David J. Stevenson, who is George Van Osdol Professor of Planetary Science at CalTech, has an entertaining way with titles. The average scientific paper has a title whose tone is dry, direct and frequently off-putting. Stevenson gives us these, as any scientist must, but give him the chance and he produces the Swiftian “A Modest Proposal: Mission to Earth’s Core,” which ran in Nature in 2003 (available here). He has also written, fascinatingly, on the possibilities of oceans on worlds other than Europa; thus his 1999 essay “An Ocean in Callisto?” (The Planetary Report Vol. 19 No 3, pp. 7-11), and “An Ocean in Uranus?” (The Planetary Report Vol. 6 No 16, 1986).
At the recent American Geophysical Union meeting, Stevenson produced the ultimate in what we writers call the ‘omniscient viewpoint’ by presenting “How to Build a Planetary System.”
So how do you build a planetary system? It turns out that we know surprisingly few answers. Stevenson argues that while such systems are common (at least based on current observational data), the question of Earthlike planets is still unresolved. “In all likelihood, the diversity of outcomes is large and non-deterministic and our particular outcome is correspondingly uncommon despite the abundance of systems,” Stevenson writes in the abstract to his AGU presentation.
So don’t expect the average solar system to look like ours. What we do believe at this juncture is that the formation of a planetary disk provides the raw materials for planet building, and seems unavoidable. But the aggregation of this matter into small planetesimals is poorly understood. If they do form, they should produce planetary bodies, but even so, the subsequent formation of gas giants is mysterious, though it is clear they are common. So given the gaps in our knowledge, we should expect to find more and more extrasolar systems that diverge widely from our Solar System.
Stevenson’s interest in oddball planet formation was evident in his 1999 paper on interstellar planets in Nature called “Possibility of Life-Sustaining Planets in Interstellar Space.” Here he argues that planets capable of sustaining life may exist far from any star; their presence will be correspondingly hard to detect. From the paper:
We thus see that bodies with water oceans are possible in interstellar space. The “just right” conditions are plausibly at an earth mass or slightly less, fortuitously similar to the expected masses of ejected embryos during giant planet formation. For a 50/50 ice-rock body, the ocean is very deep and may be underlain by high pressure phases of water ice with a rock core at still greater depths, but bodies with earthlike water reservoirs may have an ocean underlain with a rock core. Either way, these bodies are expected to have volcanism in the rocky component and a dynamo-generated magnetic field leading to a well-developed (very large) magnetosphere. Despite thermal radiation at microwave frequencies that corresponds to the temperatures deep within their atmospheres (analogous to Uranus) and despite the possibility of non-thermal radio emission, they will be very difficult to detect. If, as many have suggested, life can develop and be sustained without sunlight (but with other energy sources, plausibly volcanism or lightning in this instance) then these bodies may provide a long-lived stable environment for that life (albeit one where the temperatures slowly decline on a billion year timescale). It is even conceivable that these are the most common sites of life in the Universe.
If that doesn’t give science fiction writers something to work with, what does? The complete text of this Nature paper can be found here (PDF warning). You’ll find some descriptions of Stevenson’s recent research, including more on the possible Callisto ocean, at his Caltech page, which also includes a complete list of his publications.