For much of the 20th century, models of Solar System formation emphasized energetic, collision-dominated accretion processes (Bottke and Norman, 2017; Haskin et al., 1998). Within these frameworks, late-stage catastrophic impacts were viewed as natural outcomes of prolonged dynamical instability, providing explanatory contexts for the Moon-forming giant impact and for scenarios invoking a Late Heavy Bombardment (LHB) (Haskin et al., 1998; Bottke and Norman, 2017; Crockett, 2019). Such models provided a natural explanatory context for several widely cited hypotheses, including the Moon-forming giant impact involving a Mars-sized body commonly referred to as Theia, as well as scenarios invoking a Late Heavy Bombardment of the inner Solar System.

Over the past 2 decades, however, advances in observational astronomy and numerical modeling have substantially altered prevailing views of planet formation (Tobin et al., 2020; Karnath et al., 2020; McClure, 2025; Ciesla, 2025; Shoshi et al., 2025). In many respects, this shift echoes earlier cautions by Urey, who emphasized that planetary origin theories must remain grounded in the physical and geochemical properties of accreting materials (Urey, 1952). High-resolution imaging of protostellar disks now reveals early, complex disk substructures and rapid condensation of refractory solids, indicating that planet formation begins earlier and under more orderly conditions than assumed in classical models (Tobin et al., 2020; McClure, 2025; Shoshi et al., 2025). Increasingly, contemporary models favor cold or pebble accretion paradigms, in which planetary growth occurs through the gradual accumulation of small solids within protoplanetary disks, mediated by gas dynamics and characterized by lower typical collision energies. These models suggest a more orderly and less violent assembly process than previously assumed, particularly during the later stages of planetary growth.

Despite this paradigm shift, many canonical Solar System impact hypotheses continue to be discussed largely within earlier collision-dominated frameworks (Haskin et al., 1998; Bottke and Norman, 2017). This raises the possibility of a growing disconnect between contemporary formation theory and downstream explanatory models. In particular, assumptions regarding the frequency, timing, and energetic consequences of late catastrophic impacts are often carried forward without explicit reassessment in light of newer accretion paradigms.

In this Perspective, we examine the implications of modern cold-accretion and disk-mediated paradigms for two widely cited impact hypotheses: the Moon-forming giant impact and the Late Heavy Bombardment, with attention to consequences for planetary thermal histories and early habitability. Rather than seeking to invalidate these hypotheses, we aim to assess the extent to which their underlying assumptions remain compatible with evolving formation paradigms. Particular attention is given to the consequences for planetary thermal histories, crustal stability, volatile retention, and early habitability. We further outline observational and theoretical constraints that may help distinguish between competing formation–impact scenarios as planetary science continues to develop.

Classical accretion models envisioned hierarchical growth dominated by high-energy collisions between planetary embryos, following early frameworks of runaway and oligarchic growth (Wetherill, 1978; Kokubo and Ida, 1998). More recent work favors cold-accretion and pebble-accretion paradigms in which planetary growth is regulated by disk dynamics, gas drag, and efficient capture of small solids (Andrews et al., 2018; Birnstiel et al., 2018; Dullemond et al., 2018; Zhang et al., 2018). Within this framework, late-stage giant impacts were considered an inevitable outcome of dynamical instability, naturally producing features such as large satellites, extreme axial tilts, and widespread resurfacing events. While such models successfully reproduced certain aspects of Solar System architecture, they were developed at a time when observational constraints on protoplanetary disks and extrasolar planetary systems were limited.

More recent work has significantly revised this picture. Observations of protoplanetary disks now reveal complex, long-lived disk structures capable of regulating planetary growth and migration. In parallel, numerical studies have demonstrated that pebble accretion — the efficient accumulation of millimeter-to centimeter-sized solids — can dramatically accelerate planetary growth while simultaneously damping relative velocities through gas drag. These processes favor gentler accretion regimes, particularly during the later stages of planet formation, and may reduce the prevalence of high-energy collisions between large bodies.

High-resolution observations from ALMA and related facilities reveal ubiquitous disk substructures — rings, gaps, spirals — that strongly influence mass transport and growth (Andrews et al., 2018; Huang et al., 2018; Isella et al., 2018). These structures support models in which planetary assembly proceeds in comparatively low-velocity environments, limiting late-stage catastrophic collisions.

Magnetohydrodynamic processes play a central role in this evolution. Simulations and observations indicate that magnetic fields regulate angular-momentum transport, turbulence, and disk fragmentation, thereby shaping the pathways of planet formation (Deng et al., 2020; Wang et al., 2025; Ohashi et al., 2025). Such processes may also contribute to disk fragmentation and planet formation under specific conditions, without requiring uniformly destructive late-stage impact histories (Deng et al., 2020; Wang et al., 2025). Laboratory and theoretical work further shows that electrostatic charging and magnetic effects enable efficient dust growth past classical barriers, accelerating planetesimal formation under non-violent conditions (Steinpilz et al., 2019).

Observational constraints from the Kuiper Belt object Arrokoth demonstrate that at least some primordial bodies formed through gentle accretion with no evidence of significant collisional processing (McKinnon et al., 2020; Grundy et al., 2020). Collectively, these results indicate that cold, disk-regulated accretion is not only viable but likely common.

Exoplanet surveys further support a diversity of planetary system architectures that are difficult to reconcile with uniformly violent accretion histories. Many systems exhibit closely packed, dynamically stable configurations that appear inconsistent with extensive late-stage scattering or frequent giant impacts. While such systems are not direct analogues of the Solar System, they suggest that relatively quiescent assembly pathways may be common outcomes of disk-mediated planet formation.

It should be noted that pebble accretion remains actively debated for Earth’s formation specifically, and recent work has argued that classical embryo-impact growth may still dominate the inner Solar System (Morbidelli et al., 2024). At the same time, recent commentary has emphasized that classical embryo growth and pebble accretion need not be mutually exclusive, and that hybrid formation pathways may provide a more realistic account of terrestrial planet assembly (Johansen et al., 2024; Yzer et al., 2025). The purpose of this Perspective is therefore not to privilege a single formation pathway, but to emphasize that evolving accretion paradigms broadly motivate renewed scrutiny of late-stage impact assumptions.

Collectively, these developments indicate that planet formation may proceed through a broader range of dynamical environments than previously assumed, with cold accretion pathways playing a more prominent role than classical models allowed. This does not preclude the occurrence of catastrophic impacts altogether; rather, it suggests that their frequency, timing, and energetic significance may be more constrained than implied by earlier paradigms. Re-evaluating Solar System impact hypotheses within this updated formation context is therefore a necessary step toward maintaining internal consistency across planetary science.

The giant-impact hypothesis remains the leading explanation for the Moon’s origin (Haskin et al., 1998). Its canonical form presumes a late, high-energy collision between proto-Earth and a Mars-sized body, Theia. The resulting impact is hypothesized to have ejected sufficient material into Earth orbit to form a debris disk from which the Moon subsequently accreted. Variants of this scenario have been developed to address geochemical and isotopic constraints, and the hypothesis continues to be actively refined. Such models implicitly assume a dynamical environment conducive to rare but extreme late-stage impacts (Bottke and Norman, 2017).

It is important to emphasize that the giant impact hypothesis remains widely discussed in part because alternative Moon-formation scenarios face substantial difficulties. The Earth–Moon system’s angular momentum, the Moon’s unusually large mass ratio relative to Earth, and multiple geochemical constraints are often interpreted as supporting a high-energy origin (Hartmann and Davis, 1975; Canup, 2004; Canup, 2012). Evidence of lunar volatile depletion and high-temperature processing likewise presents challenges for purely quiescent co-formation models. At the same time, the hypothesis continues to face unresolved questions relating to isotopic homogeneity, volatile retention, angular momentum evolution, and siderophile constraints, motivating ongoing refinements such as high-angular-momentum and synestia-type impact formulations (Wisdom, 2006; Saal et al., 2008; Ćuk and Stewart, 2012; Mastrobuono-Battisti et al., 2015; Lock and Stewart, 2017).

Implicit in most versions of the giant impact hypothesis is the assumption that late-stage terrestrial planet formation occurred within a dynamical environment conducive to rare but extremely energetic collisions between large planetary embryos. Such an environment is typically associated with models in which the dissipation of the protoplanetary gas disk leads to orbital instability, increased eccentricities, and eventual high-velocity impacts among remaining bodies. Within this framework, the Moon-forming event is treated as an expected, if statistically uncommon, outcome of late accretion.

However, modern cold-accretion and pebble-accretion paradigms suggest alternative evolutionary pathways in which relative velocities among growing bodies are more effectively damped by disk interactions, and large-scale dynamical instability may be reduced or delayed (Andrews et al., 2018; Deng et al., 2020). If late-stage accretion proceeds under such conditions, the probability of a high-energy impact between Earth-sized bodies may be correspondingly lower than in classical collision-dominated scenarios. Evidence from primordial bodies such as Arrokoth reinforces the plausibility of gentle accretion pathways (McKinnon et al., 2020; Grundy et al., 2020). This raises questions regarding the assumed likelihood and timing of a Moon-forming giant impact under updated formation frameworks. A reduction in the expected frequency of late catastrophic impacts does not, however, preclude rare events such as the Moon-forming collision, which may remain a low-probability but consequential outcome of terrestrial planet evolution.

Beyond the formation of the Moon itself, the giant impact hypothesis carries broader implications for Earth’s early thermal and geological history. High-energy impacts of the magnitude required by canonical Theia scenarios are typically associated with extensive melting, potential global magma oceans, and large-scale resetting of crustal and geochemical records. While such consequences are not inherently incompatible with the emergence of life, they impose constraints on the continuity of habitable conditions and the preservation of early biosignatures. The degree to which these consequences are required, or merely one of several possible evolutionary pathways, remains an open question when considered in the context of gentler accretion regimes.

It is important to emphasize that reassessing the assumptions underlying the Moon-forming impact hypothesis does not necessitate its rejection. Rather, it invites a more explicit consideration of how sensitive the hypothesis is to the broader dynamical context of planet formation. As accretion models evolve, clarifying the conditions under which a giant impact remains a probable outcome — as opposed to a low-probability contingency — becomes increasingly relevant for understanding both the Moon’s origin and Earth’s early habitability.

The concept of a Late Heavy Bombardment (LHB) has long played a central role in interpretations of early Solar System evolution. In its classical formulation, the LHB refers to a hypothesized spike in impact rates affecting the inner Solar System several hundred million years after planet formation, inferred primarily from radiometric ages of lunar impact basins. This episode has been invoked to explain widespread resurfacing, volatile redistribution, and potential biological bottlenecks on the early Earth.

The LHB hypothesis relies on interpretations of lunar impact ages and dynamical instability among giant planets (Haskin et al., 1998; Bottke and Norman, 2017). However, alternative analyses question whether the lunar chronology requires a discrete, narrow bombardment spike at all, arguing instead that apparent clustering of basin ages may reflect sampling biases or a more extended decline in impact flux rather than a single cataclysmic episode (Boehnke and Harrison, 2016; Bottke and Norman, 2017). The distinction bears directly on environmental severity and early habitability (Crockett, 2019).

From the perspective of early habitability, the difference between a concentrated bombardment episode and a prolonged, lower-intensity impact regime is significant. Repeated sterilizing events could delay or disrupt the emergence of stable surface environments, whereas a declining or punctuated impact history may permit greater continuity in crustal and hydrospheric conditions. As with the Moon-forming impact, reassessing LHB assumptions within updated formation paradigms does not require abandoning impact-based explanations entirely, but it does suggest that their timing, magnitude, and environmental consequences may be more tightly constrained than traditionally assumed.

The Solar System clearly preserves abundant evidence for major impacts and large basin-forming events; the question addressed here is therefore not whether impacts occurred, but how inevitable and how frequent the most extreme late-stage catastrophes must have been under modern formation paradigms.

Re-evaluating late-stage impact hypotheses within the context of modern planet formation models has important implications for understanding early planetary environments and the conditions under which habitability may arise. Classical collision-dominated scenarios typically imply repeated episodes of extreme heating, widespread resurfacing, and global or near-global magma oceans, particularly following large impacts such as those invoked in Moon formation or during a hypothesized Late Heavy Bombardment. These conditions are often assumed to reset planetary surfaces and impose strong constraints on the timing and continuity of habitable environments (Schiller et al., 2020; Onyett et al., 2023).

Impacts may also play a complex dual role in habitability. While large collisions can impose episodic environmental stress, they may also contribute to volatile delivery, geochemical cycling, and the creation of transient ecological niches. Thus, reassessing impact frequency should not be interpreted as arguing against impacts per se, but rather as refining their probabilistic role within planetary evolution (Lammer et al., 2018; Mrnjavac et al., 2023).

In contrast, cold-accretion paradigms and more orderly dynamical histories suggest the possibility of greater thermal and geological continuity during early planetary evolution. Reduced frequencies of late catastrophic impacts would allow planetary crusts to stabilize earlier, retain volatiles more effectively, and preserve localized environments capable of supporting liquid water. While transient magma oceans may still occur, particularly during early stages of accretion, their duration and spatial extent may be more limited than envisioned in uniformly violent models.

Isotopic and geochemical evidence provides important constraints on early planetary thermal histories. Zircon crystals recovered from terrestrial and lunar materials preserve signatures consistent with early crustal differentiation and interaction with liquid water at surprisingly early epochs. These observations constrain the duration and spatial extent of global magma ocean conditions and indicate that at least portions of planetary crusts cooled sufficiently to support hydrospheric activity relatively soon after formation (Schiller et al., 2020; Onyett et al., 2023).

Complementary evidence from mantle geochemistry, meteorites, and asteroid samples indicates that terrestrial planets acquired and retained significant volatile inventories early in their histories. Studies of Earth’s deep mantle suggest the presence of primordial water, while isotopic analyses of Martian meteorites and samples from asteroid Ryugu indicate early acquisition of chondritic volatiles and saline water in the outer Solar System (Hallis et al., 2015; Péron et al., 2022; Matsumoto et al., 2024; Barrett et al., 2025).

Reduced reliance on late-stage catastrophic impacts alters models of volatile delivery to the early Earth. In classical collision-dominated scenarios, extensive impact heating and resurfacing are often invoked to explain volatile loss, necessitating later delivery via comets and volatile-rich asteroids. By contrast, gentler accretion pathways allow more efficient retention of primordial water and hydrogen, broadening the range of plausible pathways by which habitable conditions could arise on Earth and on terrestrial planets more generally. While exogenous delivery undoubtedly contributed to Earth’s volatile inventory, a reduced dependence on late catastrophic resetting events broadens the range of plausible pathways by which habitable conditions could arise.

Recent triple oxygen isotope measurements of a large suite of lunar samples likewise suggest that meteorites associated with late bombardment could only have supplied a small fraction of Earth’s total water inventory, further supporting models in which substantial volatiles were retained or acquired early in planetary evolution (Gargano et al., 2026).

These considerations have broader implications beyond Earth. If reduced late-stage impact frequencies and earlier crustal stabilization are common outcomes of planet formation, then habitable conditions may arise earlier and more robustly on terrestrial planets than previously thought. This perspective also informs interpretations of habitability on other bodies, including Mars and rocky exoplanets, where assumptions of extensive early resurfacing may warrant re-examination.

A key strength of reassessing Solar System impact hypotheses within evolving accretion paradigms is that the resulting interpretations are amenable to empirical testing, particularly as modern disk observations and pebble-accretion theory continue to develop (Johansen and Lambrechts, 2017; Andrews et al., 2018). Rather than relying solely on retrospective narrative reconstructions, updated formation–impact scenarios generate specific, testable predictions that can be evaluated using existing and future observational data.

One important class of constraints arises from isotopic systematics. If late-stage catastrophic impacts were less frequent or less globally disruptive than traditionally assumed, isotopic signatures within planetary crusts and mantles may exhibit greater continuity across early epochs. High-precision isotopic measurements of terrestrial, lunar, and meteoritic materials can therefore help distinguish between scenarios involving prolonged global melting and those permitting earlier stabilization of crustal reservoirs. In particular, heterogeneities preserved in ancient materials would be more consistent with limited thermal resetting than with repeated large-scale magma ocean events.

Rotational and orbital properties also offer potential tests. Giant impacts capable of producing large satellites or extreme axial tilts are expected to impart distinctive angular momentum signatures. Comparative studies of spin–orbit distributions among terrestrial planets, both within the Solar System and in exoplanetary systems, may help constrain the prevalence of such events. If relatively quiescent accretion pathways are common, extreme outcomes may represent special cases rather than typical evolutionary endpoints.

Cratering records provide an additional avenue for evaluation. Refinements in crater chronology, particularly on the Moon and other airless bodies, continue to inform debates over the existence and intensity of a Late Heavy Bombardment. Distinguishing between a sharp impact spike and a more extended decline in impact flux remains challenging, but improved sample dating and surface modeling may further constrain impact histories. These constraints, in turn, bear directly on assumptions regarding the environmental severity of late-stage bombardment.

Finally, exoplanet observations increasingly offer a broader comparative context for assessing Solar System evolution. The diversity of observed planetary system architectures, including dynamically compact and long-lived configurations, suggests that late-stage violent rearrangements are not an inevitable outcome of planet formation. As observational capabilities improve, identifying analogues to the Solar System — and assessing their inferred accretion and impact histories — may provide critical insights into the range of plausible evolutionary pathways.

Taken together, these observational constraints offer multiple, independent means of evaluating the compatibility of impact hypotheses with modern formation models. Continued integration of dynamical theory, geological evidence, and comparative planetology will be essential for refining our understanding of how planetary systems evolve and how habitable environments emerge.

While modern cold-accretion paradigms motivate a reassessment of late-stage impact assumptions, significant uncertainties remain. Planet formation models continue to evolve, and the relative importance of pebble accretion, planetesimal accretion, and dynamical instability likely varies across systems and epochs. As such, no single formation pathway can be assumed to apply universally, and catastrophic impacts may still occur under specific initial conditions.

Similarly, interpretations of geological and isotopic evidence are subject to inherent limitations. Ancient materials such as zircons provide valuable constraints on early thermal histories, but their preservation is necessarily incomplete and potentially biased toward more stable environments. The absence of evidence for extreme heating in certain records does not, by itself, constitute definitive evidence of its absence on a global scale. Care must therefore be taken to avoid overinterpreting sparse datasets.

Uncertainties also persist regarding the detailed mechanics of the Moon-forming impact itself. Numerous variants of the giant impact hypothesis have been proposed, some of which seek to minimize thermal disruption or reconcile isotopic similarities between Earth and the Moon. Evaluating the plausibility of these models within updated accretion frameworks remains an active area of research and underscores the need for continued theoretical and experimental work.

Finally, the relationship between impact histories and habitability is complex and multifaceted. Impacts can both hinder and facilitate habitability by delivering volatiles, driving geochemical cycling, and creating ecological niches, even as they impose episodic stress on planetary environments. Reassessing impact frequency and magnitude should therefore be understood not as an argument against impacts per se, but as an effort to better characterize their role within a broader evolutionary context.

Recognizing these limitations is essential for maintaining a balanced perspective. The goal of this reassessment is not to replace one rigid narrative with another, but to encourage continued integration of formation theory, geological evidence, and habitability studies as new data become available.

Contemporary models of planet formation increasingly emphasize disk-mediated, low-energy accretion processes that differ substantially from the collision-dominated paradigms that shaped earlier interpretations of Solar System evolution. As these formation frameworks have evolved, it has become necessary to re-examine the assumptions embedded in downstream explanatory models, particularly those invoking late, catastrophic impacts as central drivers of planetary history.

In this Perspective, we have explored the implications of modern cold-accretion paradigms for two widely cited impact hypotheses: the Moon-forming giant impact and the Late Heavy Bombardment. Without disputing the plausibility of impact-driven processes outright, we suggest that their presumed frequency, timing, and environmental consequences may warrant reassessment in light of updated dynamical and geological constraints. Evidence from planetary accretion theory, comparative exoplanet studies, and early geological records collectively point toward the possibility of greater thermal and environmental continuity during early planetary evolution than traditionally assumed.

Reconsidering impact assumptions has important implications for understanding early habitability, not only on Earth but across terrestrial planets more broadly. If late-stage catastrophic impacts were less frequent or less globally disruptive, habitable conditions may have arisen earlier and persisted more robustly than classical models suggest. As observational capabilities and theoretical models continue to improve, integrating evolving formation paradigms with geological and habitability evidence will remain essential for building internally consistent narratives of planetary system evolution.