The comet being closely studied by the Rosetta spacecraft was once two separate objects that fused together in a low-impact collision billions of years ago, scientists say.
That's the reason for its odd configuration, described variously as a "rubber duck" or dumbbell shape, they explain.
The strange double-lobed shape has puzzled astronomers since the first images were sent back by the European Space Agency's Rosetta spacecraft as it approached Comet 67P/Churyumov-Gerasimenko last year.
Some researchers suggested a cosmic collision scenario, while others suggested a process of erosion may have worn away a central "neck" between the comet's "head" and "body."
Now, an international research team has announced the results of their study, confirming that comet 67P is in fact a "contact binary," or two conjoined comets. Puzzle solved.
"Now we have answered it — it's a contact binary," says ESA Rosetta mission scientist Matt Taylor.
"We conclude that gentle, low-velocity collisions occurred between two fully formed kilometer-sized cometesimals (mini-comets) in the early stages of the Solar System," the researchers say in their study appearing in the journal Nature.
Astronomers used images from Rosetta's camera to observe layers, or strata, in the larger "body" area of the comet that are independent of similar layers in the "head."
The layers in each "lobe" of the comet are well-ordered around the two regions' centers of gravity, suggesting they formed independently within each lobe rather than around a center of mass of a single object.
In other words, they would have formed when each lobe was a separate object, before they came together to form the "dumbbell" comet.
That coming together would have been at a very low speed, just a few meters per second, as anything faster would have caused enormous damage to the two sections.
No such signs of damage are observable on 67P, the scientists say.
The finding suggests there is still much to learn about comet formation, they say.
"It shows that comets were forming into large sizes and that they were then still colliding in this grown form to make even larger comets," says Rosetta team member Stephen Lowry from Kent University in Britain.
So, comets we see today were not all formed completely and independently from centimeter- or meter-sized planetesimals, he says; "conjoined comets" may represent a large portion of them.
A number of other comets have exhibited similar lobed or joined shapes, the researchers point out.
"And that's really quite fascinating, because it could be quite a ubiquitous process," Lowry suggests.
