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You cannot see a rainbow on an exoplanet with your eyes, a backyard telescope, or an ordinary photograph. Astronomers instead look for tiny changes in a planet’s light as it orbits its star, then test whether the pattern fits a particular kind of atmospheric scattering. The leading reported example, WASP-76b, is a possible glory—a rainbow-related optical effect, not an ordinary rainbow—and the interpretation remains unconfirmed.
What does it mean to “see” a rainbow on an exoplanet?
It means detecting and interpreting a light signature, not resolving a colored arc in an eyepiece. Even direct images of exoplanets generally separate a faint point of planet light from the much brighter star; they do not show a planet’s weather as a detailed, colorful landscape. Astronomers infer atmospheric properties from measurements such as brightness at different orbital phases and the way light varies with wavelength or polarization.
That distinction matters because the proposed signal from WASP-76b is subtle and indirect. It is evidence that may be consistent with a glory, not a photograph of one.
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No. A conventional rainbow forms when light refracts through droplets or another medium, with different wavelengths bending by different amounts. A glory is a different scattering effect: colored, concentric rings can appear around the point opposite the light source when light interacts with small particles under particular conditions. The terms are related through atmospheric optics, but a candidate glory should not be described as an ordinary rainbow.
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Glories are also highly dependent on viewing geometry and particle properties. From Earth, a familiar example is the glory seen around an aircraft’s shadow on clouds; the European Space Agency has also described a glory observed at Venus when the Sun, Venus’s clouds, and the Venus Express spacecraft were aligned appropriately.
What is the evidence for a glory on WASP-76b?
In an article dated 5 April 2024, the European Space Agency reported potential signs of a glory on WASP-76b, a planet about 637 light-years away. The analysis used observations from ESA’s Cheops mission alongside data from other ESA and NASA missions, including TESS, Hubble, and Spitzer. The reported signal was a brightness increase associated with the planet’s eastern terminator—the boundary between its day and night sides.
Cheops monitored the planet 23 times over three years in the study described by ESA. Those repeated measurements helped researchers examine how its brightness changed with orbital position; they are not a universal observing recipe or a guarantee that the same number of observations would reveal such a feature elsewhere.
The interpretation is a hypothesis, not a confirmed detection of an exoplanetary glory. ESA reported that the glow “could be caused” by a strong, localized, direction-dependent reflection. If the explanation is confirmed, the signal could provide information about cloud particles. The cloud substance has not been established, so it should not be assumed to be water.
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How do astronomers search for rainbow-like scattering?
Repeated brightness measurements and phase curves
Photometry measures how bright a star-and-planet system appears. By repeating measurements as the planet moves through its orbit, astronomers can build a phase curve and look for changes in reflected light. A reflective cloud region could make a planet appear brighter at some viewing angles than at others. The combined Cheops and TESS observations were part of the evidence examined for WASP-76b.
Spectroscopy
Spectroscopy separates light by wavelength. During a transit, some starlight passes through the planet’s atmosphere before reaching us; the atmosphere can absorb particular wavelengths, leaving patterns that help identify atmospheric constituents. Spectroscopy is a way to characterize an atmosphere, not a camera view of a rainbow. Reflected-light spectra can also be studied, but the planet’s faint signal makes the measurement challenging.
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Polarimetry and spectropolarimetry
Polarimetry measures how light is polarized, while spectropolarimetry tracks polarization across wavelengths. Scattering can leave characteristic polarization patterns, so combining polarization with brightness and wavelength measurements may help distinguish cloud-scattering behavior. A 2023 study of a proposed Habitable Worlds Observatory considered polarized as well as unpolarized reflected-light features. It noted that some features may be more distinct in polarized light even though the planet-to-star contrast is lower.
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A coronagraph blocks much of a star’s light so that a nearby planet can be observed more easily. This can provide planet light without depending solely on a transit, but the planet must be sufficiently separated from its star in the apparent sky, and the instrument must be sensitive enough to the contrast. Even when successful, a direct image may amount to a point of light or a spectrum, not a resolved view of a colored cloud system.
| Approach | What it measures | What it can reveal | Main constraint |
|---|---|---|---|
| Phase-resolved photometry | Brightness changes across the orbit | Potentially localized, direction-dependent reflection | Signals can be small and open to more than one interpretation; repeated observations are useful |
| Spectropolarimetry | Wavelength-dependent brightness and polarization | Scattering behavior and clues about cloud properties | Faint planet light, observing angle, and star–planet contrast |
| Coronagraphic direct imaging | Planet light separated from much of the starlight | Reflected planet light and potentially its spectrum | Instrument contrast limits and the planet’s apparent separation from its star |
Why do the viewing angle and cloud particles matter?
A glory requires a suitable alignment among the illumination source, cloud particles, and observer. ESA’s account of the WASP-76b candidate quotes study lead Olivier Demangeon describing the demanding conditions: “First, you need atmospheric particles that are close-to-perfectly spherical, completely uniform and stable enough to be observed over a long time. The planet’s nearby star needs to shine directly at it, with the observer – here Cheops – at just the right orientation.”
The cloud interpretation remains conditional too. ESA said that, if the candidate is confirmed as a glory, it would imply spherical droplets that persist for at least three years or are continually replenished, with temperatures stable enough to allow them to persist. These are implications of the proposed explanation, not established measurements of the clouds’ composition.
Why can a planet’s orbital phase determine whether a feature is observable?
Reflected-light features are not equally visible from every direction. A 2023 study of the proposed Habitable Worlds Observatory found that rainbow scattering would occur away from quadrature—the viewing geometry in which a planet appears half illuminated—with a modeled phase angle around 40 degrees in the study’s conventions. Whether that angle is observable depends on the system’s orbital inclination and on the coronagraph’s inner working angle, the closest apparent separation from the star at which the instrument can detect a planet.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsThe same study estimated that water-cloud rainbow phase angles would be accessible in about 28% of its target systems under its stated assumptions. That is a modeled accessibility fraction for a proposed observatory, not a prediction that the observatory will detect rainbows, not a statement that 28% of planets have them, and not a measure of a currently operating mission’s performance.
Can you try to observe one yourself?
No consumer telescope or binoculars can resolve an atmospheric rainbow on a planet hundreds of light-years away. The relevant evidence comes from spacecraft measurements and statistical analysis of extremely faint light. A backyard observer can study exoplanets indirectly through carefully collected transit data, but that is a different project and would not reveal a rainbow or glory as a visible feature.
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