Trans-Neptunian objects (TNOs) are icy planetesimals beyond Neptune's orbit, preserved nearly unchanged for 4 billion years. Their size distribution, derived from the luminosity function, serves as a direct test of planetesimal formation models. Ground-based surveys are limited to brightnesses around magnitude 27, while the Hubble Space Telescope reached only magnitude 28 over an area of 0.02 square degrees, finding just three objects. This work stands apart: the nearest neighbors in the archive study atmospheres of small bodies, dark matter lensing, and stellar cataclysms, but no one has delved this deep into the TNO luminosity function. Webb's data open for the first time the size range of 10–30 km, where the streaming instability hypothesis with collapsing pebble clouds is tested.
Observations were made with the NIRCam camera in ultra-wide filters F150W2 and F322W2 from January 24 to February 4, 2023. A mosaic of 20 fields covered 0.05 square degrees near the ecliptic plane, with each field imaged three times at intervals of about five days. To search for moving objects, the shift-and-stack technique was used: images were aligned according to the candidate's presumed motion, enhancing its signal against stationary stars. A million algorithm detections were run through an ensemble of three residual neural networks trained on artificially injected sources, reducing false tracks to 50,000. Each candidate was then vetted by scene modeling: fitting a moving point-source model on a static background and requiring flux consistency across exposures.
27 reliable TNOs were detected, 18 of them new in three epochs and 7 in two epochs, plus one centaur. The faintest object has a brightness of 29.3 magnitude in F150W2, corresponding to a diameter of about 10 km at 15% albedo. The 40% detection efficiency threshold reached 28.8 magnitude in F150W2, a full magnitude deeper than Hubble's previous record. The luminosity function of the cold and hot populations is described by a single power law with slope α = 0.29 (+0.07/−0.06), with no significant difference between groups. More complex models, including an exponentially tapered power law, offer no advantage according to the Bayesian information criterion.
This work differs from neighboring studies in that it measures the TNO luminosity function slope over a brightness range from 25 to 29 magnitude, a regime neither Hubble nor ground-based surveys have probed. The equally shallow slope for cold and hot populations indicates that the planetesimal formation mechanism is insensitive to local disk conditions, such as temperature and density. This aligns with the streaming instability scenario with collapsing pebble clouds, which predicts a power-law tail without an exponential cutoff down to ~10 km. The shallow luminosity function also means that the bulk of the cold Kuiper Belt's mass resides in bodies around 200 km, not in small debris.
The upcoming LSST survey will increase the TNO sample by an order of magnitude, but its brightness limit of about 8 absolute magnitude will not reach the faint end explored here. Further progress into the realm of even smaller bodies will require either new deep JWST fields or dedicated missions. Of particular interest is testing whether the albedos of cold and hot TNOs converge at small sizes, which would require thermal observations in the mid-infrared.
The result directly constrains planetesimal formation models in protoplanetary disks and refines the total mass of the cold Kuiper Belt, now estimated at 0.01 Earth masses, with the uncertainty set by albedo and density rather than the luminosity function.
The immediate next step is to obtain thermal observations of small TNOs to break the degeneracy between size and albedo. It is also important to expand the sample of distant objects beyond 80 AU, where only one member has been found so far.
It remains unclear why the cold and hot populations share the same slope despite such different dynamical histories. If hot TNOs underwent intense collisional erosion, their luminosity function should be steeper, but the data do not show this. Possibly collisional evolution was weaker than assumed, or the albedos of the two populations converge at small sizes, masking the difference in size distribution.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This work presents the deepest survey of trans-Neptunian objects to date, carried out with NIRCam on JWST. Over 10 days of observations, 27 reliable TNOs down to 10 km in size were detected, using shift-and-stack techniques and machine learning to weed out false candidates. The luminosity function of the cold and hot populations is described by a single power law with a slope of 0.29, pointing to a common formation mechanism for planetesimals.
Where its strength is
For the first time, the slope of the TNO luminosity function has been measured in the magnitude range 25–29, which was previously inaccessible. The equally shallow slope for the cold and hot populations provides a direct observational constraint on planetesimal formation models, in particular on the streaming instability scenario.
Where it could go next
Worth trying
Why this may matter
Measuring the slope of the luminosity function in a new size range directly constrains planetesimal formation models, and the shallow dependence points to a universal mechanism that does not depend on local disk conditions. This is a key step toward understanding the initial mass distribution in the Kuiper Belt and, possibly, in other planetary systems.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.55, against a usual 0.64. It looks like few have drilled nearby.
The faintest reliably detected object, JPB24, is 100 times fainter than the brightest in the survey, 2015 GK56, and its brightness is comparable to a candle flame at the distance of the Moon.