Even before the Infrared Astronomical Satellite was put into Earth orbit last January, the scientists who designed its heat-seeking telescope were discussing the possibility that it might observe planets being formed in the neighborhoods of young stars about the size of our sun.

"We were going to observe roughly a dozen stars spread across the constellation Ursa Major, which are on the order of a few tens of million years old," Dr. Fred Gillett of Arizona's Kitt Peak National Observatory said yesterday by telephone from Chilton, England, where the IRAS satellite is controlled. "It never once occurred to us that we'd strike pay dirt at Vega, which is the star we use to calibrate our telescope."

When the satellite's telescope was first pointed at Vega in April, it saw more than just the third-brightest star in the heavens. It saw what appeared to be a ring of pieces of opaque, solid material surrounding the star out to a distance of almost 7 billion miles.

"The first thing we said was, 'My God, look at that,' " Gillett said yesterday. "We didn't know what to make of it."

At first, the scientists thought the satellite was looking at a strange galaxy far beyond Vega but in the same line of sight. In May they devised a plan to have the satellite observe Vega from two different points in orbit to see if there was a distant galaxy behind the star. There was not.

Then they looked carefully at half a dozen stars similar to Vega in size, age and brightness to see if they had the same opaque rings. They did not.

Again and again, the scientists pointed their orbiting telescope back at Vega, and again and again they saw the opaque ring. What was it?

One theory was that Vega was encircled by a huge ring, like the planet Saturn is, but the ring was far too large to support itself. Another theory was that behind Vega was a giant planet so large that it was bigger than the star. Still another theory was that a giant planet might be circling Vega and generating its own heat and light, as Jupiter does.

"The theories were interesting, but the physical models just didn't fit," Gillett said. "The only model that did fit was that we were witnessing a planetary system not unlike our own in the process of formation."

Will that model stand up? Has the IRAS satellite really seen the first evidence of a second solar system in the cosmos?

Gillett said yesterday that no one has questioned the satellite's basic observations: that there is something solid in orbit around Vega. What is sure to be questioned is the makeup of the material. Its average temperature is very cold, no warmer than 300 degrees below zero Fahrenheit. But that doesn't mean there isn't at least one body, among what may be thousands of solid bodies, that is warmer than that.

"The spirit we're taking it in right now is that we think we see another solar system in formation, but one reason we've gone public with it now is that we want to put it up to real scrutiny," Gillett said. "Can we test it out further? Or can we shoot it down? Those are the questions we have to answer."