Earth as it might have been

kw: book reviews, nonfiction, science, astronomy, analysis

The book title asks the first question — What if the Earth Had Two Moons? And Nine Other Thought-Provoking Speculations on the Solar System, by Neil F. Comins. He is too modest in his title; these essays are not so much speculations as analyses, bringing 400 years of discoveries and knowledge to bear on ten "what if" subjects. In the title chapter, for example, he examines how a planet with a large moon could obtain a second large satellite, smaller but closer-in than the first moon, and the effects on tides, Earth-system history, evolution, and even the cosmology of sentient inhabitants. This question and some of the others require a bit of investigation into the cosmic billiards that might lead to capture of a moon into a specific orbit, and whether having a second moon around would keep the Moon from being in a phase-locked orbit.

Suppose the Moon rotated the opposite direction? or suppose Earth were a moon to a larger planet, say the size of Neptune? The consequences of each variation on the existing system are carefully examined. Some scenarios make it harder for life to get started, or to last long enough to become sentient, or even larger than germs. For example, if the Sun were smaller, Earth would have to be much closer to it to stay warm. This would almost certainly mean Earth would be phase-locked, with one side always facing its star. That would cause all the water to distill into ice on the forever dark side, away from the star, making one hemisphere a desert as dry as our Moon, and the other too ice locked for any liquid water to form. No water, no life. If, however, the orbit were more elliptical, and could stay that way long enough, the "wobble" of the planet could be enough to keep some water liquid, making life possible, just.

The last chapter asks, What if another galaxy collided with the Milky Way? This is actually less speculative, because this one is certain to happen. A small galaxy is currently being cannibalized by our Galaxy, and the Andromeda galaxy is scheduled to sideswipe us in, say, two billion years. Much of the analysis in this chapter concerns the chances that stars will collide. Only if the centers of the galaxies pass through one another, or if globular clusters collide, is there much chance for stars to actually collide. On a galactic scale, stars are tiny and the space between them is very large (although, the presence of one or two young-appearing blue stars in most globular clusters indicates that star mergers do occur in these dense environments. This is something I've known, which is not mentioned in the book).

This book is a follow-on to an earlier book, What if the Moon Didn't Exist?, which also asks the question, what if the Sun were larger and brighter? I'll have to get the volume to find out, as my own speculations are certain to be less informed than those of Dr. Comins. Fascinating reading.