Fall 2014

Complex Planetary Systems: an astrobiologist attends a dynamics conference in Belgium

By Russell Deitrick
UW Astrobiology Graduate Student

Namur is a small city in southern, French-speaking Belgium, where the Sambre River joins with the Meuse. Like many European towns, Namur has a rich history that extends back to pre-Roman times and includes both world wars.

I came to Belgium for Complex Planetary Systems, an International Astronomy Union symposium, hosted by the Université de Namur. With my handful of French words and my smart phone (which, thankfully, had data service), I took the train from Brussels airport to Namur, wandered that city’s quiet streets admiring the architecture, braved strange hors d’ouevres, and dined with astronomers from all over the world—while fighting a 9-hour jet lag.

Talks kicked off with Jacques Laskar, an eminent dynamicist from Observatoire de Paris, who notably calculated the history of the planets in our Solar System and helped develop the theory of the Earth’s ice ages. Laskar reviewed some highlights of his work (spanning 30 years) and discussed chaos in the Solar System.

Tides
Several moons in the outer Solar System, including Europa and Titan, are believed to contain subsurface oceans, leading many to speculate on the possibilities for life there. These oceans are present because of tidal forcing, and Tim Van Hoolst, from the Royal Observatory of Belgium, stressed that accurate tidal models are needed to understand them. On the same note, Andy Richard, from Observatoire de Paris, whose tidal models include material properties like elasticity, pointed out the difficulty in conclusively determining whether Titan’s interior contains an ocean or not.

Exoplanets
While we are still several years away from measuring the atmospheric spectra of small, rocky exoplanets, we can ask if the dynamics preclude or affect habitability by altering the global climate. In this vein, Ramon Brasser, from the Institute for Astronomy and Astrophysics in Taipei, presented his study of the potential climate fluctuations on the planet HD 40307 g (now known to be a gaseous planet, though no one knew that at the time), finding that orbital variations could cause the planet to have temperature fluctuations 4 times greater than the Earth’s.

Another habitability issue is stability. Emeline Bolmont, now at Université de Namur, investigated the stability of two systems,Kepler-62 and Kepler-186, both of which have potentially Earth-like planets. She uses an N-body model that includes tidal and general relativity, finding that stability of these systems is sensitive to these effects and to the masses of the planets.

Speaking of Kepler—this space telescope uncovered a wealth of planetary systems that are packed in close to their host star and coplanar. Darin Ragozzine, at Florida Institute of Technology, is trying to understand coplanarity bias in Kepler systems. He pointed out that aliens observing our Solar System would see at most 3 planets in transit (the technique Kepler uses to detect planets), yet we see as many as 5 in some Kepler systems, suggesting that these are a fundamentally different class of system.

Formation scenarios
The formation of planetary systems is a difficult problem,computationally, because it involves processes on both large and small scales. Sebastien Charnoz, from Institut de Physique du Globe in Paris, gave a galvanizing lecture on this problem, which requires combining models of small scale processes such as friction/viscosity, with models of large things like planets and asteroids.

Alessandro Morbidelli, a top theorist in planetary formation and dynamics at Observatoire de la Cote d’Azur, showed how Earth’s impact history is best explained by a “Late Heavy Bombardment” scenario that occurred 500+ million years after formation, caused by the migration of Jupiter and Saturn. Billy Quarles, a scientist at NASA Ames, is using dynamical models to determine the mass and original location of Theia, a planetary embryo that collided with the Earth about 4.5 billion years ago and led to the formation of the Moon.

A UW alumnus, Sean Raymond, now at Laboratoire d’Astrophysique de Bordeaux, discussed strengths of weaknesses of the Grand Tack model. This formation scenario demonstrates that Mars’ small mass can also be explained by migration of Jupiter and Saturn.

As I left Belgium, exhausted and overwhelmed with information, I realized that the future had many possibilities for planetary dynamics and astrobiology. I began to see unsolved problems as opportunities, rather than roadblocks. That alone, I think, is worth a trip across the Atlantic.

Photos: (Top) Namur’s Citadel overlooks the confluence of the Sambre and Meuse rivers. (Middle) Belgium’s weather, not unlike Seattle’s, makes good beer a necessity. (Bottom) Houses along the Sambre river in Namur. 

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