HWO could study nearby gas giants—and search for their moons

Beyond the search for another Earth

NASA’s developing Habitable Worlds Observatory, or HWO, is intended to directly image Earth-sized planets around nearby stars and investigate their atmospheres. Beck Dacus and colleagues argue that the capabilities needed for that demanding task could also make nearby gas giants valuable scientific targets.

Their study forecasts observations rather than reporting new planet or moon discoveries. It was published in the Journal of Astronomical Telescopes, Instruments, and Systems on October 24, 2026; the supplied arXiv version was revised on November 5.

Gas giants reflect more light than otherwise comparable smaller planets, making detailed measurements possible in less time. Their atmospheric chemistry could help researchers investigate how planets accumulated solid material and gas during formation.

Simulating planets around 164 stars

The researchers used the EXOSIMS mission simulator to place a hypothetical Jupiter-radius planet around each of 164 nearby stars identified as promising HWO targets. They modeled two environments: planets receiving Earth-like amounts of starlight and colder planets receiving approximately Jupiter-like illumination.

The atmospheric models included clouds and absorption by methane, water and ammonia. Simulated observations accounted for photon noise, foreground and circumstellar dust, detector noise, and residual starlight left after suppression by a coronagraph—an instrument that reduces the star’s glare.

The assumed telescope had a 7.87-meter primary-mirror diameter. Every planet was placed at quadrature, a half-illuminated phase at maximum projected separation in the simulated geometry. These are controlled comparisons, not a prediction that every target star actually hosts such a planet.

Under the modeled high-performance conditions, Jupiter-sized planets in both illumination regimes exceeded a broadband visible-light signal-to-noise ratio of five after just five minutes. This measures the strength of the planet’s signal relative to the modeled noise; it does not establish how many planets a survey would find.

Chemistry and changing clouds

Ten-hour near-infrared observations could provide substantial information about giant-planet atmospheres. The team explored spectral resolving powers from 100 to 10,000, where larger numbers mean the instrument distinguishes finer wavelength details.

Rather than relying only on individual spectral measurements, the analysis combined the expected absorption pattern of each molecule across the observing band. This template-matching approach also incorporated wavelength-correlated noise from residual starlight.

Methane and ammonia were promising targets. Water was less straightforward in cold Jupiter twins: condensation removed most water vapor from the atmosphere above their ammonia clouds, weakening its spectral signature. The detailed results therefore qualify the abstract’s broader description of detecting all three molecules.

Finer spectral resolution could help separate overlapping molecular features, but spreading light across more detector pixels increases the importance of detector noise. Detecting an absorption pattern also does not by itself establish how precisely a molecule’s abundance can be measured.

Repeated brightness measurements offer another opportunity. One-hour observations reached approximately one-percent precision or better for all simulated Earth-illuminated giants and for Jupiter twins within seven parsecs. Monitoring over tens of hours could reveal rotation-related variability comparable to Jupiter’s, potentially constraining rotation periods and atmospheric structure.

Detecting moons through shadows

The researchers also simulated brightness changes when a moon casts a shadow on its planet or enters the planet’s shadow. Such events could reveal a moon without separately resolving its image.

For an Earth-sized moon on a roughly 42-hour orbit around an Earth-illuminated Jupiter, one suitably observed shadow event could be detectable within about ten parsecs. Approximately 400 hours of monitoring could cover nine orbits. If the relevant events occurred, were captured and were accurately modeled, combining them could make Mars-sized moons detectable within roughly ten parsecs.

These are conditional sensitivities, not discovery guarantees. Large-moon occurrence rates are unknown, and orbital alignment and reflectivity affect detectability. The study examined different sources of brightness variability separately rather than jointly modeling their interference.

An ambitious atmospheric possibility

For a hypothetical Earth-like moon with an oxygen-rich atmosphere, roughly 400 hours of spectroscopy at resolving power 1,000 might reveal oxygen within about seven parsecs, despite photon noise from the brighter parent planet.

That prospect requires the giant planet’s spectrum to be modeled extremely accurately. It is not evidence that such moons exist or host life.

Another major limitation concerns the coronagraph’s usable field: the simulations imposed no outer working-angle cutoff. Some nearby giants with strong predicted signals could fall outside the region where starlight is adequately suppressed. The authors’ central recommendation is therefore a design consideration: preserving access to gas giants could broaden HWO’s scientific return, but actual discoveries will depend on instrument choices and the planetary systems available.