Habitable Planets for Man

Back in 1964, RAND scientist Stephen H. Dole wrote a book titled “Habitable Planets for Man“. In it, he made a serious effort to first, define what was needed for a planet to be habitable, and second, to estimate the number of stars that might actually support such planets. Today, we had our first good test of his ideas.

This exercise is a lot more tractable than the Drake SETI equation, because most of the parameters are known — what the habitable zone around a given star would look like, how many of each class of star there are, and so forth. One of the constraints is that the star not be too large/hot, because such stars burn out very rapidly, too rapidly for life to develop. Another constraint is that the star not be too small/cool, because then its habitable zone is so close that any planet would become tidally locked, the way the Moon is on Earth. Then there are the less easily quantified measures — not so big that humans couldn’t move, yet not so small that it would lose its atmosphere.

As many have concluded since, only stars in the FGK sequence are likely to be suitable hosts. The Sun is a G-class star. Class F stars are larger and hotter, and class K are smaller and cooler. The odd lettering sequence comes because early modern astronomers classified the stars ABC…MNO before the discovery of that stellar mass was the key driver of stellar class, and the sequence was reordered OBAFGKM, with RNS holdovers of special stars, leading to the mnemonic Oh, Be A Fine Girl, Kiss Me Right Now (Smack), or the less common Oh Brave And Fearless Gorilla, Kill My Roommate Next Saturday.

Dole calculated that 3.7% of the stars in classes F2-K1 had habitable planets, and that there were therefore, perhaps as many as 50 habitable planets within 100 light-years of Earth. Now that we have better data, this result looks surprisingly close.

Today, the blog Next Big Future reports on a paper in the Astrophysical Journal (.pdf), by JPL scientist Wesley A. Traub, reporting preliminary results for the planet search by the Kepler astronomy satellite. Looking at stars in the F5-K2 classes, slightly cooler than Dole’s subjects, Traub found that roughly 34% of these stars had terrestrial planets in the habitable zone — a result almost ten times larger than Dole’s. This is one of the few times that reality has turned out to be more favorable than our original, pessimistic, estimates.

Except that terrestrial does not mean habitable. Traub defines terrestrial planets as “those with 0.5 ≤ r ≤ 2.0, corresponding to roughly 0.1-10 Earth masses”. Dole estimated that the largest a planet could be and still allow of permanent human habitation was a radius of 1.25, with a mass of 2.35 that of Earth. Similarly, Dole calculated that the smallest a planet could be and still retain an oxygen atmosphere was one that had a radius of 0.8 and a mass of 0.4 that of Earth. These constraints are quite a bit tighter than those Traub used.

We can still be cheerful, however. Assuming that the distribution of planets is linear in the region under discussion, we can make a crude estimate that perhaps 25% of the terrestrial planets found by Kepler are potentially habitable by the Dole criteria. This means that roughly 8% of FGK stars have potentially habitable planets, based on surface temperature and mass alone. That’s about twice the total that Dole came up with. Given the crudity of even today’s data, we can say that he was in the ballpark.

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