Planets rich in carbon, including so-called diamond planets, may lack oceans, according to NASA-funded theoretical research.
Our sun is a carbon-poor star, and as result, our planet Earth is
made up largely of silicates, not carbon. Stars with much more carbon
than the sun, on the other hand, are predicted to make planets chock
full of carbon, and perhaps even layers of diamond.
By modeling the ingredients in these carbon-based planetary systems,
the scientists determined they lack icy water reservoirs thought to
supply planets with oceans.
"The building blocks that went into making our oceans are the icy
asteroids and comets," said Torrence Johnson of NASA's Jet Propulsion
Laboratory in Pasadena, Calif, who presented the results Oct. 7 at the
American Astronomical Society Division of Planetary Sciences meeting in
Denver. Johnson, a team member of several NASA planetary missions,
including Galileo, Voyager and Cassini, has spent decades studying the
planets in our own solar system.
"If we keep track of these building blocks, we find that planets around carbon-rich stars come up dry," he said.
Johnson and his colleagues say the extra carbon in developing star
systems would snag the oxygen, preventing it from forming water.
"It's ironic that if carbon, the main element of life, becomes too
abundant, it will steal away the oxygen that would have made water, the
solvent essential to life as we know it," said Jonathan Lunine of
Cornell University, Ithaca, N.Y., a collaborator on the research.
One of the big questions in the study of planets beyond our solar
system, called exoplanets, is whether or not they are habitable.
Researchers identify such planets by first looking for those that are
situated within the "habitable zone" around their parent stars, which is
where temperatures are warm enough for water to pool on the surface.
NASA's Kepler mission has found several planets within this zone, and
researchers continue to scrutinize the Kepler data for candidates as
small as Earth.
But even if a planet is found in this so-called "Goldilocks" zone,
where oceans could, in theory, abound, is there actually enough water
available to wet the surface? Johnson and his team addressed this
question with planetary models based on measurements of our sun's
carbon-to-oxygen ratio. Our sun, like other stars, inherited a soup of
elements from the Big Bang and from previous generations of stars,
including hydrogen, helium, nitrogen, silicon, carbon and oxygen.
"Our universe has its own top 10 list of elements," said Johnson, referring to the 10 most abundant elements in our universe.
These models accurately predict how much water was locked up in the
form of ice early in the history of our solar system, billions of years
ago, before making its way to Earth. Comets and/or the parent bodies of
asteroids are thought to have been the main water suppliers, though
researchers still debate their roles. Either way, the objects are said
to have begun their journey from far beyond Earth, past a boundary
called the "snow line," before impacting Earth and depositing water deep
in the planet and on its surface.
When the researchers applied the planetary models to the carbon-rich
stars, the water disappeared. "There's no snow beyond the snow line,"
said Johnson.
"All rocky planets aren't created equal," said Lunine. "So-called
diamond planets the size of Earth, if they exist, will look totally
alien to us: lifeless, ocean-less desert worlds."
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