JWST Ruled Out Finding Tiny Moons Around An Exoplanet — And That's Actually Great News
The James Webb Space Telescope (JWST) has fundamentally transformed our understanding of the cosmos since it commenced full science operations in mid-2022, delivering breathtaking imagery of distant galaxies, probing the atmospheric chemistry of alien worlds, and peering back toward the very dawn of the universe. Yet amid this cascade of discoveries, one tantalizing prize has remained stubbornly out of reach: the detection of an exomoon — a natural satellite orbiting a planet in another star system entirely.
Exomoons remain, for now, a wholly theoretical class of object in astronomy. While our own solar system boasts over 290 confirmed moons, including scientifically celebrated worlds like Europa, Enceladus, and Titan — all considered prime candidates in the search for extraterrestrial life — not a single moon beyond our solar system has been unambiguously confirmed. JWST was widely expected to change that, yet so far it has returned precisely zero confirmed exomoon detections. A growing concern among astronomers has been whether instrumental noise represents an insurmountable barrier for the telescope, effectively imposing a detection "noise floor" that masks the subtle signals these tiny worlds would produce.
Now, a compelling new study by Dr. David Kipping, an astronomer at Columbia University and the creator of the widely acclaimed Cool Worlds YouTube channel, offers not only a diagnosis of the problem but a genuinely promising path forward. The paper, currently available in pre-print form on arXiv, demonstrates that JWST is, in fact, physically capable of detecting small exomoons — it simply needs to look at the same target repeatedly, across multiple transits, rather than relying on a single observation.
The Elusive Signal of an Exomoon
To understand why finding exomoons is so extraordinarily difficult, it helps to appreciate the detection method astronomers rely upon. The most productive technique for finding exoplanets — and by extension, their moons — is the transit method. When a planet passes in front of its host star from our vantage point on Earth, it causes a tiny, measurable dip in the star's brightness. A moon orbiting that planet would cause an additional, even tinier dip in light — a signal so faint it can easily be buried beneath the background hiss of instrumental and astrophysical noise.
The scale of this challenge is almost humbling. Earth's Moon, if observed transiting the Sun from a distant star system, would block only about 0.007% of the Sun's light. Detecting a signal of that magnitude across interstellar distances, amid the thermal fluctuations of a space-based detector and the seething activity of a distant star's surface, demands extraordinary precision and, critically, extraordinary patience.
"JWST doesn't have a noise floor that prevents it from detecting relatively small moons — it's very capable of finding moons the size of our own, provided astronomers decide to observe multiple transits." — Dr. David Kipping, Columbia University
Red Noise: The Hidden Enemy of Exomoon Hunters
A central villain in this story is a phenomenon known as "red noise" — a type of correlated, low-frequency noise that is particularly insidious in astronomical observations. Unlike white noise, which is random and averages out relatively quickly with repeated measurements, red noise is structured and slow-varying, mimicking genuine astrophysical signals in ways that can thoroughly confuse the algorithms designed to detect them.
Red noise in JWST data can arise from several sources:
- Detector drift: As the telescope's infrared detectors thermally settle or experience slight temperature fluctuations, the measured brightness of a target star can wander gradually over time.
- Pointing jitter: Tiny, slow drifts in the telescope's orientation can shift the position of a star's image on the detector, causing apparent brightness changes.
- Stellar variability: Starspots — the analogues of sunspots on distant stars — rotate across the stellar disk, causing brightness variations that can masquerade as a transiting moon signal.
- Instrument systematics: Each detector pixel has slightly different sensitivity characteristics; as a star drifts slowly across pixels, the recorded brightness changes accordingly.
This red noise problem came sharply into focus when JWST observed the gas giant Kepler-167 e in a dedicated exomoon hunt. Despite high hopes, the red noise so thoroughly corrupted the data models that astronomers found themselves unable to detect any moon substantially smaller than Earth itself — a deeply discouraging result for a telescope of JWST's otherwise remarkable capability. For context, the moons most astrobiologists are excited about — worlds like Europa and Enceladus — are considerably smaller than Earth, making this an especially painful limitation.
LP 890-9 c: A Proof-of-Concept Target
To rigorously demonstrate his multi-transit approach, Dr. Kipping turned to a fascinating exoplanet called LP 890-9 c, also catalogued as SPECULOOS-2c. This rocky world orbits an ultra-cool red dwarf star located approximately 105 light-years from Earth — a stellar neighbor by cosmic standards, though still an almost incomprehensible 990 trillion kilometers away.
LP 890-9 c has several properties that make it scientifically compelling, quite apart from the moon-hunting exercise:
- Its orbital period is just 8.46 days, meaning it completes a full year in less time than it takes for a typical business meeting to recur monthly.
- Despite its scorchingly close orbit — approximately 0.04 astronomical units (AU) from its star, less than one-tenth the distance between Mercury and the Sun — it is theoretically located within the habitable zone of its extremely cool host star. Red dwarfs emit far less heat and light than our Sun, so their habitable zones crowd much closer in.
- JWST had observed the planet across twelve separate transits, providing Dr. Kipping with an unusually rich dataset to analyze — a rarity for exoplanet targets, where observing time is fiercely rationed.
It is worth noting, however, that Dr. Kipping was candid about the moon-hunting prospects for this particular planet from the outset. At 0.04 AU, the tidal forces exerted by the host star on any hypothetical moon would be ferocious. Over geological timescales of billions of years, such tidal interactions would almost inevitably either strip a moon away entirely, flinging it into deep space, or drag it inward until it disintegrates into a ring system. In short, LP 890-9 c is almost certainly moonless by now, regardless of what it may have hosted early in its history. The target was chosen not because it was likely to yield a discovery, but because it was ideal for demonstrating and stress-testing a methodology.
The Power of Averaging: How Multiple Transits Cut Through the Noise
The core insight of Dr. Kipping's paper is elegantly grounded in fundamental physics and statistics. Exomoons must obey Newtonian orbital mechanics. At any given transit, a moon will occupy a predictable location relative to its host planet — governed by its orbital period, inclination, and eccentricity. It cannot randomly teleport to wherever an instrumental glitch happened to deposit a spurious signal. This physical determinism is the key that unlocks the multi-transit approach.
Instrumental artifacts and stellar variability, by contrast, are largely stochastic — they do not repeat with the same character from one transit to the next. A detector pixel that behaved anomalously during one observation will not necessarily do so in the next. A starspot that rotated across the stellar disk during one transit will be in a different position — or absent entirely — the next time the planet crosses. When data from multiple transits are combined and averaged, these incoherent noise sources begin to cancel one another out, while a genuine, physically consistent moon signal accumulates and strengthens.
Strikingly, even one of the twelve LP 890-9 c transits in Dr. Kipping's dataset showed conspicuous red noise contamination. Yet when that single problematic transit's data was combined with just one clean transit, the team's sensitivity to detecting an exomoon rose significantly — a powerful demonstration of the technique's efficiency. The noise does not need to be perfect in every individual observation; the averaging process is remarkably forgiving.
A Null Result That Changes Everything
Despite the methodological success, the scientific conclusion for LP 890-9 c itself was a null result — as Dr. Kipping expected. The analysis confirmed, with 95% statistical confidence, that LP 890-9 c hosts no moons larger than 0.1 Earth radii. To put that in physical terms: that rules out any moon as large as Io, Europa, or even the much smaller Enceladus — Saturn's geologically active, ocean-harboring moon that measures just 0.04 Earth radii in radius.
But in science, a well-constrained null result can be just as valuable as a positive detection. This represents, by a considerable margin, the most sensitive exomoon search ever conducted beyond our solar system. Previous constraints from other telescopes and methodologies have been far less stringent. By establishing that JWST possesses no inherent noise floor preventing small-moon detection, Dr. Kipping's work clears the path for future targeted searches — searches aimed at planets where moons are actually expected to survive.
The implications extend beyond exomoon science. The multi-transit stacking technique has potential applications across a wide range of subtle transit phenomena, including the detection of exoplanetary rings, trojan asteroids, and even the atmospheric hazes of small rocky worlds. Learn more about JWST's broader scientific mission at the NASA JWST Mission Page and the ESA Webb Space Telescope portal.
Where Should JWST Look Next?
The obvious next question is: which exoplanets are the most promising targets for a genuine exomoon detection campaign? Several criteria emerge naturally from Dr. Kipping's analysis and from broader theoretical work on moon stability:
- Larger orbital separation: A planet orbiting farther from its star experiences weaker tidal stripping, allowing moons to persist over longer timescales. Gas giants in wide orbits — analogues to Jupiter or Saturn — are prime candidates.
- Multiple available transits: The multi-transit technique demands repeated JWST observations, so shorter-period planets — despite their other drawbacks — offer more frequent transit opportunities within a given observing season.
- Quiet host stars: Choosing stars with minimal starspot activity reduces the astrophysical component of red noise, simplifying data interpretation.
- Large moon Hill sphere: A planet's Hill sphere — the region within which it can gravitationally retain a moon against the tidal pull of its star — must be large enough to accommodate a stable lunar orbit. This generally favors massive planets in wide orbits.
The Kepler-1625 system and the Kepler-1708 system have both produced tantalizing but unconfirmed exomoon candidate signals in archival Kepler telescope data, also from Dr. Kipping's research group. These systems — featuring giant planets in wide orbits around sun-like stars — represent natural next targets for the kind of dedicated multi-transit JWST campaign this new paper advocates. You can explore the broader landscape of confirmed exoplanets and candidate systems through the NASA Exoplanet Archive.
The Precious Currency of JWST Time
There is an unavoidable tension at the heart of this exciting prospect. Observing time on JWST is among the most fiercely competed resources in all of modern science. The telescope's schedule is carved up through a rigorous peer-review process, balancing demands from cosmologists studying the epoch of reionization, planetary scientists tracking solar system bodies, exoplanet atmospheric specialists, and galaxy evolution researchers — among dozens of other communities. Dedicating the multiple transits required for a sensitive exomoon search represents a substantial time investment that must compete with all of these other pressing scientific priorities.
Yet the payoff of a first confirmed exomoon detection would be extraordinary. Such a discovery would not merely add a new class of object to the astronomical catalogue — it would open entirely new lines of inquiry into the formation and evolution of planetary systems, the prevalence of potentially habitable environments beyond Earth, and the dynamical histories of alien worlds. Moons may be among the most common abodes for liquid water in the universe, sheltered by tidal heating even in the frozen outer reaches of planetary systems far from their stars — much as Europa and Enceladus maintain subsurface oceans in our own solar system. For a deeper look at the science of potentially habitable moons, visit the NASA Astrobiology Institute.
A Door Opened, Not Closed
It would be easy to read a headline about JWST failing to find exomoons and conclude that the telescope has disappointed. The reality, as Dr. Kipping's meticulous analysis makes clear, is almost the precise opposite. JWST has not failed to find exomoons because it is incapable — it has not found them yet because, until now, astronomers were not using the right strategy. The noise that frustrated earlier single-transit searches is not a fundamental physical barrier; it is an engineering and observational challenge that the multi-transit averaging approach can systematically overcome.
The null result at LP 890-9 c is, in the most literal sense, a proof of concept. It proves the concept works. The method is sound, the telescope is capable, and the universe almost certainly contains exomoons waiting to be found — statistical arguments based on the abundance of moons in our own solar system suggest they should be extraordinarily common. We are not asking whether JWST can find an exomoon. We are now asking, simply, when it will be given the chance to look in the right place, with the right strategy, for long enough.
And when that moment comes, the discovery could rank among the most profound in the history of planetary science.
