Carbon worlds may be waterless, finds NASA study

Carbon worlds may be waterless, finds NASA study
This artist's concept illustrates the fate of two different planets: the one on the left is similar to Earth, made up largely of silicate-based rocks with oceans coating its surface. The one on the right is rich in carbon—and dry. Chances are low that life as we know it, which requires liquid water, would thrive under such barren conditions. New theoretical findings show that planetary systems with carbon-rich stars would host waterless rocky planets. On Earth, it is believed that icy asteroids and comets are the main suppliers of Earth's ocean. But, in star systems rich in carbon, the carbon would snag up oxygen to make carbon monoxide, leaving little oxygen to make water ice. In those systems, the asteroids and comets would be dry. The most extreme carbon-rich stars, with much more carbon than our sun, are thought to create carbon-based planets, as depicted in this illustration. Those planets would lack oceans due to a lack of icy asteroids and comets serving as water reservoirs. Credit: NASA/JPL-Caltech
( —Planets rich in carbon, including so-called diamond planets, may lack oceans, according to NASA-funded theoretical research.

Our sun is a -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 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 reservoirs thought to supply planets with oceans.

"The 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 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 , 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, -less desert worlds."

The computer model results supporting these conclusions were published in the Astrophysical Journal last year. The implications for habitability in these systems were the focus of the Division of Planetary Sciences meeting.

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Journal information: Astrophysical Journal

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Nov 05, 2013
This seems really sketchy to me. They used a model which can only be tested by comparing it to our own solar system, so there's no way to really validate whether the model actually works the way it should under diverse conditions. Then they used the model to make predictions about a type of planet that may not exist.

There are a lot of unknown factors regarding exo-planetary systems at this point. I tend to take stories like the one above with a grain of salt because they are so speculative. We can almost guarantee that nearly every theory regarding exoplanets is either completely wrong, missing some major points, or still needs modification at the very least. If you change the ratio of carbon in the accretion disk, there's no telling what that does to the chemistry in the disk, or how it ends up getting distributed in the disk, and how that affects other chemicals.

If there was a way to validate the model, then okay, but we can't do that yet.

Nov 05, 2013
I thought all along that the chemistry of a world would be even more critical than its physics -- temperature, gravity, pressure, period of rotation, inclination, orbital eccentricity etc. Between physics and chemistry there is a lot that can go wrong so far as producing life goes and most estimates I've seen err on the side of optimism by considering only a subset of the possible problems.

Nov 05, 2013
and most estimates I've seen err on the side of optimism by considering only a subset of the possible problems

I agree. If you look at Earth as an example, which we think of as being in the habitable zone, it has really only been somewhat habitable throughout history. The atmosphere has changed several times, and don't forget the snowball Earth periods (not to mention ice ages, which were far less severe than the snowball Earth periods). We don't even know what causes that, so I think it is majorly misleading when they pretend to be able to predict what kinds of planets may or may not be habitable.

Nov 05, 2013
"there is a lot that can go wrong so far as producing life goes and most estimates I've seen err on the side of optimism by considering only a subset of the possible problems."

We don't know what can go wrong yet. We do know that life clades within geochemistry, and specifically hydrothermal vents, which even Mars has and Venus likely had before its, ahem, carbon demise (as a CO2 GW hothouse). Worlds that are not optimal, like Mars, would have much lower productivity of its biosphere, but reasonably not full extinction.

Also, it would be a very unnatural, finetuned process that results in one or even a few objects. More likely we should be optimistic, then when we find actual problems lower the estimates.

Not having water would be a huge problem. =D

Nov 05, 2013
That's a possibility, even a good probability, but I wouldn't bet the farm on its being the rule for planets around carbon rich stars. A lot of possibilities factor into this equation, of which the outcome described here is just one plausible one. Applying this idea to the theory of evolution of planet Earth, as this article does, is irresponsible, IMO.

Nov 06, 2013
Yeah, we haven't even confirmed the theory of carbon planets yet. The object they are simulating may not even exist. Based on our limited observations of proto-planetary accretion disks, it appears that the composition of the disk affects the distribution of materials in the disk and the chemistry that happens in it. We don't have any idea what types or sizes of planets might form in a disk with more carbon than our system. It might make very little difference, since the term "carbon rich" is only a relative term. The system is still dominated by hyrdrogen and helium, just like any other place, with just a bit more carbon. I don't see any reason to think a rocky planet would capture that much more carbon rather than silicon. The gas giants would have more carbon, certainly, but maybe not the rocky ones.

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