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Co-orbital systems contain two or more bodies sharing the same orbit around a planet or star. The best-known flavors of co-orbital systems are tadpoles (in which two bodies' angular separations oscillate about the L4/L5 Lagrange points $60^\circ$ apart) and horseshoes (with two bodies periodically exchanging orbital energy to trace out a horseshoe shape in a co-rotating frame). Here, we use N-body simulations to explore the parameter space of many-planet horseshoe systems. We show that up to 24 equal-mass, Earth-mass planets can share the same orbit at 1 au, following a complex pattern in which neighboring planets undergo horseshoe oscillations. We explore the dynamics of horseshoe constellations, and show that they can remain stable for billions of years and even persist through their stars' post-main sequence evolution. With sufficient observations, they can be identified through their large-amplitude, correlated transit timing variations. Given their longevity and exotic orbital architectures, horseshoe constellations may represent potential SETI beacons., 10 pages, 10 figures. Published in MNRAS. YouTube playlist with animations of horseshoe constellation systems here: https://www.youtube.com/playlist?list=PLelMZVM3ka3F335LGLxkxrD1ieiLJYQ5N . Blog post here: https://planetplanet.net/2023/04/20/constellations-of-co-orbital-planets/

We analyze 5108 AFGKM stars with at least five high precision radial velocity points as well as Gaia and Hipparcos astrometric data utilizing a novel pipeline developed in previous work. We find 914 radial velocity signals with periods longer than 1000\,d. Around these signals, 167 cold giants and 68 other types of companions are identified by combined analyses of radial velocity, astrometry, and imaging data. Without correcting for detection bias, we estimate the minimum occurrence rate of the wide-orbit brown dwarfs to be 1.3\%, and find a significant brown dwarf valley around 40 $M_{\rm Jup}$. We also find a power-law distribution in the host binary fraction beyond 3 au similar to that found for single stars, indicating no preference of multiplicity for brown dwarfs. Our work also reveals nine sub-stellar systems (GJ 234 B, GJ 494 B, HD 13724 b, HD 182488 b, HD 39060 b and c, HD 4113 C, HD 42581 d, HD 7449 B, and HD 984 b) that have previously been directly imaged, and many others that are observable at existing facilities. Depending on their ages we estimate that an additional 10-57 sub-stellar objects within our sample can be detected with current imaging facilities, extending the imaged cold (or old) giants by an order of magnitude., 39 pages, 14 figures, 5 tables, published in The Astrophysical Journal Supplement Series

The inner solar system's modern orbital architecture provides inferences into the epoch of terrestrial planet formation; a ~100 Myr time period of planet growth via collisions with planetesimals and other proto-planets. While classic numerical simulations of this scenario adequately reproduced the correct number of terrestrial worlds, their semi-major axes and approximate formation timescales, they struggled to replicate the Earth-Mars and Venus-Mercury mass ratios. In a series of past independent investigations, we demonstrated that Mars' mass is possibly the result of Jupiter and Saturn's early orbital evolution, while Mercury's diminutive size might be the consequence of a primordial mass deficit in the region. Here, we combine these ideas in a single modeled scenario designed to simultaneously reproduce the formation of all four terrestrial planets and the modern orbits of the giant planets in broad strokes. By evaluating our Mercury analogs' core mass fractions, masses, and orbital offsets from Venus, we favor a scenario where Mercury forms through a series of violent erosive collisions between a number of ~Mercury-mass embryos in the inner part of the terrestrial disk. We also compare cases where the gas giants begin the simulation locked in a compact 3:2 resonant configuration to a more relaxed 2:1 orientation and find the former to be more successful. In 2:1 cases, the entire Mercury-forming region is often depleted due to strong sweeping secular resonances that also tend to overly excite the orbits of Earth and Venus as they grow. While our model is quite successful at replicating Mercury's massive core and dynamically isolated orbit, the planets' low mass remains extremely challenging to match. Finally, we discuss the merits and drawbacks of alternative evolutionary scenarios and initial disk conditions., Comment: 23 pages, 9 figures, 3 tables, accepted for publication in Icarus

Advances in computing power and numerical methodologies over the past several decades sparked a prolific output of dynamical investigations of the late stages of terrestrial planet formation. Among other peculiar inner solar system qualities, the ability of simulations to reproduce the small mass of Mars within the planets' geochemically inferred accretion timescale of, 16 pages, 7 figures, accepted for publication in Icarus

In recent years a paradigm shift has occurred in exoplanet science, wherein low-mass stars are increasingly viewed as a foundational pillar of the search for potentially habitable worlds in the solar neighborhood. However, the formation processes of this rapidly accumulating sample of planet systems are still poorly understood. Moreover, it is unclear whether tenuous primordial atmospheres around these Earth-analogs could have survived the intense epoch of heightened stellar activity that is typical for low-mass stars. We present new simulations of in-situ planet formation across the M-dwarf mass spectrum, and derive leftover debris populations of small bodies that might source delayed volatile delivery. We then follow the evolution of this debris with high-resolution models of real systems of habitable zone planets around low-mass stars such as TRAPPIST-1, Proxima Centauri and TOI-700. While debris in the radial vicinity of the habitable zone planets is removed rapidly, thus making delayed volatile delivery highly unlikely, we find that material ubiquitously scattered into an exo-asteroid belt region during the planet formation process represents a potentially lucrative reservoir of icy small bodies. Thus, the presence of external ~Neptune-Saturn mass planets capable of dynamically perturbing these asteroids would be a sign that habitable zone worlds around low-mass stars might have avoided complete desiccation. However, we also find that such giant planets significantly limit the efficiency of asteroidal implantation during the planet formation process. In the coming decade, long-baseline radial velocity studies and Roman Space Telescope microlensing observations will undoubtedly further constrain this process., 19 pages, 9 figures, 3 tables, accepted for publication in ApJ