Water is an essential ingredient for life as we know it, but its origins on Earth, or any other planet, have been a long-standing puzzle. Was most of our planet’s water incorporated in the early Earth as it coalesced out of the material orbiting the young sun? Or was water brought to the surface only later by comet and asteroid bombardments? And where did that water come from originally?
A study published on March 7 in the journal Nature provides new evidence to bolster a theory about the ultimate origins of water—namely, that it predates the sun and solar system, forming slowly over time in vast clouds of gas and dust between stars.
”We now have a clear link in the evolution of water. It actually seems to be directly inherited, all the way back from the cold interstellar medium before a star ever formed,” says John Tobin, an astronomer studying star formation at the National Radio Astronomy Observatory and lead author of the paper. The water, unchanged, was incorporated from the protoplanetary disk, a dense, round layer of dust and gas that forms in orbit around newborn stars and from which planets and small space bodies like comets emerge. Tobin says the water gets drawn into comets “relatively unchanged as well.”
Astronomers have proposed different origins story for water in solar systems. In the hot nebular theory, Tobin says, the heat in a protoplanetary disk around a natal star will break down water and other molecules, which form afresh as things start to cool.
The problem with that theory, according to Tobin, is that when water emerges at relatively warm temperatures in a protoplanetary disk, it won’t look like the water found on comets and asteroids. We know what those molecules look like: Space rocks, such as asteroids and comets act as time capsules, preserving the state of matter in the early solar system. Specifically, water made in the disk wouldn’t have enough deuterium—the hydrogen isotope that contains one neutron and one proton in its nucleus, rather than a single proton as in typical hydrogen.
An alternative to the hot nebular theory is that water forms at cold temperatures on the surface of dust grains in vast clouds in the interstellar medium. This deep chill changes the dynamics of water formation, so that more deuterium is incorporated in place of typical hydrogen atoms in H2O molecules, more closely resembling the hydrogen-to-deuterium ratio seen in asteroids and comets.
“The surface of dust grains is the only place where you can efficiently form large amounts of water with deuterium in it,” Tobin says. “The other routes of forming water with deuterium and gas just don’t work.”
While this explanation worked in theory, the new paper is the first time scientists have found evidence that water from the interstellar medium can survive the intense heat during the formation of a protoplanetary disk.
The researchers used the European Southern Observatory’s Atacama Large Millimeter/submillimeter Array, a radio telescope in Chile, to observe the protoplanetary disk around the young star V883 Orionis, about 1,300 light-years away from Earth in the constellation Orion.
Radio telescopes such as this one can detect the signal of water molecules in the gas phase. But dense dust found in protoplanetary disks very close to young stars often turns water into ice, which sticks to grains in ways telescopes cannot observe.
But V883 Orionis is not a typical young star—it’s been shining brighter than normal due to material from the protoplanetary disk falling onto the star. This increased intensity warmed ice on dust grains farther out than usual, allowing Tobin and his colleagues to detect the signal of deuterium-enriched water in the disk.
“That’s why it was unique to be able to observe this particular system, and get a direct confirmation of the water composition,” Tobin explains. ”That signature of that level of deuterium gives you your smoking gun.” This suggests Earth’s oceans and rivers are, at a molecular level, older than the sun itself.
“We obviously will want to do this for more systems to make sure this wasn’t just that wasn’t just a fluke,” Tobin adds. It’s possible, for instance, that water chemistry is somehow altered later in the development of planets, comets, and asteroids, as they smash together in a protoplanetary disk.
But as an astronomer studying star formation, Tobin already has some follow up candidates in mind. “There are several other good candidates that are in the Orion star-forming region,” he says. “You just need to find something that has a disk around it.”