# The Link between Athor and EL Meteorites Does Not Constrain the Timing of the Giant Planet Instability

André Izidoro, Rogerio Deienno, Sean N. Raymond, Matthew Clement

► To cite this version:

André Izidoro, Rogerio Deienno, Sean N. Raymond, Matthew Clement. The Link between Athor and EL Meteorites Does Not Constrain the Timing of the Giant Planet Instability. The Astrophysical Journal Letters, 2025, 990 (2), pp.L59. 10.3847/2041-8213/adfdc9. hal-05353449

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# The Link between Athor and EL Meteorites Does Not Constrain the Timing of the Giant Planet Instability

André Izidoro¹, Rogerio Deienno², Sean N. Raymond³, and Matthew S. Clement⁴

¹Department of Earth, Environmental and Planetary Sciences, 6100 MS 126, Rice University, Houston, TX 77005, USA

²Solar System Science & Exploration Division, Southwest Research Institute, 1301 Walnut St. Suite 400, Boulder, CO 80302, USA

³Laboratoire d'Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, Pessac, 33615, France

⁴Johns Hopkins APL, 11100 Johns Hopkins Rd., Laurel, MD 20723, USA

Received 2024 April 16; revised 2025 August 15; accepted 2025 August 20; published 2025 September 8

## Abstract

The asteroid Athor, residing today in the inner main asteroid belt, has been recently identified as the source of EL enstatite meteorites to Earth. It has been argued that Athor formed in the terrestrial region—as indicated by a similarity in isotopic compositions between Earth and EL meteorites—and was implanted in the belt $\ge 60$ Myr after the formation of the solar system. A recently published study modeling Athor's implantation in the belt further concluded, using an idealized set of numerical simulations, that Athor could not have been scattered from the terrestrial region and implanted at its current location unless the giant planet dynamical instability occurred after Athor's implantation ($\ge 60$ Myr). In this work, we revisit this problem with a comprehensive suite of dynamical simulations of the implantation of asteroids into the belt during the terrestrial planet accretion. We find that Athor-like objects can in fact be implanted into the belt long after the giant planets' dynamical instability. The probability of implanting Athor analogs when the instability occurs at $\lesssim 15$ Myr is at most a factor of $\sim 2$ lower than that of an instability occurring $\sim 100$ Myr after the solar system formation. Moreover, Athor's implantation can occur up to $\ge 100$ Myr after the giant planet instability. We conclude that Athor's link to EL meteorites does not constrain the timing of the solar system's dynamical instability.

*Unified Astronomy Thesaurus concepts: Solar system (1528); Solar system terrestrial planets (797); Asteroid belt (70); Solar system gas giant planets (1191); Dynamical evolution (421)*

## 1. Introduction

It is widely accepted that the solar system's giant planets formed in a more compact orbital configuration and evolved to their current orbits via a dynamical planetary instability (R. Gomes et al. 2005; H. F. Levison et al. 2011; D. Nesvorný & A. Morbidelli 2012; R. Deienno et al. 2017). The timing of this envisioned event, however, remains difficult to constrain (P. Boehnke & T. M. Harrison 2016; N. E. B. Zellner 2017). While it was originally suggested to have happened relatively late, about 500–800 Myr after the formation of the solar system (see R. Gomes et al. 2005; K. Tsiganis et al. 2005 and references therein), recently derived dynamical and cosmochem- ical constraints indicate that the instability happened within the first $\sim 100$ Myr of the solar system history (A. Morbidelli et al. 2018; D. Nesvorný et al. 2018; S. J. Mojzsis et al. 2019; R. de Sousa Ribeiro et al. 2020). The exact timing of the instability has dramatic implications for the early evolution of the inner solar system. If it happened earlier in this window (potentially as early as just after the dispersal of the Sun's gaseous disk; B. Liu et al. 2022), it would have had a strong impact on the middle phases of terrestrial planet growth (N. A. Kaib & J. E. Chambers 2016; M. S. Clement et al. 2018). On the other hand, if it occurred later in this window, it might have coincided with the timing of the Moon-forming event (e.g., Q. Yin et al. 2002; T. Kleine et al. 2009; S. R. DeSouza et al. 2021; M. S. Clement et al. 2023b).

The timing of the giant planet instability also has important implications for the early evolution of the asteroid belt, Kuiper Belt, and other small-body reservoirs (e.g., D. Nesvorný 2018). Thus, strongly constraining the instability's timing has the potential to confirm or refute a number of different models and theories. It remains a central challenge to advance our understanding of solar system formation and evolution.

It was recently proposed that the inner main-belt asteroid Athor ($a \approx 2.38$ au; $e \approx 0.14$; $i \approx 9°$) constrains the instability's timing to $\ge 60$ Myr after the formation of the solar system (C. Avdellidou et al. 2024a).

Athor is a member of an asteroid family. Athor's family has recently been identified via spectroscopic observations as the source of rare EL-type enstatite meteorites (C. Avdellidou et al. 2022). Asteroid family identification and reconstruction techniques were used to suggest that Athor's parent body was originally about 64 km in diameter (Athor is today about 44 km) and that an impact event within the inner main belt about $3_{-0.4}^{+0.5}$ Gyr ago (M. Delbo et al. 2019) created its current asteroid family.

Thermal evolution models and thermochronometric data of EL meteorites suggest that they are fragments of an even larger planetesimal, with a diameter of about 240–420 km. In order to reconcile the proposed link between EL meteorites and Athor with these thermochronometer models, C. Avdellidou et al. (2022) suggested that Athor's parent body was a fragment of this same large planetesimal. The study further suggested that the breakup of the initial $D \sim 240–420$ km planetesimal that produced Athor's parent body occurred no earlier than 60 Myr after the condensation of calcium-aluminum-rich inclusions (CAIs; C. Avdellidou et al. 2022; M. Trieloff et al. 2022). An

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Original content from this work may be used under the terms of the [Creative Commons Attribution 4.0 licence](https://creativecommons.org/licenses/by/4.0/). Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
earlier fragmentation would not be consistent with thermo-
chronological data of EL meteorites (M. Trieloff et al. 2022).

If Athor's parent body indeed formed from the breakup of a
*D* ~ 240–420 km object, it is highly unlikely that the
formation event occurred within the modern main-belt region.
That would result in an X-complex asteroid family population
much larger than the current one or produce relics of a “lost”
asteroid family, which are features not observed today
(C. Avdellidou et al. 2022). Therefore, the most likely
scenario, according to C. Avdellidou et al. (2022), is that
Athor's parent body formed when a large planetesimal
(*D* ~ 240–420 km) was fragmented on an impact occurring
in the terrestrial planet-forming region (e.g., <1.8 au) no
earlier than 60 Myr after the CAIs and was subsequently
implanted into the belt. Athor itself is the largest surviving
remnant of a second collision that occurred about 1 ~Gyr after
Athor's parent body's implantation into the belt (M. Delbo
et al. 2019). This secondary event formed the Athor family and
made possible the delivery of the EL-type meteorites to Earth
(C. Avdellidou et al. 2022; M. Trieloff et al. 2022).

As discussed in C. Avdellidou et al. (2022), Athor's
implantation from the terrestrial region seems to be further
supported by the strong isotopic similarities between EL
meteorites and Earth (M. Javoy et al. 2010; N. Dauphas
2017). The implantation of Athor from the terrestrial
planet formation region into the asteroid belt is also broadly
aligned with new solar system formation models suggesting
that terrestrial planets formed from a narrow ring of
planetesimals (B. M. S. Hansen 2009; A. Izidoro et al. 2022;
A. Morbidelli et al. 2022) and that the asteroid belt may have
been born either “empty”—that is, devoid of planetesimals
(S. N. Raymond & A. Izidoro 2017; A. Izidoro et al.
2022, 2024)—or very depleted in mass (R. Deienno et al.
2024). Such an implantation scenario is also supported by the
similarities between the collisionally evolved size-frequency
distribution (SFD) of terrestrial planet leftover planetesimals
(akin to the S-complex taxonomic class; F. E. DeMeo &
B. Carry 2014) and the SFD of S-complex main asteroid belt
asteroids (R. Deienno et al. 2025). Athor is potentially the best
evidence that these models may be plausible.

Building on the arguments and results of C. Avdellidou
et al. (2022), C. Avdellidou et al. (2024a) suggested that Athor
could not have been implanted from the terrestrial region into
the belt—near its current location—if the giant planet
dynamical instability occurred earlier than ~60 Myr after the
formation of the solar system. To rule out a dynamical
instability happening earlier than 60 Myr, C. Avdellidou et al.
(2024a) performed a set of numerical simulations modeling the
implantation of planetesimals in the asteroid belt considering
different idealized dynamical scenarios. In their nominal
simulations, they did not model the accretion of terrestrial
planets as we do here but instead assumed that the terrestrial
planets were fully formed and interacting with a leftover
population of planetesimals crossing the asteroid belt. A set of
their simulations also included the effects of a leftover embryo
on a moderately eccentric orbit crossing the asteroid belt,
which they refer to as Theia. From these simulations,
C. Avdellidou et al. (2024a) concluded that the only plausible
way to implant Athor into the belt at such a late epoch
(between ~60 and ~100 Myr after the formation of the solar
system) is if the planetary dynamical instability occurred after
Athor's implantation (i.e., >60 Myr after CAI formation). We

will further discuss the differences between our simulations
and theirs later in the Letter.

In this Letter, we demonstrate that a much broader range of
instability times are consistent with Athor's implantation into
the belt after 60 Myr from CAIs. Plausible scenarios include
dynamical instabilities that took place during the first 15 Myr
of the solar system evolution. We show that the simulations of
C. Avdellidou et al. (2024a) failed to capture key dynamical
processes that commonly take place during the accretion of
terrestrial planets and that influence the capture of planetesi-
mals in the belt. Furthermore, we discuss the challenges
involved in using metrics derived from a single small body to
constrain key aspects of the solar system's early evolution
such as the timing of the giant planet instability. We also
address the comments by C. Avdellidou et al. (2024b), which
were posted on arXiv in response to the early preprint of
this work.

Our Letter is structured as follows. In Section 2, we describe
our methods and numerical simulations. In Section 3, we
present our results; in Section 4, we discuss the comments of
C. Avdellidou et al. (2024b); and in Section 5, we present our
final conclusions.

**2. Methods**

Our simulations model the accretion of terrestrial planets
within a narrow ring of planetesimals. These simulations were
first presented and analyzed in A. Izidoro et al. (2024); here we
explore the relevance in the context of Athor’s implantation
specifically. They were conducted in two steps. First, we modeled
the growth from planetesimals to planetary embryos (E. Kokubo
& S. Ida 2000) using the LIPAD code (H. F. Levison et al. 2012).
Each ring was assumed to have a specific radial surface density
profile, proportional to Σ(r) = r⁻¹, and different simulations
considered different values of x (0, 1, and 5.5). We also
performed simulations with an “upside-down U-shaped” ring
profile, represented by Σ(r) = (-200(r/au - 1)² + 24) g cm⁻²
(A. Izidoro et al. 2022). Our LIPAD simulations started with
3000 planetesimals (A. Izidoro et al. 2022), which collisionally
evolved following the LIPAD prescription. We neglected the
effects of gas-driven planet migration in all simulations, but we
accounted for the effects of aerodynamic gas drag (I. Adachi
et al. 1976), as well as inclination and eccentricity gas-tidal
damping (P. Cresswell & R. P. Nelson 2006). The gas disk
lifetime was set to 5 Myr (e.g., J. P. Williams & L. A. Cieza
2011), and it dissipated following an exponential decay with an
e-fold timescale of 2 Myr. Our simulations were integrated using
a time step of 4 days.

In the second step, we used the outcome of our LIPAD
simulations as inputs to simulate the growth from planetary
embryos to final planets (J. E. Chambers & G. W. Wetherill
1998). For step 2, we used the Mercury integration package
(J. E. Chambers 1999) and assumed that the Sun’s natal disk
was already dissipated. Objects more massive than the Moon
were treated as self-interacting particles. Lower-mass objects
did not self-interact but gravitationally interacted with the Sun
and other massive objects. Our step 2 simulations started with
approximately 300–500 planetesimals and 20–30 planetary
embryos, depending on the planetesimal ring surface density
profile (see A. Izidoro et al. 2024 for details).

The timing of the instability was treated as a free parameter.
We performed simulations with instability times (t<sub>inst</sub>) of 0, 10,
50, and 100 Myr relative to the time of gas disk dispersal
(start of step 2). Assuming that the Sun’s natal disk dissipated 5 Myr after the condensation of CAIs, our instability times correspond to 5, 15, 55, and 105 Myr after the solar system’s formation, i.e, the time of CAI condensation (Y. Amelin et al. 2010). Unlike our previous work (A. Izidoro et al. 2024), in this Letter, we exclusively express time relative to the birth of the solar system formation (CAI formation). We performed 50 simulations, starting with slightly different distributions of planetary embryos and planetesimals, for each combination of instability time and ring surface density profile.

We model the giant planet instability assuming the giant planets instantaneously “jump” from preinstability orbits to their current ones. We refer to this evolution as “instantaneous instability.” This approach is common in the literature (e.g., W. F. Bottke et al. 2012; R. Brasser et al. 2013; R. Deienno et al. 2016) and allows us to circumvent potential specific effects associated with weakly constrained variables when modeling the giant planet dynamical instability. For instance, the initial resonant configuration of the giant planets, the total mass assumed of the primordial planetesimal disk beyond Neptune, the extension of the planetesimal disk, etc. are weakly constrained parameters in giant planet instability models (e.g., D. Nesvorny 2011). Different assumptions for these parameters may lead to very different dynamical evolution pathways for the giant planets themselves. The main advantage of invoking the instantaneous instability approach is that it allows us to mitigate any specific dynamical effect coming from the assumed instability itself without affecting the quality of the results of this work. Our instantaneous instability simulations were integrated at least up to 200 Myr following gas disk dispersal. Note that in A. Izidoro et al. (2024), we tested two different types of dynamical instabilities, “instantaneous” and “interpolated.” The interpolated case corresponds to a well-tested instability evolution that reproduces multiple constraints of the present-day solar system (R. Deienno et al. 2018; see their Figures 1 and 3). In this case, the orbital evolution of the giant planets is prescribed via interpolation from the original simulation of R. Deienno et al. (2018). Compared to the interpolated instability, the instantaneous instability approach is less efficient at implanting objects into the asteroid belt (A. Izidoro et al. 2024). Nevertheless, we employ the instantaneous instability here to enable a direct and consistent comparison with C. Avdellidou et al. (2024a).

Before the dynamical instability, Jupiter and Saturn were initially placed in the 3:2 mean-motion resonance (F. Masset & M. Snellgrove 2001; A. Morbidelli & A. Crida 2007; see also R. Deienno et al. 2017) with $a_J = 5.4$ au, $a_S = 7.3$ au, $e_J = e_S = 0$, $i_J = 0°$, and $i_S = 0.°5$. After the instability, their orbits are represented by $a_J = 5.25$ au, $a_S = 9.54$ au, $e_J = 0.048$, $e_S = 0.056$, $i_J = 0°$, and $i_S = 1.°5$.

# 3. Results

## 3.1. Examples of Implantation

In this section, we analyze the implantation of planetesimals into the Athor region (hereafter defined as 2.3 au $< a < 2.48$ au, $e \le 1-2/a$, $i \le 15°$ as in C. Avdellidou et al. 2024a). Altogether, the initial number of planetesimals in our simulations at 5 Myr is ~80,000 planetesimals. We report probabilities and implantation efficiencies relative to this initial number of planetesimals.

Out of 200 simulations, three planetesimals were implanted in the Athor region (one planetesimal per simulation). This corresponds to an implantation efficiency of $\approx 3.7 \times 10^{-5}$ at 200 Myr. These three simulations produced fairly good solar system analogs, with small Mars analogs and at least three terrestrial planets. Planetesimals were implanted in the Athor region after the instability took place ($t_{inst} = 5$ Myr). We have verified that the three implanted planetesimals of these simulations entered the asteroid belt at about 28, 31, and 40 Myr after CAI formation.

We extended the integration time of these simulations with $t_{inst} = 5$ Myr to 1 Gyr, and only one of them produced a long-term stable implanted object. We will refer to this specific simulation in this section as “Aθor-1.” As no planetesimal was implanted in the Athor region after 60 Myr in any of these 200 simulations, we can already expect that the probability of implanting Athor after 60 Myr is relatively low and roughly of the order of $\lesssim 1.2 \times 10^{-5}$. Later in this Letter, we will confirm that Athor’s implantation probability after 60 Myr for an early-instability scenario (e.g., $t_{inst} = 5$ Myr) is about $\approx 2 \times 10^{-5}$. Note that there are also many different ways to compute and report implantation probabilities. For instance, one may decide to account only for the total number of planetesimals in a specific simulation (or set of simulations) with successful implantation. Our simulations typically start with 300–500 planetesimals. If a single planetesimal is implanted in a specific simulation, its implantation could be given as 1/500, which is a much higher value than our average probability, reported over our full sample of simulations (with 80,000 planetesimals). That being said, because we have only a single Athor asteroid, any nonzero probability is in fact quite reassuring.

With the goal of improving the statistics of our simulations when probing the long-term stability of Athor-like objects, we performed additional simulations in which we cloned selected simulations that produced good solar system analogs. Mars analogs are defined as planets with the following orbital and physical parameters: semimajor axes in the range 1.3 au $< a < 1.8$ au, orbital eccentricities lower than $\le 0.1$, orbital inclinations lower than $i \le 10°$, and masses in the range 0.05 $M_⊕ < M < 0.3 M_⊕$. Earth analogs are defined as planets with masses higher than 0.5 $M_⊕$, orbital eccentricities lower than $\sim 0.1$, and orbital inclinations lower than $\sim 5°$. We do not define Mercury and Venus analogs, but some of our selected simulations also produced reasonable analogs of these planets, with *a*, *e*, *i*, and mass within a factor of $\sim 2$ of the real planets.

We cloned our selected systems by changing planetesimals’ semimajor axes by a tiny fraction ($\pm 0.001\%$). This cloning process allows us to increase our simulation sample size and save CPU time instead of running new simulations from the beginning to increase our simulation number.

We start by cloning our Athor-1 simulation. We focus on it exclusively in the subsequent paragraphs and analyses. We cloned this simulation 100 times at 70 Myr, when there were about 30 leftover planetesimals in the system and the terrestrial planets were not fully formed yet. We integrated each clone simulation up to 200 Myr. For the most successful cases (i.e., when an object is implanted in the Athor region), we further extended the simulation to 1 Gyr.

From this set of 100 clone simulations, four simulations implanted an object in the Athor region (one planetesimal per simulation). Three out of the four implanted planetesimals
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Implantation of a planetesimal from the terrestrial region into the asteroid belt during the final accretion phase of terrestrial planets in our Athor-1 simulation. The planetesimal is implanted in the inner main belt, in the Athor region. The x-axes show time, and the y-axes show, from top to bottom, semimajor axis, orbital eccentricity, and inclination. Dashed lines delimit the Athor region, defined as $2.3 \text{ au} < a < 2.48 \text{ au}$, $e < 1-2/a$, and $i \le 15^\circ$. This simulation corresponds to an example where the instability takes place at the time of the disk dispersal, i.e., $t_{\text{inst}} = 5 \text{ Myr}$ after the solar system formation. At 5 Myr, the gray-line planetesimal is at about 1.4 au, and it finally enters the asteroid belt at $\sim 100 \text{ Myr}$ ($a \gtrsim 1.8 \text{ au}$). Its effective implantation occurs at $\sim 120 \text{ Myr}$, when a planetary embryo (which will be later referred to as Krios; light brown) is ejected from the system.</figcaption></figure>

were the same object implanted in our original Athor-1 simulation. In all three cases, this object survived in the belt for 1 Gyr. However, one new planetesimal (which was ejected in the original Athor-1 simulation) was implanted in the Athor region in one of the clone simulations. Figure 1 shows the implantation of this object and its long-term dynamical evolution for 1 Gyr. This clone simulation produced three terrestrial planets (analogs 1, 2, and 3). Note that the planetesimal implanted in the Athor region, shown in gray, is at about 1.4 au at 5 Myr and finally reaches the asteroid belt at about $\sim 100 \text{ Myr}$. This late implantation in the belt makes this object consistent with Athor (C. Avdellidou et al. 2024a); i.e., it lived in the terrestrial region ($a < 1.8 \text{ au}$) for $> 60 \text{ Myr}$ before implantation. We can also use this set of clone simulations to reassess the probability of implanting such an object at late times.

Recalling that each of our clone simulations would have started with about 500 planetesimals within 1.5 au (see Section 2 and A. Izidoro et al. 2024), as in our original Athor-1 simulation, the overall probability of implanting this object is $\approx 2 \times 10^{-5}$ (1 object implanted/(100 clones $\times$ 500 planetesimals)). This represents the

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Implantation of a planetesimal from the terrestrial region into the asteroid belt during the final accretion phase of terrestrial planets in our Athor-2 simulation. The planetesimal is implanted in the inner main belt, in the Athor region. The x-axes show time, and the y-axes show, from top to bottom, semimajor axis, orbital eccentricity, and inclination. Dashed lines delimit the Athor region, defined as $2.3 \text{ au} < a < 2.48 \text{ au}$, $e < 1-2/a$, and $i \le 15^\circ$. Like Figure 1, this simulation corresponds to another example where the instability takes place at the time of the disk dispersal, i.e., $t_{\text{inst}} = 5 \text{ Myr}$ after the solar system formation. At 5 Myr, the gray-line planetesimal is at about 1.6 au, and it finally enters the asteroid belt at $\sim 100 \text{ Myr}$ ($a \gtrsim 1.8 \text{ au}$). Its effective implantation occurs at $\sim 290 \text{ Myr}$, when Krios (orange) is ejected from the system.</figcaption></figure>

probability of implanting an Athor-like object at $\sim 100 \text{ Myr}$ for an instability taking place at $\sim 5 \text{ Myr}$.

We performed a second set of simulations where we selected another simulation with $t_{\text{inst}} = 5 \text{ Myr}$ that produced an Athor analog and cloned it 120 times. We refer to this simulation as “Aθor-2.” We cloned this system at the 25 Myr point of our original simulation, with the goal of improving our statistics and motivated by the fact that the system at 25 Myr looks like a good candidate for a solar system analog. Out of our 120 simulations, one implanted a planetesimal in the Athor region. The dynamical evolution of this system is shown in Figure 2. This clone simulation produced four terrestrial planets (analogs 1–4). In Figure 2, the implanted planetesimal finally entered the asteroid belt at about 105 Myr, but it did not attain its final, fully implanted semimajor axis and eccentricity until $\sim 290 \text{ Myr}$. It started initially at about 1.4 au at 5 Myr, reached the asteroid belt region ($a > 1.8 \text{ au}$) for the first time between 15 and 20 Myr, left the main-belt region again between 65 and 100 Myr, and finally reentered the main belt at
about 105 Myr. This late implantation makes this object consistent with Athor (C. Avdellidou et al. 2024a).

A few other clone simulations of Athor-2 implanted planetesimals in the belt with slightly higher orbital eccentricities and inclinations than our favorite case (and Athor). We do not consider these cases when computing implantation probabilities. The implantation probability of our best Athor analog by itself, accounting for all 120 clone simulations (1 implanted planetesimal/(120 clones × 500 planetesimals), is about ≈1.7 × 10⁻⁵ for an instability with $t_{inst}$ = 5 Myr. Accounting for these additional slightly less appreciable implantation cases, the overall implantation efficiency could be up to 2–3 times higher.

Our Athor-1 and Athor-2 simulations produced our best examples of solar system analogs containing Athor-like objects. Both our Athor-1 and Athor-2 simulations match our defined solar system analog constraints. We have verified that the two Mars analogs of Figures 1 and 2 grew relatively fast and were almost fully formed (reaching >90%–95% of their final masses) during the gas disk phase. This makes them broadly consistent with the estimated Mars accretion timescale (e.g., N. Dauphas & A. Pourmand 2011).

Until now, we have dedicated this section to analyzing and discussing the results of simulations with early instability times, i.e., $t_{inst}$ = 5 Myr. Now we analyze the implantation probability in simulations with late instabilities, $t_{inst}$ = 55 and 105 Myr.

Out of our 200 simulations with $t_{inst}$ = 55 Myr, four planetesimals were implanted in the Athor region, resulting in an implantation efficiency of ≈5 × 10⁻⁵. However, these four planetesimals first entered the asteroid belt ($a > 1.8$ au) between 15 and 40 Myr after CAIs, with an average entrance time of ~25 Myr, making them inconsistent with Athor (M. Trieloff et al. 2022).

Out of our 200 simulations with $t_{inst}$ = 105 Myr, a total of 13 planetesimals were implanted in the Athor region of the belt. Out of these 13 planetesimals, four were implanted in the asteroid belt after 60 Myr, with an average time of entrance into the belt of ~70 Myr after CAIs. This yields an implantation probability of Athor analogs of about ≈5 × 10⁻⁵. These results also suggest that implanting objects in the Athor region at ~100 Myr with $t_{inst}$ = 105 Myr has a probability of the order of ≲1.2 × 10⁻⁵, as no such late implantation was observed.

## 3.2. The Implantation Mechanism

Athr analogs implanted after the giant planet instability all have similar origins. This is true regardless of whether they are implanted as late as those in Figures 1 and 2 or earlier (see examples in the Appendix). They are typically planetesimals scattered from ≈1 to 1.7 au into the Mars region that undergo multiple scattering events by a Mars analog and a leftover unstable planetary embryo on an asteroid-belt-crossing orbit. We refer to this embryo as Krios, the brother of Theia, the name commonly given to the Moon-forming impactor. These objects are shown in Figures 1, 2, and 3. We also found implantation examples in our sample of simulations where Theia assumes the role of “Krios.” In these specific cases, Theia, rather than being ejected from the solar system like Krios, collides with the Earth analog while also promoting Athor’s implantation.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Evolution of semimajor axis, pericenter, and apocenter of all planetary bodies shown in Figure 1, using the same color-coding in both figures. The implantation of the planetesimal in the belt (gray lines) at about 120 Myr occurs due to its gravitational interaction with Krios and Mars. The high eccentricity and inclination phases of Krios before its ejection induce secular effects on the planetesimal that reduce its orbital inclination and eccentricity, ultimately implanting it in the belt around Athor’s modern orbit.</figcaption></figure>

Figure 3 shows the evolution of the semimajor axis, pericenter, and apocenter of all objects shown in Figure 1. Although orbital overlap between the planetesimal and the asteroid belt exists for <60 Myr, this planetesimal may have avoided breakup to 80 Myr or so, when it may have finally fragmented near its pericenter. Note that at 85 Myr, it is no longer in the belt ($a < Q < 1.8$ au). In this case, as all fragments would be in terrestrial planets crossing orbits, all but one (the lucky Athor) would have been lost.

In our simulations, Athor-like objects were typically implanted into the asteroid belt after experiencing a secular torque from Krios (or Theia), typically during Krios’s (Theia’s) final high-eccentricity/inclination phase (see Figure 3 and an additional example in the Appendix).

Our simulations show that planetary embryos like Krios play a fundamental role in implanting Athor-like objects in the asteroid belt. The simulations of C. Avdellidou et al. (2024a) did not capture the effect of this type of object during the final phase of accretion of terrestrial planets. Their simulations did not implant planetesimals in the Athor region for early instability times due to assumptions in their design and the very limited number of simulations. C. Avdellidou et al. (2024a) tested four different implantation scenarios: (i) implantation after terrestrial planet formation, where the terrestrial planets and giant planets are assumed to be fully formed and in their current orbits; (ii) implantation during terrestrial planet formation, using a single simulation outcome from D. Nesvorný et al. (2021); (iii) implantation due to the gravitational effects of a leftover planetary embryo envisioned to be Theia; and (iv) implantation during the giant planet instability. C. Avdellidou et al. (2024a) ruled out scenarios (i), (ii), and (iii) and concluded that only (iv) is plausible. Our results support their conclusions that scenario (i) cannot implant Athor-like objects. However, we show here that scenarios (ii) and (iii) are also consistent with Athor. Their simulation of scenario (ii) probably failed to implant planetesimals in the Athor region because they only tested a single accretion history of the terrestrial planets. Their scenario (iii) failed to implant planetesimals in the Athor region
because they assumed an ad hoc initial orbit for the putative
leftover embryo.

3.2.1. Which Instability Timing Is More Likely to Implant Athor?

Our simulations with an instability occurring within 15 Myr
after CAI formation yield a probability of implanting Athor
after 60 Myr of at least $\approx 2 \times 10^{-5}$. Simulations with an
instability occurring 105 Myr after CAI formation produce an
implantation probability of $\approx 5 \times 10^{-5}$. This shows that the
probability of implanting Athor in the early-instability scenario
(M. S. Clement et al. 2018; D. Nesvorný & A. Morbidelli 2012; M. S. Clement
et al. 2021). If one were to make judgments on past events in
solar system history based on factors of 2, the instability itself
would be thrown out.

Our implantation probabilities—and the factor of ~2
difference for Athor’s implantation in a late-versus-early
instability—can be more easily understood if we recall that, to
first order, derived implantation efficiencies are a function of
the total number of planetesimals available for implantation at
different times. We have computed the ratio of the number of
planetesimals existing in the terrestrial scattered disk in
simulations where the giant planets are kept in preinstability
orbits to those with instabilities occurring at 5 Myr. We define
planetesimals in the terrestrial scattered disk as those located
in the terrestrial region (1 au < *a* < 1.8 au) or the asteroid belt
region (1.8 au < *a* < 3.5 au) with very high orbital
eccentricities (*e* > 0.4) and inclinations (*i* > 35°). The number
of planetesimals in the scattered disk in the preinstability case
can be taken as a proxy for late-instability cases. Our results
are presented in Figure 4.

Figure 4 shows that the inferred factor of 2 difference in
implantation efficiencies between our early- and late-instabil-
ity simulations is largely in agreement with the evolution of
the ratio of the number of planetesimals in the terrestrial
scattered disk in these simulations. As one can see, simulations
with giant planets in preinstability orbits tend to have a factor
of ~1.4–2 more planetesimals in the terrestrial scattered disk
(ready to be implanted) between 60 and 100 Myr than those
where they have attain their modern orbits at 5 Myr after CAIs.
This is because once Jupiter and Saturn are on their present-
day orbits, they slightly accelerate the process of accretion and
depletion in the terrestrial region. Thus, the number of
planetesimals available to be implanted on Athor-like orbits
is reduced quicker than when the giant planets are on their
preinstability orbits. Note that this is true independent of the
injection method. C. Avdellidou et al. (2024a) claim that,
regardless of the size of the population in the scattered disk,
objects cannot be injected into the Athor region unless Jupiter
raises their pericenters as they cross the belt as a result of its
dynamical evolution during the instability. We show that
interaction with nonsurviving embryos is another potential
mechanism even after the instability.

Figure 4 supports our findings and suggests that the inferred
mere factor of ~2 difference in the probabilities of implanting
Athor with an instability occurring before 15 Myr compared to

<figure><img src="image_7.png" /></figure>

**Figure 4.** Temporal evolution of the ratio of the average number of planetesimals in the terrestrial scattered disk in simulations where the instability takes place at 5 Myr (giant planets are in their current orbits) after solar system formation and simulations where the giant planets are kept in preinstability orbits. Planetesimals in the terrestrial scattered disk are defined as those in the terrestrial region (1 au < *a* < 1.8 au) and those in the belt region but with very high orbital eccentricities and inclinations (1.8 au < *a* < 3.5 au, *e* > 0.4, and *i* > 35°). We have tested several different cutoffs for the definition of planetesimals in the terrestrial scattered disk, and the results were similar to the presented case. For instance, accounting only for planetesimals in the terrestrial region (1 au < *a* < 1.8 au) leads to broadly similar results. The number of planetesimals in the terrestrial scattered disk for each giant planet configuration is averaged over 50 simulations modeling the accretion of terrestrial planets. The gray lines show 1σ standard deviation.

an instability after 60 Myr is reasonable and robust. From our
results, we cannot firmly exclude the possibility that these
probabilities are virtually the same, in particular for implant-
ation times ≫70 Myr.

We conclude this section by reemphasizing that the link
between Athor and EL meteorites suggested by C. Avdellidou
et al. (2022, 2024a) is not sufficient to constrain the timing of
the giant planet dynamical instability.

4. Discussion

In a response to a preliminary version of this work posted to
arXiv, C. Avdellidou et al. (2024b) levied two critiques against
the implantation via unstable embryo mechanism. We respond
to each of these below.

4.1. Terrestrial Planet Excitation

C. Avdellidou et al. (2024b) correctly point out that systems
with an unstable embryo ejection near the end of the process of
terrestrial accretion are likely more prone to overexcitation.
Indeed, the low rate of replicating the remarkably circular and
coplanar orbits of Earth and Venus in terrestrial accretion and
giant planet instability models is a well-documented problem
in the literature (R. Brasser et al. 2009; S. N. Raymond et al.
2009; A. Izidoro et al. 2014; N. A. Kaib & J. E. Chambers
2016; M. S. Clement et al. 2023b; D. Nesvorny et al. 2025).

To make the case that systems able to implant Athor via our
proposed mechanism will be systematically overexcited in a
manner inconsistent with the modern terrestrial system,
C. Avdellidou et al. (2024b) plot the angular momentum
deficits (AMDs; J. Laskar 1997) and time of last embryo
ejection of 330 terrestrial analogs from the combined works of
D. Nesvorný et al. (2021) and J. M. Y. Woo et al. (2024). As
only 2 out of 50 simulations with an embryo ejection after
60 Myr have AMDs consistent with the modern inner solar
system, the authors suggest that the probability of Athor's capture via the mechanism we propose here must be much less than we stated.

Aside from the fundamental flaw in attempting to “prove a negative” by relying on a 96% probability of failure, this criticism is flawed for three main reasons.

1. Figure 1 of C. Avdellidou et al. (2024b) insufficiently captures the effects of the actual mechanism in dispute. More important quantities to measure would be the time of last embryo ejection from the 1.5 au $\lesssim a \lesssim 2.0$ au region, the time after which no embryo crosses 2.0 au, or the change in system AMD during an actual embryo ejection event. There are many reasons other than a late embryo ejection as to why any given system's AMD might be too high, and this reality clouds any meaningful interpretation of their Figure 1. Moreover, AMD is known to be a highly degenerate metric for measuring the success of terrestrial analog systems (D. Nesvorný et al. 2021; M. S. Clement et al. 2023b). Indeed, a system with the wrong number of planets or one where Mars's and Mercury's eccentricities are low but Earth's and Venus' are high could both possess good AMDs. If anything, their Figure 1 only serves to highlight the continued shortcomings of all contemporary terrestrial planet formation models in terms of the AMD metric.

2. The arguments of C. Avdellidou et al. (2024b) also rely on terrestrial planet formation simulations that, while state-of-the-art, are highly limited in mass and particle resolution. In order to make the 200 Myr epoch of terrestrial planet formation computationally tractable, modelers must simplify the state of the terrestrial disk around the time of gas disk dispersal to a population of 20 or so massive embryos and $\sim 10^3$ planetesimals that are unreasonably massive and do not gravitationally interact with one another (the models we presented in A. Izidoro et al. 2024 make similar simplifications). However, high-resolution simulations of embryo growth during the gas disk phase (e.g., R. Deienno et al. 2019; K. J. Walsh & H. F. Levison 2019; J. M. Y. Woo et al. 2021) predict that a large population of smaller embryos with $0.001 M_{\oplus} \lesssim m \lesssim 0.01 M_{\oplus}$ should also permeate the region between 1.0 and 2.0 au when the gas disk dissipates. In the simulations at the heart of this debate, this class of object might only be represented by a single, overly massive embryo (hence the need to make probabilistic arguments). Yet, in reality, these smaller embryos are likely more than capable of implanting Athor-like planetesimals in the inner belt via our proposed scattering mechanism and also potentially far less likely to overexcite the terrestrial planets. This is a central flaw of the AMD argument of C. Avdellidou et al. (2024b). The number of embryos in the simulations being analyzed is far too small to quantify the total fraction of systems that can eject an object massive enough to secularly torque a $\sim 50$ km planetesimal after $t = 60$ Myr. While future work will be required to accurately quantify this, we speculate that the probability of a late embryo ejection in a higher-resolution simulation that produces a good AMD is likely much higher than postulated by C. Avdellidou et al. (2024b).

3. Finally, to directly and conclusively address and refute the arguments and criticisms of C. Avdellidou et al. (2024b),

we evaluated the change in AMD during Athor-implanting embryo ejections in our simulations. Each system experiences a change in AMD as a result of the dynamical exchange; however, that change is not necessarily in the wrong direction. Our first analog in Figure 1 possess a very high AMD of 0.0132—an order of magnitude larger than that of the solar system—before the embryo ejection at about 290 Myr. As a consequence of angular momentum exchange between Mars and the unstable embryo, the postejection AMD of the system is only 0.0067 (3 times higher than the solar system AMD of 0.0018). Similarly, the AMD of the system shown in Figure A1 evolves from 0.0062 to 0.0019 (AMD<sub>ss</sub> = 0.0018) as a result of the dynamics that occur during Athor's implantation. The damping of the eccentricities and inclinations of Earth, Venus, and Mars can be seen in Figure A2 as well. Given our very limited number of Athor analogs, the identification of at least one simulation with embryo ejection at about 90 Myr where the AMD matches that of the solar system lends credibility to our interpretation.

## 4.2. Giant Planet Residual Migration

The second concern raised in the response letter of C. Avdellidou et al. (2024b) relates to the stability of objects in the inner main belt during the giant planet's residual migration phase. The authors reference the work of M. S. Clement et al. (2020), which shows how an absence of asteroids in the modern asteroid belt with precession rates between 24" and 28" yr<sup>-1</sup> is a consequence of resonant cycling during residual migration. That paper also demonstrates how the residual migration preferentially depletes high-inclination asteroids in the inner belt (Athin is not high-inclination in this sense) and removes excessive objects with large inclinations that are produced by the giant planet instability (A. Morbidelli et al. 2010; R. Deienno et al. 2016). The authors go on to conclude that this result implies that no mechanism can inject asteroids in the inner belt after the giant planet instability.

We disagree with this interpretation. The main reason for this is that the implantation probabilities involved with these mechanisms are too low to irreparably alter the belt's orbital architecture. More specifically, there are three possible timings of Athor's implantation with respect to residual migration, and we do not view any as being overly problematic.

1. *Athin is implanted before residual migration.* As pointed out in C. Avdellidou et al. (2024a), this possibility is entirely consistent with the residual migration models of M. S. Clement et al. (2020). To confirm this, we investigated the fates of Athor analogs in M. S. Clement et al. (2020) and found that 82% survive within the Athor region.

2. *Athin is implanted during the residual migration phase.* Though potentially quite likely, this case is not addressed in the arguments of C. Avdellidou et al. (2024b) or in their original paper (C. Avdellidou et al. 2024a). The actual duration of the epoch of residual migration is poorly constrained but loosely related to the amount of leftover planetesimal mass after the instability. However, in most simulations, the bulk of it lasts for between 10 and 100 Myr (D. Nesvorný & A. Morbidelli 2012; R. Deienno et al. 2017). For an instability occurring at,
for example, 70 Myr, it is entirely possible that Athor could be implanted several tens of Myr later, still during the resonant reversal stage of residual migration. This could also be the case for a $t = 10$ Myr instability if residual migration persists for more than 50 Myr. To our knowledge, no one has explicitly tested how the efficiency of asteroid implantation from the terrestrial region is effected by unique resonant perturbations that occur as a consequence of ongoing residual migration. We recall that our instantaneous instability approach does not include this effect by design. While we acknowledge it as a possible mechanism that could lead to the implantation of Athor-like objects, its magnitude remains unquantified.

3. Athor is implanted after residual migration is complete. It is our interpretation that this possibility lies at the core of the criticism raised in C. Avdellidou et al. (2024b), and it corresponds to the standard scenario considered in our work. Their argument primarily concerns the ratio of asteroids with inclinations above to those below the $v_6$ resonance in the inner asteroid belt (0.08 in the modern belt). Simulations of the giant planet instability generally struggle to reproduce this ratio, with typical outcomes yielding values of $\ge 1.0$ (A. Morbidelli et al. 2010; R. Deienno et al. 2016, 2018). M. S. Clement et al. (2020) demonstrated that this discrepancy can be naturally resolved by the preferential removal of high-inclination asteroids during residual migration. Athor itself is not a high-inclination asteroid; therefore, its implantation after the completion of residual migration would actually help reduce the $v_6$ ratio and bring modeled outcomes closer to the observed solar system. We therefore suspect that C. Avdellidou et al. (2024b) is more concerned with the potential implantation of high-inclination asteroids via our embryo scattering mechanism. This is indeed possible, and we do observe some cases of high-inclination capture ($i > 15^\circ$) in our simulations. Notably, there are only two large ($D \ge 45$ km) S-type asteroids—25 Phocaea and 234 Barbara—with inclinations above $15^\circ$, corresponding roughly to the region above the $v_6$ secular resonance (C. Froeschle & H. Scholl 1986). We analyzed the high- to low-inclination ratio in the simulations of A. Izidoro et al. (2024) using the instantaneous instability approach, measured at 200 Myr. Our results show no evidence that postinstability implantation is significantly more efficient for high-inclination objects. For example, in simulations with an instability at 5 Myr, the number of low-inclination objects ($2.3 \text{ au} < a < 2.48 \text{ au}$, $e < 0.35$, $i < 15^\circ$) is four, while the number of high-inclination objects in the same a-e range but with $15^\circ < i < 35^\circ$ is five, yielding a ratio of 1.2. All these objects entered the main belt after the instability (see top panels of Figure 2 in A. Izidoro et al. 2024), suggesting that the probability of postinstability implantation is comparable for low- and high-inclination objects.

For an instability at 15 Myr, however, some implantation into the inner main belt occurs even before the instability (see second row of Figure 2 in A. Izidoro et al. 2024), and these preinstability objects are typically low-inclination. The probability of implanting low-inclination bodies in the inner main belt before the

instability—for instance, when $t_{\text{inst}} \ge 15$ Myr—is up to 2 orders of magnitude higher ($\sim 10^{-3}$; see Table 1 and Figure 2 of A. Izidoro et al. 2024) than the efficiencies we find for our embryo scattering mechanism ($\sim 10^{-5}$). This naturally favors small ratios for the high- and low-inclination populations. Indeed, for an instability at 15 Myr, the high-to-low-inclination ratio at 200 Myr is 0.125 (1/8), which is less than twice the observed $v_6$ value (0.08). For instabilities at 55 Myr and 105 Myr, these ratios are 0.136 (3/22) and 0.262 (11/42), respectively. These results indicate that postinstability implantation does not inherently produce an excessive population of high-inclination objects or an anomalously high $v_6$ ratio. Instead, this ratio is primarily determined by the preinstability existing belt population, their inclinations prior to the instability, and the local inclination excitation driven by the instability itself, which may vary from case to case (e.g., as discussed in R. Deienno et al. 2018). We also note that our reported ratios are based on simulations that stop at 200 Myr; given that high-inclination objects are more likely to be removed over the subsequent $\sim 4$ Gyr of solar system evolution (e.g., D. A. Minton & R. Malhotra 2009, 2011), the final $v_6$ ratio would likely be even lower.

Finally, it is worth noting that the simulations of C. Avdellidou et al. (2024a; see their Figure 2, panels (A), (B), and (C)) implant objects that lie borderline with the Athor region, showing that their results are not wholly incompatible with Athor-like implantation. The absence of a single definitive implantation instead reflects the limitations of their idealized approach and the specific boundaries they adopted for the Athor region, rather than a fundamental dynamical impossibility.

5. Conclusion

In this Letter, we studied the implantation of the asteroid Athor from the terrestrial region into the asteroid belt. We were motivated by the recent results of C. Avdellidou et al. (2024a) suggesting that Athor could not be implanted in the belt if the giant planet dynamical instability occurred before 60 Myr after the formation of the solar system. To revisit this problem, we performed and analyzed a suite of numerical simulations modeling the accretion of terrestrial planets from a narrow ring of planetesimals (A. Izidoro et al. 2024), with a focus on the implantation of planetesimals from the terrestrial region into the asteroid belt.

We modeled the giant planet dynamical instability by assuming that the giant planets formed in a more compact, circular, and coplanar configuration before evolving to their current dynamical state. The timing of the instability was treated as a free parameter in this work. We performed simulations assuming that the giant planet instability occurred at 5, 15, 55, and 105 Myr after the formation of the solar system.

Our simulations showed that Athor-like objects can be implanted in the asteroid belt up to $\ge 100$ Myr after the solar system dynamical instability occurred. We compared the probability of implanting Athor after 60 Myr in scenarios where the instability occurs “early,” within 15 Myr, and relatively “late,” $\sim 100$ Myr after the solar system formation. Our results show that the probability of implanting Athor-like objects when the instability occurs at $\sim 100$ Myr is at most a
factor of 2 higher than that from the scenario where the instability occurs within 15 Myr after the formation of the solar system.

We conclude this work by stating that Athor's origin alone cannot be used to constrain the time of the solar system dynamical instability. Its possible connection with EL meteorites should be regarded as just one line of evidence, to be evaluated alongside other constraints.

If the instability indeed occurred earlier than proposed by C. Avdellidou et al. (2024a), the asteroid Athor's implantation in the asteroid belt is simply one of many low-probability dynamical phenomena that have taken place in the solar system's 4.5 Gyr history. Recent asteroid family analyses also suggest that the primary source of EL chondrites may not be the Athor family (M. Brož et al. 2024), which, if correct, would undermine the main motivation of C. Avdellidou et al. (2024a) and our own.

A dynamical instability occurring $\geqslant$15 Myr after the formation of the solar system remains a plausible scenario to explain both the implantation of the asteroid Athor and the total mass carried by S-complex asteroids (taxonomic class; i.e., F. E. DeMeo & B. Carry 2014) in the belt (see A. Izidoro et al. 2024; R. Deienno et al. 2025). An early instability could have been triggered by dynamical interactions between the giant planets themselves (R. de Sousa Ribeiro et al. 2020) or by dispersal of the gaseous disk (B. Liu et al. 2022). Such a scenario has already been shown to be compatible with thermochronological measurements of asteroidal meteorites (G. Harper Edwards et al. 2024), and the dynamical influence of an early instability could explain a number of features of Earth, the terrestrial planet system, and the asteroid belt (M. S. Clement et al. 2019, 2023a; D. Nesvorný et al. 2021; S. Joiret et al. 2023).

## Acknowledgments

We thank the anonymous reviewer for the constructive comments and suggestions, which have significantly improved the clarity and quality of this work. The work of R.D. was supported by the Center for Lunar Origin and Evolution (CLOE), a team in NASA's SSERVI program (cooperative agreement 80NSSC23M0176). M.S.C. is supported by NASA Emerging Worlds grant 80NSSC23K0868 and NASA's CHAMPs team, supported by NASA under grant No. 80NSSC21K0905 issued through the Interdisciplinary Con- sortia for Astrobiology Research (ICAR) program. S.N.R acknowledges funding from the Programme Nationale de Planetologie (PNP) of the INSU (CNRS), and in the frame- work of the Investments for the Future programme IdEx, Universite de Bordeaux/RRI ORIGINS. A.I. thanks Debjeet Pathak for several discussions and proofreading the Letter.

# Appendix

## Additional Examples of Implantation of Planetesimals in the Athor Region

Examples presented in this section show implantation of planetesimals in the Athor region in simulations that also produced good solar system analogs. Although in both Figure A1 (and Figure A2) and Figure A3, planetesimals were only effectively implanted in the belt at about 100 Myr, both implanted objects arrived in the belt region too early (before 60 Myr after CAI formation), making them incon- sistent with Athor.

<figure><img src="image_8.png" /></figure>

**Figure A1.** Representative example of implantation of a planetesimal that arrives relatively early in the belt, at about ~40 Myr. The early arrival of this object in the asteroid belt makes it most likely inconsistent with Athor. As in Figure 1, the x-axes show time, and the y-axes show, from top to bottom, semimajor axis, orbital eccentricity, and inclination. Dashed lines delimit the Athor region, defined as $2.3 \text{ au} < a < 2.48 \text{ au}$, $e < 1-2/a$, and $i < 15^\circ$. This simulation corresponds to an example where the instability also takes place at the time of the disk dispersal, $t_{\text{inst}} = 5$ Myr, relative to the solar system formation time. The final implantation of this planetesimal happens at about 85 Myr, when Krios is ejected from the system.
<figure><img src="image_9.png" /><figcaption>**Figure A2.** Evolution of the semimajor axis, pericenter, and apocenter of all planetary bodies shown in Figure A1, using the same color-coding for both figures. The implantation of the planetesimal in the belt (gray lines) at about 85 Myr occurs due to its gravitational interaction with Krios and Mars, but note that it enters the asteroid belt region (a ≳ 1.8 au) between 30 and 40 Myr. As in Figure 3, the high eccentricity and inclination phases of Krios before its ejection induce secular effects on the planetesimal, reducing its orbital inclination and eccentricity and ultimately implanting it in the belt and Athor's region.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Figure A3.** Representative example of implantation of a planetesimal that arrives relatively early in the belt, at about ~40 Myr. The early arrival of this object in the asteroid belt makes it most likely inconsistent with Athor. As in Figure 1, the x-axes show time, and the y-axes show, from top to bottom, semimajor axis, orbital eccentricity, and inclination. Dashed lines delimit the Athor region, defined as 2.3 au < *a* < 2.5 au, *e* < 0.15, and *i* < 15°. This simulation corresponds to an example where the instability also takes place at the time of the disk dispersal, *t*<sub>inst</sub> = 5 Myr, relative to the solar system formation time. The final implantation of this planetesimal happens at about 85 Myr, when Krios is ejected from the system.</figcaption></figure>

## ORCID iDs

André Izidoro https://orcid.org/0000-0003-1878-0634

Rogerio Deienno https://orcid.org/0000-0001-6730-7857

Sean N. Raymond https://orcid.org/0000-0001-8974-0758

Matthew S. Clement https://orcid.org/0000-0001-8933-6878

## References

Adachi, I., Hayashi, C., & Nakazawa, K. 1976, *PThPh*, 56, 1756

Amelin, Y., Kaltenbach, A., Iizuka, T., et al. 2010, *E&PSL*, 300, 343

Avdellidou, C., Delbo, M., Morbidelli, A., et al. 2022, *A&A*, 665, L9

Avdellidou, C., Delbo', M., Nesvorný, D., Walsh, K. J., & Morbidelli, A. 2024a, *Sci*, 384, 348

Avdellidou, C., Delbo', M., Nesvorný, D., Walsh, K. J., & Morbidelli, A. 2024b, *arXiv:2404.19390*

Boehnke, P., & Harrison, T. M. 2016, *PNAS*, 113, 10802

Bottke, W. F., Vokrouhlický, D., Minton, D., et al. 2012, *Natur*, 485, 78

Brasser, R., Morbidelli, A., Gomes, R., Tsiganis, K., & Levison, H. F. 2009, *A&A*, 507, 1053

Brasser, R., Walsh, K. J., & Nesvorňy, D. 2013, *MNRAS*, 433, 3417

Brož, M., Vernazza, P., Marsset, M., et al. 2024, *A&A*, 689, A183

Chambers, J. E. 1999, *MNRAS*, 304, 793

Chambers, J. E., & Wetherill, G. W. 1998, *Icar*, 136, 304

Clement, M. S., Chambers, J. E., Kaib, N. A., Raymond, S. N., & Jackson, A. P. 2023a, *Icar*, 394, 115445

Clement, M. S., Deienno, R., & Izidoro, A. 2023b, *Icar*, 389, 115260

Clement, M. S., Kaib, N. A., Raymond, S. N., & Walsh, K. J. 2018, *Icar*, 311, 340

Clement, M. S., Morbidelli, A., Raymond, S. N., & Kaib, N. A. 2020, *MNRAS*, 492, L56

Clement, M. S., Raymond, S. N., & Kaib, N. A. 2019, *AJ*, 157, 38

Clement, M. S., Raymond, S. N., Kaib, N. A., et al. 2021, *Icar*, 355, 114122

Cresswell, P., & Nelson, R. P. 2006, *A&A*, 450, 833

Dauphas, N. 2017, *Natur*, 541, 521

Dauphas, N., & Pourmand, A. 2011, *Natur*, 473, 489

Deienno, R., Gomes, R. S., Walsh, K. J., Morbidelli, A., & Nesvorný, D. 2016, *Icar*, 272, 114

Deienno, R., Izidoro, A., Morbidelli, A., et al. 2018, *ApJ*, 864, 50

Deienno, R., Izidoro, A., Nesvorný, D., et al. 2025, *ApJ*, 986, 146

Deienno, R., Morbidelli, A., Gomes, R. S., & Nesvorný, D. 2017, *AJ*, 153, 153

Deienno, R., Nesvorný, D., Clement, M. S., et al. 2024, *PSJ*, 5, 110

Deienno, R., Walsh, K. J., Kretke, K. A., & Levison, H. F. 2019, *ApJ*, 876, 103

Delbo, M., Avdellidou, C., & Morbidelli, A. 2019, *A&A*, 624, A69

DeMeo, F. E., & Carry, B. 2014, *Natur*, 505, 629

de Sousa Ribeiro, R., Morbidelli, A., Raymond, S. N., et al. 2020, *Icar*, 339, 113605

DeSouza, S. R., Roig, F., & Nesvorný, D. 2021, *MNRAS*, 507, 539

Froeschle, C., & Scholl, H. 1986, *A&A*, 166, 326

Gomes, R., Levison, H. F., Tsiganis, K., & Morbidelli, A. 2005, *Natur*, 435, 466

Hansen, B. M. S. 2009, *ApJ*, 703, 1131

Harper Edwards, G., Brenhin Keller, C., Newton, E. R., & Stewart, C. W. 2024, *NatAs*, 8, 1264

Izidoro, A., Dasgupta, R., Raymond, S. N., et al. 2022, *NatAs*, 6, 357

Izidoro, A., Deienno, R., Raymond, S. N., & Clement, M. S. 2024, *arXiv:2404.10831*

Izidoro, A., Haghighipour, N., Winter, O. C., & Tsuchida, M. 2014, *ApJ*, 782, 31

Javoy, M., Kaminski, E., Guyot, F., et al. 2010, *E&PSL*, 293, 259

Joiret, S., Raymond, S. N., Avice, G., et al. 2023, *Icar*, 406, 115754

Kaib, N. A., & Chambers, J. E. 2016, *MNRAS*, 455, 3561

Kleine, T., Touboul, M., Bourdon, B., et al. 2009, *GeCoA*, 73, 5150

Kokubo, E., & Ida, S. 2000, *Icar*, 143, 15

Laskar, J. 2000, *A&A*, 317, L75

Levison, H. F., Duncan, M. J., & Thommes, E. 2012, *AJ*, 144, 119

Levison, H. F., Morbidelli, A., Tsiganis, K., Nesvorný, D., & Gomes, R. 2011, *AJ*, 142, 152

Liu, B., Raymond, S. N., & Jacobson, S. A. 2022, *Natur*, 604, 643

Masset, F., & Snellgrove, M. 2001, *MNRAS*, 320, L55

Minton, D. A., & Malhotra, R. 2009, *Natur*, 457, 1109

Minton, D. A., & Malhotra, R. 2011, *ApJ*, 732, 53
Mojzsis, S. J., Brasser, R., Kelly, N. M., Abramov, O., & Werner, S. C. 2019, ApJ, 881, 44

Morbidelli, A., Baillié, K., Batygin, K., et al. 2022, NatAs, 6, 72

Morbidelli, A., Brasser, R., Gomes, R., Levison, H. F., & Tsiganis, K. 2010, AJ, 140, 1391

Morbidelli, A., & Crida, A. 2007, Icar, 191, 158

Morbidelli, A., Nesvorny, D., Laurenz, V., et al. 2018, Icar, 305, 262

Nesvorny, D. 2011, ApJ, 742, L22

Nesvorný, D. 2018, ARA&A, 56, 137

Nesvorný, D., & Morbidelli, A. 2012, AJ, 144, 117

Nesvorny, D., Morbidelli, A., Bottke, W. F., Deienno, R., & Goldberg, M. 2025, AJ, 170, 180

Nesvorný, D., Roig, F. V., & Deienno, R. 2021, AJ, 161, 50

Nesvorný, D., Vokrouhlický, D., Bottke, W. F., & Levison, H. F. 2018, NatAs, 2, 878

Raymond, S. N., & Izidoro, A. 2017, SciA, 3, e1701138

Raymond, S. N., O'Brien, D. P., Morbidelli, A., & Kaib, N. A. 2009, Icar, 203, 644

Trieloff, M., Hopp, J., & Gail, H.-P. 2022, Icar, 373, 114762

Tsiganis, K., Gomes, R., Morbidelli, A., & Levison, H. F. 2005, Natur, 435, 459

Walsh, K. J., & Levison, H. F. 2019, Icar, 329, 88

Williams, J. P., & Cieza, L. A. 2011, ARA&A, 49, 67

Woo, J. M. Y., Grimm, S., Brasser, R., & Stadel, J. 2021, Icar, 359, 114305

Woo, J. M. Y., Nesvorný, D., Scora, J., & Morbidelli, A. 2024, Icar, 417, 116109

Yin, Q., Jacobsen, S. B., Yamashita, K., et al. 2002, Natur, 418, 949

Zellner, N. E. B. 2017, OLEB, 47, 261