Mariana Sanchez

The scarcity of giant planets around M dwarfs raises important questions about the physical processes that regulate planet formation around the least massive stars. In this work, we investigate planet formation around very low-mass stars using N-body simulations that incorporate pebble accretion, gas accretion, planet-disk interactions, star-planet tidal interactions, and stellar evolution during the pre-main-sequence phase. We explore how disk properties influence planetary growth and migration by assuming a compact dust disk of 30 au.Our simulations show that low-viscosity disks naturally favor the formation of close-in super-Earths, while the scarcity of larger planets around M dwarfs emerges as a natural outcome of the planet formation process. In cases where planet-planet collisions efficiently assemble cores of 2–3 Earth masses, close-in mini-Neptunes and Neptune-mass planets may form. Moreover, if planetary cores reach masses of ~5 Earth masses within the first ~1 Myr, cold giant planets with masses between Saturn and Jupiter can form at distances of 1–5 au, provided that the disks are massive (~10% of the stellar mass), weakly turbulent (αt = 10-4), and long-lived (10 Myr).We further investigate the role of fragmentation in the formation of planetary cores by introducing an evolving pebble flux linked to different fragmentation velocities associated with distinct grain compositions across the disk. These results provide new insights into the diversity of planetary systems around M dwarfs and the conditions required for giant planet formation in the low-stellar-mass regime.

Context . Close-in rocky planets are the most common type of exoplanets around late M dwarfs, ranging from more temperate worlds to highly irradiated lava planets with molten surfaces, and many theoretical studies have attempted to explain their formation. However, the origin of rocky planets with orbital periods shorter than one day, known as ultra-short-period (USP) planets, remains uncertain. Aims . We aim to investigate whether the formation and survival of USP planets is connected to the location of the inner edge of the protoplanetary disk, considering different disk edge prescriptions. Methods . We used N -body simulations that include planet-disk interactions, star-planet tidal interactions, and relativistic corrections, applied to a sample of lunar-mass planetary seeds growing via pebble accretion in a low-viscosity disk ( α _t = 10 ⁻⁴ ). The inner edge of the disk was modeled in three ways: as a fixed close-in edge, as an outward-evolving edge set by the magnetospheric truncation radius, and as an inward-evolving edge defined by the corotation radius. Results . Ultra-short-period planet formation appears to be tightly controlled by the location of the disk’s inner edge. Our simulations show that only the close-in-fixed-edge scenario and the inward-evolving-edge scenario are capable of producing USP planets, as planets tend to follow the movement of the disk’s inner edge. This suggests that USP planet formation is favored when the inner edge remains close to the corotation radius of a rapidly rotating star.

Context . Modeling how cold giant planets form around M dwarfs remains a challenge, both because their protoplanetary disks can lack sufficient mass and because such planets are expected to migrate inward while interacting with the disk. Moreover, it remains unknown whether inner rocky planets can survive in systems that host a cold giant around very low-mass stars, which could have important implications for the habitability of rocky worlds. Aims . We investigated the conditions required for the formation of giant planets at large orbital distances (1−3 au) around a 0.1 M _⊙ star, and explored the circumstances under which a close-in rocky planet can survive. Methods . We performed N -body simulations in which planetary embryos grow through pebble accretion, followed by gas accretion during the disk lifetime. Assuming a local disk turbulent viscosity (α _t ) of 10 ⁻⁴ , we included planet-disk interactions throughout the disk evolution, using a new prescription that accounts for the onset of outward migration when the planet-to-star mass ratio ( q ) exceeds 0.002. Results . We find that a cold giant planet can form around a late M dwarf, even with an initial pebble mass of only 6 M _⊕ , provided the disk gas mass is 10% of the stellar mass. This outcome requires a compact 20 au disk in which the inner, viscosity-dominated region has a high gas surface density set by a low accretion viscosity (α _g =10 ⁻⁴ ), that planet–planet collisions assemble a ∼ 5 M _⊕ core within 1 Myr, and that the gas disk survives for 10 Myr. In addition, an inner rocky planet can survive in a close-in orbit if it migrates into the inner disk cavity before the outer body grows into a giant. Conclusions . The initial dust mass required for giant planet formation around very low-mass stars does not need to be as extreme as previously thought. A combination of planet–planet collisions, efficient pebble accretion, and a long disk lifetime plays a key role in enabling the formation of cold giant planets with masses between those of Saturn and Jupiter.

Context . The migration of giant planets remains a complex and rich topic. While significant progress has been made in understanding migration in discs with high viscosity, the migration of planets with high planet-star mass ratios in low-viscosity environments is still not fully understood. Aims . Our aim is to study the migration of planets with high planet-to-star mass ratios embedded in low-viscosity protoplanetary discs, characterised by the viscous parameter α = 10 ⁻⁴ , and to derive analytical prescriptions that could be used to study planet formation across a range of stellar masses, spanning from Sun-like stars to M dwarfs. Methods . We performed hydrodynamical simulations using the FARGO3D code, exploring the migration of planets with high planet-star mass ratios (10 ⁻³ ≤ q ≤ 2 × 10 ⁻² ) under different disc conditions, including variations in gas surface density, scale height, and density slope. Results . Our simulations reveal a change in the migration direction at a mass ratio of q ≈ 0.002, with planets exhibiting outward migration for q > 0.002. Additionally, for planets undergoing outward migration, we find that the migration speed depends on the unperturbed local gas density. In most cases, outward migration is driven by a positive torque related to the fact that the planet’s eccentricity remains below e < 0.2. However, under certain disc parameterisations, planets with q > 0.01 can develop higher eccentricities in the range 0.2 < e < 0.45, which can lead to stalled migration. Conclusions . Our findings suggest that outward migration is a viable mechanism for massive planets in low-viscosity discs, which has implications for the formation and distribution of super-Jupiter planets around Sun-like stars and planets more massive than Neptune around very low mass stars. Given the challenges in detecting such planets, improving our theoretical understanding of their migration is essential for interpreting exoplanet demographics and guiding future observational efforts.

Aims. Most of the exoplanets discovered in our Galaxy to date orbit low-mass stars, which tend to host small disks in their early stages. To better elucidate the link between planet formation and disk substructures, observational biases should be reduced through observations of these small, faint disks at the highest resolution using the Atacama Large Millimeter Array (ALMA). Methods. We present new high-resolution (0.03–0.04″) ALMA observations at 1.3 mm of 33 disks located in the Lupus star-forming region that have total dust continuum fluxes of Results. We show that 67% of Lupus protoplanetary dust disks have dust radii smaller than 30 au and characterize the newly discovered substructures in 11 disks with some of the shortest separation gaps. The size–luminosity relation, when accounting for the smallest disks, aligns well with a drift-dominated dust evolution scenario for the Class II Lupus disks. For the most compact disks, with radii of less than 30 au, we compared measured sizes and fluxes with a grid of radiative transfer models to derive millimeter-emitting dust masses, which ranged from 0.3 to 26.3 M_⊕. Assuming that the detected substructures were dynamical effects of planets, we approximated the results of an interpolation to estimate planet masses and found a range of 20–2000 M_⊕ with separations between 2 and 74 au. Conclusions. Our results indicate that two-thirds of the protoplanetary disks in Lupus are smooth, on scales larger than 4 au, and compact, with substructures being more prominent in the few larger disks. These compact disks are consistent with drift-dominated evolution, with their masses and optical depths suggesting that they may have already experienced some planet formation, with most of the small solids converted into planetesimals and planets. This makes them prime candidates, with optimal conditions, for explaining the formation and origin of super-Earths.

Context. Due to the improvements in radial velocity and transit techniques, we know that rocky or rocky-icy planets, in particular close-in super-Earths in compact configurations, are the most common ones around M dwarfs. On the other hand, thanks to the high angular resolution of ALMA we know that many disks around very low-mass stars (between 0.1 and 0.5 M_⊙) are rather compact and small (without observable substructures and radius less than 20 au), which favors the idea of an efficient radial drift that could enhance planet formation in the terrestrial zone. Aims. Our aim was to investigate the potential formation paths of the observed close-in rocky exoplanet population around M dwarfs, especially close-in super-Earths, assuming that planet formation could take place in compact disks with an efficient dust radial drift. Methods. We developed N-body simulations that include a sample of embryos growing by pebble accretion exposed to planet-disk interactions, star-planet tidal interactions, and general relativistic corrections that include the evolution of the luminosity, radius, and rotational period of the star. For a star of 0.1 M_⊙, we considered different gas disk viscosities and initial embryo distributions. We also explored planet formation by pebble accretion around stars of 0.3 M_⊙ and 0.5 M_⊙. Lastly, for each stellar mass, we ran simulations that include a sample of embryos growing by planetesimal accretion instead of pebble accretion. Results. Our main result is that the sample of simulated planets that grow by pebble accretion in a gas disk with low viscosity (α = 10⁻⁴) can reproduce the close-in low-mass exoplanet population around M dwarfs in terms of multiplicity, mass, and semi-major axis. Furthermore, we found that a gas disk with high viscosity (α = 10⁻³), and thus lower pebble accretion rates, cannot reproduce the observed planet masses as no planet more massive than 0.5 M_⊕ could be formed in our simulations. In addition, we show that planetesimal accretion favors the formation of smaller planets than pebble accretion does. Whether this planet population truly exists remains unknown with the current instrumental sensitivity. Conclusions. Rocky planet formation around M dwarfs can take place in compact and small dust disks driven by an efficient radial drift in a gas disk with low viscosity (α = 10⁻⁴). This result points toward a new approach in the direction of the disk conditions needed for rocky planet formation around very low-mass stars.

M dwarfs are the most common stars in our solar neighborhood. Due to the improvements in radial velocity and transit techniques, we know that rocky planets, in particular close-in super-Earths, in compact configurations are the most common ones around M dwarfs. On the other hand, thanks to the high angular resolution of ALMA we know that most disks around very low mass stars are rather compact and small, which favors the idea of an efficient radial drift that could enhance planet formation in the terrestrial zone. Motivated by these results, we have investigated rocky planet formation around M dwarfs driven by pebble accretion through N-body simulations. We assumed that planet formation took place in compact dust disks caused by efficient dust radial drift. In the simulations we incorporated planet-disk interactions and tidal and relativistic corrections that include the evolution of the luminosity, radius and rotational period of the star. For our standard model we used different gas-disk viscosities and initial embryo distributions. For different stellar masses we also studied planet formation by planetesimal accretion. Our main result is that the sample of simulated planets that grow by pebble accretion in a gas-disk with low viscosity can reproduce the low-mass exoplanet population around M dwarfs in terms of multiplicity, masses and semi-major axis. Furthermore, we found that a gas disk with high viscosity can not reproduce the observed planet masses. Also, we show that planetesimal accretion favors the formation of water worlds and small planets that so far have not been detected. This work points towards a new approach for the disk conditions needed to study rocky planet formation around M dwarfs.

Context. The confirmed exoplanet population around very low mass stars is increasing considerable through data from the latest space missions and improvements in ground-based observations, particularly with the detection of Earth-like planets in the habitable zones. However, theoretical models need to improve in the study of planet formation and evolution around low-mass hosts. Aims. Our main goal is to study the formation of rocky planets and the first 100 Myr of their dynamical evolution around a star with a mass of 0.08 M_⊙, which is close to the substellar mass limit. Methods. We developed two sets of N-body simulations assuming an embryo population affected by tidal and general relativistic effects, refined by the inclusion of the spin-up and contraction of the central star. This population is immersed in a gas disk during the first 10 Myr. Each set of simulations incorporated a different prescription from the literature to calculate the interaction between the gas-disk and the embryos: one widely used prescription which is based on results from hydrodynamics simulations, and a recent prescription that is based on the analytic treatment of dynamical friction. Results. We found that in a standard disk model, the dynamical evolution and the final architectures of the resulting rocky planets are strongly related with the prescription used to treat the interaction within the gas and the embryos. Its impact on the resulting close-in planet population and particularly on those planets that are located inside the habitable zone is particularly strong. Conclusions. The distribution of the period ratio of adjacent confirmed exoplanets observed around very low mass stars and brown dwarfs and the exoplanets that we obtained from our simulations agrees well only when the prescription based on dynamical friction for gas-embryo interaction was used. Our results also reproduce a close-in planet population of interest that is located inside the habitable zone. A fraction of these planets will be exposed for a long period of time to the stellar irradiation inside the inner edge of the evolving habitable zone until the zone reaches them.