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A New Duo of Exoplanet Questions

Yesterday’s successful launch of a SpaceX Falcon, and the subsequent safe return of the Dragon spacecraft after a three hour ride, puts an exclamation point on Dana Andrews’ paper on space and commercial viability, which was discussed here yesterday. We’re a long way from a sustainable space infrastructure — many reports note the fact that what SpaceX did yesterday roughly parallels where humans were in space about fifty years ago, with the early Soviet and American flights — but we are seeing the most promising signs yet of a viable launch business emerging from the commercial sector, with all that implies about eventual use of space resources and future colonization.

Exoplanetary Puzzles

Now we wait for news from NanoSail-D, whose sail deployment should, if my sources are right, be today, but the @NanoSailD Twitter feed has grown quiet. We’ll think good thoughts and, while waiting, move on to the exoplanet hunt, the latest news from which is the discovery that the planet WASP-12b, found last year by the Wide-Angle Search for Planets project, has an atmosphere dominated by molecules containing carbon. That’s a sharply different scenario from our Earth, which contains much smaller amounts of carbon and a great deal more oxygen.

The find makes for interesting speculation about other planets that might exist in the same system. Although WASP-12b is a ‘hot Jupiter,’ orbiting its star in a mere 26 hours, it’s possible that smaller, rocky worlds could form around the star, which is about 1200 light years away. Just what such worlds might be like is a question that intrigues Joe Harrington (University of Central Florida), who analyzed the Spitzer data for the study:

“A rocky planet in such a planetary system could have an interior abundant in diamonds and a surface littered with graphite and diamonds. The theorists will have fun with this one. Could life thrive in such an environment, with little oxygen or water? That might not be so far-fetched given last week’s announcement by NASA of bacteria that can survive by using arsenic in place of phosphorus, previously thought to be essential.”

A brief pause to note that the NASA work on arsenic is taking flak from many quarters (see the comments on Centauri Dreams‘ recent story on the announcement), leaving significant doubts about its credibility. The broader principle — that life may surprise us, though perhaps not in this case — is obviously still in play.

It took an analysis of WASP-12b’s infrared spectrum through Spitzer Space Telescope data to flag the significance of carbon, revealing that the world is the first planet found where the oxygen/carbon ratio is reversed over what we see in our own Solar System. Our Sun’s carbon-to-oxygen ratio is about one to two (half as much carbon as oxygen), and at least among the inner planets, none have been found to have more carbon than oxygen (the ratio for the outer planets has not yet been established). WASP-12b is the first planet to have a carbon-to-oxygen ratio greater than one measured, a significant change from Earth normal.

“When the relative amount of carbon gets that high, it’s as though you flip a switch, and everything changes,” said Marc Kuchner, an astronomer at NASA Goddard Space Flight Center, Greenbelt, Md., who helped develop the theory of carbon-rich rocky planets but is not associated with the study. “If something like this had happened on Earth, your expensive engagement ring would be made of glass, which would be rare, and the mountains would all be made of diamonds.”

A Riddle Much Closer to Home

Not that WASP-12b wasn’t unusual to begin with. It’s close enough to its star that gravity has elongated it, pulling mass off the planetary atmosphere into a thin disk that orbits with the planet. Temperatures in the range of 2600 Kelvin (2325 degrees Celsius) make it one of the hottest exoplanets yet discovered. But the next step in the WASP-12b story may take place in a colder place. For to understand what’s happening there, we need to know whether planets like Jupiter have a carbon/oxygen ratio we can measure. It’s tricky to make that reading because much of Jupiter’s oxygen is trapped in water and thus not available for spectroscopic observation because it has condensed out of the atmosphere due to the cold at Jupiter’s orbital distance.

All eyes thus turn to Juno, a Jupiter mission that will launch in 2011 and reach the giant planet five years later, where it will map water and oxygen to determine whether Jupiter is as carbon-rich as WASP-12b. Getting the answer for Jupiter will tell us just how unusual WASP-12b really is, as this background story in Nature makes clear. The paper is Madhusudhan et al., “A high C/O ratio and weak thermal inversion in the atmosphere of exoplanet WASP-12b,” published online by Nature 8 December 2010 (abstract).

A Challenge to Formation Theories

Be aware as well of the interesting gas giant found around HR 8799. HR 8799e is one of four gas giants in this system, orbiting its star at 14.5 AU and in the process distinguishing itself from its more distant brethren (at 24, 38 and 68 AU respectively). Gravitational instability can be invoked to explain the formation of the three outer gas giants, but HR 8799e is in too warm a position at 14.5 AU to make it likely, leaving us with core accretion as the formation model.

But core accretion doesn’t fit the outer worlds, the process taking so long to complete that solid planetary cores would not have had sufficient gas from the dissipating protoplanetary disk to form a gas giant. Different formation models in the same system? It seems unlikely, because the masses of these planets are similar and they’re showing clear signs of orbital resonance, leading Christian Marois (Herzberg Institute of Astrophysics) to suggest the planets got where they are today through orbital migration. HR 8799, some 129 light years from Earth, is going to preoccupy planetary formation theorists for some time to come. The paper is Marois et al., “Images of a fourth planet orbiting HR 8799,” published online in Nature 8 December 2010 (abstract).


Comments on this entry are closed.

  • andy December 9, 2010, 14:17

    For those without a Nature subscription, the paper was also posted to the arXiv and can be obtained here. Would be interesting to know what the star’s C/O ratio is for comparison.

  • JohnHunt December 10, 2010, 3:04

    IMO, we are seeing low-cost access to orbit in the form of SpaceXs Falcon 9 and Falcon 9 Heavy. We’d love to see space access be even cheaper but $78 million for 32,000 kg is cheap enough to begin a bootstrapping process which itself will dramatically reduce the mass which needs to be transported to LEO by providing orbiting fuel from lunar water ice. This reduced mass will lower the cost of space development as much as cheap access to space will. This PowerPoint from the Colorado School of Mines makes this point quite well.

    The first off-Earth resource to be exploited will be lunar polar ice for propellant. There will be a bootstrap succession of increasing supplies which first will be briefly used to facilitate transport of more equipment to the lunar poles.

    NASA will probably be the major first customer in very much the way that COTS/CCDev has been very successfully done. SpaceX and others will receive funding when they achieve milestones and will have a guaranteed initial market (i.e. NASA) for delivering a certain quantity of L1 and LEO fuel.

    Eventually the amount of Lunar Ice to LEO (LITL) will increase to quantities sufficient to fuel orbital transfer vehicles which will be able to service the LEO to GEO market. At the same time, orbital servicing will completely transform how satellites are built.

    NASA will purchase ongoing quantities to fuel Earth Departure Stages for rather large planetary missions and for a cost-effective and hence a vigorous manned Mars mission.

    At this point, we will have the ability to begin making more plausible arguments for developing a lunar-based infrastructure which could result in the sort of beamed energy systems that interests those of us who would like to see interplanetary missions.

    Lunar teleoperations will be a routine part of lunar mining operations and so the idea of robotically constructing rectenna on the moon won’t be so far fetched. Existing power generation facilities on Earth already produce sufficient terrawatts for a decent sized interstellar probe if the microwave transmitters could be constructed on Earth. Unused night time power production capacity of these power plants is considerable.

    So, I see what’s happening with SpaceX as being a turning point for us.

    The depletion of fossil fuels will simply drive up their costs until nuclear, wind, solar, and biofuels become competitive. Easily within a hundred years, fusion will be a viable option as well. And yes, with lunar operations, space-based power sats will provide ever increasing amount of power.

    I would also agree that species survival could be a compelling argument but not due to resource depletion so much as because of the need to establish a self-sustaining off-Earth colony in case someone invents a self-replicating existential threat.

  • andy December 10, 2010, 19:38

    Also have to wonder about how this result ties in with the observations of high levels of carbon in the Beta Pictoris disc. May well be the case that gas giants are typically carbon-rich.

  • bigdan201 December 10, 2010, 20:54

    The SpaceX developments are very promising. It’s nice to see the commercial sector being seriously involved in space.

    It’s likely that we’ll find exoplanets with a different chemical makeup. I’m looking forward to the variety of terrestrial worlds – maybe we’ll find carbon planets, the wiki of which I linked, or something else.

  • Eniac December 11, 2010, 1:00

    The diamond quote does not make sense to me. If there is insufficient oxygen, carbon will bond with silicon to form silicon carbide instead of diamond. There would have to be an excess of carbon over both oxygen and silicon combined for there to be “mountains of diamond” on a rocky planet. That does not seem likely to me…

    Besides, an “atmosphere dominated by molecules containing carbon” is hardly special, both Mars and Venus have such atmospheres, and I believe the Earth’s does not only because of the presence of life.

    Lastly, I do not understand how it can be that we have to wait for Juno to find out something about Jupiter that we have already been able to find for an exoplanet. Can’t we take better spectra of Jupiter than of an exoplanet without sending a probe?

  • andy December 11, 2010, 11:38

    Eniac: regarding the question of diamond on such planets, one question is what happens in the high-pressure, high-temperature regions in the planet’s interior, and how much material from the interior is expected to be brought to the surface via volcanic processes. IIRC the “crust” of a terrestrial-size carbon planet is indeed expected to be composed of carbides, with deeper layers of the planet containing various allotropic forms of carbon.

    Regarding why we don’t know the C/O ratio for Jupiter, as I understand it the problem is that Jupiter is a cold planet, so various molecules tend to condense out of the atmosphere. This includes water, which is probably the dominant oxygen-containing molecule, thus the spectrum of the upper atmosphere does not provide a good representation of the carbon-to-oxygen content of the planet as a whole. WASP-12b on the other hand is a very hot planet, so the dominant oxygen and carbon-containing species should remain in the atmosphere.

  • Eniac December 14, 2010, 2:53

    Thanks, Andy, that makes sense, on the C/O ratio for Jupiter.

    If by a “carbon planet” one means a planet containing stochiometric excess of carbon (more carbon than 2 times oxygen and silicon combined), you are right. Otherwise, the planet will be made up of carbides and silicates with a CO2 atmosphere, I am pretty sure.

    Carbon will preferentially bind oxygen. If there is no more oxygen left, it will bind silicon. Any remaining silicon will be metallic. If we run out of silicon, too, only then will there be elemental carbon. Things might be different in the interior, but my feeling is not different enough to change the basic stochiometry.

    On Venus, Mars, and Earth, carbon is in the minority, thus we find neither carbides nor elemental carbon, only CO2 (except on Earth, where photosynthesis disrupts the chemical balance).