The Hunt for Moons of Rogue Planets express
In the vast darkness of interstellar space, the rogue planet WISE 0855 flickers with a barely perceptible heat glow, like a cooling ember. Its temperature is only a few dozen degrees above freezing — if life existed there, it would see a sunless sky, in an eternal icy haze. The James Webb telescope stared intently at this faint light for 11 hours, hoping to catch the shadow of a passing moon — as if a microscopic soot particle momentarily eclipsed a smoldering ember.
Astronomers built a model predicting the “ember’s” wavering and hunted in the data for dips — moon traces. No direct finds occurred, but the researchers tested their method by injecting artificial moons into the data. If a shadow dimmed the light by at least 0.5%, the detector triggered in 96% of cases, corresponding to a Titan-sized moon.
Original abstract
as written by the authors arXiv:2510.24575
JWST is collecting time-series observations of many free-floating planets (FFPs) to study their weather, but these light curves are the ideal datasets to search for exomoons that transit the FFP during observations. In this paper, we present observations of the planetary-mass Y dwarf ($$T=250-285K$$, $$M = 6.5\pm3.5 M_{Jup}$$, d = 2.3$$\,$$pc) WISE J085510.83-071442.5 (WISE 0855), whose proximity and brightness make it ideal for a transiting exomoon search. We examine 11 hours of time-series spectra from the JWST Near-Infrared Spectrograph (NIRSpec) whose sensitivity, in combination with Gaussian process (GP) modeling, allows for the disentanglement of exomoon transits from WISE 0855's variability. We do not find statistically significant evidence of an exomoon transit in this dataset. Using injection and recovery tests of artificial transits for depths ranging between 0.1-1% (0.35-1.12 $$R_{\oplus}$$) we explore the exomoon parameter space where we could successfully detect transits. For transit depths $$\geq 0.5\%$$ (1.96$$\,R_{\text{Titan}}$$), our detection rate is 96%, which, for WISE 0855, corresponds to a moon with a companion-to-host mass ratio similar to that of Titan and Saturn. Given our sensitivity, transit probabilities, and our observational duration, we determine a $$\sim$$91% probability of detecting a Titan mass analog exomoon after 18 such observations if every observed system hosts a Titan mass analog exomoon in a Galilean-like system. This suggests that JWST observations of dozens of FFPs could yield meaningful constraints on the occurrence rate of exomoons. This paper is the first demonstration that JWST is sensitive to Galilean moon mass analogs around FFPs.
WISE 0855 is one of the coldest objects known beyond the Solar System: its temperature is just a few dozen degrees above freezing. If intelligent life were there, it would see a sky without any sun, perpetually shrouded in water-ice fogs.
Rogue planets with moons are almost a ready-made illustration for Isaac Asimov’s story “The Wanderer”: the characters live on a moon of an orphan planet, warmed only by internal heat.