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WASHINGTON (PTI): Asteroids from the inner solar system may be the source of the majority of Earth's water, a new study has claimed, contradicting the belief that most of our planet's water originated in the outer solar system.

The prevailing theories suggest that water was delivered on Earth from outer solar system by comets that coalesced beyond Jupiter's orbit, then migrated inward.

"Our results provide important new constraints for the origin of volatiles in the inner solar system, including the Earth," said lead researcher Conel Alexander of the Carnegie Institution of Washington.

"And they have important implications for the current models of the formation and orbital evolution of the planets and smaller objects in our solar system," Alexander was quoted as saying by SPACE.com.

For their study, published in the journal Science, the researchers analysed samples from 86 carbonaceous chondrites.

The primitive meteorites are thought to be key sources of the early Earth's volatile elements like hydrogen and nitrogen.

The team measured the abundance of different hydrogen, nitrogen and carbon isotopes in the chondrite samples. Isotopes are versions of an element that have different numbers of neutrons in their atomic nuclei. For example, the isotope deuterium -- also known as "heavy hydrogen" -- contains one neutron, while "normal" hydrogen has none.

The amount of deuterium in celestial bodies' water ice sheds light on where the objects formed in the solar system's early days. In general, bodies that took shape farther from the sun have relatively higher concentrations of deuterium, the researchers said.

The 86 chondrite samples' deuterium content -- which the team gleaned from clays, the remnants of water ice -- suggest the meteorites' parent bodies formed relatively close to Sun, perhaps in the main asteroid belt between Mars and Jupiter.

Comets, by contrast, have much higher deuterium ratios.

As a result, scientists think most of them were born in the solar system's frigid outer reaches.

The isotopic composition of the bulk Earth appears to be more consistent with chondrites than with comets, they said.

There are many different types of chondrites and no single group is a perfect match. So our planet probably accreted its water and other volatiles from a variety of chondrite parent asteroids, they added.