Alone in the Universe? Quirks of Oasis Earth

Article excerpt

A list of major factors points toward Earth's rarity. Our planet is not at all mediocre in terms of the type of galaxy and solar system it occupies, its location within them, its own history, and the present time within that history [see summary in "Rare Earth Factors," p. 000].

A few examples will demonstrate the detailed analysis that bolsters the rare Earth hypothesis.

The Moon

The Moon is abnormally large. In a way, it and Earth are almost sister planets. The large Moon plays a remarkable role in stabilizing the tilt of Earth's spin axis. If it were not there, smaller, or farther away, the tilt of Earth's spin axis would wander due to the gravitational effects of other bodies in our solar system. This tilt angle drift would make the planet's climate very unstable, possibly causing more frequent mass-extinction events. While the Moon might not be a critical factor, it surely helps make Earth an oasis for life.

Plate tectonics

Plate tectonics, movement of large segments of the planetary crust over tens of millions of years, does not appear to occur on the other Earthlike planets or satellites. The plate tectonic cycling of material--which forms at seafloor spreading centers and disappears from the surface along subduction zones at continental margins--is an odd process that may require just the right planet mass, core, mantle, crust, and evolutionary history. …