Interplanetary Habitable Zone: Calculations for the Solar System

The Interplanetary Habitable Zone: A Multi-Parameter Approach to Assessing Space Settlement

The Interplanetary Habitable Zone (IHZ) defines a planetary system's ability to support life that has spread beyond its planet of origin. This multi-parameter criterion considers energy, transportation costs, radiation risks, and resources. Unlike the classical circumstellar habitable zone (CHZ), the IHZ analyzes not only planetary surfaces but also orbital transfers, asteroids, and moons. The model is applied to the Solar System and the exoplanetary system TRAPPIST-1, revealing a migration sequence: Earth → Moon → Mars → asteroid belt.

Agent-based simulation confirms the Solar System's advantage over compact systems like TRAPPIST-1 due to a balance of resources and risks. Δv costs and radiation limit expansion, but resource distribution creates internal zones of stability.

Key Factors of the IHZ

Energy Availability

Energy is a fundamental factor. Stellar flux at distance r is given by f_ = L_ / (4πr²), where L_* is stellar luminosity. Efficiency η depends on temperature: η = η_ref (1 - α(T_eq - T_ref)), with α ≈ 0.003–0.005 K⁻¹ for photovoltaics. Inner planets benefit from solar energy, outer ones from low temperatures for thermonuclear systems.

Transportation Costs (Δv)

Orbital transfers require Δv. In the Solar System, minimum Δv from Earth: Moon (5.8 km/s), Mars (5.7 km/s), Ceres (7.0 km/s). In TRAPPIST-1, high orbital density increases interplanetary costs, reducing migration efficiency.

Radiation Risks

Solar storms dominate within 2 AU, galactic radiation beyond Mars. A risk factor exponentially reduces habitability: H_rad = exp(-β D_rad), where D_rad is dose, β is species tolerance.

Resources

Weight coefficient R_i for body i: metals, volatiles, gravitational material. Earth leads in total reserves, the asteroid belt in metals (iron, nickel), outer moons in water.

Formalizing the IHZ Model

Overall habitability index for location i:

H_i = w_E E_i w_Δv exp(-γ Δv_i) w_rad H_rad_i w_R * R_i

where w are weights normalized to sum 1. Systemic IHZ is integrated H across all bodies, with a threshold for stability.

For the Solar System:

• Inner IHZ: Earth-Moon-Mars (high E, low Δv).
• Middle: asteroid belt (resources).
• Outer: limited by radiation and Δv.

System Comparison

| Parameter | Solar System | TRAPPIST-1 |

|-----------|--------------|------------|

| Zone radius | 0.3–5 AU | 0.02–0.06 AU |

| Δv migration | Low gradient | High |

| Resources | Distributed | Concentrated on planets |

| Energy | High inside | Uniform but weak |

Spread Simulation

Agent-based model simulates populations migrating according to H_i. Parameters:

• Start: Earth (H=1).
• Step: choose target with probability ∝ H_j * exp(-c / P), P is population.
• 10⁴ iterations.

Results for the Solar System:

• First wave: Moon (Δv=5.8 km/s, resources).
• Second: Mars and vicinity.
• Third: Ceres, Vesta (metals).
• Stabilization: 70% within 2 AU.

For TRAPPIST-1:

• Migration limited to 2–3 planets due to Δv >10 km/s.
• IHZ shrinks toward the star.

Evolution graph shows exponential growth until resource saturation.

Key Takeaways

• The Solar System has an advantage due to resource gradients and low Δv for inner bodies.
• IHZ depends on technological level: biological life is limited by radiation, techno-life by Δv.
• TRAPPIST-1 is vulnerable to stellar flares, reducing overall habitability.
• The model is applicable for searching technosignatures in exoplanetary systems.
• Future extensions: accounting for O'Neill cylinders and self-replicating systems.

Applications in Space Economy

IHZ helps prioritize missions: Moon for staging, Mars for bases, asteroids for mining. In the Solar System, the optimal strategy is phased expansion, minimizing Δv. For exoplanets, the model predicts technosignature zones around stars with Earth-like planets.

— Editorial Team