PLANETARY DISCOVERIES RATE A 10.

Byline: Robert S. Boyd Knight-Ridder Tribune News Wire

Just one year ago this month, astronomers announced the discovery of the first planet circling a sunlike star outside our solar system. Now the count of alien planets is up to 10 - surpassing the nine in our own back yard - and rising fast.

``It's sort of the Planet-of-the-Month Club,'' joked George Gatewood, a planet hunter at the University of Pittsburgh's Allegheny Observatory.

So far, none of the newly detected heavenly bodies - giant gas balls similar to Jupiter or Saturn - resembles the Earth or is likely to harbor life.

And so far none of them has been observed directly. Rather, their presence has been inferred from tiny fluctuations, or wobbles, that an invisible companion causes in the motion of a star.

Nevertheless, multiple observations by at least five separate teams of astronomers greatly increase the probability that Earth-like planets will be found elsewhere in our galaxy in the next decade or two.

The Clinton administration and the National Aeronautics and Space Administration have made the search for extraterrestrial life one of their highest priorities. A series of robotic expeditions to Mars, checking for signs of life on that nearby planet, will start next month.

``The extraordinary discovery'' of these planets ``has completely transformed the field of extra-solar planet detection,'' said Alan Boss, an astrophysicist at the Carnegie Institution of Washington.

According to Boss, two stars - one in the Big Dipper and the other in the constellation Cancer (the Crab) - are believed to have ``planetary systems'' composed of two or more orbiting objects. This has created ``tremendous hope among astronomers that planetary systems similar to ours do indeed exist,'' he said.

Another star in the Big Dipper, or Ursa Major, has one planet around it. And other stars believed to be hosting at least one giant planet are located in the constellations Andromeda, Bootes, Cygnus, Pegasus and Virgo.

As more powerful telescopes come into use, scientists hope to find smaller planets.

Some of the planets found so far are of a size and temperature that would permit them to harbor water, clouds and complex organic molecules, according to Geoffrey Marcy, a leading planet hunter at San Francisco State University.

In addition to the 10 planets around sunlike stars, astronomers have identified four other planets orbiting neutron stars - known as pulsars - in the constellation Virgo. Three planets were discovered around one pulsar and one around the second.

Pulsars are former stars that exploded and collapsed into extremely dense, fast-spinning, highly radioactive neutron stars. Because of the intense radiation, life there is out of the question.

``You'd need a lead umbrella at the beach,'' said Alexander Wolszczan, the Pennsylvania State University astronomer who discovered the pulsar planets four years ago.

Furthermore, at least half a dozen examples of a third class of celestial objects known as ``brown dwarfs'' have been detected in the past year. These are embryonic stars that fail to shine because they aren't big enough to generate enough heat to ignite their hydrogen fuel.

Travel to any of these extra-solar planets is impossible because of the enormous distances to even the nearest stars. The closest planet spotted so far is circling the star known as Lalande 21185, eight light-years (48 trillion miles) from Earth.

Despite the obstacles and uncertainties, Boss said astronomers ``will continue to discover new gas-giant planets and brown dwarfs, perhaps at the breakneck rate of a planet a month.''

STARS WITH PLANETS

Here is a list of the stars that have been identified with one or more surrounding planets. Each star is identified by a number or Greek letter followed by the name of its constellation.

51 Pegasi

47 Ursae Majoris

70 Virginis

16 Cygni

Tau Bootis

Upsilon Andromedae

55 rho Cancri (two planets)

Lalande 21185 (two planets around an unnamed star in the Big Dipper)

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Box: STARS WITH PLANETS (see text)