Good news for hunters of extraterrestrial life: Water may be more widespread in planetary systems than previously thought. A team of researchers studying the origin of the water in our solar system has concluded that up to half of it formed before the sun itself was born—that is, in the cloud of dust and gas that was the progenitor of our solar system. If water can form in abundance in such clouds, then it may be found everywhere.
Our solar system is awash with water. Apart from Earth, water is found on the moon, Mars, Mercury, comets, and the icy moons of the giant planets. But where did it come from? Water is known to form in the clouds of gas and dust of the interstellar medium (ISM) from which planetary systems coalesce, but is it destroyed when the newly formed sun starts pumping out heat and light, only to be formed again later? Or does that primordial water survive star formation and remain around us today?
To answer that question, a team led by astronomer L. Ilsedore Cleeves of the University of Michigan, Ann Arbor, focused on deuterium, a heavy form of hydrogen that was created in the big bang along with normal hydrogen. There are about 26 deuterium atoms for every million hydrogen atoms across the universe, but it is six times as prevalent in the water on Earth and in other solar system bodies. Scientists conclude that when the water formed, the reaction creating deuterium-rich “heavy water” was slightly faster than the one creating normal water, so the proportion of deuterium in water increased.
But that enrichment of deuterium happens only under certain conditions: It has to be very cold (only a few tens of degrees above absolute zero), plus you need oxygen and some sort of ionizing radiation to get the reaction going. All of those things are available in the ISM. The ionizing radiation there is cosmic rays, particles from distant sources that zip through space at high speed. And astronomers have observed water in the ISM that is highly enriched in deuterium, so that could be source of the solar system’s water.
Still, there’s a question mark over whether this interstellar water could survive the violence of the sun’s birth. To find out, Cleeves and her colleagues sought to determine whether the same water-forming reactions could have occurred after the sun’s formation, in the protoplanetary disk of gas and dust from which planets form. Such a disk would offer low temperatures and an oxygen supply just as the ISM does, but would there be enough ionizing radiation?
The team constructed a detailed model of the chemical processes creating water in a protoplanetary disk. Much of the cosmic rays are fended off by the young star’s magnetic field and particles streaming out from the star, but there are other sources of radiation: x-rays from the star and short-lived radionuclides in the disk. As the researchers report online today in Science, those sources of radiation just don’t produce heavy water fast enough. “We found that heavy water didn’t form in any abundance over a million years,” Cleeves says.
In fact, the team estimates that as much as 50% of the water now on Earth may have existed since before the birth of the sun 4.5 billion years ago. And that’s good news for other planetary systems. The conditions in the ISM are far more uniform across space than those in protoplanetary disks, so it’s likely that there is water everywhere waiting for planets to form. “As the number of confirmed planetary systems increases, it’s reassuring that … water is available,” Cleeves says.
“This is a very interesting result. We’ve been debating this for years, whether or not the ices have an interstellar heritage,” says astrophysicist Karen Willacy of NASA’s Jet Propulsion Laboratory in Pasadena, California. She says that other groups have tried to model the collapse of clouds in the ISM into planetary systems to see if ice would survive, but “with various results, that don’t always agree,” Willacy says. “This is much more simple approach, just using the chemistry which is well understood.”