Tides Influence Habitability

Tides Influence Habitability

“My own personal feeling is that the chances of life on this planet are 100 percent…I have almost no doubt about it.”

So said the UC-Santa Cruz astrophysicist who discovered an earth-sized planet orbiting a nearby star called Gliese 581. His optimism stemmed from the fact that the planet orbits in the liquid water habitable zone. But new research puts a serious damper on the idea that these types of planets could host life.

The star in question, Gliese 581 is an M-dwarf—a star with a mass 75 percent smaller than the Sun. Consequently, the liquid water habitable zone sits much closer than Earth’s orbit around the Sun. While Gliese 581g (the planet) does receive enough radiation from its star to keep water liquid, the close location causes other problems, and all three relate to tides.1

Like the amount of energy received from a star, the strength of the tides increases the closer a planet orbits to its star. However, where the energy increases as the square (twice as close results in four times the energy), the tides increase as the fourth power. In other words, a planet three times closer to a star would experience tidal forces 81 times larger.

Gliese 581g orbits roughly one-sixth the Earth-Sun distance around a star with three-tenths the mass of the Sun. Thus, compared to Earth, Gliese 581g is subject to tidal forces around 400 times more powerful. Such strong interactions change the dynamics of the planet so that it becomes uninhabitable.

First, the strong tidal forces cause any tilt in the planet’s axis to quickly (in astronomical terms) decay. Earth’s rotation axis makes a 23.5 degree angle with the plane in which it orbits. This tilt causes the seasons and, more importantly, distributes the Sun’s radiation more evenly around the globe. Without seasons, the Earth’s poles would turn into a perpetual deep freeze and the equator would experience hellish temperatures. This scenario would generate powerful winds and large storms and eventually evaporate the atmosphere into space. Only planets orbiting stars greater than nine-tenths the mass of the Sun maintain a tilted axis for longer than one billion years.

Second, the tides cause the planet to stretch and contract, which heats the planet. For a planet close to its star (like Gliese 581g and other planets orbiting M-dwarf stars), this heating is sufficient to drive plate tectonic activity. More-distant planets like Earth experience less tidal heating and require an additional source of heat. However, for Earth, the additional heat comes from the decay of radioactive isotopes acquired during formation and from a collision with a Mars-sized object (which also made the Moon).

Third, the increased tidal forces on planets orbiting M-dwarf stars cause the orbital period to synchronize with the rotation period. Known as tidal locking, such a state means that the same side of the planet always faces the star. As a result, the planet’s daytime side sees scorching temperatures whereas the nighttime side permanently freezes. Some have hypothesized the existence of a region between the two extremes where liquid water might exist. However, winds generated between the hot and cold sides would transport any water in the more temperate region onto the cold side where it would freeze.

The slow rotation (one day for every year) also means that planets orbiting in liquid water habitable zones around M-dwarf stars have no magnetic fields. Without the magnetic field, the flares and winds generated by the star will sputter any planet’s atmosphere into space (although somewhat counterintuitive, M-dwarf stars are more active than the Sun). The Earth’s magnetic field protects its atmosphere so that it still exists—even a few billion years after formation.

Astronomers will continue to detect planets outside our solar system that resemble Earth in some fashion, and the popular announcements will often herald the find as “discovering a habitable planet.” Such pronouncements assume a “minimalist” model—where finding a planet that might have liquid water almost assures the presence of life.

However, this research supports a model like Reason To Believe’s, where life requires a far greater set of criteria than simply liquid water. In fact, the latest extrasolar planet discoveries affirm that Earth is rare (if not unique) in its capacity to host life. We would expect that, as these discoveries continue, detailed analysis of the results will support the notion that Earth appears to be the product of a purposeful, intelligent Agent.

  1. R. Heller, J. Leconte, and R. Barnes, “Tidal Obliquity Evolution of Potentially Habitable Planets,” Astronomy and Astrophysics 528 (April 2011): A2