The Prevalence of Planets And the Probability of Life

Science fiction crawls with intelligent BEMs (bug-eyed monsters) and humanoids (manlike creatures) that live on planets revolving about distant stars. Most fact-fancying scientists are far more skeptical. Somewhere in the universe, they say, there may indeed be a scattering of planets with salubrious atmospheres and temperatures, and something like earthly life. But the planets are probably few, and the odds against finding or communicating with their far-out creatures, say the skeptics, are exceedingly high.

Caltech Geochemist Harrison Brown demurs. It is a good bet that populated planets are spotted throughout the universe, says he, and their civilized inhabitants may well be trying to talk to earth.

Invisible Bodies. In Science, Dr. Brown spells out his reasons for believing in the prevalence of populated planets. The stars that man sees in the sky, he says, are masses of matter big enough to support thermonuclear reactions that give off a great deal of light. Objects with less than 7% of the mass of the sun do not shine.

According to Brown's estimate, the 10,000 cubic parsecs* of space around the sun contain about 1,000 visible stars. Most of them are comparatively small; the smaller and dimmer being the most numerous. But stars that are too small and dim cannot be seen un less they are very near the earth, so their apparent number is low. If they are slightly smaller still, they give no light and cannot be seen at all.

This does not mean that they do not exist. There is good evidence, says Brown, that when such visible stars condensed out of primitive gas and dust, many smaller bodies were also formed at the same time. Seven invisible objects somewhat bigger than Jupiter have already been detected by the wobbling motion that they cause in the stars around which they revolve. Dr. Brown estimates that 10,000 cubic parsecs of space contain 12,730 invisible bodies with sizes ranging from one-sixth the mass of the sun down to "earth equivalent"--the mass that the earth would have if its original hydrogen and other gases had not been driven off by the heat of the nearby sun. Small bodies down to "Mars equivalent" number 60,330, which makes them 60 times as common as luminous, visible stars.

Life Zones. Of this enormous heavenly host only the sun's planets are near enough to be seen by reflected light, but most of the rest are probably organized in planetary systems too. Dr. Brown believes that nearly half of these groups contain no central body large enough to be self-luminous. The other systems contain an average of about 50 members each, but nearly all of them are too cold or too hot to support life. Perhaps two planets per luminous star have conditions suitable for life as it is known on earth.

This adds up to a vast number of "life-zone" planets--at least 200 billion in the Milky Way galaxy alone--and if planets smaller than Mars are included, the number is greater still. "If planetary systems are indeed extremely abundant," says Dr. Brown, "one might conclude with equal conviction that man is not alone--that his equivalents may occupy hundreds or even thousands of bodies within our galaxy. Listening for evidence of the existence of such forms may indeed prove a profitable and exciting pursuit."

*A parsec is equal to 3.26 light-years or 19 trillion miles.

This file is automatically generated by a robot program, so reader's discretion is required.