitable zones of red dwarfs are tidally-locked and don’t rotate. In those cases, orbital phase variations can reveal the differences in reflected light from solid surfaces and liquid surfaces. Detecting oceans on these planets could require special techniques, but it could also be easier because surface features remain fixed.
“Given that discovering an ocean on an exoplanet would confirm its status as a habitable world, this science case is literally the raison d’etre of the
Habitable Worlds Observatory.” – Cowan et al. 2025.
This figure shows planetary reflectivity (top; total flux) and polarization (bottom; polarized flux) as a function of scattering angle (or phase angle) and wavelength (colors). Glories, rainbows, Rayleigh scattering, and ocean glint generate strong polarization signatures and subtle reflectance signatures. The tell-tale shoulder of glint at crescent phases is apparent on the right. Image Credit: Vaughan et al. 2023.
There’s more complexity to detecting exoplanet oceans. The quality of the reflected light can depend on the exoplanet’s orbital phase. “Surface oceans can therefore be identified as the dark regions when the surface of a planet is rotationally mapped near quadrature or gibbous phases, and then appear reflective —and linearly polarizing— at crescent phases,” the researchers explain. A gibbous phase is when an exoplanet is more than half illuminated by the Sun, but not fully illuminated.
However, as in many things in exoplanet science, this specular reflection doesn’t immediately jump out and announce itself. The signal is there to be detected, but it’s not necessarily so easy to discern.
The HWO water-detecting techniques outlined in the paper take time to get right. “All of the proposed techniques for identifying surface water rely on precise, time-resolved measurements of the planet’s reflectance,” the authors write. To do that, we need accurate rotational mapping of the exoplanet’s surface. Since we don’t have these measurements for exoplanets, the researchers use Earth as a case study.
“Rotational mapping requires instrument stability on the timescale of the planet’s rotational period,” the researchers explain. Earth’s is 24 hours and scientist can plausibly expect that exoplanets’ rotational periods are within an order of magnitude of that. “Although instrument stability on these shorter timescales should not pose a challenge for HWO, the photometric precision may be a challenge, especially for rapidly-rotating worlds,” the authors write.
That means that the HWO would need at minimum four integrations per rotation to measure an exoplanet’s rotational period. That gets more complicated when cloud cover is present. “Clouds present a challenge in all cases: one can only map the surface of cloud-free regions of a planet, and time varying clouds greatly complicate the mapping exercise,” Cowan and his co-authors write.
The researchers say that mapping the surface of exoplanets with HWO would benefit from greater photometric precision and simultaneous multi-band photometry.
On the left is a one-dimensional color map of Earth based on 24 hours of disk-integrated multi-band photometry. On the right is a two-dimensional surface map recovered from simulated full-orbit multi-band observations of a cloud-free Earth twin; in this simulation the observer was at northern latitudes and hence could not map the southern portion of the planet. In either of these maps, one can identify the major landforms and oceans of Earth, plausibly identifying this planet as habitable. Image Credit: Cowan et al. 2025.
The HWO won’t be alone in its efforts to detect habitable exoplanets, if it does get built. The ESO’s Extremely Large Telescope (ELT) will see first light in the next few years. Hopefully, if US science doesn’t suffer deep cuts, so will an American ELT, though not for a few years after that. These telescopes should be able to search for biosignatures on at least a small number of terrestrial planets orbiting nearby red dwarfs. The Large Interferometer For Exoplanets (LIFE), another proposed mission, could indirectly detect oceans on exoplanets orbiting Sun-like stars.
The authors explain that LIFE may be able to detect how a large ocean would moderate seasonal temperature variations on an exoplanet if it observed it for a long time. “Such climatological constraints would be complementary to the ground truth provided by HWO,” the authors write.
The reliable detection of a large ocean on an exoplanet would be an exciting discovery. It would open the door to a new understanding of exoplanets, their potential habitability, and even how water gets delivered to exoplanets.
“Indeed HWO is uniquely capable of identifying surface liquid oceans via their optical properties. Given that discovering an ocean on an exoplanet would confirm its status as a habitable world, this science case is literally the raison d’etre of the Habitable Worlds Observatory,” the authors conclude.