Habitable Zones

An article at Salvo (subscription required) drives a spike into the notion that the galaxy, and perhaps the universe, are filled with habitable planets and that life elsewhere in the cosmos is an inevitability.

The article, by astronomer Hugh Ross, observes that the conviction that there simply must be life out there somewhere and that our earth is not unique in being fit for sustaining life is based on four assumptions:

1. The density and kinds of planets throughout our galaxy and all other galaxies in the universe are roughly the same as what we observe in the vicinity of our solar system.
2. About 20 percent of all planets are habitable.
3. Life inevitably will arise on all habitable planets.
4. The probability of a technologically advanced civilization arising from simple life forms is better than one chance in 10 billion.

Each of these assumptions, especially #3, are deeply problematic, but Ross focusses in this article on #2. The belief that 20% of all planets are habitable is based on the fact that of the 3000+ planets discovered so far, 20% of them lie at a distance from their star that would permit water to exist in the liquid state. Since water is a necessary condition for life, it's assumed that these planets could sustain living things.

Ross points out, however, that though water is a necessary condition for life, it's not a sufficient condition. There are, in fact, nine different "habitable zones" and all nine must overlap in order for life to exist on a planet.

In addition to the water habitable zone there are also the following:

1. Ultraviolet habitable zone
2. Photosynthetic habitable zone
3. Ozone habitable zone
4. Planetary rotation rate habitable zone
5. Planetary obliquity habitable zone
6. Tidal habitable zone
7. Astrosphere habitable zone
8. Electric wind habitable zone

Ross explains each of these zones in his recently released book Improbable Planet, but in the Salvo article he gives this example:

Typically, these zones do not overlap. For example, the distance a planet must be from its host star so that it receives enough ultraviolet radiation to enable the synthesis of many life-essential compounds, but not so much as to kill living things, is rarely the same distance that a planet must be from its host star for liquid water to possibly exist on its surface.

For 97 percent of all stars, the liquid water habitable zone does not overlap the ultraviolet habitable zone.

Thus,

A planet is a true candidate for habitability only if it simultaneously resides in all nine habitable zones....So far, astronomers have measured the characteristics of 3,484 planets. Only one of all these 3,484 planets resides in all nine known habitable zones. That one is Earth.

For all we know there may be other habitable zones in addition to these nine, but there are in any case several conclusions to be drawn from the information Ross provides us. First, the principle of mediocrity - the principle that the earth is not exceptional in any significant way - is ludicrous.

Second, the notion that residence of a planet in the water habitable zone is sufficient to justify hopes that life could exist on that planet is naive.

When the necessity for all nine habitable zones overlapping is combined with the dozens of other parameters that any planet must possess in order to be suitable for life suggests that Earth-like planets are probably extremely rare.

In fact, if it turns out that such planets are not uncommon that finding in itself would be so astonishing as to point to intelligent, purposeful engineering of the universe.