According to new research published today in Nature Astronomy, water may have developed in the first 200 million years of the universe’s existence. The deaths of the universe’s initial stars may have accelerated the creation of the life-giving chemical. The study also discovered that rocky planets might be formed in the water-rich environment left behind long before the earliest galaxies emerged.
Principal scientist Daniel Whalen of Portsmouth University in the United Kingdom and his colleagues discovered that a rare sort of supernova triggered by the first stars produced enough water to saturate the surrounding regions where the future generations of stars and planets would be born.
After the Big Bang, the cosmos was filled with hydrogen and helium, which generated the first stars. These suns were not in galaxies, which did not yet exist, but rather at the points where cobweblike threads of dark matter strung between empty voids intersected. Gravity attracted gas to these crossings, and when the density reached a certain threshold, the first stars formed.
These stars were immense, up to 300 times the mass of our Sun. Their temperatures were tremendous, and they used up their fuel quickly. And they died in supernovae, which sprayed new elements throughout the galaxy.
Whalen and colleagues replicated two types of supernovae considered to be common in the earliest generation of stars by monitoring the lives of stars with 13 and 200 solar masses. “We watched primordial stars form … and then they blew up,” Whalen recalls.
The first type of supernova is a core collapse supernova. These occur in stars that are at least eight to ten times more massive than the Sun. Stars age and deplete the hydrogen in their core. They then evolve by succes
sive cycles of fusion burning in an onion-like structure surrounding the center, fusing increasingly heavier elements into new ones in a series of thin layers.
Iron was the final element generated since it cannot be melted to produce energy. Then, in the constant conflict between gravity and nuclear fusion (which produces photons that hold the star up), gravity triumphs and the core collapses, resulting in a neutron star that cannot be compressed further. The rest of the star collapses; it collides with the core and bounces back, causing a shock wave. Within the shock wave, additional fusion occurs, this time producing elements heavier than iron.
At the same time, the core’s tremendous pressures generate neutrinos, which further supercharge the rebounding material, eventually splitting the star apart. And all of that material, including newly produced metals (astronomers refer to any element other than hydrogen and helium as a metal), is flung outward, leaving only the dense star core.
Alternatively, pair-instability supernovae occur exclusively in stars with masses several times greater than the Sun’s. These stars’ cores can reach temperatures high enough to convert light into particles — pairs of electrons and positrons. The conversion of energy into matter reduces pressure in the core, causing it to constrict abruptly. This generates enough energy to immediately initiate additional thermonuclear burning, resulting in a shock wave that rips the star apart.
Whalen compares the effect to that of a massive thermonuclear bomb. The explosion is so tremendous that it entirely disintegrates the star, leaving no core behind. Every trace of star material is ejected into space.
A pair-instability supernova can unleash as much as 100 times the energy of a core-collapse explosion. “They were the first super, super powerful factories of heavy elements in the universe,” Whalen says.
After the explosion
What happened after the supernovae amazed Whalen and his colleagues.
When the first stars exploded, they were surrounded by residual hydrogen gas from the star. Whalen’s simulations revealed that the residual material consisted of microscopic clumps held together by gravity. As hot ejecta from the supernova sped outward, metals within it, including oxygen, interacted with the hydrogen, hastening their gravitational collapse. The additional metals also helped the clumps cool, allowing oxygen to mix with hydrogen to make water.
However, the two incidents were not exactly the same. While the material from a core-collapse supernova flowed reasonably smoothly outward, the material ejected by a pair-instability supernova was chaotic due to the stronger explosion. The higher turbulence in the second example resulted in more clumps—and because the ejecta did not have to go as far, it formed water more faster. Furthermore, the higher pressures and temperatures of the pair-instability supernova resulted in more metals than the core-collapse explosion, allowing the clumps to cool faster.
The consequence is a large time divergence. While core-collapse supernovae produce water within 30 million to 90 million years of their explosion, pair-instability supernovae can hydrate their environs in just three million years.
The concentration of metals also made a significant influence. According to Whalen, a core-collapse supernova produces only a few tenths of a solar mass in metals. Pair-instability supernovae, on the other hand, produce around 100 sun masses of metals, including 30 to 50 solar masses of oxygen. This can result in a lot more water.
The clusters surrounding a core-collapse supernova were 10 to 30 times more water-rich than dispersed clouds of gas in the Milky Way today. Pair-instability supernovae formed even richer clusters, but a few times less water-rich than the solar system today.
Because they are more massive, stars that produce pair-instability supernovae have an advantage over their cousins. These suns are born, burn, and die every 2.5 million years. The stars that cause core-collapse supernovae evolve over a period of around 12 million years. So, water was first introduced into the region surrounding the most massive stars.
The first planets?
In both cases, the clumps left behind are projected to produce the next generation of stars. But what about the planets? There have been doubts that planets may originate from the debris left by the earliest stars. However, some simulations indicate that these aggregates could indeed be the site of future planet formation.
Guideo De Marchi, an astronomer at the European Space Research and Technology Center who was not involved in the new study, discovered protoplanetary disks in settings similar to those found in the early cosmos, notably those with low metallicity (few heavy elements). He claims that in 2003, Hubble discovered evidence of a large planet in a globular cluster with extremely low metallicity.
The type of supernova may also influence the types of planets that form. Gas giants are mostly composed of hydrogen and helium in their outer layers, whereas rocky planets require more silicates and other heavy elements to develop. Because the material produced by a pair-instability supernova has a higher metallicity (more complex elements) than that produced by a core-collapse supernova, stars born from a core-collapse supernova have enough material to form Jupiter-like planets, whereas the descendants of a pair-instability explosion may be surrounded by rocky worlds.
All of this may have occurred before the first galaxies emerged.
Can we see it?
There’s a caveat: Whalen and his colleagues only looked into individual explosions. Several first-generation stars are most likely to arise where cosmic filaments cross. Each star would emit radiation that might shatter newly formed water molecules. At the same time, each zone would have dust, which would protect the water from the radiation.
“It is a delicate balance,” of those two forces, says De Marchi. But he remains optimistic. “The fact that we see the presence of water in protoplanetary disks in low metallicity environments [like] the Small Magellanic Cloud means that some of it can be preserved,” he says.
Searching for detectable traces of water in the early universe remains difficult. Observing the initial generation of planets orbiting solitary stars is also important. Even groupings of stars would be too dim to detect with today’s technology. So Whalen and his colleagues are attempting to determine what kind of signal those initial water-rich stars and planets would emit as a whole.
“An entire population [of water-rich stars and planets] in the early universe might create this hazy water … emission background,” Whalen says. That emission could potentially be detected in the coming decade by either the Atacama Large Millimeter Array in Chile or the Square Kilometer Array under construction in Australia and South Africa.
