Sean N. Raymond

Venus possesses no natural satellite, raising the question of whether a formed moon could have survived. We explore the tidal evolution of a Venus-moon system, coupling Venus’s spin to the satellite’s orbit under tides from the moon and Sun. We survey spin period ( P ₀ = 5–100 hr), moon mass ( M _m = 0.01–10 M _Moon ), eccentricity, quality factor, and initial semimajor axis under both constant- Q and constant time lag models. Survival depends on competition between outward migration (∝ M _m ) and synchronous radius expansion ( ∝ M m 2 ): for circular orbits around a rapidly spinning Venus ( P ₀ ≲ 12 hr), a lunar-mass satellite survives the age of the solar system in both models. For P ₀ ≲ 10 hr, eccentricity pumping can destabilize low-mass satellites, while for P ₀ ≳ 15 hr or M _m ≳ 2 M _Moon , the synchronous radius overtakes the orbit and drives Roche destruction within ∼0.03–1.7 Gyr in the constant- Q model. The constant time lag model instead permits quasi-synchronous survival for massive moons at fast spin. Explaining Venus’s present state requires satisfying two constraints simultaneously: loss of the satellite and despinning of an initially rapid rotator. Both are met only within a restricted region of parameter space, favoring moderate postimpact spin periods and lunar-to-super-lunar masses. Giant impact simulations predict spin periods ≳12 hr for Venus’s present rotation, placing a lunar-mass satellite at the survival boundary. For last-impact conditions within this region, the present absence of a Venusian satellite arises through tidal evolution alone; a subsequent catastrophic stripping event, while capable of removing a moon, is not required.

As they orbit within the Galaxy, stars swim through a vast population of interstellar objects (ISOs). In this paper, we use N -body simulations to show that a fraction of ISOs within ∼1 pc of the Sun (its tidal radius) may be captured during the flyby of another star – a mechanism that requires no planets. Capture is most efficient when the impulse imparted by the flyby is comparable to the escape speed at the widest stable orbit, which is roughly 0.1 km/s for the Sun. The ISO capture is dominated by the few highest-impulse stellar flybys, typically involving relatively slow encounters with massive stars. Most ISOs are captured in the outer parts of the Oort cloud, with semimajor axes greater than ∼50000 au. Using Monte Carlo simulations, we show that the Sun underwent only a small number of ISO-capturing flybys in its history (median [mean] of 1 [1.7]). Using the Ō tautahi-Oxford population model, we estimate that a few times ∼10 ⁴ ‘Oumuamua-sized ISOs were likely captured by the Sun. This only represents a ∼10 ^−8±1 contribution to the total Oort cloud population, yet it contains roughly as many present-day captured ISOs as Jupiter-assisted capture provides. Given that flybys are unavoidable in the Galactic field, most stars should host sparse Oort clouds populated with ISOs captured during stellar flybys. Massive stars are both the main drivers of capture when they fly by a given star, and more efficient at capturing ISOs around themselves than low-mass stars.

The ubiquity of free-floating planets inferred from microlensing and direct imaging surveys suggests that planetary ejection—a process in which planets initially born encircling a stellar host become gravitationally unbound—is common. Four overarching mechanisms have been proposed to induce planetary ejection: close approaches of neighboring planets, instabilities in binary or multi-star systems, stellar and planetary flybys, and post-main-sequence stellar evolution. Here we review the mechanisms underlying planetary ejection, as well as predictions derived from each. Current and upcoming microlensing surveys offer the potential to test existing models and distinguish between planetary ejection mechanisms, providing further insight into the demographic-level architectures of exoplanets across stellar environments.

It is generally presumed that the tidal field of the Milky Way’s disk is the main perturbation that has driven observed long-period comets (LPCs) from the Oort cloud into the inner solar system. The tide’s influence on the Oort cloud should produce a distinct anisotropy in the arguments of perihelion ( ω ) of dynamically new LPCs with semimajor axes ( a ) over 10 ⁴ au. Simulating LPC production dominated by the Galactic tide, we find that observed dynamically new LPCs are more isotropic than expected. Meanwhile, our simulation exhibits much better agreement between simulated and observed “returning” LPCs that have made a handful of passages through the inner solar system prior to discovery. The isotropy of new LPCs can be explained if the Oort cloud is much less centrally concentrated than the conventional Oort cloud formation model predicts. However, a second possibility also exists. Additional simulations we perform show that the observed ω distributions of new and returning LPCs can both be well replicated if the star HD 7977 passed within ∼6000–10,000 au of the Sun ∼2.5 Myr ago. In such a scenario, our solar system is still undergoing the later stages of a comet shower. These simulations imply that the modern observed LPC flux is approximately twice as high as the longer-term (tide-dominated) rate. This also implies that estimates of the Oort cloud’s population should be revised downward by a factor of ∼2. Our LPC analysis predicts that the upcoming Gaia data release will favor an HD 7977 impact parameter of ∼6000–10,000 au.

Context. Free-floating planetary mass objects – worlds that roam interstellar space untethered to a parent star – challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation. Aims. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects that formed in these environments), these low-mass substellar binaries represent the most sensitive probe of the mechanisms at play during the star formation process. Methods. We use the Hubble Space Telescope and its Wide Field Camera 3 and the Very Large Telescope and its ERIS adaptive optics facility to search for visual companions among a sample of 77 objects, members of the Upper Scorpius and Taurus young nearby associations, with estimated masses in the range between approximately 6–66 M _Jup . Results. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9±0.4 mas or ∼18 au assuming a distance of 160 pc, with an estimated primary mass in the range between 3–6 M _Jup and a secondary mass between 2.6– 5.2 M _Jup , depending on the assumed age. This corresponds to an overall binary fraction of 1.8 _−1.3 ^+2.6 % among low-mass brown dwarfs and free-floating planetary mass objects over the separation range ≥7 au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below ∼30–50 M _Jup between Upper Sco and Taurus. In Taurus, a binary fraction of 5.6 _−2.3 ^+3.2 % is found for objects with masses below 30M _Jup , and of 7.8 _−2.4 ^+3.0 % for objects with masses below 50M _Jup , while no binary was found among 80 objects over the matching luminosity range in Upper Sco, corresponding to an upper limit of ≤1.2%. Conclusions. This difference may point to intrinsically distinct formation conditions, with warmer parental molecular clouds originally present in Upper Sco potentially inhibiting fragmentation into the lowest-mass brown dwarfs and free-floating planets compared to cooler environments such as Taurus.

Context. Free-floating planetary-mass objects (FFPs) have been detected through direct imaging in several young, nearby star-forming regions. The properties of circumstellar disks around these objects may provide a valuable probe into their origin but are currently limited by the small sample sizes explored. Aims. We aim to perform a statistical study of the occurrence of circumstellar disks down to the planetary-mass regime. Methods. We performed a systematic survey of disks among the population identified in the 5–10 Myr-old Upper Scorpius association, restricted to members outside the younger, embedded Ophiuchus region, and with estimated masses below 105 M _Jup . We took advantage of unWISE photometry to search for mid-infrared excesses in the WISE (W1–W2) color. We implemented a Bayesian outlier detection method, which models the photospheric sequence and computes excess probabilities for each object, enabling a statistically sound estimation of disk fractions. Results. We explored disk fractions across an unprecedentedly fine mass grid, reaching down to objects as low as ~6 M _Jup assuming 5 Myr or ~8 M _Jup assuming 10 Myr, thus extending the previous lower boundary of disk fraction studies. Depending on the age, our sample includes between 17 and 40 FFPs. We confirm that the disk fraction steadily rises with decreasing mass and exceeds 30% near the substellar-to-planetary mass boundary at ~13 M _Jup . We find hints of a possible flattening in this trend around 25–45 M _Jup , potentially signaling a transition in the dominant formation processes. This shift in trend should be considered with caution and needs to be confirmed with more sensitive observations. Our results are consistent with the gradual dispersal of disks over time, as disk fractions in Upper Scorpius appear systematically lower than those in younger regions.