5.5 - Planetary Habitability

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

We have barely scratched the surface of the total field of general planetology in this brief survey, and most if not all of the discussions have been simplifications of vastly more complicated processes. The concept of habitable zones, for instance, is a very old and respected idea but one which should not be engraved in stone and rendered sacred. Countless ways can be imagined to “beat the heat.” Some of the more obvious of these are surface effects on the planet itself and have nothing to do with the stellar class of the primary.

For example, the greenhouse effect adds about 30 K to Earth’s temperature, and about 500 K to that of Venus. In Titan’s air, methane and hydrogen might trap solar energy and heat the planet significantly. Calculations indicate that if the surface pressure is on the order of 0.1-0.4 atm, the greenhouse effect could easily add 60-110 K. This would raise the temperature at the surface of Titan to 150-200 K.^1280,1281 Were Titan at the distance of Jupiter instead of Saturn, another 30 K or so increase could probably be arranged -- putting it very close to Mars, temperature-wise. There are indications that even chilly Neptune may have a greenhouse amounting to some 80-90 K.²⁰⁴⁶

A second warming factor is the presence of small-particle smog suspended in the air of Titan. These darkened organic dust motes can absorb sunlight and transfer still more heat to the surrounding atmosphere.²⁰⁴⁶ So we see that perfectly valid arguments may be made to extend the outer reach of the habitable zone of Sol as far Jupiter and possibly even Saturn!

What are the limits of mass for habitable planets? Again, the answers don’t come easily. In selecting worlds that might be habitable for human life, Dole set forth the following values: Mass should be greater than 0.4 M_earth, to ensure that a heavy enough atmosphere can evolve and remain trapped, and should be less than 2.35 M_earth, to keep the force of gravity below 1.5 Earth-gees.²¹⁴ Planetary mass will also affect the likelihood of finding planetwide oceans (Figure 5.11).

While these are useful estimates, they are clearly rather conservative when applied to all ET lifeforms instead of just to humans. Rasool expects that in a few eons, Mars’ atmosphere will thicken sufficiently for it to begin evolving towards a more Earth-like clime.²⁰⁶⁵ The mass of Mars, however, is only 0.11 M_earth. And while human life may be uncomfortable at more than 1.5 gees, there is absolutely no rationale for using this as the cutoff for all carbon-based intelligent life. Accretion models suggest that terrestrial worlds may form with masses as high as 5-10 M_earth,¹²⁵⁸ with surface gravity reaching at least 2.2 gees.

Another factor we have not really considered is the tides caused by satellites (or by the primary). Tides may occur in the lithosphere and atmosphere, but are most effective when they arise in the hydrosphere -- the ocean. A moon which is very massive, or quite close, will tug at its primary much more insistently and raise higher tides (Figure 5.12).

The tides are important because they will alter the erosion of continents, wave motions in the sea, the weather, and so forth. Larger tides will slow the rotation of the planet, depending on the distribution of land masses, and may have enormous implications in the emergence of life from the sea.

Figure 5.12 Tides Raised on an Earthlike Planet by Satellites of Various Masses and Distances

Assuming a very homogenous pair of fluid bodies, the tidal height H may be expressed mathematically as:

H = constant x	M_sat RP⁴ -- -- -- -- -- -- M_pr³	, or
H = constant x	r_satR_sat³R_p -- -- -- -- -- -- - r_pr³

where M_p and M_sat are planetary & satellite mass, R_p and R_sat are planetary and satellite radius, r_p and r_sat are the respective densities, and r is the average distance between the two bodies.¹⁹⁸⁰ (Those equations are based on a highly oversimplified model -- for fuller treatment see Alfvén and Arrhenius¹⁹⁸⁰ or Goldreich and Soter.¹²⁴³)

There are additional complicating factors. Peculiar tidal resonances are known to occur. For instance, we now know that Mercury is not a one-face planet as was once thought. Instead, it turns on its axis exactly three times for every two trips around the sun. (A case of “spin-orbit coupling.”²⁰⁴⁸) Venus also appears to be “tidally locked” -- but to Earth.²⁰⁴¹ The sun must similarly be taken into account. Sol is responsible for only about one-third of Earth’s oceanic tides, but a planet in the habitable zone of a K2 star would experience far greater tides even if it had no moon.

The tilt of the planet’s axis is likewise significant with respect to habitability.* All of the ecospheres computed in this and the previous chapter were based on the assumption of a relatively low inclination to the orbital plane. (Earth is about 23°, which is fairly typical.) A planet with high inclination will have more extreme seasonal temperature variations across its surface. Large tracts of land may become totally uninhabitable, although marginal livability apparently can be retained for tilts as high as 81°.²¹⁴

The tilt of a world is responsible for its seasons. Planets with 0° inclination should have relatively humdrum, monotonous climates all year long (although an especially eccentric orbit might produce season like effects). With no seasons, there would be no regularly changing weather patterns, no cycles of autumnal death and vernal rebirth in the plant kingdom, no migrations of fish and fowl. The entire rhythm of existence would be lacking, and the influence on culture, religion, philosophy, and the agricultural sciences must necessarily be enormous.

Many rare and exotic environments for life may exist in our Galaxy.²¹⁴ A “superjovian orbiter” might derive life-giving heat from the gas giant it circled. Inhabitants of this terrestrial world on the side that permanently faced away from the superjovian would scoff at tales of a giant Thing in the sky and reports of strange native religions brought back by intrepid explorers who had visited the other side. (The auroras there should be fantastic, if Io turns out to have beautiful yellow displays as many believe.^2047,2090)

The Earth-Moon system is for all practical purposes a double planet, and it is not unreasonable to suppose that in many stellar systems across the Galaxy two Earths orbit one another. A world with two habitable belts, which might be found nearer the inside edge of the stellar ecosphere, is also a distinct possibility. Only the polar regions could be livable -- the tropics would be unbearably hot.

There may be starless worlds, as the late astronomer Harlow Shapley suggested, bodies which lie alone out in the cold of interstellar space.⁸¹⁶ Life is possible only if these planets are self-heating.^18,2061 (Hal Clement used this idea in his science fiction story entitled “The Logical Life.”)

Perhaps we will find pelagic worlds, or terrestrials with Saturn-like (or Uranus-like) rings, or planets with large liquid bodies at the surface maintained near the triple point of the thalassogen. The ocean would boil furiously while gleaming icebergs floated and tossed on the frothy sea. The possibilities are as limitless as the imagination.

* Orbital eccentricity is also important -- e must be less than 0.2 if at least 10% of the surface is to remain human-habitable.²¹⁴