Jean-Marc Petit

The Colours of the Outer Solar System Origins Survey (Col-OSSOS) measured the optical/near-IR colors of a brightness-complete sample of trans-Neptunian objects (TNOs). Like previous surveys, this one found a bimodal color distribution in TNOs, categorized as red and very red. Additionally, this survey proposed an alternative surface classification scheme: FaintIR and BrightIR. Cold classical TNOs mostly have very red or FaintIR surfaces, while dynamically excited TNOs show a mixture of surfaces. This likely indicates that formation locations and proximity to the Sun influenced surface characteristics and color changes. Our study combines the data from Col-OSSOS with two dynamical models describing the formation of the Kuiper Belt during Neptune’s migration. We investigate the proposed surface-color-changing line and explore the distribution of different surfaces within the primordial disk. By comparing radial color transitions across various scenarios, we explore the origins of surface characteristics and their implications within the context of BrightIR and FaintIR classifications. Moreover, we extend our analysis to examine the distribution of these surface classes within the present-day Kuiper Belt, providing insights into the configuration of the early solar system’s planetesimal disk prior to giant planet migration. We find that the most likely primordial disk compositions are inner neutral/outer red (with transition 30 . 0 − 1.2 + 1.1 au) and inner BrightIR/outer FaintIR (with transition 31 . 5 − 1.2 + 1.1 au).

We report the discovery and careful orbital determination of 64 new irregular moons of Saturn found in images taken using the Canada–France–Hawaii Telescope from 2019 to 2021, bringing the total number of Saturnian irregulars to 122. By more than doubling the sample of Saturnian irregular moon orbits, including pushing to smaller sizes, we can now see finer detail in their orbital distribution. We note the emergence of potential subgroups associated with each of Siarnaq and Kiviuq within the Inuit group. We find that in the inclination range 157 ^∘ –172 ^∘ , the ratio of smaller moons (diameters less than 4 km) to larger moons (diameters greater than 4 km) is significantly larger than that of any other inclination range in the retrogrades. We denote this subset of the Norse group as the Mundilfari subgroup after its largest member. The incredibly steep slope of the Mundilfari subgroup’s size distribution, with a differential power law index of q = 6, strengthens the hypothesis in E. Ashton et al. that this subgroup was created by a recent catastrophic collision, 8 yr ago.

The outer Solar System preserves a reservoir of material from the formation of our planetary system and provides evidence of the distant past. Objects in the Trans-Neptunian region have experienced little to no thermal or collisional processing, and objects classified as dynamically cold are expected to still be on primordial orbits. The orbital dynamics and physical structure of the objects found in the Trans-Neptunian region allow us to learn how the Solar System formed and developed. This distant region allows us to study the primordial building blocks of the Solar System that no longer exist in an unmodified form in the inner Solar System.Objects in Mean Motion Resonance (MMR) with Neptune are one of the more significant populations in the distant solar system (beyond distances of 70 au). Resonant populations have very specific orbital characteristics (angular orbital elements, semi-major axis, etc.) that result in varying but quantifiable detectability for each resonance. We define distant resonances as those beyond, but not including, the 2:1 resonance. We include all resonances at semimajor axes greater than 48 au, and with a sufficient number of detections to be statistically relevant. Using recent improvements in the determination of the size-frequency distribution of the Kuiper belt we provide new estimates of the population sizes of these distant resonant orbits. These population estimates provide a key input into understanding the expectation for the discovery of objects at large heliocentric distances.Our knowledge of the Trans-Neptunian region is incomplete. As we examine larger heliocentric distances our detection efficiency rapidly decreases. Trans-Neptunian Objects (TNOs) can only be directly observed in reflected sunlight which varies with an r-4 relationship. Due to the extreme faintness of TNOs at large heliocentric distances, an extremely significant population of objects could exist. At distances as close as 90 au [1] populations could rival the size of the known Kuiper belt while still escaping detection, see Figure 1. If every object in the Kuiper belt (Hr < 9) were concentrated into a narrow, low inclination ring, this ring would be on the cusp of detection at 100 au. It is clear that significant populations with novel structure and valuable insights into the Solar System could exist in the regions beyond 70 au where detection rapidly becomes increasingly difficult.In order to examine this region, it is of vital importance to understand what we already know to exist in this distant region. While probing this region directly is very difficult, the population that resides there is not completely unknown. Known objects with semi major axis beyond ~70 au are usually either very large, on eccentric orbits, or both. Large objects are easier to detect as they reflect more light. Objects with eccentric orbits have perihelia much closer to the sun and therefore become much brighter allowing easier detection near their perihelion passage. The most prominent known populations with these large eccentricities are objects in resonance with Neptune, with many possessing large eccentricities and aphelion distances >70 au. As we probe fainter magnitudes, and therefore greater distances, it is important to be able to disentangle known populations from possible new populations.The Vera C. Rubin Observatory Legacy Survey of Space and Time will revolutionize our understanding of the Solar System, however this survey will be less sensitive to the most distant Trans-Neptunian regions. LSST will increase the number of known TNOs by an order of magnitude. However, despite massively increased sky coverage, it will have a similar limiting magnitude as the OSSOS survey (~24.5). This results in an effective distance limit of 80-90 au for all but the most massive TNOs. See Figure 2 for a survey simulation of the OSSOS survey (orange points) and note the steep drop off in detections in the 80-90 au range. These results will be analogous to the depth of LSST, except with a much larger sky coverage for LSST. Furthermore, the currently planned LSST Deep fields are not on the ecliptic and will be insensitive to objects on low inclination orbits. This highlights the need for a dedicated deep drilling field on the solar ecliptic to enable the exploration of possible in-situ formed component in the distant Solar System.The regions of the Solar System beyond 70 au are a fascinating frontier. In the examination of this region it is important to be able to distinguish between known “excited” populations like resonant objects and their large eccentricities and the possible discovery of unknown “cold” populations with low inclination and eccentricity distributions. An entire second cold classical belt could exist at 90 au and it would have escaped detection by modern surveys. As currently planned, LSST, while incredibly powerful, is not the right mechanism to explore this region and dedicated search efforts will be required if we wish to learn more about the distant reaches of our Solar System. Study of objects in resonance with Neptune are the first step toward doing so.Figure 1 – Graph of the upper limit on the number of objects with Hr < 9 before detection would be likely at increasing heliocentric distance. Each ring represents a simplified orbital toy model. The number of objects was determined by the OSSOS Survey Simulator and characterization data for several modern surveys. Horizontal lines represent the number of objects in the cold Kuiper belt (blue), hot Kuiper belt (orange), and both combined (purple).Figure 2 – Figure demonstrating oversampled OSSOS Survey Simulator output for several modern surveys and how their detection numbers drop off with distance for a >70 au heliocentric distance ring. Note the (orange) OSSOS++ points that correspond to a detection limit of ~24.5, which is broadly similar to the LSST survey limit. LSST will have...

Here we present results of CLASSY: the Classical and Large-A Solar SYstem survey. Running into its third year, this 2-year CFHT Large Program was allotted 75 nights from 2022B through 2024A inclusive, with a 16 night extension in 2024B and 2025A to accommodate time lost due to equipment failure. Using shift’n’stack techniques, CLASSY has been surveying 5 independent pointings (10.1 square degrees total) of the cold classical belt’s forced midplane in search of Kuiper Belt Objects and very distant extreme trans-Neptunian Objects. Field opposition locations are chosen to be spaced as evenly as possible in ecliptic longitude and each spans a two-month window (AS: Aug-Sept, ON, JF, MJ, JA). The survey design involves 5 visits to each field across the first year (discovery), with observations of these fields at a second opposition one year later for some fields, and 2 years later for others (tracking). The survey has achieved limiting magnitudes of r~26.5, and we expect to have an >80% recovery rate during tracking. Due to the nature of the observations which by design are drifted at rates that approximate typical main-belt Kuiper Belt Objects, highly distant and/or inclined objects tend to sheer off the tracking pointings, resulting in a reduced recovery rate for objects on those orbits. To compensate, a pointed recovery effort at Magellan and supplemented at Palomar has accompanied the main CFHT program, successfully recovering many of our discoveries on more extreme orbits. To date, all discovery fields have been acquired, and all fields have received a complete year 2 or year 3 follow-up, thereby completing the main CLASSY observing program. The main science goals of CLASSY are to measure the size distribution of the cold classical belt to absolute magnitudes as faint as H_r~10, and to provide a census of extreme TNOs with minimal and well measured bias in ecliptic longitude. By nature of the robust pre/recovery during the first year, free inclinations can be calculated with sufficient accuracy to provide a surprisingly robust separation between members of the cold classicals and the more excited TNO populations, thereby enabling robust measurements of the luminosity functions of each population separately. We find that both populations exhibit the same luminosity functions to within the precision of the observations. Our observations reveal that the cold classical Kuiper Belt Objects exhibit an absolute magnitude distribution that is somewhat shallower than a direct extrapolation of the tapered exponential that accurately describes the size distribution measured for larger and brighter objects (see Figure 1). The inferred differential power-law slope over the range of CLASSY discoveries q~-2.5, revealing a decreasing mass per size, and suggests that what ever formation process resulted in these objects, it preferentially resulted in objects with diameters D~150 km, where the size distribution rolls over to this shallower slope. Even though our analysis clearly demonstrates sensitivity to objects at distances as far as 200 au, CLASSY has found a surprising dearth of eTNOs, discovering no more than 1 in the three fields we have searched to date (AS, ON, JF) though at time of writing, the year 2 and 3 follow-up observations have not been included in our analysis resulting in large uncertainties in orbital semi-major axis. At the distances over which most eTNOs are found, CLASSY is sensitive to objects with sizes smaller than the roll-over size of objects in the more proximate main Kuiper Belt, D~150 km. This preliminary result implies that eTNOs either exhibit a different size distribution and thus, possess relatively fewer large bodies than the main belt, or that these objects have subdued albedos compared with main belt objects, or both. We will conclude with a discussion of these and other prospects that can address the observed paucity of eTNOs. Figure 1: The preliminary absolute magnitude distribution of the cold classicals derived from the observations of AS (blue) and JF (orange) blocks. The thin black outline marks the cold classical Kuiper Belt Object H-distribution presented by Kavelaars et al. (2021) and the thick grey curve displays the best-fit tapered exponential of the cold classicals presented by Napier et al. (2024). ReferencesKavelaars et al. (2021) OSSOS Finds an Exponential Cutoff in the Size Distribution of the Cold Classical Kuiper Belt ApJ, 920L, 28KNapier et al. (2024) The DECam Ecliptic Exploration Project (DEEP). V. The Absolute Magnitude Distribution of the Cold Classical Kuiper Belt, PSJ Vol 5, 270N.

We highlight the announcement by the IAU Minor Planet Center of 128 new irregular moons of Saturn discovered in our CFHT imaging acquired in 2023. We were able to link 83 of the new moons to detections from our earlier 2019–2021 study. Of the 58 new retrograde moons with multiyear arcs, 46 are likely in the Mundilfari subgroup. This large fraction is in line with the steep size distribution of the subgroup and the idea that it was created by a recent collision. Also, by more than tripling the number of Kiviuq subgroup members, the new discoveries have confirmed that this subgroup is a very tightly grouped collisional family, although likely much older.

A proposed Vera C. Rubin Observatory Deep Drilling micro-survey of the Kuiper Belt will investigate key properties of the distant solar system. Utilizing 30 hours of Rubin time across six 5-hour visits over one year starting in summer 2026, the survey aims to discover and determine orbits for up to 730 Kuiper Belt Objects (KBOs) to an $r$-magnitude of 27.5. These discoveries will enable precise characterization of the KBO size distribution, which is critical for understanding planetesimal formation. By aligning the survey field with NASA's {\it New Horizons} spacecraft trajectory, the micro-survey will facilitate discoveries for the mission operating in the Kuiper Belt. Modeling based on the Outer Solar System Origin Survey (OSSOS) predicts at least 12 distant KBOs observable with the {\it New Horizons} LOng Range Reconnaissance Imager (LORRI) and approximately three objects within 1~au of the spacecraft, allowing higher-resolution observations than Earth-based facilities. LORRI's high solar phase angle monitoring will reveal these objects' surface properties and shapes, potentially identifying contact binaries and orbit-class surface correlations. The survey could identify a KBO suitable for a future spacecraft flyby. The survey's size, depth, and cadence design will deliver transformative measurements of the Kuiper Belt's size distribution and rotational properties across distance, size, and orbital class. Additionally, the high stellar density in the survey field also offers synergies with transiting exoplanet studies.

Following the Pluto flyby of the New Horizons spacecraft, the mission provided a unique opportunity to explore the Kuiper Belt in situ. The possibility existed to fly by a Kuiper Belt object (KBO), as well as to observe additional objects at distances closer than are feasible from Earth-orbit facilities. However, at the time of launch no KBOs were known about that were accessible by the spacecraft. In this paper we present the results of 10 yr of observations and three uniquely dedicated efforts—two ground-based using the Subaru Suprime Camera, the Magellan MegaCam and IMACS Cameras, and one with the Hubble Space Telescope—to find such KBOs for study. In this paper we overview the search criteria and strategies employed in our work and detail the analysis efforts to locate and track faint objects in the Galactic plane. We also present a summary of all of the KBOs that were discovered as part of our efforts and how spacecraft targetability was assessed, including a detailed description of our astrometric analysis, which included development of an extensive secondary calibration network. Overall, these efforts resulted in the discovery of 85 KBOs, including 11 that became objects for distant observation by New Horizons and (486958) Arrokoth, which became the first post-Pluto flyby destination.

Context. We report the exploitation of a sample of Solar System observations based on data from the third Gaia Data Release (Gaia DR3) of nearly 157 000 asteroids. It extends the epoch astrometric solution over the time coverage planned for the Gaia DR4, which is not expected before the end of 2025. This data set covers more than one full orbital period for the vast majority of these asteroids. The orbital solutions are derived from the Gaia data alone over a relatively short arc compared to the observation history of many of these asteroids. Aims. The work aims to produce orbital elements for a large set of asteroids based on 66 months of accurate astrometry provided by Gaia and to assess the accuracy of these orbital solutions with a comparison to the best available orbits derived from independent observations. A second validation is performed with accurate occultation timings. Methods. We processed the raw astrometric measurements of Gaia to obtain astrometric positions of moving objects with 1D sub-mas accuracy at the bright end. For each asteroid that we matched to the data, an orbit fitting was attempted in the form of the best fit of the initial conditions at the median epoch. The force model included Newtonian and relativistic accelerations to derive the observation equations, which were solved with a linear least-squares fit. Results. Orbits are provided in the form of state vectors in the International Celestial Reference Frame for 156 764 asteroids, including near-Earth objects, main-belt asteroids, and Trojans. For the asteroids with the best observations, the (formal) relative uncertainty σ_a/a is better than 10⁻¹⁰. Results are compared to orbits available from the Jet Propulsion Laboratory and MPC. Their orbits are based on much longer data arcs, but from positions of lower quality. The relative differences in semi-major axes have a mean of 5 × 10⁻¹⁰ and a scatter of 5 × 10⁻⁹.