The Earth did not get its water from asteroids — it may have had it all along

• The Earth may not have been born dry, but wet, with water reservoirs from the Big Bang stored within its foundational rocks.
• A new study uses data from the European Space Agency's (ESA) Herschel telescope to explain how water was formed as ice on tiny dust particles wrapped inside cold tenuous interstellar clouds.
• When the clouds burst to form a solar system, water saved itself by anchoring to pebble-sized dust particles.

The prevailing theory on how the Earth got its water is that the planet was born dry and water was added later as asteroids rained down and delivered ice from the outer solar system. But that may not be the only explanation.

It’s possible that Earth was actually born wet and water was a part of its very core since the Big Bang, according to a new study published in Astronomy & Astrophysics.

"It's fascinating to realise that when you drink a glass of water, most of those molecules were made more than 4.5 billion years ago in the cloud from which our sun and the planets formed," said Ewine van Dishoeck of Leiden University and lead author of the paper.

According to him, this is the first study to dive into the journey of water throughout the entire star and planet formation process, including the steps in between. It combines past studies based on data by the European Space Agency’s (ESA) Herschel — a far-infrared space telescope to research the presence of water in space — while adding its own insights as well.

Most solar systems are born with enough water to fill several thousand oceans

The Earth was born immediately after the Big Bang. At first, there were only interstellar clouds and clumps of rocks spinning around the Sun.

The prevailing theory on how Earth got its water assumes that the Sun's heat was so strong that it boiled away any existing water in the area. It was only when asteroids came along and bashed into the planet that oceans formed.

But, data from Herchel suggests that water may have been there all along, preserved within the foundational rocks of the Earth. In fact, solar systems across the universe may be born with enough water to fill several thousand oceans.

The Earth was born wet

The study proposes that most of the water was formed as ice on tiny dust particles wrapped inside cold tenuous interstellar clouds. The ice forms primarily formed in the pre-stellar stage, when temperatures were cooler and stars have not yet come into existence.

It’s only when these clouds collapse that they form new stars and planets.

The heat that is generated does boil away some of the water. But, most of the water is preserved in reservoirs within planet-forming disks — a spiral containing dust grains that eventually become bigger planets.

Water saves itself by anchoring to pebble-sized dust particles in the disks around stars that have just been formed. And these pebbles serve as the building block for new planets — like Earth.

“Water is transported from clouds to disks mostly as ice, with no evidence for strong accretion shocks,” says the study.

The ice continues to grow on dust particles layer by layer. “Even at abundances that are somewhat lower than expected, many oceans of water are likely present in planet-forming regions,” it notes.

Not only does this explain how water came to be on Earth, but it also proposes that new planetary systems are likely to be born with enough water to become habitable. Herschel data shows that water gas and ice are commonly associated with star-forming regions and that this conclusion is independent of the ‘environment’ or location of the Milky Way galaxy.

A 2015 theoretical study based on data collected by the National Aeronautics and Space Administration's (NASA) Hubble Space Telescope believes that only 8% of the potentially habitable planets that will ever form in the universe existed at the time of the Big Bang, 4.6 billion years ago. Over the next 6 billion years, the other 92% will be born.

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