Astrobiology

Astrobiology is a discipline that seeks to identify habitats for life in the universe. Obviously extrasolar planets are a good place to look for life, and my work is relevant to the field of astrobiology. The large fraction of systems near instability (see my Stability Page) led to the Packed Planetary Systems (PPS) hypothesis that I developed along with Sean Raymond, Thomas Quinn and Richard Greenberg. This hypothesis predicts that any gap between planets that are wide enough to support an additional planet have planets in those orbits, and was summarized in an article by Steve Soter in American Scientist in September 2007: Are Planetary Systems Filled to Capacity? We have mapped out regions of known planetary systems that might contain an additional planet (Barnes & Raymond 2004; Raymond & Barnes 2005; Raymond, Barnes & Kaib 2006). In some of those cases, the proposed planet may lie in the habitable zone, i.e. H₂O would be in liquid form on the surface.

To demonstrate where planets may lie, we will focus on the system 55 Cnc. Fig. 1 shows results of where a saturn-mass planet could be stable for 100 million years. Such a planet is still below the detection limit of modern planet surveys.

Fig. 1 - The most likely location of a putative saturn-mass planet in the 55 Cnc system. The dashed lines designate the habitable zone. If a satellite like Europa was in orbit about a gas giant planet in that location, it would have a liquid water surface.

Sean and I, along with Nathan Kaib, found that 55 Cnc is an ideal place for a habitable planet (right mass, orbit and water content). In Fig. 2 we show results for 10 simulations of terrestrial planet formation in this system. The dashed lines are the habitable zone, and color corresponds to water content. About 30% of the simulations we ran produced habitable planets. But don't write your congressman about a mission to this planet just yet. 55 Cnc is about 41 light years away, so we won't be visiting any time soon.

Fig. 2 - Possible orbits and compositions of putative terrestrial planets in 55 Cnc. Orange and green planets (note the color of the Earth in the Solar System row) with horizontal lines inside the dashed vertical lines would probably be habitable (they'd have liquid water on the surface, if they had an atmosphere like the Earth's). Also note that a lot of planets form in the habitable zone, but without enough water initially. If a later source could douse these planets with water (like comets), they might become habitable.

This latter work received a lot of press. I was interviewed live by Ted Simons on KTAR radio, Phoenix. You can hear the whole interview here (for the record, 55 Cnc is in the constellation Cancer; the danger of live radio I guess).

Extrasolar planets are telling us a lot about how planets form, and how our Solar System fits into the Universe. Research like this can tell us where to find habitable planets and life, maybe even intelligent life!


Last Update: 1 Aug 2007