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5. Habitable zones of stars

A. Hazards; orbital stability, radiation and lifetimes

Planetary Orbital Stability - Most stars are members or binary pairs or multiple star systems, unlike our Sun. It is estimated that 70% of all stars may be in such systems. There are only a few cases where a planetary orbit in such a system might be stable enough for life to develop: 1) the planet is in an orbit which is large compared to the separation of the stars - this orbit is likely to be too far outside the "comfort" zone, see below; 2) the planet is close to one star and far from the other(s) - this orbit may be too close for comfort. There exists a meta-stable figure-8 orbit around two stars, but this is likely to range in and out of the habitable zones of the two stars in a single orbit, and long-term stability in the presence of other stars or planets would be very doubtful.

Stellar Radiation - The hottest stars, of spectral class O and B, emit radiation in a distribution of energies which peak at short wavelengths or high frequencies, in the ultraviolet. This radiation easily disrupts the molecular bonds which hold complex pre-biotic molecules such as amino acids and nucleotide bases together. Thus life could not easily emerge on surfaces exposed to the radiation of O and B stars. This does not preclude to origin of life in sheltered locations, such as underground or even in shallow ponds or oceans.

Stellar Lifetimes - The lifetime T of a star is proportional to its mass M and inversely proportional to its luminosity L: T = const. x M/L. The O and B stars are 10-20 times more massive than the Sun, but 10,000-20,000 and more times as luminous, so they only last 1/1000 as long as the Sun, or less than about 10 or 20 million years. These really short-lived stars do not last long enough for the evolution of life as we know it. They are useful, however, as they convert Hydrogen and Helium to heavier elements, particularly Carbon, Oxygen and Nitrogen, plus a little Sulfur and Phosphorus, which, together with Hydrogen, form the CHON(SP) elements necessary for life.

The lower mass K, M and R stars may last billions of years, which is fine for evolution, except that the universe is only some 14 billion years, give or take  1 or 2 billion. Some of these stars formed too long ago for there to to have been much CHON(SP) in their solar nebulae and thus in any resulting planets.


B. Habitable zones for some stellar types

Size of Habitable Zone - The "habitable" or "comfort" zone around a star has generally taken to be that zone in which an Earth-like planet would have surface temperatures allowing some liquid water. This is very conservative and biased, but reflects the conditions for the only life that we know about yet. Clearly, the evidence for Jupiter's moon Europa possibly having some liquid water beneath its frozen surface will force a reevaluation of the "habitable zone" concept. While we might compare volumes of habitable zones, it probably makes more sense to compare areas on the star's equatorial plane, as it appears that most planets will be found in such planes. Here is a table of habitable zones for various star types, from big, hot O stars to small, relatively cool M stars.

Type Typical Star Temperature Mass, rel Lifetime, Billion Yr Min, AU Max, AU Area, rel
O rare 30,000+ K 20 <0.001 N/A N/A N/A
B Rigel 20,000 K 10 0.002 N/A N/A N/A
A Sirius A 10,000 K 2.3 1      
F Procyon 8,000 K 1.2 7 2.8. 6.4 2.0
G Sun 6,000 K 1 10 0.85. 2.0 (1.1?) 1.0
K 61 Cygni 4,000 K 0.2 100+     ~0.05
M Proxima Centaurus 3,000 K 0.02. 1000 0.06. .0032 0.001

Stars follow a general law of nature; there are many more small ones than big ones. Most of the stars detected nearest to us are K and M stars, see the table of nearest stars.

[Note calculations indicating a much smaller "habitable zone" over the time needed to develop advanced life.]


C. Effects of Atmospheres and Gravitational tugging  

A planet's atmosphere has a lot to do with habitability: there are three planets in the "comfort" zone of our G star; Venus, Earth and Mars, but only one appears to be able to support life at present. Venus' atmosphere is too thick and results in a hellishly hot surface temperature, while Mars' atmosphere is now too thin, so there is no liquid water, at least on the surface. In addition, there are other factors: it now appears that Jupiter's moons Europa and possibly Callisto have liquid oceans beneath their frozen surfaces. Europa and Callisto are in orbit around Jupiter at 5.2 AU from the Sun. How can it be warm enough for liquid water? Gravitational tugging by Jupiter and its other large "Galilean" moons keeps the center of Europa warm, in the same way that it keeps the center of the innermost Galilean moon, Io, above the melting temperature of Sodium, resulting in Io's very active volcanos. Both of these factors make the "habitable zone" around a star larger.

Note the implications of the  recent discovery of extrasolar planets - most of the first ones are Jupiter-sized or bigger in extremely close orbits, closer than Mercury to our Sun. This is not at all in accord with the classical models for planet formation. One disturbing conclusion is that planetary disks drain down into young stars very quickly, until there is little material in the outer portion of the disks. In many cases this seems to include moving one or more Jupiter-sized objects through the orbital range that is considered "habitable" for these stars. Why does our solar system look so different? Is it sheer luck that a terrestrial planet like the Earth ended up where it is?


[Marcy observations & web site, other observers of extrasolar planets.]


Further Reading:

E. Chaisson & S. McMillan, Astronomy - A Beginner's Guide to the Universe (ch 21), Prentice Hall, 1995.

 E. Chaisson Universe - an Evolutionary Approach to Astronomy (ch 29), Prentice Hall, 1988.

IMAGES TO BE ADDED: Animated drain down to star?

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