EXOA16
We cordially invite all experts and interested colleagues to submit abstracts for oral and poster contributions in all areas related to theoretical, observational, and experimental studies of terrestrial planet formation in our solar system and extrasolar planets. We strongly encourage contributions from early career scientists.
Session assets
How to cite: Jacobson, S. A., Nathan, G., and Nguyen, H. D.: Theia can arrive late and be oxidized, but not if it is large compared to proto-Earth, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1131, https://doi.org/10.5194/epsc-dps2025-1131, 2025.
A surprising discovery in exoplanetary science has been compact systems of Earth to super-Earth sized planets orbiting within ∼ 10-2 to 10-1 au from their star, a region lacking planets in our Solar System. While compact systems are common, their origin is debated. A prevalent assumption is that compact systems formed after the infall of gas and solids to the circumstellar disk ended. However, observational evidence suggests accretion commences earlier. We propose that compact systems are surviving remnants of planet accretion during the end stages of infall (Rufu & Canup, 2025, Nature Communications, in press). This early accretion condition leads to a characteristic planetary system-to-star mass ratio, offering a compelling explanation for the remarkably similar ratios observed in known compact systems (Fig. 1).
We simulate the accretion of planets using an N-body code that includes a time-dependent infall of gas and solids, with a rate that decays exponentially on a timescale, τin, as well as gas disk interactions modeled with an exponentially decaying surface density characterized by a timescale τg.
The mass of a planet that accretes within a disk region undergoing infall is regulated by a balance between mass growth due to accretion of infalling solids, and inward gas-driven orbital migration that becomes faster as the planet grows. This balance selects for similarly-sized planets whose mass is a function of infall and disk conditions. We show that the infall-produced planets survive until the gas disk disperses and migration ends, and that across a broad range of conditions, the mass of surviving systems is regulated to a few ⊗10-5 to 10-4 times the host star's mass (Fig. 2). Additionally, these simulations predict a relatively weak dependence of planet and planet system mass on stellar metallicity. This appears consistent with the nearly equal occurrence of warm super-Earths around stars of wide-ranging metallicities (Petigura et al, 2018) that has been unexplained.
Whether planets that form during infall survive depend on the radial extent of disk infall, rc, and the ratio of the gas disk lifetime to the infall timescale, τg /τin. Here we focus on small rc, and small τg /τin cases, consistent with compact systems. However, rc depends sensitively on the angular momentum of the parent cloud core and interactions of infalling material with the disk and stellar magnetosphere, so that even stars with similar masses may have substantially different rc values, resulting in different system architectures. For larger rc, long accretion timescales in the outer infall region may promote the formation of giant planets after infall has ended. Larger systems will also have larger τg /τin due to their longer gas disk lifetimes (Schib et al. 2021), implying that surviving inner planets that accreted during infall would have a lower total mass compared to the star. Most broadly, our results suggest that the long-standing assumption that planet accretion commences only after infall has ended may not be valid for all systems, and consideration of this early accretionary phase is warranted.
Fig 1 caption - Estimated total mass of transiting compact systems, Mtot, scaled to the stellar mass, M*. a, (Mtot/M*) for compact systems having ≥3 known planets that orbit a single star within a<0.5 au (circle markers, blue box). Points are ordered left-to-right by ascending stellar mass. Data are from www.exoplanetarchive.ipac.caltech.edu. For cases without mass estimates, we use the observed planet radius, increase the estimated radius uncertainty by a factor of 2, and then apply a radius vs. mass relation (Weiss & Marcy 2014). Light [medium] blue circles are systems with all [some] planetary masses estimated from this relation, while dark blue circles are systems with measured planetary masses. We include only systems with resulting errors ΔMtot /Mtot<1. b, Distribution of compact system mass ratios. Over a wide range of stellar masses ( M*~0.1 to 1.3 stellar mass), compact multi-planet systems display a common mass ratio, with 90% of systems having 3⊗10-5<(Mtot/M*)<3⊗10-4 (gray shaded region). This mass ratio is more similar to that of the gas giant satellite systems (square markers, yellow box) than to the inner or outer planets in our Solar System (triangle markers, red box).
Fig 2 caption - Results of planet accretion simulations with varied disk and infall properties. Final planetary system mass scaled to the stellar mass (1Msun ) as a function of (αε/ƒ) (α is the viscosity parameter, ε is the fraction of infalling solids incorporated into planets, and ƒ is the infall gas-to-solids ratio). The infall rate decays with timescale τin =5⊗105 yr, while the gas disk disperses over a longer timescale, τg =1.3 to 2τin (colors, legend). The simulations assume either an inner disk cavity with radius rcav=20R*~0.13 au (triangles) or no cavity (circles). Grey region shows range for 90% of observed compact systems shown in Fig. 1. Dashed lines show analytical predictions for the no-cavity case with τg/ τin=1.3 (light blue) and τg/ τin=2 (dark blue). Horizontal bars show plausible viscosity ranges based on observed stellar accretion rates (Hartmann et al. 1998), models of magnetorotational instability (MRI, Jankovic et al. 2019) and infall-driven shocks (Lesur et al. 2015), assuming ƒ/ε=102.
Fig.1
Fig. 2
How to cite: Rufu, R. and Canup, R.: Origin of compact exoplanetary systems during disk infall, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1153, https://doi.org/10.5194/epsc-dps2025-1153, 2025.
Introduction:
How the parent bodies of chondritic meteorites, and the chondrules they host, formed is one of the most hotly debated questions in planetary sciences. The curious properties of chondrules have presented significant challenges to theoretical models that have tried to understand their formation and there is currently no consensus on the ability of the various proposed mechanisms to form chondrules consistent with the physicochemical constraints.
Here, we present the newly proposed IVANS (Impact Vapor Plumes and Nebula Shock) model for chondrule and chondrite formation (Stewart et al. 2025) and discuss how particles are size sorted and collected into chondritic mixtures (Lock et al.).
Figure 1: Schematic overview of the hydrodynamics of impact vapor plumes and nebular shocks (Stewart et al. 2025).
The Impact Vapor Plumes and Nebula Shock (IVANS) model:
A schematic of the IVANS model (Stewart et al. 2025) is shown in Figure 1. The IVANS model is based on vaporizing collisions between planetesimals composed of dust and ice in the presence of the nebular gas. Above about 1 km/s, the impact velocity is great enough to drive disruption and vaporization of a fraction of the colliding bodies (Figure 1A). Such collisions are frequent during planet formation before gas disk dispersal, particularly in epochs in which giant planets were growing rapidly and/or migrating (Carter & Stewart 2020). Each impact generates a cloud of cooling water vapor from the planetesimals which expands supersonically (inner blue region in Figure 1B), driving a warmer shock wave into the solar nebula (outer ring with warm colors). The elongated expanding shell of shocked nebula is hotter in the principal impact direction and cooler in the opposite and lateral dimensions (indicated by the color gradient). Portions of the shocked nebula are warm enough to melt free-floating nebular dust and form chondrules. Expansion of the vapor plume eventually leads to a pressure low in the solar nebula and subsequent hydrodynamic reversal in the flow field (Figure 1C). The nebular gas, dust, and chondrules flow into the low pressure region, mixing materials that were processed in different regions. The collapsed mixture has the characteristics observed in chondritic mixtures: quenched chondrules mixed with dust and ice. The mixed region is orders of magnitude larger in scale than the original planetesimals and the timescale of the collapse is order 10s hours.
Breakup and coupling of particles in the IVANS model:
We calculated the drag force on individual dust grains and aggregates to determine which size particles can couple to the gas in impact-driven nebular shocks. The nebular shock and the vapor plume-nebular interface were both treated as step changes in gas velocity. The acceleration of particles were then calculated using drag formulations for the relevant regime (Brown & Lawler 2003; Probstein 1969). The stability of particles to shear was assessed using experimentally-determined criteria (Theofanous 2011), or comparison between the drag force and surface tension (liquid droplets) or tensile strength (aggregates).
Using a Monte-Carlo approach, we varied the densities of the nebular gas, plume gas, nebular shock velocity, nebular shock length scale, and plume length scale over a large range. We found that the dominant parameter that determines the maximum size particle that is coupled to the nebular shock or plume front is the density of the nebular gas (Figure 2). Larger particles simply pass through the shock and are then not collected in the collapsing system. In this sense, the system acts as a ‘reverse-sieve’.
Under the conditions of impact-driven nebular shocks, the maximum size of coupled particles agrees well with the maximum size range of chondrules for the plausible range of nebular gas densities (chondrules typically span 0.1 to a few mm in size; Metzler, 2018). Different chondrite groups have different maxima and mean size chondrules (Jones 2012), which is interpreted to mean that they formed in distinct nebular environments. Within the framework of the IVANS model, the variation in the nebula pressures/densities at the time/place the chondrule forming impact occurred is responsible for the difference in chondrule sizes.
Figure 2: The maximum sizes of chondrules collected in the IVANS model are a strong function of the nebular gas density. Points are the maximum size of particles stopped in the nebular shock for each of our Monte-Carlo simulations for both dust grains or (partially) molten droplets (orange) and dust aggregates (blue). The size of aggregates is given as the equivalent size if the aggregate melted to form a chondrule.
Conclusions:
We find that processing of material by vaporizing collisions in the solar nebula produces a size-sorted assembly of melted silicates with the first-order physical and chemical properties of chondrules. The IVANS model therefore provides a promising new explanation for the origin of chondrules and chondrites.
In the IVANS model there is a mechanistic link between vaporizing collisions and chondrites. Chondrites are not only precious time capsules of primitive nebular materials but also key records of the wider dynamics of planet formation. The different degrees of thermal processing through nebular shocks experienced by different meteorite groups reflects the collisional histories in different locations and at different times in the solar nebula (Carter & Stewart 2020). The chondritic record can therefore place constraints on the formation and migration of the gas giants and so the dynamics of giant and terrestrial planet formation.
References:
Brown PP, Lawler DF. 2003. J. Environ. Eng. 129(3):222–31
Carter PJ, Stewart ST. 2020. Planet. Sci. J. 1(2):45
Jones RH. 2012. Meteorit. Planet. Sci. 47(7):1176–90
Lock SJ, et al. In prep.
Metzler K. 2018. Meteorit. Planet. Sci. 53(7):1489–99
Probstein RF. 1969. In Problems of Hydrodynamics and Continuum Mechanics, pp. 568–83.
Stewart ST, et al. In press. Planet. Sci. J. doi: 10.3847/PSJ/adbe71. arxiv: 2503.05636
Theofanous TG. 2011. Annu. Rev. Fluid Mech. 43(1):661–90
How to cite: Lock, S., Carter, P., Stewart, S., Davies, E., Petaev, M., and Jacobsen, S.: Coupling and breakup of particles in the impact vapor and nebular shocks (IVANS) model of chondrule formation, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–13 Sep 2025, EPSC-DPS2025-1705, https://doi.org/10.5194/epsc-dps2025-1705, 2025.