Astronomers have always felt uncertain about Pluto, the outermost planet in the solar system. It is suspiciously small, with less than half of the earth's diameter, and its orbit is peculiar. Instead of revolving in a near-circle around the sun as the other planets do, Pluto follows an eccentric ellipse, cutting across the orbit of Neptune, its sunward neighbor (which is 39 times the size of the earth). These deviations suggest that Pluto may not be a real planet.
Last week Astronomer Gerard Peter Kuiper (rhymes with piper) of the University of Chicago made another move toward demoting Pluto. Recent observations have proved that its period of rotation on its own axis is more than six days (TIME, Feb. 6). For a planet, says Scientist Kuiper, this is too slow.
Most astronomers now think that the sun and its planets were once a great cloud of gas and dust which gradually condensed around a central mass. That mass became the sun. As the gas cloud grew smaller and denser, some of its material spun out to form a flat disk. After a billion years or so, the disk broke up into loose blobs called protoplanets. Each of these contracted independently, forming its own core. Any material left outside eventually turned into satellites revolving around a planet.
Solar Cleanup. While the protoplanets were still in existence, about 4.5 billion years ago, the sun became dense and hot enough to support nuclear reactions that made it glow brightly. Its light and heat blew gases away from the nearer protoplanets (proto-earth, proto-Mars, etc.), leaving little more than rocky cores. The more distant protoplanets, which became Jupiter, Saturn, Uranus and Neptune, retained a good deal of their gases, as they do today. They did grow smaller, however, and as their gravitation decreased, their satellites tended to escape like dogs that have slipped their leashes.
Dr. Kuiper thinks that Pluto is an escaped satellite that once revolved around Neptune. The other satellites of Neptune, Triton and Nereid, may have escaped too, but eventually were recaptured. They tangled with the gaseous envelope that still surrounded the mother planet and were reduced again to the satellite status. Pluto, however, managed to keep its freedom until the sun had dissipated most of Neptune's gaseous envelope. Now it is probably safe for the life of the solar system.
Servile Birth. If Pluto were a real planet, says Dr. Kuiper, its orbit could not be so eccentric. Best proof, however, of Pluto's humble origin is its slow rotation. Planetary satellites turn only fast enough to present the same face to their planet. The earth's moon does this, rotating once for each turn around its orbit. Dr. Kuiper believes that Pluto used to revolve around Neptune once in about 6½ days, rotating on its own axis in the same period. Now, on its lonely orbit around the sun, it rotates just as fast as when it was attached to Neptune.