Reasons to Believe

The Tidal Habitable Zones

Habitable zones support an important line of evidence for the supernatural design of life-friendly planets. Two such locations include the water and the ultraviolet radiation habitable zones. A planet must be neither too distant from, nor too near, its star; otherwise water will not exist in all three states (frozen, liquid, and vapor) on the planet's surface. Likewise, a planet's distance from its star must be just-right to receive the just-right amounts and wavelengths of ultraviolet radiation so as to sustain the possibility of efficient plant photosynthesis.

There are also two different galactic habitable zones. A planet must not orbit the center of a galaxy at too great nor too close of a distance so that it can be endowed with the just-right mix of heavy elements. It also must orbit at a distance from the center of the galaxy where the planet crosses spiral arms no more frequently than about once every billion years.

Recently four American astronomers discovered yet another set of habitable zones pointing to fine-tuning design: the tidal zones.1 The team focused particularly on whether life-supportable planets could orbit stars less than about half the mass of the Sun because such stars make up about ninety percent of all the stars in the Milky Way Galaxy.

However, these low-mass stars are so dim that the water habitable zone is quite close to the star. The problem in this case is that the tidal force a star exerts on a planet is inversely proportional to the fourth power of the distance between the star and its planet. Thus, shrinking the distance to one half increases the tidal force by sixteen times!

If a planet gets too close to its star, it becomes tidally locked with one hemisphere pointing permanently toward its star in the same manner that one hemisphere of the Moon points permanently toward Earth. Tidal locking means that one face of the planet will be blistering hot while the opposite hemisphere will be frigid. The only place on such a planet where life is conceivably possible would be the twilight zone–that line between permanent light and permanent dark. However, it would be very rare for such a twilight line to be stable enough for life.

The American team pointed out another tidal problem for planets orbiting dim stars: tidal heating. For example, Jupiter's moon Io is so close to Jupiter that tidal heating engenders volcanism sufficient to resurface Io at least once every million years. Such extreme heating would render any form of life impossible. On the other hand, without some minimum level of tidal heating, planets orbiting dim stars in the water habitable zone will lack the plate tectonic activity necessary to recycle carbon dioxide and other greenhouse gases so that a runaway greenhouse does not permanently sterilize the planet. The researchers demonstrated that the tidal habitable zone for such stars is surprisingly narrow.

Earth possesses an internal composition and structure that guarantees a just-right level of plate tectonic activity apart from tidal heating. However, it, too, manifests a tidal habitable zone. If Earth were even a tiny bit closer to the Sun, it would become tidally locked. However, if it were just a little farther away from the Sun, ocean tides would be substantially different. It is the complex combination of tidal effects from the Moon and the Sun that permits Earth to sustain such a huge biomass and biodiversity at its seashores. Different tides would lower the potential for such a rich and abundant ecology.

The team's research adds to mounting fine-tuning in favor of a supernatural, super-intelligent explanation for Earth's capacity to support advanced life.

1. Rory Barnes et al., "Tidal Limits to Planetary Habitability," Astrophysical Journal Letters 700 (July 20, 2009): L30–L33.

Subjects: Extrasolar Planets, Solar System Design

Dr. Hugh Ross

Reasons to Believe emerged from my passion to research, develop, and proclaim the most powerful new reasons to believe in Christ as Creator, Lord, and Savior and to use those new reasons to reach people for Christ. Read more about Dr. Hugh Ross.