Quasi-Interstellar Objects (quasi-ISOs)

• Quasi-ISOs are small bodies with ambiguous origins, defined either as returning Solar-system ejecta or captured extrasolar objects with near-hyperbolic orbits.
• They exhibit distinct kinematic signatures, notably very low hyperbolic excess speeds (<1 km/s) that set them apart from bona fide interstellar objects.
• Studying quasi-ISOs offers a unique probe into past Oort-cloud erosion and the Solar System’s dynamical history influenced by Galactic tides and stellar encounters.

Searching arXiv for recent and relevant papers on quasi-interstellar objects and related ISO capture/return dynamics. Quasi-interstellar objects, or quasi-ISOs, denote a class of small bodies that occupy an intermediate or ambiguous position relative to the standard category of bona fide interstellar objects. In the 2026 dynamical-return formulation, they are unbound objects that originated in the Solar System’s own Oort cloud, were removed by Galactic tides and/or passing stars, and later returned to the Solar neighborhood on a second hyperbolic passage, thereby appearing interstellar in orbital energy despite being Solar-System-native in composition (Forbes et al., 5 Jul 2026). Other papers use the same label for captured extrasolar bodies now on bound heliocentric orbits, or for marginally unbound low-velocity ejecta, so the term is context-dependent and not yet uniform across the literature (Dehnen et al., 2021, Marquardt-Demen et al., 7 Sep 2025, Albrow et al., 4 Dec 2025).

1. Terminological scope and competing definitions

The most explicit distinction in the recent literature is between a bona fide ISO and a quasi-ISO. In the returnee framework, a bona fide interstellar object is any small body observed on a hyperbolic orbit whose origin lies in another stellar system, whereas a quasi-ISO is an ISO that originated in the Solar System’s own Oort cloud, was unbound, and returned after orbiting in the Galactic potential for $10$–$100$ Myr (Forbes et al., 5 Jul 2026). By construction, such objects sample Solar System small-body compositions but appear interstellar in orbital energy.

This is not the only usage. Dehnen, Hands & Schönrich define a quasi-ISO as an ISO that has suffered a gravitational slingshot with one of the giant planets, lost enough energy to become bound, and now moves on an elliptical orbit about the Sun (Dehnen et al., 2021). Marquardt-Demen et al. use the same term, also calling these objects “Type 8 ISOs,” for planetesimals that formed around another star, traversed interstellar space, and have since lost enough energy to become temporarily bound to the Solar System (Marquardt-Demen et al., 7 Sep 2025). A broader kinematic usage introduces “quasi-ISOs” as planetesimals with marginally unbound speeds from their host star, specifically $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$, with $1<e\lesssim1.02$ (Albrow et al., 4 Dec 2025). In survey simulation work, the phrase can also function as a practical label for detected interstellar objects with eccentricities just above unity, such as $1<e\le1.01$ (Engelhardt et al., 2017).

Usage	Definition	Representative paper
Solar returnee	Solar-System-born, ejected, later returns on a hyperbolic orbit	(Forbes et al., 5 Jul 2026)
Bound captured ISO	Extrasolar body captured onto an elliptical orbit by planetary scattering	(Dehnen et al., 2021)
Type 8 ISO	Temporarily bound extrasolar planetesimal	(Marquardt-Demen et al., 7 Sep 2025)
Marginally unbound ejectum	Low-$v_\infty$, $1<e\lesssim1.02$ object	(Albrow et al., 4 Dec 2025)

This suggests that any technical discussion of quasi-ISOs must specify whether it is referring to returning Solar ejecta, bound captures, or merely low-excess-speed hyperbolic bodies.

2. Solar-origin quasi-ISOs: ejection from the Oort cloud and Galactic return

In the returnee model, quasi-ISOs arise primarily from erosion of the outer Oort cloud. Outer Oort cloud comets at semimajor axes $a\sim2\times10^4$ AU are weakly bound to the Sun and can be unbound when perturbations from the Galactic tidal field or stellar flybys change their orbital energy by $\Delta E \gtrsim |E|$ (Forbes et al., 5 Jul 2026). The characteristic ejection velocity is written as $v_{\rm ej}\sim \sigma_{\rm ej}$, with $100$0 set by the orbital speed at $100$1 AU and by weak stellar impulses. The simulations adopt an isotropic Gaussian ejection distribution,

$100$2

with $100$3–$100$4, and place particles on a shell of radius $100$5 pc beyond the Solar tidal radius,

$100$6

Once unbound, the object moves on a Galactic-orbit epicycle. Relative to the Sun it executes oscillations in $100$7 and $100$8. In the vertical direction,

$100$9

so perturbations $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$0 and $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$1 at ejection time $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$2 yield midplane crossings at

$$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$3

with $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$4, corresponding to a fundamental period of $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$5 Myr (Forbes et al., 5 Jul 2026). Return is most probable at the first few midplane crossings, $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$6, so $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$7–$$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$8 Myr. Objects ejected with higher $$0.5\;\mathrm{km/s}\lesssim v_\infty \lesssim 3\;\mathrm{km/s}$$9 lose phase coherence more quickly and rarely return.

The same work finds that quasi-ISOs primarily depart the Solar System through erosion of the outer Oort cloud in the past few hundred Myr, excluding the most recent $1<e\lesssim1.02$0 Myr, and that the much larger population of ISOs produced by the Solar System early in its life is too spread out in the Galaxy to contribute significantly to the observed sample (Forbes et al., 5 Jul 2026).

3. Rarity, velocity structure, and observational discrimination

The return probability per ejected ISO is defined as

$1<e\lesssim1.02$1

with a peak near $1<e\lesssim1.02$2–$1<e\lesssim1.02$3 Myr for realistic heating models and $1<e\lesssim1.02$4 (Forbes et al., 5 Jul 2026). Convolution with Oort-cloud erosion histories gives

$1<e\lesssim1.02$5

For plausible erosion histories, normalized either to $1<e\lesssim1.02$6 or to $1<e\lesssim1.02$7, the predicted flux of quasi-ISOs passing within $1<e\lesssim1.02$8 AU is $1<e\lesssim1.02$9–$1<e\le1.01$0 (Forbes et al., 5 Jul 2026). Even in optimistic cases this is $1<e\le1.01$1 of the expected detection rate of bona fide ISOs, quoted as $1<e\le1.01$2 for $1<e\le1.01$3I-sized objects.

The most diagnostic observable is the hyperbolic excess speed. Quasi-ISOs in this sense have

$1<e\le1.01$4

typically, up to $1<e\le1.01$5, whereas bona fide ISOs from other stars are expected to have $1<e\le1.01$6–$1<e\le1.01$7 (Forbes et al., 5 Jul 2026). Their sky radiants are predicted to cluster in a few spots in the Galactic plane, near $1<e\le1.01$8 and $1<e\le1.01$9, rather than near the Solar apex direction favored by extrasolar ISOs. Their osculating eccentricities satisfy $v_\infty$0, and their pericenter distributions are nearly flat in $v_\infty$1 because of gravitational focusing.

The operational consequence is straightforward: any inbound object with $v_\infty$2 is almost certainly a quasi-ISO or a hyperbolic Oort-cloud comet, not an extrasolar ISO (Forbes et al., 5 Jul 2026). In this specific sense, quasi-ISOs do not seriously contaminate the genuinely Galactic ISO sample because they are intrinsically rare and kinematically distinct.

4. Constraints on Oort-cloud erosion and the recent Galactic environment

The scientific interest of a detected returnee quasi-ISO lies less in its abundance than in what it would imply about Solar System history. The sheer rarity of such objects means that a detection would require either a substantially larger original Oort cloud, for example $v_\infty$3, or a recent catastrophic erosion event removing a large fraction of the Oort cloud $v_\infty$4–$v_\infty$5 Myr ago (Forbes et al., 5 Jul 2026). The candidate perturbations explicitly discussed are a stellar flyby or a cluster shock.

Because Gaia’s reach for precise back-integrations of stellar encounters is limited to $v_\infty$6 Myr, a quasi-ISO discovery could reveal evidence for a major past perturbation otherwise invisible today (Forbes et al., 5 Jul 2026). A plausible implication is that quasi-ISOs provide a dynamical probe of Oort-cloud mass loss on timescales that are poorly constrained by direct reconstruction of the Solar neighborhood’s encounter history.

The same argument also sharpens the contrast with ordinary extrasolar ISOs. In the returnee model, the observed ISO sample remains “truly Galactic” precisely because Solar-origin returnees contribute $v_\infty$7 of detections and are easily distinguished by their very small $v_\infty$8 (Forbes et al., 5 Jul 2026). Detection would therefore be exceptional evidence for anomalous Oort-cloud depletion rather than evidence against the extrasolar interpretation of the broader ISO population.

5. Bound quasi-ISOs: capture of extrasolar objects into heliocentric orbits

A distinct literature uses quasi-ISO to denote captured extrasolar bodies that are now bound to the Sun. Dehnen, Hands & Schönrich show that capture of interstellar objects into the Solar System is dominated by $v_\infty$9, primarily through Jupiter and, to a lesser extent, Saturn (Dehnen et al., 2021). With $1<e\lesssim1.02$0, they estimate $1<e\lesssim1.02$1, or about $1<e\lesssim1.02$2 captures per $1<e\lesssim1.02$3 yr into orbits with $1<e\lesssim1.02$4 au, while the steady-state resident population within $1<e\lesssim1.02$5 au is $1<e\lesssim1.02$6. In their phase-space treatment, most bound orbits crossing those of Jupiter and Saturn are fully mixed with unbound phase space, so long-term capture is balanced by ejection; on timescales $1<e\lesssim1.02$7, no permanent traps exist at $1<e\lesssim1.02$8 au (Dehnen et al., 2021).

Marquardt-Demen et al. examine the same broad class with direct $1<e\lesssim1.02$9-body simulations using REBOUND with REBOUNDx, including solar gravity, the four giant planets, radiation forces, Yarkovsky effect, general relativity, and cometary outgassing (Marquardt-Demen et al., 7 Sep 2025). They report a Jupiter capture rate of $a\sim2\times10^4$0 bound capture per $a\sim2\times10^4$1 yr, Saturn at $a\sim2\times10^4$2 per $a\sim2\times10^4$3 yr, and a steady-state bound population of $a\sim2\times10^4$4 from Jupiter alone or $a\sim2\times10^4$5 including Saturn. Bound ISOs captured by Jupiter almost always satisfy $a\sim2\times10^4$6, have eccentricities peaking between $a\sim2\times10^4$7 and $a\sim2\times10^4$8, semimajor axes $a\sim2\times10^4$9, perihelia near Jupiter’s orbit, and inclinations modestly biased toward the prograde direction (Marquardt-Demen et al., 7 Sep 2025). For binary interstellar objects, the Hills mechanism is viable but less common and tends to produce isotropic inclinations and perihelia $\Delta E \gtrsim |E|$0 au.

These studies identify a second major meaning of quasi-ISO: not a returning Solar ejectum, but a captured extrasolar object whose origin is interstellar even though its present orbit is bound. The contrast with the returnee definition is fundamental. One class is Solar-System-native in composition and unbound in the observed state; the other is extrasolar in composition and bound in the observed state.

6. Observational ambiguities, survey context, and classification problems

Survey simulations underscore how easily weakly hyperbolic and weakly bound populations can overlap with conventional comet classifications. Engelhardt et al. generated a steady-state synthetic ISO population and found that some synthetic detected ISOs had eccentricities as small as $\Delta E \gtrsim |E|$1, in the range of the largest eccentricities of several known comets (Engelhardt et al., 2017). For a canonical size-frequency distribution, the fraction with $\Delta E \gtrsim |E|$2 is empirically $\Delta E \gtrsim |E|$3–$\Delta E \gtrsim |E|$4 of all detections, and near $\Delta E \gtrsim |E|$5 the cumulative distribution is fit by

$\Delta E \gtrsim |E|$6

with $\Delta E \gtrsim |E|$7 and $\Delta E \gtrsim |E|$8 (Engelhardt et al., 2017). This establishes an observational background of barely hyperbolic objects that can masquerade as “quasi-ISOs” in a purely orbit-based sense.

A related review by Siraj & Loeb describes quasi-ISOs as a third dynamical class: interstellar by origin but no longer on hyperbolic trajectories, typically with $\Delta E \gtrsim |E|$9 and $v_{\rm ej}\sim \sigma_{\rm ej}$0 AU (Siraj et al., 2021). That review emphasizes two broad capture channels, capture in the Sun’s birth cluster and capture in the field via present-day planets, and argues that upcoming surveys such as LSST will enlarge the sample of both freely traveling ISOs and bound quasi-ISOs (Siraj et al., 2021). Marquardt-Demen et al. similarly identify orbital-element regions of interest for candidate selection, especially $v_{\rm ej}\sim \sigma_{\rm ej}$1 for Jupiter captures and isotropic inclinations with $v_{\rm ej}\sim \sigma_{\rm ej}$2 au for Hills-mechanism captures, while noting that subsequent compositional studies would be required to confirm interstellar provenance (Marquardt-Demen et al., 7 Sep 2025).

Taken together, these results show that “quasi-ISO” is not a single settled dynamical category. In current usage it can denote recycled Solar System ejecta on a second hyperbolic passage, extrasolar objects captured onto bound heliocentric orbits, or low-excess-speed objects near the boundary $v_{\rm ej}\sim \sigma_{\rm ej}$3. The 2026 returnee framework is the sharpest definition of the term as a physically distinct subclass: dynamically unbound, chemically Solar-System-native, kinematically slow, and diagnostically informative about Oort-cloud erosion $v_{\rm ej}\sim \sigma_{\rm ej}$4–$v_{\rm ej}\sim \sigma_{\rm ej}$5 Myr in the past (Forbes et al., 5 Jul 2026).