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Tuning Darwin’s Target List

Barrie Jones (The Open University, UK) will have an interesting job in ESA’s planned Darwin mission. Darwin’s goal is to find Earth-like worlds around other stars. Jones will prioritize planetary systems so the mission’s telescopes — three instruments mounted on separate spacecraft — can concentrate on those most likely to have Earth-like planets in their habitable zones. He’s already done computer modeling on many currently known planets, finding that half of them occur in systems where a stable orbit for a terrestrial world exists in the habitable zone.

But much depends on how you deal with planetary migration, says Jones, discussing his computer modeling in a recent interview. A gas giant moving through the inner system to become a ‘hot Jupiter’ may or may not preclude the presence of an Earth-like planet — recent work gives us hope that it will not, but the issue is still undecided. Says Jones:

If Earths can form after the giants migrate toward the star and become “Hot Jupiters,” then about 50 percent of the planetary systems could have Earths in the habitable zone. These Earths wouldn’t have been ejected subsequently by the giant’s gravity. However, if we rule out the possibility of forming Earths after migration of the giants across the habitable zones, then we end up with 16 or 17 percent. There’s a huge drop because so many of the systems discovered to date have Hot Jupiters! So understanding whether terrestrial planets can form after migration is important.

Darwin will survey about a thousand nearby stars in its hunt for rocky planets, working in the mid-infrared, where the brightness contrast between star and planet drops from a thousand million to one to a more manageable (but still extraordinarily difficult) million to one. The method will be nulling interferometry, combining the light from the separate 3-4 meter space telescopes and canceling out the light of the central star. And Jones will have plenty of time to refine his techniques — Darwin’s technology is very much in development, its launch date 2015 or later.

Meanwhile, Leonard David reports from a media workshop in Boulder sponsored by the University of Colorado’s Center for Astrobiology. The debate about how to find planets around other stars gets more intense as we move to the terrestrial planet level. And there’s an interesting riff in Leonard’s article on the interplay of theory and observation, one caught classically by Jack Lissauer: “The standard theorist line is never believe in an observation unless it has been confirmed theoretically. I’m trained as a theorist…but I’m interested in observations.”

Lissauer, a familiar name in these pages, works at NASA Ames and, among other accomplishments, has done key work with Elisa Quintana on the formation of planets in binary systems. And he’s right in finding a synergy here. Researchers have to be constrained by observation, but theorists have to come up with good ideas about where to point the telescope. Which is just what Barrie Jones is doing with Darwin, and Margaret Turnbull (Space Telescope Science Institute) with HabCat, a catalog of nearby stars thought amenable to Earth-like worlds.

We’re on the edge of breakthrough. With the instrumentation now in the pipeline, we should be finding planets like ours over the next ten years, many of them perhaps in surprising places. And even before we get a Kepler or a Darwin into space, there is the real possibility of finding terrestrial worlds via transit methods around red dwarfs. We caught a glimpse of that possibility yesterday with GEMSS. Both theorists and observers are going to have more work than they can handle, and soon.

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