Long ago, Neptune’s moon Triton was probably a planet in its own right. But somehow Neptune captured it into orbit, and scientists for years have been trying to figure out how this happened. ‘‘For capture, the object has to slow down as it approaches the capturing object,’’ said planetary scientist Craig Agnor of the University of California at Santa Cruz. ‘‘In Triton’s case, something that’s not there now had to be there then.’’

Agnor and University of Maryland astronomer Douglas Hamilton think they have figured out what the something was: Triton once had a companion, and when the ‘‘binary’’ neared Neptune, Neptune’s gravity broke the two apart, kept Triton and let the companion whirl off into space. Their results were reported in last week’s edition of the journal Nature. Agnor said in a telephone interview that an alternative ‘‘collision’’ model, in which a lone Triton would have crashed into another Neptune satellite, requires a chance encounter that would have been ‘‘geometrically unlikely’’. And an ‘‘aerodynamic drag’’ theory would need Neptune’s atmosphere to reach out and grab Triton and bring it into orbit. ‘‘But then the gas has to go away,’’ Agnor said, or Triton, like a spent satellite orbiting Earth, would lose so much speed it would fall into Neptune. In the binary model, Triton and its companion would have rotated around a gravitational point, or center of mass. As they approached Neptune, it pulled them apart to claim one of the companions—Triton—for its own. Binaries are common in the asteroid belt, and the planet Pluto actually forms a binary with its large moon, Charon. (LAT)

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