Hi All

Many exoplanet searches have found "hot Jovians" - Jupiter-like planets that may have migrated in from outer orbits. About 5% of sun-like stars have them and they might be common about red-dwarfs - if anyone could be bothered looking. Most workers in the science assume that migration will perturb any inner planets so utterly that they will be either ejected into interstellar space or at least into outer-planet orbits. However a new article actually looked at simulations of what really happens...

http://au.arxiv.org/abs/astro-ph/0307512

Survival of Terrestrial Planets in the Habitable Zone in the Presence of Jovian Migration
Authors: Avi M. Mandell, Steinn Sigurdsson (Penn State University)
Comments: 7 pages, 3 figures, submitted to ApJL

The presence of ``Hot Jupiters'', Jovian mass planets with very short orbital periods orbiting nearby main sequence stars, has been proposed to be primarily due to the orbital migration of planets formed in orbits initially much further from the parent star. The migration of giant planets would have profound effects on the evolution of inner terrestrial planets in these systems, and previous analyses have assumed that no terrestrial planets survive after migration has occurred. We present numerical simulations showing that a significant fraction of terrestrial planets could survive the migration process, eventually returning to circular orbits relatively close to their original positions. A fraction of the final orbits are in the Habitable Zone, suggesting that planetary systems with close-in giant planets are viable targets for searches for Earth-like habitable planets around other stars.

...hence the outcome for inner planets is not nearly so dire as previously believed.

Contrary to my former opinions red-dwarfs aren't such bad targets for exo-life searches either. Firstly a planet might retain a liquid water ocean further out from its star than in our Earth's case because (a) more infra-red and (b) super-greenhouse gases methane and ammonia last longer because there's a LOT less UV. Hence liquid-water ocean planets might escape glaciation and avoid the tidal-lock radius. While there's less UV there's still enough visible light for decent photosynthesis.

But even if tidal lock happens along it's not the end of the atmosphere as previously believed. Global weather simulations show that even a fraction of a bar is enough to keep heat flow going between the hot and cold hemispheres. Hence no freeze out, and there's still the oceanic conveyor-belt to keep things going too.

So if red-dwarfs are nicer places to be and are 20 times more common than stars like the Sun or hotter, then ETIs have more places to call home. So why aren't they calling us???

Adam