Assessing Planetary Stability and Long-Term Habitability in Nearby Stellar Binaries: 70 Oph, 36 Oph, $γ$ Leo

Abstract: Binary stars are common and have the potential to host habitable planets, which may reside in more complex habitable zones as compared to planets orbiting single stars. In this work, we use numerical simulations to assess the possibility that bright, nearby stellar multiples 36 Oph, 70 Oph, and $γ$ Leo could host habitable planets. We find that for the 36 Oph A/B system and for the 70 Oph A/B system, the stars can support planets residing in permanently habitable zones with low ejection rates and moderate eccentricity oscillations. The habitable zones around the red giants in the $γ$ Leo system exhibit severe dynamical instability due to the high binary eccentricity, eliminating the habitable zones around both stars. In these two systems, we find that planets in the habitable zone with orbits coplanar to that of the binary become uninhabitable due to interactions with the binary only 1.5% - 1.8% of the time, while planets with orbits 45 degrees misaligned to the plane of the binary experience larger oscillations in orbital eccentricity and as a result become uninhabitable 4.8% - 5.4% of the time. Our results identify 36 Oph and 70 Oph as promising targets for future missions such as the Habitable Worlds Observatory and SHERA, while suggesting that the stars in $γ$ Leo are unlikely to host any habitable planets. Our methods can be applied more generally to other binary stellar systems to refine target lists for upcoming habitable planet searches.

• This paper evaluates the habitability of nearby stellar binaries 36 Oph, 70 Oph, and γ Leo through N-body simulations, finding that 36 Oph and 70 Oph can support stable, habitable-zone planets while γ Leo cannot due to dynamical instability.
• Are the most likely long-term stable observational target could be excluded due to Kozai oscillations this must be especially explored for α Leo, Therefore, To determine if binaries such as 70 will have a Habitable-Planets need to prioritize relative positions of inertial frames, using P1 or **SHERA observational data.
• The simulations highlight the need of wide ensemble tests, joint astrometric/RV observations, complete orbital mapping, and a more complete incorporating oscillating, flexible, options for KL time scales shorter than 1 Myr.

Motivation and scope

Roughly half of Sun-like stars reside in stellar multiples, yet only a small fraction of confirmed exoplanets are found in such systems, owing to detection biases in transit and radial velocity surveys as well as the dynamical hazards that companions pose to planet formation and survival (2608.13243). This paper by Jankowski, Becker, Clark, Mamajek, Christiansen, and collaborators presents N-body assessments of whether three nearby ($d \lesssim 40$ pc) stellar multiples — 36 Ophiuchi, 70 Ophiuchi, and $\gamma$ Leonis — could host habitable-zone (HZ) planets, with the explicit goal of informing target selection for the Habitable Worlds Observatory (HWO), the Roman Coronagraph Instrument, and the proposed SHERA astrometry mission. The work is motivated in part by recent JWST evidence for a candidate giant planet in the HZ of $\alpha$ Cen A, which renewed interest in binaries as habitable-planet targets.

System parameters and habitable zone definitions

The authors adopt state-of-the-art orbital solutions for each binary: a new joint astrometric/RV solution for 36 Oph A/B ($P = 501$ yr, $a = 74.9$ AU, $e = 0.8999$), the century-plus solution of Li et al. for 70 Oph A/B ($P = 88.126$ yr, $a = 23.232$ AU, $e = 0.50015$), and the Romanenko & Kiselev E1 solution for $\gamma$ Leo ($\gamma$0 yr, $\gamma$1 AU, $\gamma$2). Because the binary separations are large, secondary-star insolation is neglected and single-star HZ boundaries are computed using the eccentricity-corrected flux prescription of Bolmont et al., with inner/outer limits at $\gamma$3 and $\gamma$4. The resulting HZs span 0.36–0.87 AU (36 Oph A) and 0.43–1.03 AU (36 Oph B); 0.55–1.32 AU (70 Oph A) and 0.29–0.70 AU (70 Oph B); and 12–29 AU ($\gamma$5 Leo A) and 6–14 AU ($\gamma$6 Leo B).

The paper is candid about orbit-solution systematics: literature solutions for 36 Oph differ dramatically in eccentricity ($\gamma$7 in Izmailov et al. versus $\gamma$8 in Irwin et al.), and the adopted solution's high eccentricity aligns with the latter. For $\gamma$9 Leo, the adopted stellar-evolution masses ($\alpha$0 combined) conflict substantially with earlier dynamical estimates ($\alpha$1), an inconsistency the authors acknowledge while preferring the former.

Simulation methodology

Simulations use REBOUND with the IAS15 integrator over 1 Myr, sampling binary orbits from Gaussian posteriors around best-fit values. One hundred massless test particles are injected linearly across each star's HZ, initialized either coplanar with the binary or at 45° mutual inclination (with small random offsets up to 0.6°). The authors verify via the Antognini quadrupole Kozai-Lidov timescale formula that 1 Myr captures more than ten KL cycles in all systems ($\alpha$2–$\alpha$3 yr). Planets are classified as dynamically unstable (collision or ejection, with ejection defined as $\alpha$4 outer HZ edge or $\alpha$5), uninhabitable (flux ever outside HZ limits), or habitable (permanently within the Permanently Habitable Zone).

Results: 36 Oph and 70 Oph support stable habitable zones

For 36 Oph, both stars retain the majority of test planets in their HZs. Maximum ejection rates reach only 10% (around A) and 5% (around B), confined to the far HZ edge in the non-coplanar case. Uninhabitability concentrates near the inner HZ edge — 100% of planets interior to 0.449 AU (A) or 0.456 AU (B) fail — but drops to 0–2% beyond these radii. Across all simulations, coplanar planets become uninhabitable only 1.5–1.8% of the time, versus 4.8–5.4% at 45° inclination due to Kozai-Lidov-driven eccentricity oscillations. The authors conclude both components can readily host stable HZ planets, making 36 Oph a strong observational target.

For 70 Oph, results are similarly favorable. Comparison against the Holman & Wiegert critical semi-major axis (using Quarles et al. coefficients) yields stability limits of $\alpha$6 AU (coplanar) and $\alpha$7 AU (45°), comfortably exceeding the outermost HZ edge at 1.32 AU — so the dominant failure mode is flux variation rather than dynamical instability. Kozai oscillations render uninhabitable a subset of inclined planets near the inner HZ boundary (roughly 0.565–0.580 AU for A; 0.300–0.308 AU for B). An important observational implication follows: if SHERA or HWO detects an HZ planet around 70 Oph A, the survival constraint on misaligned orbits could bound the planet's inclination and hence its mass.

Results: γ Leo is dynamically inhospitable

In stark contrast, every test particle injected into the HZs of either $\alpha$8 Leo star was ejected; no simulated planet remained habitable under any configuration. The cause is the binary's extreme eccentricity ($\alpha$9): all sampled stellar orbits physically intersect both HZs, so even the non-coplanar case — treated as the best case, since coplanar configurations were not simulated — cannot avoid orbit crossing. Additionally, as evolved red giants, the stars' HZs have migrated outward into regions that were likely unstable during the main-sequence lifetime, meaning planets probably never formed there and would not have remained long enough for biological development even if they had. The simulations also bear on the planetary content: the confirmed giant $P = 501$0 Leo Ab survives in roughly 90% of runs, but the unconfirmed $P = 501$11340-day candidate from Han et al. never remained stable, leading the authors to argue the signal is more plausibly attributable to stellar activity than to a planet. Follow-up of this system should therefore focus on refining the binary orbit and characterizing Ab rather than searching for habitable worlds.

Limitations and open questions

Several caveats qualify these conclusions. The test particles are massless and non-interacting, so multi-planet architectures and mutual perturbations are not modeled; flux-based habitability uses instantaneous insolation without climate modeling or atmospheric buffering. Orbital parameters are drawn independently from normal distributions rather than from full covariant posteriors, and for 36 Oph the adopted orbit relies on a solution still in preparation. The 36 Oph C tertiary (~4400 AU) is assumed dynamically decoupled despite reported astrometric acceleration suggesting a possible unseen companion, and undetected companions in any of the systems could introduce unmodeled perturbations. Only two inclinations (0° and 45°) are probed, leaving intermediate geometries unexplored, and the 1 Myr integration, while capturing many KL cycles, does not address gigayear-scale secular evolution relevant to actual biosphere timescales.

Conclusion

This study applies a reproducible simulation framework to three nearby binaries and reaches clear, differentiated verdicts: 36 Oph and 70 Oph can sustain permanently habitable zones around both components with low ejection rates, particularly for coplanar planets, and should be prioritized for HWO/SHERA observations; $P = 501$2 Leo's high-eccentricity orbit renders its habitable zones dynamically untenable, and its putative second planet is likely spurious. The methodology — posterior-sampled binary orbits, test-particle HZ injection, and KL-timescale-validated integration lengths — constitutes a template for vetting additional multiple-star systems ahead of next-generation direct-imaging and astrometric surveys.