Chapter 38 — The Shield Above

Jupiter is the largest planet in the solar system—approximately 318 times Earth's mass. Its gravitational influence extends throughout the solar system and significantly affects the trajectories of small bodies, including comets and asteroids. This gravitational influence operates as a deflection shield for the inner solar system.

In 1994 astronomer George Wetherill of the Carnegie Institution published a simulation study estimating what would happen to the inner solar system in the absence of Jupiter. His conclusion: without Jupiter, Earth would be struck by comets approximately 1,000 times more frequently than it currently is. Jupiter deflects long-period comets before they can reach the inner solar system by gravitationally perturbing their orbits on their way in.

A planet bombarded at 1,000 times the current rate would not maintain the stable surface conditions required for complex life. Mass extinction events would follow each other in rapid succession. The development of complex multicellular life over geological time would not be possible under such bombardment. Jupiter's existence in its specific orbital position is a prerequisite for Earth's surface stability—a prerequisite that planetary systems around other stars need not have.

Wetherill, G.W. "Possible Consequences of Absence of Jupiters in Planetary Systems." Astrophysics and Space Science 212 (1994): 23–32. Gonzalez, G. and Richards, J.W. The Privileged Planet. Regnery Publishing, 2004. Chapter 6. Ward, P.D. and Brownlee, D. Rare Earth. Copernicus Books, 2000.

Chapter 39 — The Stabilizer

Earth's rotational axis is tilted at approximately 23.5 degrees relative to the plane of its orbit around the Sun. This tilt produces the seasons. Equally important, the tilt is stable: it oscillates between approximately 22.1 and 24.5 degrees over a 41,000-year cycle—modulating season intensity but never varying enough to produce catastrophic climate swings.

The stability of Earth's axial tilt is maintained by the Moon. In a 1993 paper, Jacques Laskar and colleagues at the Paris Observatory ran computer simulations of Earth's axial behavior without the Moon. Without the Moon's gravitational stabilization, Earth's axial tilt would vary chaotically from near 0 degrees to as much as 85 degrees over geological time. The climate consequences of such variation would be severe and likely incompatible with the maintenance of complex life.

Mars provides a case study. Mars has no large moon. Its axial tilt currently stands at about 25 degrees, similar to Earth's. Laskar's simulations show that Mars's tilt has varied between approximately 0 and 60 degrees over geological time—consistent with geological evidence of past polar ice migration and repeated global climate shifts. The Moon is anomalously large relative to Earth. No other rocky planet has a satellite of comparable relative size. Its presence at its current orbital distance produces exactly the stabilizing effect that Earth's climate requires.

Laskar, J., Joutel, F., Robutel, P. "Stabilization of the Earth's obliquity by the Moon." Nature 361 (1993): 615–617. Ward, P.D. and Brownlee, D. Rare Earth. Copernicus Books, 2000. Chapter 9.