Until now, scientists thought the "neck" might be a place where erosion had occurred. The comet is constantly sending dust and gas into space, so if one spot in particular — the neck — had been more active than other parts of the comet, it could have whittled the icy rock into an uneven shape.
But by studying high-resolution images from Rosetta and tracing the patterns of the onion-like layers that form the body of the comet, the researchers were able to determine that the two halves had formed and grown independently.
“The big news is that the layers of each lobe wrap independently, indicating that the two lobes grew separately and only joined together later in their history, gently ‘docking’ together during an ancient encounter that apparently did little damage to either partner,” Jay Melosh, a professor of planetary science at Purdue University in Indiana who wasn’t involved in the study, told the Associated Press. “This growth is believed to have occurred about 4.5 billion years ago, contemporaneous with the formation of the solar system.”
At a news briefing on Monday, the BBC reports, researchers involved in the study suggested that a slow, gentle collision had brought the lobes together.
"I would say it's a quite beautiful love story," lead study author Matteo Massironi said.