Properties of the Planets and Habitable Zones

Key points: What a habitable zone is; basic required conditions for possible life

We turn to the origin of life. From what we have learned, what makes a planet suitable for life in the first place?

A "habitable" planet should:

• Orbit a star that remains stable in output for billions of years
• Be at a distance from the star that results in its achieving a suitable temperature so its surface water is liquid, not frozen
• Have a circular orbit, so constant conditions prevail for its entire "year"

These requirements lead to the concept of a "habitable zone". A low mass, cool star can only sustain life on planets close-in, while higher-mass, hotter stars make their planets too hot close-in and life can only be sustained farther out. Stars more than about twice the mass of the sun do not last long enough for life to form on their planets. If a planet is too close to a star, its spinning will synchronize with its orbital motion and it will always point the same side to the star - it will be "tidally locked", like the earth's moon is locked to the earth. This behavior may or may not be bad news for the formation of life. (From F. M. Walter, http://www.ess.sunysb.edu/fwalter/AST101/habzone.html)

We can add more general requirements for life in general:

• Not orbit a star that is too close to a cosmic explosion like a supernova
• Be far enough from massive planets that they do not continually divert asteroids to hit it or perturb its orbit strongly
• Probably not be so massive that it retains hydrogen and becomes a "gas giant"
• Perhaps it is also essential to have a massive planet well outside its orbit, like Jupiter, to divert potential devastating asteroids away, or to make them destroy themselves (as in the asteroid belt).

There are distinct trends among the planets that indicate still more requirements for life:

• We have already discussed how the distance from the sun and mass of a planet determine whether it can retain "light" gases (or any gases) in its atmosphere. An atmosphere like that of the earth, dominated by nitrogen and oxygen, requires the planet not to be too massive or it might also retain hydrogen and helium like Jupiter, or too low in mass so the oxygen and nitrogen escape and only heavier nucleus gases like argon can be retained (as on Mars).
• Massive atmospheres generally trap heat at the surface of the planet, due to the heating of the surface of the planet by sunlight and the efficient absorption of the infrared radiation they emit by certain gases in their atmospheres, such as carbon dioxide. If a planet is close to its star, this extra heating can become large, and drive water away and lead to more carbon dioxide in the atmosphere, increasing the heating for what is called a runaway greenhouse effect, like on Venus. If the planet is farther out, the greenhouse effect can just heat it up mildly to make its climate more pleasant, like on Earth.
• In fact, large amounts of water must be available on the planet for many reasons. Water is essential for the chemical reactions leading to and sustaining life on Earth. Water appears to be critical in controlling the amount of carbon dioxide in the atmosphere and avoiding runaway greenhouse effects. Water also probably plays a role in forming the relatively low density rocks that make continents that can "float" over the surface and create plate tectonics.
• Smaller planets cool relatively fast, and their cores solidify. Once that happens, the planet no longer can drive large scale motions of continents by plate tectonics. If the planet retains an atmosphere and has "weather" with water, for example, then its continents will be eroded down and will not be rebuilt, so it will slowly approach more closely to perfectly round. The amount of land above its oceans will decrease, providing less opportunity for advanced life to roam. Thus, the whole pattern of geological activity and the nature of the surface of a planet is strongly influenced by the state of its interior - liquid or solid.

The first possibly habitable planet (besides Earth) was discovered by measuring radial velocities of the nearby red dwarf star Gliese 581. The planet is in an orbit of only 0.073 AU radius and orbits the star in 13 days. The star has a mass of 0.3 M; those with sharp eyes will see that puts the planet just inside the habitable zone in the graph above, but close enough to be exciting.  The mass of the planet could be as low as five times that of the earth (or as high as 30 times the earth). If it is near the lower end and rocky like the earth, its size could be only 50% larger than the earth. 

There are a lot of conditions for a habitable planet, but to form life, we are not done! For example, the sun has aged substantially since the earth formed. As a main sequence star ages, its interior pressure rises and it becomes more luminous. The energy output of the sun has probably increased about 30% since the earth formed - enough to make conditions change enough to be very challenging for life to persist.

However, it appears from a number of lines of evidence that the temperature on the surface of the earth was much more stable than the output of the sun would indicate! Carbon dioxide emitted in intense early volcanic activity may have resulted in just enough extra greenhouse effect at the beginning to warm the surface temperature to about its current levels, allowing life to evolve for billions of years in a relatively constant environment. Still, obtaining just the right conditions for such a long time must further narrow the habitable zone.

Comparing with the other planets, it is clear that rather special conditions on Earth make life possible here! If all these conditions are met, is life inevitable or does it require something else?? Or are we being too restrictive in our ideas, too tied to our particular forms of life and their requirements

Test your understanding before going on

Woolly Mammoth, symbol of the ice age, from http://news.nationalgeographic.com/news/2001/11/1101_WoolyMammoth.html		God gives life to Adam, on the ceiling of the Sistine Chapel, by Michelangelo.
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