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Planets form in the presence of abundant interstellar water
inherited as ices from the parent molecular cloud.
Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)/ESO/J.
Emerson/VISTA/Cambridge Astronomical Survey Unit |
Much of the water on Earth and elsewhere in the solar system likely predates the birth of the sun, a new study reports.
The finding suggests that water is commonly incorporated into newly forming planets throughout the
Milky Way galaxy and beyond, researchers said — good news for anyone hoping that Earth isn't the only world to host life.
"The implications of our study are that interstellar water-ice
remarkably survived the incredibly violent process of stellar birth to
then be incorporated into planetary bodies," study lead author Ilse
Cleeves, an astronomy Ph.D. student
at the University of Michigan, told Space.com via email. [
7 Theories on the Origin of Life]
"If our sun's formation was typical, interstellar ices, including
water, likely survive and are a common ingredient during the formation
of all extrasolar systems," Cleeves added. "This is particularly
exciting given the number of confirmed extrasolar planetary systems to
date — that they, too, had access to abundant, life-fostering water
during their formation."
Astronomers have discovered nearly 2,000
exoplanets so
far, and many billions likely lurk undetected in the depths of space.
On average, every Milky Way star is thought to host at least one planet.
Water, water everywhere
Our solar system abounds with water. Oceans of it slosh about not only on Earth's surface but also beneath the icy shells of
Jupiter's moon Europa and the Saturn satellite
Enceladus. And water ice is found on Earth's moon, on comets, at the
Martian poles and even inside shadowed craters on Mercury, the planet
closest to the sun.
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Artist's concept showing the time sequence
of water ice, starting in the sun's parent
molecular cloud, traveling through the
stages of star formation, and eventually being
incorporated into the planetary system itself.
Credit: Bill Saxton, NSF/AUI/NRAO |
Cleeves and her colleagues wanted to know where all this water came from.
"Why is this important? 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
protoplanetary disks around
forming stars," study co-author Conel Alexander, of the Carnegie
Institution for Science in Washington, D.C., said in a statement.
"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," Alexander added.
Heavy and 'normal' water
Not all water is "standard" H2O. Some water molecules contain
deuterium, a heavy isotope of hydrogen that contains one proton and one
neutron in its nucleus. (Isotopes are different versions of an element
whose atoms have the same number of protons, but different numbers of
neutrons. The most common hydrogen isotope, known as protium, for
example, has one proton but no neutrons.)
Because they have different masses, deuterium and protium behave
differently during chemical reactions. Some environments are thus more
conducive to the formation of "heavy" water — including super-cold
places like interstellar space.
The researchers constructed models that simulated reactions within a
protoplanetary disk, in an effort to determine if processes during the
early days of the
solar system could
have generated the concentrations of heavy water observed today in
Earth's oceans, cometary material and meteorite samples.
The team reset deuterium levels to zero at the beginning of the simulations, then watched
to see if enough deuterium-enriched ice could be produced within 1
million years — a standard lifetime for planet-forming disks.
The answer was no. The results suggest that up to 30 to 50 percent of
Earth's ocean water and perhaps 60 to 100 percent of the water on comets
originally formed in interstellar space, before the sun was born.
(These are the high-end estimates generated by the simulations; the
low-end estimates suggest that at least 7 percent of ocean water and at
least 14 percent of comet water predates the sun.)
While these findings, published online today (Sept. 25) in the journal
Science, will doubtless be of interest to astrobiologists, they also
resonated with Cleeves on a personal level, she said.
"A significant fraction of Earth's water is likely incredibly old, so
old that it predates the Earth itself," Cleeves said. "For me,
uncovering these kinds of direct links between our daily experience and
the galaxy at large is fascinating and puts a wonderful perspective on
our place in the universe."
Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.
