Customer Reviews

Habitable Planets for Man

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This is the most valuable reference book a science fiction writer can have. Period. It's also wonderful fun to read.
Pair it with the 2000 book RARE EARTH for contrast. Dole is still more interesting, and doesn't really have an agenda, despite the title.

What a smart guy he must have been!

"Habitable Planets for Man" offers in-depth speculation about extrasolar planets. We learn that Earth-like worlds are reasonably likely only around a rather narrow range of stellar types which exclude every bright star except Alpha Centauri. Even if a planet with abundant oxygen and water is found, its weather may be unsuitable; tides or seismic activity may be extreme; narcosis due to excessive carbon dioxide or nitrogen may occur; day lengths may be difficult to tolerate. The chapter "An Appreciation of the Earth" is practically redundant, because after studying the earlier text, the reader is sure to be glad of living on the third rock from the Sun!

This book was written in 1970 as an analysis of what sort of planet would be inhabitable by humans on a permanent basis (colonization), and what are the chances of finding such planets around nearby stars. The book looks at human requirements, such as temperature, gravity, atmospheric composition, etc. Then other factors are reviewed, such as solar system organization, stellar properties, satellite relationships, special properties of binary star systems, etc.

This book makes for some very interesting reading. It is rather dated, though, with the mass of Pluto being placed at equivalent to the Earths, and with water oceans being speculated about for Venus. Admittedly, this probably does mean that some of the conclusions are suspect. However, the depth of information in this book does make it an interesting resource for science-fiction authors, and other interested in speculating about extra-solar planets for man.

This glorious book is virtually identical to the first edition, published in 1964. And while it is a little dated, it is simply terrific. I wish I had read it when it first came out.

Even the opening page is a treat, showing where our Sun would appear if we looked at the night sky from the vicinity of Alpha Centauri. It would be the brightest star in Cassiopeia! But what if we were in the vicinity of, say, Eta Cassiopeiae? Well, in that case, our Sun would be in the Southern Cross.

Now we get into the meat of the book, looking for habitable planets. What do we mean by "habitable?" Even before the book was written, the term "habitable zone" was used to refer to a region around a star in which a rocky planet could have liquid water on the surface, and that's not a bad definition. But Stephen Dole wants a little more than that. He wants the mean temperature extremes in every season to be between 14 degrees F and 104 degrees F so we poor humans won't freeze or get overheated (personally, I think we could survive a little outside this zone: we do on Earth). He wants between 0.2 and 30 lumens of light per square centimeter so we can grow our plants.

What about gravity? Well, sure, we'd get uncomfortable at much over 1.5g. But as Dole points out, there may be a stronger limit. Once a planet is above about 3.2 earth masses, it is likely to capture plenty of helium, so much that it will then capture hydrogen and become a gas giant. Too much atmosphere for us!

How about oxygen? I might settle for a place that lacked oxygen and try to "terraform" it. Not Dole: he wants us to be able to breathe on that planet! And he computes some acceptable partial pressures of oxygen.

Of course, we'll need water. Oceans! But we don't want more than about 90% of the surface to be covered by oceans: we need some land as well. And there are other requirements: not too much dust or wind, not too much radiation, a mass of at least 0.4 earth masses (so it can retain a breathable atmosphere), a day of less than about 100 hours so it does not get too cold at night or too hot during the day, decent values of inclination and eccentricity, and so on. We also need to be careful about "tidal locking," with a year slowly becoming equal to a (sidereal) day on the planet. That planet might have part of its surface overheat to the extent that the oceans start to boil, with the water being lost by photodecomposition followed by a loss of the hydrogen to space (with the exception, perhaps, of some ice retained on the "dark side" of the planet).

Next comes perhaps the finest part of this book, namely a table showing, for each star spectral type: the mean star mass, luminosity, radius, number of such stars per cubic parsec, years of residence on main sequence, and habitable zone boundaries. He eventually calculates (or estimates, by each spectral type) the total number of habitable planets in the Milky Way to be 645 million!

That is a worthy and pioneering piece of work.

Dole then gives a table of the stars within 22 light-years that he thinks might be candidates to have habitable planets. Since then, we've looked at some of these stars a little more carefully, and in the next few years, I think we'll know much more about them and the potential for habitable planets orbiting them. My guess is that we'll find at least one such planet.

I highly recommend this classic on the habitability of planets in the Galaxy. It got me to remember a popular motto (which Dole, to his credit, does not use): "The meek will inherit the Earth. The rest of us shall go to the stars."