March 19, 2008 | David F. Coppedge

Planet Formation: Just Add Water?

The Spitzer Space Telescope found evidence of water in a dust disk around a star.  Does this mean we understand how the earth, with all its water, formed?
    Using the Spitzer infrared instrumentation, John Carr (Naval Research Laboratory) and Joan Najita (National Optical Astronomy Observatory) found spectra of organic molecules and water in the planet-forming region of star AA Tauri.  Reporting in Science,1 they said this extends the water-bearing region out to 3 AU (astronomical units), previously observed only one-tenth that range (0.3 AU).  Earth, by definition, is 1 AU from our sun.  The presence of water is important not only for oceans but for the oxidizing state of the gas and minerals that might make up a rocky planet.
    Commenting on this paper in the same issue of Science,2 Fred Ciesla of the Carnegie Institution of Washington was excited.  His excitement centered not so much on finding answers as much as sorting out possibilities.  Chondrites (meteoritic material) support a wide range of isotopic ratios.  This means they are not much help defining the conditions in planet-forming regions.  “Although agreements of this type between the models and the chondrites hint that we are beginning to understand how our solar system formed, they are far from definitive,” he said.  “Alternate models have been proposed and have equal success in explaining the properties of chondritic materials.”
    The debate remains unsettled, even though it is important: “Identifying which of these models is correct is critical to furthering our understanding of how planetary systems form because it has implications on other issues, ranging from the origin of cometary grains to the manner by which giant planets form.”
    So does the news about water at AA Tauri settle the question?  Not exactly.  Dust disks around other stars show a wide range of available water vapor, from wet to dry: “Interestingly, water appears to be depleted in SVS 13 relative to what is predicted in stagnant disk models,” he said.
    Does the degree of observed variation confuse models of planetary evolution or constrain them?  It depends on one’s optimism, imagination, and creativity:

To date, these observations do not distinguish which of the models developed for our solar nebula is correct but rather lend support to recent models for the dynamic evolution of water and other volatiles in protoplanetary disks.  However, as the techniques used by Carr et al. are applied to other disks, correlations between their chemical compositions and their physical properties can be identified.  Models for water evolution predict that the enhancement of water in inner disks should be followed by periods of depletions, so systematic variations with age are expected.  Also, larger disks would provide more water ice to drift inward and thus would produce greater enhancements in the inner disk.  Searching for such correlations will thus allow us to test models developed for our own solar nebula and determine whether it evolved in a similar way as other disks in our galaxy or if, instead, our planetary system is the result of one or multiple unique circumstances.  Right now, these new results, combined with the discovery of high temperature grains in comets and in the outer regions of protoplanetary disks, suggest that the manner by which our solar system formed may have been the rule.

The presence of high-temperature grains in comets, he didn’t mention, was a complete surprise (01/25/2008, bullet 1, and 12/27/2007).


1.  John S. Carr and John R. Najita, “Organic Molecules and Water in the Planet Formation Region of Young Circumstellar Disks,” Science, 14 March 2008: Vol. 319. no. 5869, pp. 1504-1506, DOI: 10.1126/science.1153807.
2.  Fred Ciesla, “Planetary Science: Observing Our Origins,” Science, 4 March 2008: Vol. 319. no. 5869, pp. 1488-1489, DOI: 10.1126/science.1155858.

May, might, would, could, should – perhaps water content goes up and down; maybe planets form by core accretion and then again, maybe they don’t; maybe earth formed by one (or more) unique circumstances or maybe planet formation is common, maybe there is a rule here and maybe not – but we are getting warmer!  “Observing our origins,” Ciesla calls this.  Any number of models can be “developed” that fit the same observations or any new ones that come along.  There are enough holes in this line of reasoning to drive a starship freight transport fleet through.  Ciesla has a case of ingrown eyeballs.  The only thing he is observing is his imagination, projected on the back of his skull.

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