by Mike Wall
September 25, 2014
Planets form in the
presence of abundant interstellar water
inherited as ices from the parent
R. Hurt (SSC-Caltech)/ESO
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
Ancient Heritage of Water Ice in the Solar System).
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
"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,"
"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
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.
Artist's concept showing
the time sequence of water ice,
starting in the sun's parent
traveling through the stages of
and eventually being incorporated
into the planetary system itself.
Credit: Bill Saxton, NSF/AUI/NRAO
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
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.
Ilse 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
water" - including super-cold places like
The researchers constructed models that simulated reactions within a
protoplanetary disc, 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
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
(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
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