Forming Uranus and Neptune through giant impacts: accretion scenarios with large and small impactor mass ratios in the early Solar System

¹São Paulo State University, FEG, Department of Mathematics, Guaratinguetá, São Paulo, Brazil (leandro.esteves@unesp.br)

• ²Rice University, Department of Earth, Environmental and Planetary Sciences, Houston, Texas, USA

The formation of Uranus and Neptune remains one of the outstanding problems in planetary science. Unlike Jupiter and Saturn, the ice giants possess relatively low masses and significantly tilted spin axes, with obliquities of ~98° and ~30°, respectively. These characteristics suggest that they experienced one or more giant impacts during their formation. However, the specific nature of these collisions—the number, mass ratio, and dynamical conditions of the impactors—remains debated.

Previous studies have explored the accretion of Uranus and Neptune through giant impacts between planetary embryos with comparable masses, typically around 5 M_⊕. These scenarios can successfully reproduce the current masses and mass ratio between the two planets, as well as their large obliquities, assuming stochastic impacts (Izidoro et al. 2015). However, these impacts often lead to the formation of planets with excessively rapid rotation, due to the large angular momentum delivered in approximately equal-mass collisions. This inconsistency with the present-day rotation periods of Uranus (~17.2 hours) and Neptune (~16.1 hours) presents a significant challenge.

An alternative hypothesis involves impacts between bodies with large mass ratios—for instance, a proto-Uranus (~13 M_⊕) and a much smaller embryo (~1 M_⊕). Smooth Particle Hydrodynamics simulations indicate that such large mass ratio collisions can dissipate more angular momentum and result in slower rotating planets, with spin periods more consistent with Uranus and Neptune (Reinhardt et al. 2020). In addition, depending on the impact geometry and location, these impacts can still generate large obliquities, particularly for Uranus, without significantly altering the mass ratio or total mass of the planets.

In this work, we explore both scenarios using a large suite of N-body simulations that incorporate key processes relevant to planet formation in a gaseous protoplanetary disk. Our simulations start with a population of planetary embryos with masses ranging from ~1 to 13 M_⊕ and include the effects of type-I migration, as well as eccentricity and inclination damping from the gas disk. We investigate how the mass distribution of impactors and the dynamical environment influence the frequency and outcomes of collisions that can reproduce the observed characteristics of Uranus and Neptune. The high mass ratio scenario (HMR) simulations start with two massive protoplanets (~13 M_⊕) and several small embryos (0.5–3 M_⊕). The I15 scenario, based on Izidoro et al. (2015), involves only similar-mass embryos (~5 M_⊕).

Figure 1 illustrates the initial conditions used in simulations. The top panel shows the gas surface density as a function of radial distance, with the blue curve denoting how the giant planets shape the protoplanetary disk, following Morbidelli & Crida 2007. The vertical lines mark the approximate orbits of Jupiter, Saturn, and the range of distribution for embryos. The middle panel displays the normalized resultant torque, where negative values indicate inward migration toward the Sun and positive values represent outward migration. Lines are color-coded to denote varying body masses. The bottom panel depicts the approximate initial positions of Jupiter, Saturn, and the embryos, distributed between approximately 10 and 35 AU.

Our results show that scenarios with high mass ratio impactors are more likely to yield planets with slower spin rates, alleviating the angular momentum problem present in equal-mass collision (I15) scenario. However, these same simulations exhibit a significantly reduced probability of such collisions occurring. This is because gas damping is relatively inefficient for low-mass embryos (≲1 M_⊕), which tend to be dynamically excited and scattered by more massive protoplanets instead of merging with them. Consequently, although the final spin states are more favorable, the rarity of such collisions limits the overall success rate of this formation path.

Conversely, simulations involving similar-mass impactors result in a higher frequency of collisions and a greater number of systems that match the final masses of Uranus and Neptune. Nonetheless, most of these planets end up with excess angular momentum, highlighting the trade-off between collision frequency and rotational outcomes in these different formation scenarios.

Figure 2 shows the distribution of rotation periods for Uranus/Neptune analogues from simulations. The light-blue and dark-blue vertical lines represent the actual rotation periods of Uranus and Neptune, respectively. The four upper panels display planets that collided with specific small embryos in the High Mass Ratio (HMR) scenario simulations. The bottom panels show results from the I15 scenario with embryo masses of 6 M_⊕ and 4-8 M_⊕. The percentage plotted in red indicates the fraction of simulations where at least two protoplanets collided with embryos, reached masses close to those of Uranus and Neptune, and preserved the early Solar System architecture.

Despite these contrasting dynamics, our statistical analysis shows that the overall probability of simultaneously reproducing the observed masses, mass ratio, and spin periods of Uranus and Neptune is comparable between the two scenarios, differing by no more than a factor of ~2. In both cases, the likelihood of achieving such an outcome remains low, on the order of 0.1–1%.

These findings suggest that both the large and small mass ratio impact scenarios remain viable from a planet formation perspective. The ultimate pathway may depend on additional factors such as the structure and evolution of the protoplanetary disk, the timing of giant planet migration, and stochastic dynamical interactions in the outer solar system. Future work incorporating improved models of gas disk evolution, pebble accretion, and spin-orbit coupling may further constrain the plausibility of these scenarios.

References: 

• This work: Esteves, L., Izidoro, A. & Winter, O.C., 2025. Accretion of Uranus and Neptune: Confronting different giant impact scenarios. Icarus, 429, p.116428.
• Izidoro, A. et al., 2015. Accretion of Uranus and Neptune from inward-migrating planetary embryos blocked by Jupiter and Saturn. A&A, 582, A99.
• Morbidelli, A. & Crida, A., 2007. The dynamics of Jupiter and Saturn in the gaseous protoplanetary disk. Icarus, 191(1), pp.158–171.
• Reinhardt, C. et al., 2020. Bifurcation in the history of Uranus and Neptune: the role of giant impacts. MNRAS, 492(4), pp.5336–5353.