Earth in the Zone

The Impossibly Habitable Planet

Athletes sometimes speak of "being in the zone," a mental state that makes physical exploits seem effortless to them, as they achieve personal bests. Planets too, must be "in the zone," in fact, in eight of them, if they hope to have any chance at the elusive prize: sustaining life.

It's All About Zoning

Two astrobiology research teams recently calculated that the number of habitable planets in the Milky Way Galaxy stands at over 40 billion.¹ But to reach this number, the researchers presumed that planetary systems everywhere in our galaxy are as abundantly endowed with planets as is our own solar neighborhood with its eight planets.

More importantly, they considered only one zoning criteria, the liquid water (or temperature) habitable zone criteria. For life to exist on a planet, that planet must, at a minimum, reside within a certain distance range from its host star, such that the planet's surface temperature falls between 0°-100°C, making the existence of liquid water possible. This is the liquid water habitable zone, but it is not the only zone a planet must fall into if it is to be capable of sustaining life. There are seven other habitable zones, and for life to be truly possible on a planet, that planet must reside simultaneously within all of them. These zones are:

1. The liquid water habitable zone,
2. The ultraviolet habitable zone,
3. The photosynthetic habitable zone,
4. The ozone habitable zone,
5. The planetary rotation rate habitable zone,
6. The planetary obliquity habitable zone,
7. The tidal habitable zone, and
8. The astrosphere habitable zone.

We'll discuss each of them in turn.

The Ultraviolet Habitable Zone

This zone is that region about a star in which a planet must reside if the amount of incident ultraviolet (UV) radiation arriving on the planet's surface is to be neither too great nor too little for life's survival. For life, UV radiation is a double-edged sword. Without it, the synthesis of many life-essential biochemical compounds cannot occur. Too much of it, however, will damage or kill land-based life. Both the quantity and the wavelength of incident UV radiation must be fine-tuned for life to survive, and even more so for life to flourish.

Even for primitive life, the UV and liquid water habitable zones around most stars typically do not overlap. For instance, for host stars with effective temperatures of less than 4,300°C, the outer edge of the UV habitable zone is closer to the star than the inner edge of the water habitable zone. For host stars with effective temperatures greater than 5,800°C, the inner edge of the UV habitable zone is farther from the host star than the outer edge of the water habitable zone. For older stars that have completed their hydrogen-burning phase, the UV habitable zone is about ten times more distant from the host star than the liquid water habitable zone. For comparison, our sun's effective temperature is 5,505°C. (Recall that 100°C is the boiling point.)

This requirement, that the liquid water and UV habitable zones must overlap, eliminates most stars as possible candidates to host a habitable planet. Only three percent of stars in the galactic habitable zone (the distances from the galactic center at which life is possible) are candidates for hosting a planet on which primitive life can survive, even for just a brief time period.

The Photosynthetic Habitable Zone

This zone encompasses the distance range from a host star at which photosynthetic life can survive. While life is possible in the absence of photosynthesis, such life's metabolic rates range from hundreds to millions of times lower than the rates of photosynthetic life, limiting it to microbes. Without photosynthetic life, no plants or animals are possible.

Photosynthetic life implies many more demanding constraints on the quantity, stability, and spectral range of light incident on a planet's surface. While limited photosynthetic activity is possible on a planet where the UV and liquid water habitable zones overlap, for the efficient and globally distributed photosynthetic activity that is needed for plants and animals to thrive, seven additional factors must be fine-tuned:

1. light intensity,
2. ambient temperature,
3. carbon dioxide concentration in the environment,
4. seasonal variation and stability of seasonal variation,
5. mineral availability,
6. liquid water supplies, and
7. atmospheric humidity (for land-based life).

These seven factors (of this one zone), by themselves, eliminate all known star-planet systems as candidates for habitability except the sun-earth system.

The Ozone Habitable Zone

This is that distance range from a star at which stellar radiation impinging on a planet with an oxygen-rich atmosphere will produce just-right amounts of ozone in the planet's atmospheric layers so as to allow surface life to thrive. Ozone is a molecule comprised of three oxygen atoms. It forms in a planet's stratosphere as short-wavelength UV radiation and, to a lesser degree, as stellar x-ray radiation reacts with dioxygen (2O2 3 O3 + O). Ozone is destroyed in a planet's stratosphere as it reacts with atomic oxygen (O3 + O 3 2O2). The balance between the production and destruction of ozone determines how much ozone resides in a planet's stratosphere. In a planet's troposphere, electrical discharges (lightning), in addition to UV radiation, produce ozone.

Ozone in the earth's stratosphere, for example, absorbs 97-99 percent of the sun's short-wavelength UV radiation (radiation damaging to life) while allowing much of the longer-wavelength radiation (radiation beneficial to life) to pass through to the earth's surface. For the variability of stellar UV emission to be low enough for life to thrive on a planet orbiting a host star, the host star's mass must be virtually identical to the sun's. Stars more massive than the sun exhibit greater UV emission variability, as do stars less massive. The host star's age also must be virtually the same as the sun's. Even with the just-right host star, the distance range from that star at which a planet's stratospheric ozone layer will permit life to thrive is much narrower than the UV and photosynthetic zones.

Of course, a planet is ruled out if it lacks the atmospheric oxygen needed to manufacture ozone shields. Yes, more than one shield is needed. In addition to the stratosphere, ozone shields are required in the troposphere and mesosphere. For example, too much ozone in the troposphere (for the earth, the first six miles above sea level) leads to respiratory failure in animals, reduces crop yields, and wipes out many plant species. Insufficient ozone in the troposphere leads to ever-increasing buildup of biochemical smog.

The Planetary Rotation Rate Habitable Zone

This refers to how a planet's rotation rate affects its clouds' reflectivity and, hence, how much of the host star's light penetrates to the planet's surface. Three-dimensional atmospheric circulation models show that, given the same atmospheric magnitude and composition, rapidly rotating planets will generate narrower low-altitude cloud belts than will slowly rotating planets. These narrower tropical cloud belts will reflect much less of the host star's light and, thus, make the planet's surface warmer.

A planet's rotation rate affects the position of several habitable zones. The faster the rotation rate, the more distant the water, UV, photosynthetic, and ozone habitable zones will be from the host star. In addition, the rotation rate will affect the size of all these zones in different ways.

The Planetary Obliquity Habitable Zone

This refers to how the tilt of a planet's rotation (for the earth, its 24-hour spin) axis relative to its orbital (for the earth, its yearlong revolution around the sun) axis—i.e., its obliquity—affects the planet's surface temperature. Climate simulation studies demonstrate that the higher the obliquity, the warmer a planet's surface.

Just as a planet's rotation rate affects the position of several habitable zones, so does its obliquity. The greater the obliquity, the more distant the water, UV, photosynthetic, and ozone habitable zones will be from the host star. In the same manner in which a planet's rotation rate affects the size of these habitable zones in different ways, so does the planet's obliquity.

The Tidal Habitable Zone

This zone is the distance range from a host star at which tidal forces exerted by the star permit life to exist. 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!

Tidal forces slow down a planet's rotation rate. However, if a planet gets too close to its star, it rapidly becomes tidally locked, with one hemisphere pointing permanently toward the star and the other hemisphere pointing away. Hence, tidal locking means that one face of the planet will always be blisteringly hot while the opposite hemisphere will be frigid. Only at the twilight zone—the line between permanent light and permanent darkness—would life be conceivable on a tidal-locked planet. However, atmospheric transport would move water from the day-side to the night-side of the planet, where it would get permanently trapped as ice.

A star's tidal force also erodes its planet's rotation axis tilt. A planet too close to its star will experience its rotation axis tilt driven to less than 5° and, consequently, it will fail to experience seasons. Not having seasons would be catastrophic for a planet's habitable area.

For the earth, certain low tidal force levels have benefits beyond ensuring that it attained the just-right rotation rate at the just-right time in its history for plant and animal life to develop. The complex interaction of tidal effects from the sun and the moon permits the earth to sustain a huge biomass (living matter) and biodiversity at its seashores and continental shelves. Additionally, the earth's tides are optimal for recycling nutrients and wastes. Different tides would lower the potential for such a rich and abundant ecology.

How narrow is the tidal habitable zone? Alter the sun's mass by more than one percent, or the earth's distance from the sun by more than one percent, and the earth no longer sits within the tidal habitable zone.

The Astrosphere Habitable Zone

This refers to that part of the plasma "cocoon" (safe place) carved out of the interstellar medium by a star's wind in which a planet conceivably could support life. That cocoon can act as a buffer to screen planetary atmospheres and surfaces from high-energy cosmic radiation that would be lethal to life.

The buffering, however, must be just right. A powerful stellar wind will generate a large plasma cocoon. This cocoon, though, will blast a life-supportable planet with so many stellar radiation particles as to either kill or seriously limit life spans. On the other hand, if the stellar wind produces too small of a plasma cocoon, life gets blasted by deadly cosmic radiation.

The size of a star's astrosphere depends on both the star's mass and its age. It also depends on the density of the interstellar medium in which the star resides. Since stars are on orbital paths about the galactic center, the density of the interstellar medium in a given star's vicinity will vary over the course of its orbit. A star could host a habitable planet if: (1) at any given time in the star's burning history its astrosphere is small enough to limit stellar radiation damaging to life, and (2) its astrosphere extends far enough out to include that region where the liquid water, UV, photosynthetic, and tidal habitable zones overlap.

Alone in the Zone

For none of the 1,795 planets discovered and measured outside of our solar system do all eight known habitable zones overlap. Thus, none are possible habitats for life more advanced than primitive bacteria lasting for no more than a million years. Instead of over 40 billion "habitable" planets, accumulating evidence is pointing to the existence of just one. Instead of good fortune explaining our existence, Someone who knows all about habitable zones, including ones yet to be discovered, and who knows about the changing physics of the solar system, must have designed the earth and its long history of life so that human beings could live on it during this brief epoch of time.

Note
1. Erik A. Petigura, Andrew N. Howard, and Geoffrey W. Marcy, "Prevalence of Earth-Size Planets Orbiting Sun-Like Stars," Proceedings of the National Academy of Sciences USA 110 (Nov. 2013), 19,273-19,278.