The first discovery of earthsized planets orbiting a star other than our sun was announced by Alexander Wolszczan of Pennsylvania State University in January and February of this year.1,2 Almost immediately this finding raised the hopes of those who want to believe in the possibility-or as some might boldly suggest, the probability-of abundant life sites in the universe.
Let's review some background and consider whether or not this exciting discovery realistically supports such notions.
The first evidence of planets outside our solar system arose more than twenty years ago through the research of George Van Biesbroeck. He noted that a few nearby stars move back and forth across the line of sight as if they were being tugged by the gravity of smaller, unseen bodies orbiting them. However, the quality of Van Biesbroeck's measurements was questioned, and his observations indicated "planets" several times more massive than Jupiter-objects which some astronomers would hesitate to call planets because of their special characteristics.
More recently, debris shells have been found to orbit most young stellar objects. Theoreticians have produced models explaining how the gas, dust, and rocks in such shells will eventually condense into planets. The evidence produced by Wolszczan and his team, however, is the first detection of several star-orbiting bodies roughly comparable to the mass of the earth.
Wolszczan's 1994 announcement actually represents a refinement of data from two years ago.3,4 What he saw then as evidence for at least two planets orbiting pulsar PSR B1257+12 has now been more precisely detected as three separate planets. The first planet orbits 30 million miles form the pulsar with an orbital period of 67 days, the second planet orbits 50 million miles from the pulsar with an orbital period of 98 days. Together these two planets contain three times the mass of Earth. The third planet has a mass equivalent to that of our moon (0.0123 the mass of Earth) with an orbital period of 25 days. Other small planets beyond 500 million miles from the pulsar may also exist.
That these planets orbit a pulsar is important to "life-site" considerations. A pulsar is the highly collapsed remainder of an exploded star. It is so tightly condensed that all its protons and electrons are fused together to form neutrons. A single teaspoonful of pulsar material would weigh five billion tons.
This particular pulsar, PSR B1257+12, is so highly collapsed that it rotates hundreds of times per second; thus, matter on its surface relative to its center moves at nearly the velocity of light. As a result, the pulsar sends out a rotating pencil-beam of radio wavelength radiation.
A small body orbiting a pulsar is dramatically different from such bodies as Venus, Earth, and Mars. Stars massive enough to eventually become pulsars are highly unlikely to produce, during their formative stages, planets like Earth. And if they did, the planets would be destroyed in the cataclysmic blast that results in a pulsar. The bodies Wolszczan discovered must have formed after (or perhaps as a result of) that explosion. They no doubt contain far more heavy elements than does Earth.
For this reason and because of the deadly radiation pulsars pour out, such planets are totally out of the running as life-support sites.
References:
1. Cowen, Ron, "New Evidence for Planets Orbiting a Pulsar," Science News, 145 (1994), p. 151.
2. Thorsett, Stephen E., "The Times They Are A-Changing," Nature, 367 (1994), pp. 684-685.
3. Bales, M., A. G. Lyne, S. L. Shemar, Nature, 352 (1991), pp. 311-313.
4. Wolszczan, Alexander, and D. A. Frail, Nature, 355 (1992), PP. 145-147.