Uranus and Neptune as methane planets: producing icy giants from refractory planetesimals

Abstract: Uranus and Neptune are commonly considered ice giants, and it is often assumed that, in addition to a solar mix of hydrogen and helium, they contain roughly twice as much water as rock. This classical picture has led to successful models of their internal structure and has been understood to be compatible with the composition of the solar nebula during their formation (Reynolds and Summers 1965; Podolak and Cameron 1974; Podolak and Reynolds 1984; Podolak et al. 1995; Nettelmann et al. 2013). However, the dominance of water has been recently questioned (Teanby et al. 2020; Helled and Fortney 2020; Podolak et al. 2022). Planetesimals in the outer solar system are composed mainly of refractory materials, leading to an inconsistency between the icy composition of Uranus and Neptune and the ice-poor planetesimals they accreted during formation (Podolak et al. 2022). Here we elaborate on this problem, and propose a new potential solution. We show that chemical reactions between planetesimals dominated by organic-rich refractory materials and the hydrogen in gaseous atmospheres of protoplanets can form large amounts of methane 'ice'. Uranus and Neptune could thus be compatible with having accreted refractory-dominated planetesimals, while still remaining icy. Using random statistical computer models for a wide parameter space, we show that the resulting methane-rich internal composition could be a natural solution, giving a good match to the size, mass and moment of inertia of Uranus and Neptune, whereas rock-rich models appear to only work if a rocky interior is heavily mixed with hydrogen. Our model predicts a lower than solar hydrogen to helium ratio, which can be tested. We conclude that Uranus, Neptune and similar exoplanets could be methane-rich, and discuss why Jupiter and Saturn cannot.

• The paper proposes that methane production via reactions between hydrogen and refractory planetesimals challenges the traditional water-dominated model.
• Using stochastic modeling, the study tests varied internal compositions and confirms consistency with observed planetary sizes and moments of inertia.
• The research suggests exoplanets may also exhibit methane-rich interiors, offering new pathways to understand planetary formation.

Insights on Uranus and Neptune as Methane Planets

The paper "Uranus and Neptune as methane planets: producing icy giants from refractory planetesimals" addresses a fundamental question about the composition and formation of the ice giants Uranus and Neptune. Traditional models have categorized these planets as being predominantly composed of water ice; however, this paper challenges the prevalence of water and proposes an alternative perspective where methane plays a significant role in their composition.

Key Arguments and Methodology

The research reassesses the classical understanding of Uranus and Neptune, which posits that these planets are primarily composed of water ice, mixed with rock, based on the assumption of solar-like elemental abundances during their formation. However, as highlighted by recent observations, planetesimals in the outer solar system contain mainly refractory materials, including organic-rich compounds. This presents a crucial inconsistency, as the accreted material's composition does not align with the traditionally accepted notion of ice-rich Uranus and Neptune.

The authors suggest redefining the internal structure models of these giants by proposing that interactions between hydrogen in the protoplanetary atmospheres and the organic-rich refractories led to the production of large quantities of methane. This process could naturally reshape the composition of these planets from being water-dominated to containing substantial amounts of methane.

Using stochastic modeling approaches, the authors generate numerous hypothetical internal compositions and test their viability against known physical properties of Uranus and Neptune, such as size, mass, and moment of inertia. Their models cover a wide parameter space, providing critical insights into the possible configurations of elements within these planets.

Important Findings

1. Methane Formation: The model suggests that methane can naturally arise due to chemical reactions between hydrogen and carbon-rich planetesimals. This leads to the formation of a methane-dominated icy component within the planets' interiors, reducing the need to rely solely on water as the primary component.
2. Elemental Ratios: The paper predicts a lower than solar hydrogen to helium ratio in the atmospheres of Uranus and Neptune. This is a significant theoretical assertion that can be empirically tested by future missions.
3. Compatibility with Observations: The resultant compositions from their models align with the observed physical characteristics of Uranus and Neptune, underlining the plausibility of their methane-centric hypothesis.
4. Implications for Exoplanets: The authors extend their findings to speculate that similar-sized exoplanets might also have methane-rich compositions, a detail that could refine our understanding of planetary formation and evolution beyond our own solar system.

Implications and Future Directions

The paper introduces a substantial modification to the nascent understanding of planetary compositions in the outer solar system. By illustrating that methane can dominate the internal ice budget of Uranus and Neptune, it calls for a reassessment of their accretion histories and current compositions. Additionally, the research highlights avenues for observational tests, suggesting that future space missions should prioritize assessing the hydrogen to helium ratio in these planetary atmospheres to confirm or refute their models.

The implications further stretch into an expanded framework for modeling exoplanetary systems, particularly those that exhibit similar bulk compositions and atmospheric characteristics as Uranus and Neptune. This work thereby opens a new frontier in distinguishing planetary types based not just on observable surface characteristics but also on nuanced internal chemical processes.

By challenging long-standing models, this research adds a crucial layer of complexity to our understanding of the outer planets, emphasizing the intricate interplay between accretion, chemistry, and planetary evolution. It stands as an invitation for future detailed investigations using advanced models and observational data to either bolster or refine this new paradigm.