Astronomers have found the first comet with oceanlike water, giving a major boost to the theory that celestial bodies were a significant source of water for a thirsty early Earth.

The intense heat of the planet immediately after it formed means that any initial water would have quickly evaporated; scientists believe the oceans emerged around­8 million years later. The puzzle is where the water, which is vital for life on Earth, came from.

Chemical analysis of water-ice from comets had suggested they could have delivered no more than 10 percent of the water in today’s oceans. But research by Paul Hartogh of Germany’s Max Planck Institute for Solar System Research and colleagues showed that a comet called 103P/Hartley 2 has the same chemical composition as the Earth’s oceans.

The finding substantially increases the amount of water that might have originated from comets, which are largely made up of rock and ice. Previous models of the early Earth implied that most water came from asteroids.

In the case of Hartley 2, researchers using infrared instruments found that ice on the comet has a near identical “D/H” ratio to seawater. D/H measures the proportion of deuterium, or heavy hydrogen, to ordinary hydrogen.

“It was a big surprise when we saw the ratio was almost the same as what we find in the Earth’s oceans,” Hartogh said.

“It means it is not true anymore that a maximum of 10 percent of water could have come from comets. Now, in principle, all the water could have come from comets.”

Hartogh, whose research was published online last week in the journal Nature, believes Hartley 2, whose current orbit around the sun does not extend much beyond Jupiter, started life in a different part of the solar system than other comets studied.

It probably formed in the Kuiper belt, which lies about 30 to 50 times farther from the sun than the Earth, while the others come from the Oort Cloud, some 5,000 times farther away.