Water is essential to life on Earth and likely indicator for alien life as well, which is why so much of the world’s space missions and astronomical research is directed towards locating evidence of it on another planet.
But aside from finding it elsewhere, scientists also have yet to figure out how the Earth ended up with so much of it to begin with.
Now, a new study has shown that our world’s water was deposited on the Earth as it formed and a significant fraction of it was around before our star was born.
This makes it a lot more likely that water isn’t a fluke of the specific conditions of our little corner of the Universe, raising the possibility that extra-terrestrial life could exist on exoplanets orbiting other stars in our galaxy.
An illustration of water in our Solar System through time from before the Sun’s birth through the creation of the planets.
“This is an important step forward in our quest to find out if life exists on other planets,” said Professor Tim Harries of the University of Exeter’s Physics and Astronomy Department, in a statement. “We know that water is vital for the evolution of life on Earth, but it was possible that the Earth’s water originated in the specific conditions of the early solar system, and that those circumstances might occur infrequently elsewhere.
“By identifying the ancient heritage of Earth’s water, we can see that the way in which our solar system was formed will not be unique, and that exoplanets will form in environments with abundant water. Consequently, it raises the possibility that some exoplanets could house the right conditions, and water resources, for life to evolve.”
Although liquid water has proved elusive to astronomical observations, evidence of water in some form has actually been found all over the Solar System. Icy comets and moons have frozen deposits, while mineral samples from meteorites and Mars also hold watery elements.
Comets and asteroids, as primitive remnants of the stuff the Solar System is made of, can offer a lot of information about the early days of our system’s formation and their ices are able to tell us more about the water that was around back then.
Before now, scientists theorised that the Earth’s water came from the proto-planetary disc, or solar nebula, surrounding the Sun in its youth, from which the planets formed. But they didn’t know if the ice had come from the Sun’s own parental interstellar molecular cloud, or whether it had been formed by chemical reactions in the solar nebula.
“If water in the early Solar System was primarily inherited as ice from interstellar space, then it is likely that similar ices, along with the prebiotic organic matter that they contain, are abundant in most or all proto-planetary disks around forming stars,” explained Conel Alexander of the Carnegie Institute.
“But if the early Solar System’s water was largely the result of local chemical processing during the Sun’s birth, then it is possible that the abundance of water varies considerably in forming planetary systems, which would obviously have implications for the potential for the emergence of life elsewhere.”
To find out, a team of international scientists used ‘heavy water’ ices – those with an excess of water made from hydrogen isotope deuterium – and water ice. The difference in mass between the isotopes result in subtle differences in their behaviour during chemical reactions.
Using sophisticated modelling to simulate the proto-planetary disc, they created one in which all the deuterium from the interstellar ice had been eliminated in the forming of the Sun. They wanted to see if the system could generate the ratio of deuterium to hydrogen that’s found on Earth and in meteorites and comets just from chemical reactions in the solar nebula – but it couldn’t.
According to the simulation, between 30 and 50 per cent of the water came from the molecular cloud, making it around a million years older than the Solar System.
“We let the chemistry evolve for a million years – the typical lifetime of a planet-forming disc – and we found that chemical processes in the disk were inefficient at making heavy water throughout the solar system,” said Ilse Cleeves, an astronomy Ph.D student at the University of Michigan. “What this implies is if the planetary disc didn’t make the water, it inherited it. Consequently, some fraction of the water in our Solar System predates the Sun.”
Michigan’s Ted Bergin, professor of astronomy, said that means that many star systems could have had access to water.
“Based on our simulations and our growing astronomical understanding, the formation of water from hydrogen and oxygen atoms is a ubiquitous component of the early stages of stellar birth. It is this water, which we know from astronomical observations forms at only ten degrees above absolute zero before the birth of the star, that is provided to nascent stellar systems everywhere,” he said.