An exoplanet's thermal phase curve — the variation in infrared brightness as the planet rotates, showing its day and night sides — encodes the planet's heat transport. A large day-night temperature contrast means poor circulation. A small contrast means efficient atmospheric redistribution. Song, Yang, and Ouyang show that an undetected moon can corrupt this measurement.
When an airless exomoon orbits close to its star, it develops extreme temperature swings — hundreds of degrees between its illuminated and dark hemispheres. As the moon passes behind the planet (occultation) or in front of it (transit), the combined thermal signal changes. The moon's contribution is small but systematic: it adds a periodic modulation to the planet's phase curve that, if not recognized as a separate body, gets absorbed into the planetary temperature analysis.
The bias has a specific direction. An undetected moon makes the planet appear to have a larger day-night temperature contrast than it actually does. The analysis overestimates the temperature difference and underestimates the atmospheric heat transport. A planet with efficient circulation could be misidentified as having poor heat redistribution — the phantom moon making the atmosphere look more stagnant than it is.
The signal strength depends on geometry. For close-in planets, the occultation depth can reach 100 parts per million — large enough to detect but also large enough to distort phase curve fits if the moon's existence isn't assumed. The periodic nature of the signal (following the moon's orbital period, not the planet's) offers a detection pathway, but only if observers look for it.
The absence of evidence for the moon becomes evidence against the atmosphere. A measurement that seems purely planetary is contaminated by a body nobody accounted for.