Extraterrestrial Civilizations - Isaac Asimov [61]
The Sun, therefore, slows Mercury’s rotation more effectively than the Moon slows Earth’s, but less effectively than the Earth slows the Moon’s. We might suspect, then, that Mercury rotates slowly but not so slowly as to face one side only to the Sun.
In 1890, Schiaparelli (who reported the canals on Mars thirteen years before) undertook the task of observing Mercury’s surface. This is a very difficult thing to do, since Mercury is farther from us than Mars, usually; since Mercury shows only a crescent phase, usually, whereas Mars is always full or nearly full; and since Mercury, unlike Mars, is usually close enough to the brightness of the Sun to make comfortable viewing unlikely. Nevertheless, from what faint spots Schiaparelli could make out on the surface of Mercury, he decided that it rotated only once in each revolution of 88 days, and that it faced only one side to the Sun.
In 1965, however, radar waves that were emitted from Earth were bounced off Mercury’s surface. The echo, received on Earth, told a different story. The length of the radar waves changes if they strike a rotating body, and the change varies with the speed of rotation. From the nature of the reflected radar waves, it turns out that Mercury’s period of rotation is 59 days, or just ⅔ of its period of revolution. This is a comparatively stable situation, not as stable as having its rotation equal to its period of revolution, but stable enough to resist further change through the Sun’s insufficiently strong tidal effect.
Now we can return to the imaginary situation of our midget star, with Earth circling it at a distance of 300,000 kilometers (186,000 miles) from its center. This distance is only 1/500 that of our Earth from the Sun, and even allowing for the fact that the midget star had only 1/16 the mass of the Sun, its tidal effect on Earth would be 150,000 times that of the Earth’s tidal effect on the Moon.
There is no question, then, but that if Earth were close enough to a midget star to be within its ecosphere, the powerful tidal effect of the star would slow its rotation, and quite early in its lifetime cause it to face one side forever toward the star and one side forever away.
On the side facing always toward the star, the temperature would go up past the boiling point of water. On the side facing always away from the star, the temperature would drop far below the freezing point of water. There would be no liquid water on either side.
One could imagine that there might be a “twilight zone” on the boundary between the forever-lit and the forever-dark hemispheres, in which the conditions would be mild. This would be so only if the orbit of the planet were nearly circular. Even then, the temperature on the hot side might be hot enough to result in the slow loss of the atmosphere, so that the planet would be airless and the twilight zone no more habitable than any other part.
As we imagine a larger and larger star, the ecosphere would be farther and farther from it. A planet within the ecosphere would be subjected to a smaller and smaller tidal effect. Eventually, if the star were large enough, the tidal effect will no longer be large enough to render the planet unfit for life as we know it.
We might estimate that a star should have at least ⅓ the mass of the Sun (which means it would have to be of spectral class M2 at least) before a planet in its ecosphere would be suitable for life.
Nor is the matter of tidal effect the only problem with midget stars. The width of an ecosphere depends on how much energy a star is radiating. A massive, luminous star has an ecosphere far out in space and one that is very deep; deeper than the entire width of our Solar system. A midget star has an ecosphere that is close in on itself and is very shallow. The chance of a planet’s happening to form within so shallow an ecosphere is vanishingly small.
Finally, stars smaller than spectral class M2 are very