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Although it makes up 70 per cent of our planet's surface, scientists still don't all agree on where Earth's water actually comes from.
Now, researchers claim to have found the origins of water in the earliest moments of the universe.
According to scientists from the University of Portsmouth, water first formed in the debris of supernova explosions 100 to 200 million years after the Big Bang.
These findings suggest that the ingredients for life on Earth were in place billions of years earlier than previously thought.
Using computer simulations, the researchers show that water would have formed when the very first stars in the universe died and collapsed into supernovae.
As the oxygen produced by these blasts cooled and mixed with the surrounding hydrogen, water was able to form in the clumps of material left behind.
These dense, dusty cores are also the most likely origins of the material that would go on to form the first planets.
In their paper, Dr Daniel Whalen and his co-authors write: 'Besides revealing that a primary ingredient for life was already in place in the Universe 100–200 Myr after the Big Bang, our simulations show that water was probably a key constituent of the first galaxies.'
While 70 per cent of Earth's surface is covered with water (pictured), the origins of this key ingredient for life have long baffled scientists. Now, scientists say they have identified the first source of water in the universe and it is billions of years earlier than expected
Water, which has the chemical formula H2O, is made up of two ingredients: hydrogen and oxygen.
Hydrogen was formed along with the other light elements such as helium and lithium in the first few minutes after the Big Bang as the sea of super-heated particles cooled and clumped into atoms.
However, oxygen's atoms are so large that they can't be formed in this way.
Instead, oxygen and the other heavier elements had to be forged by the nuclear reactions created by stars.
About 100 million years after the Big Bang, about 13.7 billion years ago, clouds of primordial hydrogen and helium came together under the force of gravity.
As they grew denser, the pressure at the core eventually became so great that it kickstarted nuclear fusion reactions which transformed the gas clouds into stars and brought the first light to the cosmos.
Eventually, these stars burned through their supplies of hydrogen fuel and collapsed in on themselves, triggering enormous supernovae.
Briefly reaching temperatures around 1,000,000,000°C (1,800,000,000°F), those explosions fused the raw material from hydrogen and helium atoms into larger molecules including oxygen.
Scientists say that water would have been formed in the aftermath of stellar explosions called supernovae that were hot enough to create oxygen. These are the same types of blasts which produce nebulae like the Crab Nebula (pictured)
The scientists used computer simulations to model two supernova explosions, one from a star 13 times the mass of the sun (left) and one from a star 200 times the mass of the sun (right). These images show the heat produced by those blasts with the yellow and red regions showing greater heat
The explosions scattered hydrogen and oxygen in a halo surrounding the blast. Over the next 90 million years, those elements came together to produce water. The larger supernova (red) produces more water at a greater speed than the smaller explosion (blue)
How did water form in the universe?
Water has two ingredients: hydrogen and oxygen.
Hydrogen formed in the first minutes after the Big Bang out of cooling subatomic particles.
Oxygen only formed later when the first stars collapsed in supernova explosions.
The intense heat of those nuclear reactions converted hydrogen and helium into heavier elements.
Hydrogen and oxygen then clumped together under gravity in the cloud of debris surrounding the supernova.
Over millions of years, this produced large amounts of water which could have survived to make up part of the first galaxies.
In their paper, published in Nature Astronomy, the researchers modelled what would happen in the aftermath of two supernova explosions - one from a star 13 times the mass of the Sun and the second for a star 200 times the mass of the Sun.
This simulation showed that the first and second supernovae produced 0.051 solar masses of oxygen and 55 solar masses of oxygen respectively.
After the explosion, a cloud of hydrogen and oxygen is shot out into an enormous halo surrounding the remnants of the star where they start to combine into water.
At first, the low density of the halo means water levels stay fairly low but, as the halo starts to clump together under gravity, the water levels start to increase dramatically.
After 30 to 90 million years the smaller supernova produced the equivalent of one hundred-millionth to one millionth of a solar mass of water.
The second, large explosion meanwhile produced 0.001 solar masses of water after just 3 million years.
If that water could survive the violent galaxy formation process, then it could have been one of the key components of the first galaxies.
What makes this finding particularly interesting is that this could explain how water arrived on habitable planets such as Earth.
The resulting cloud cores of the smaller (left) and larger (right) supernovae produced water which could have made its way into the first galaxies. If this is correct, it means water could have been present on planets for billions of years longer than previously thought
The clouds of debris left behind by primordial supernovae are a likely origin for small stars like our sun and the protoplanetary disks from which planets are formed
The dense 'molecular cloud cores' in which water formed most abundantly are a likely origin of protoplanetary disks, swirling clouds of dust that go on to form planets, and low-mass stars such as our sun.
In some of those disks, water levels could be almost as high as they are anywhere else in the universe today.
The researchers write: 'These disks would have been heavily enriched by primordial water, to mass fractions that were 10–30 times greater than those in diffuse clouds in the Milky Way in the CC supernova core and to only a factor of a few lower than those in the Solar System today.'
The large amount of water and high chance of a low-mass star forming raises the possibility that planets with liquid water could form in the aftermath of those first supernova explosions.
This implies that a key condition for life might have been met billions of years earlier than scientists had previously thought.
KEY DISCOVERIES IN HUMANITY'S SEARCH FOR ALIEN LIFE
Discovery of pulsars
British astronomer Dame Jocelyn Bell Burnell was the first person to discover a pulsar in 1967 when she spotted a radio pulsar.
Since then other types of pulsars that emit X-rays and gamma rays have also been spotted.
Pulsars are essentially rotating, highly magnetised neutron stars but when they were first discovered it was believed they could have come from aliens.
'Wow!' radio signal
In 1977, an astronomer looking for alien life in the night sky above Ohio spotted a radio signal so powerful that he excitedly wrote 'Wow!' next to his data.
In 1977, an astronomer looking for alien life in the night sky above Ohio spotted a radio signal so powerful that he excitedly wrote 'Wow!' next to his data
The 72-second blast, spotted by Dr Jerry Ehman through a radio telescope, came from Sagittarius but matched no known celestial object.
Conspiracy theorists have since claimed that the 'Wow! signal', which was 30 times stronger than background radiation, was a message from intelligent extraterrestrials.
Fossilised Martian microbes
In 1996 Nasa and the White House made the explosive announcement that the rock contained traces of Martian bugs.
The meteorite, catalogued as Allen Hills (ALH) 84001, crashed onto the frozen wastes of Antarctica 13,000 years ago and was recovered in 1984.
Photographs were released showing elongated segmented objects that appeared strikingly lifelike.
Photographs were released showing elongated segmented objects that appeared strikingly lifelike (pictured)
However, the excitement did not last long. Other scientists questioned whether the meteorite samples were contaminated.
They also argued that heat generated when the rock was blasted into space may have created mineral structures that could be mistaken for microfossils.
Behaviour of Tabby's Star in 2005
The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astronomers since being discovered in 2015.
It dims at a much faster rate than other stars, which some experts have suggested is a sign of aliens harnessing the energy of a star.
The star, otherwise known as KIC 8462852, is located 1,400 light years away and has baffled astonomers since being discovered in 2015 (artist's impression)
Recent studies have 'eliminated the possibility of an alien megastructure', and instead, suggests that a ring of dust could be causing the strange signals.
Exoplanets in the Goldilocks zone in 2017
In February 2017 astronomers announced they had spotted a star system with planets that could support life just 39 light years away.
Seven Earth-like planets were discovered orbiting nearby dwarf star 'Trappist-1', and all of them could have water at their surface, one of the key components of life.
Three of the planets have such good conditions, that scientists say life may have already evolved on them.
Researchers claim that they will know whether or not there is life on any of the planets within a decade, and said: 'This is just the beginning.'
