WASHINGTON, Dec. 14 — Comets are not all made of interstellar dust and ice, but instead may contain material shot from the heart of the solar system during its tumultuous birth, scientists reported Thursday after examining pristine particles of a comet that were brought back by the Stardust spacecraft.

The evidence suggests that comets did not form in isolation in the outer parts of the solar system as it coalesced from a swirling mass of primal material, the researchers said. Instead, they said, some of the hot material that formed planets around the Sun seems to have spewed off into distant areas and become a component of distant comets.

“Many people imagined that comets formed in total isolation from the rest of the solar system; we have shown that’s not true,” said Donald Brownlee, a University of Washington astronomer who is the lead scientist for the Stardust mission.

“As the solar system formed 4.6 billion years ago,” Dr. Brownlee said, “material moved from the innermost part to the outermost part. I think of it as the solar system partially turning itself inside out.”