Who Needs a Moon?

by Govert Schilling 

BOSTON—The number of Earth-like extrasolar planets suitable for harboring advanced life could be 10 times higher than has been assumed until now, according to a new modeling study. The finding contradicts the prevailing notion that a terrestrial planet needs a large moon to stabilize the orientation of its axis and, hence, its climate.

In 1993, French mathematicians Jacques Laskar and Philippe Robutel showed that Earth’s large moon has a stabilizing effect on our planet’s climate. Without the moon, gravitational perturbations from other planets, notably nearby Venus and massive Jupiter, would greatly disturb Earth’s axial tilt, with vast consequences for the planet’s climate. The steadily orbiting moon’s gravitational tug counteracts these disturbances, and Earth’s axial tilt never veers too far from the current value of 23.5°, where 0° would mean the axis was perpendicular to the plane of Earth’s orbit around the sun.

Indeed, Laskar and Robutel also showed that the axial tilt of Mars, which has only two tiny moons, has varied between 10° and 60° in the past, which caused huge climate variations that in turn could have contributed to the loss of most of the planet’s atmosphere, leaving Mars the bone-dry desert world that it is now. Since then, most astrobiologists have assumed that Earth-like planets in other solar systems would need a comparatively large moon to support complex life over long periods of time.

Given the generally accepted idea of how Earth got its big moon—through an improbable, dramatic collision with a Mars-sized body that knocked a huge chunk of Earth loose—astronomers estimate that only 1% of all Earth-like planets in the universe might actually have such a hefty companion. That would mean that planets harboring complex life might be relatively rare.

However, Jack Lissauer, a theoretical astrophysicist at NASA’s Ames Research Center in Moffet Field, California, is much more optimistic. Together with Jason Barnes, a physicist at the University of Idaho, Moscow, and John Chambers, a theoretical astrophysicist at the Carnegie Institution’s Department of Terrestrial Magnetism in Washington, D.C., he has carried out large numbers of detailed numerical simulations of "moon-less Earths," which show that the consequences are less dire than is generally assumed.

That’s because really big changes in a planet’s tilt would occur only after a very long time, so there would be more than enough time for the evolution of life, Lissauer reported yesterday here at the summer meeting of the American Astronomical Society. “The variations in Earth’s axial tilt would indeed be substantially larger if there was no large moon,” Lissauer says, “but really big excursions from the current value would only occur on time scales of billions of years.” That would leave ample time for advanced land life to evolve under relatively stable climatic conditions—although what would happen to such life during an axial shift remains unclear.

When a planet rotates in the opposite direction to its orbital motion (which happens to be the case for Venus), the effect of gravitational perturbations on its spin axis would be even smaller, the simulations indicate. And, of course, if a planetary system contains only one planet, there are no perturbations at all. Nobody knows how common such single-planet system might be.

Not everybody is overwhelmed by the importance of the new results. “I don’t think [changes in a planet’s axial tilt] would be a problem for the development of advanced life,” as any type of life would adapt to changing circumstances anyway, says planetary scientist Sara Seager of the Massachusetts Institute of Technology in Cambridge.

But Bill Borucki of NASA’s Ames Research Center, who is the principal investigator of the planet-hunting Kepler satellite mission, says he is “surprised and delighted” by Lissauer’s conclusions. “Kepler is searching for Earth-like planets orbiting other stars,” he says, “and this means much more of them might be harboring complex life. It’s a wonderful result.”