Evidence is mounting that the earth and planets were formed when a nearby star 20 times the size of the sun exploded and showered our neighborhood with the seeds of creation.

The evidence says that at the precise time the solar system was born almost 4.6 billion years ago there was a massive nuclear explosion in the not-too-distant heavens that scattered radioactive debris across billions of miles of space and into the vicinity of what is now our solar system. The evidence places the birth of the solar system and the stellar explosion no more than 5 million years apart, which, in cosmological terms, is insignificant.

"Something went off with a helluva big bang Just before the solar system was born," said Dr. Gerald J. Wasserburg of the California Institute of Technology, where much of the recent evidence has been gathered. "We're talking about a difference in time that is almost instantaneous, a difference so close that it's almost zero."

Scientists believe the exploding star sent such a strong shock wave through space that it forced the gas and dust swirling in the neighborhood to come together and begin to form planets.

The vast quantity of radioactive debris thrown into space by the blowup of the star is also believed to have had a strong role in the formation of the planets and possibly the formation of the sun It was, Wasserburg said, the "last salting of the soup."

Evidence is derived from two meteorities that fell from space to earth or were discovered in the last 10 years. One, the Allende meteorite, dropped several tons of debris onto Mexico, and the other, the Santa Clara meteorite, scattered a few hundred pounds of iron across Mexico.

Bound up inside the Allende meteorite were "marbles" of radioactive elements in amounts unlike any seen on earth or even in moon rocks. Scientists found present in Allende strange mixes of barium, calcium, strontium, neodymium and samarium. Dr. Robert Clayton of the University of Chicago found an odd mix of an isotope of oxygen known as oxygen-16, which make it appear as it Allende were the leftover of a hydrogen bomb explosion.

"There are some objects in Allende which did not come from any normal solar mix," Wasserburg said. "Everyelement it some of those little marbles in Allende looks like it was made in Oak Ridge," the atomic energy facility in Tennessee.

The Allende meteorite contained huge amounts of an isotope called magnesium-26 that is produced only by the radioactive decay of another isotope, alumimum-26. This isotope has all but vanished from the solar system because its half-life of 700,000 years is short enough to have made it extinct.

Never before have scientists seen evidence of aluminum-26 bound up in such large amounts in the rock formed at the birth of the solar system. This evidence told them that a nuclear event was responsible for the original production of aluminum-26 and that enormous heats were being generated inside the rocks from the radioactive decay of elements like aluminum-26.

"At the levels we measured, there was enough radioactivity in the dust and gas cloud at the birth of the solar system to melt a planet." said Wasserburg, who found the first evidence of aluminum-26 with Drs. Typhoon Lee and D. A. Papanastassiou of Cal Tech. "If the whole solar system had alumimum-26 as abundant as our numbers said, there was more emergy present than the binding energy of the sun," Wasserburg said.

Even fresher evidence comes from the Santa Clara meteorite, which produced an unexpected amount of an isotope known as silver-107. Again, the large amount the silver isotope meant it has produced by the radioactive decay of another extinct isotope. In this case, the isotope was from the noble metal palladium and called palladium-107, whose existence in nature has never before been confirmed.

"People have thought about palladium-107 and looked for palladium-107 for 30 years," Wasserburg said. "We claim to have found it." The claim appears in the current issue of Geophysical Research Letters under the names of Cal Tech's Dr. William R. Kelly and Wasserburg.

The evidence that palladium-107 existed in large quantities in a meteorite whose origins go back 4.6 billion years is the strongest evidence yet that the solar system was seeded with "star dust" at the time of it, birth. Palladium-107 is only made in a nuclear furnace. The suggestion of its existence means the nuclear furnace had a hand in the formation of the planets.

The discovery of aluminum-26 in the Allende meteorite told scientists there was enough heat present to melt rock. The finding of the palladium-107 in the Santa Clara meteorite meant there was enough rock being melted to make planets.

"It's confirmation," Wasserburg said, "that there was some big nuclear event making a wide spectrum of radioactive elements across the whole periodic table."

In finding the palladium-107, Kelly and Wasserburg also dated its formation as no more than 5 million years beyond the 4,5555 billion years assigned from the analysis of the moon rocks to the formation of the solar system. The date for the palladium-107 also matched the date for the aluminum-26. Thus, two separate meteorites yielded identical dates for the formation of extinet isotopes.

It was no accident that Kelly and Wasserburg went looking for palladium-107 in a meteorite rich in iron. Kelly suggested they look for the extinct palladium in the Santa Clara meteorite because real palladium metal had already been found mixed in with the iron in the meteorite.

In discovering the palladium-107. Kelly and Wasserburg stumbled on the relics of a planet no bigger than a few miles across as it melted in the process of formation. The iron and palladium in the Santa Clara meteorite could have been the core of a planet in its first few million years of life.

Among the isotopes of the heavy metals, palladium-107 is unique because its life is so short. For instance, uranium-235 is still in the earth's crust. Palladium-107 is not. Its halflife is 6.5 million years, meaning it almost disappeared 12 million years after it fell from a star and arrived in the regions of the solar system and began to make planets.

"You dodn't make iron meteorites with little dust grains. You've got to have a planet and you've got to have the planet melting iron out from rock," Wasserburg said. "To get the palladium-107, you've got to strip out the iron at the same time you've still got some radioactive palladium left. This is a very short time scale."

In the current issue of Geophysical Research Letters, Kelly and Wasserburg describe their experiment to find the ghosts of palladium-107.Kelly worked 3 1/2 years preparing the experiment. Wasserburg describes it as "nontrivial."

The experiment involved methods of analyzing fractions of the meteorite so small they measured out as nanograms, meaning they were so tiny they had to be handled under a microscope. It involved extending methods of analyzing "moon dust" to meteorite pieces. The precision of measurement doubled the accuracy of any meteorite analysis previously done.

Most scientists believe aluminum-26 and the palladium-107 could only have come from an exploding star not too far from where the solar system rests. The remnants of the explosion cannot be seen. Its first identifiable relics are the extinct isotopes of aluminum and palladium.

The idea that a supernova explosion gave birth to the solar system fits nicely with another theory of stellar evolution gaining more scientific acceptance. This theory is that stars are formed in clusters as they pass through the spiral arms of the galaxy. The largest stars that grow out of this passage burn up their nuclear fuel in just a few million years and then blow up to send the universe with their radioactive decay products. CAPTION: Map, Allende meteorite contains an abundance of an odd mix of radioactive elements, Smithsonian Institution