4.99 x 105


" Not known; rotation period is perhaps of the order of 2 weeks.

" Not known; rotation period is perhaps of the order of 2 weeks.

rotation. From Table 9 it may be deduced that values of h2 as large as 1.2 are not sufficiently high to stop a planet's rotation, while values of h2 of approximately 2.0 or more appear to be large enough to stop a planet's rotation. It probably would not be too far afield to estimate a critical value of h2 at approximately 2.0; that is, if h2 is greater than 2.0, planetary rotation rates would probably be too slow (after the braking force had been working for several billion years) to be compatible with habitability. For a habitable planet to possess the proper surface temperatures in the vicinity of a small main-sequence star, however, it must orbit within an ecosphere—so close to the primary that the tidal braking effect becomes large. Thus for stars at the low-temperature end of the main sequence, the tidal braking effect and the planetary temperature requirements for habitability are incompatible.

The inner boundary due to tidal braking effect depends on both the mass of the primary and the properties of the planet, although it is quite insensitive to the planetary properties because (R^M^)1 is very nearly a constant for habitable planets. As may be seen in Figure 26, if h2 equal to 2.0 is used as a criterion, habitable planets can exist in ecospheres only around stars having masses larger than 0.72 solar mass. A "full" ecosphere can exist around primaries of stellar mass greater than about 0.88 solar mass, but the ecosphere is narrowed by the tidal braking effect for primaries of lesser mass until it disappears when the stellar mass reaches about 0.72. The range in mass of stars that could have habitable planets is thus 0.72 to 1.43 solar masses, corresponding to main-sequence stars of spectral types F2 through Kl. There is an extension of this range down to the larger class M stars (mass greater than 0.35 solar mass) for a to o.i s

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