With comets unlikely, most astronomers now think Earth’s water came from asteroids.

The new findings, published in the journal Science, came after Rosetta arrived at Comet 67P in August, close enough for the instrument to begin detailed analysis of the molecules coming off the comet. Earlier, the same instrument discovered that the comet exuded the scents of formaldehyde and rotten eggs.

“It’s a nice start to this phase of the mission,” Matt Taylor, the project scientist, said of the water findings.

With Earth’s water a puzzle, scientists had long presumed that the planet was dry when it formed 4.5 billion years ago, and that the water came later, perhaps during the “late heavy bombardment” period more than 3.8 billion years ago. Comets, often called dirty snowballs, seemed a likely candidate.

Comets originate from two places in the solar system: the Kuiper belt, a ring of icy debris just beyond the orbit of Neptune; and the Oort cloud, a spherical shell of frozen detritus much farther out. Asteroids are rocky bodies in the inner solar system, mostly between the orbits of Mars and Jupiter.

But comets’ water turned out to be different from Earth’s. A few water molecules have a heavier version of hydrogen called deuterium that replaces one of the two hydrogen atoms, forming what is known as heavy water. On Earth, about one in 6,000 water molecules contains deuterium.