I remember wondering, while still getting acclimated to the odd existence of ‘hot Jupiters’ in those amazing first years of exoplanet discovery, what the view from a terrestrial world in one of those systems might be like. After all, a Jupiter-sized mass in close solar orbit must make for some unusual visual effects. Do terrestrial worlds exist around these stars? For that matter, what are the constraints on terrestrial planet formation in systems where gas giants orbit farther out, well past the habitable zone?
These questions are occasioned by the work of Sean Raymond (University of Colorado), whose paper on the subject will soon run in the Astrophysical Journal Letters. Raymond looks at how the presence of gas giants would affect the late stages of terrestrial world formation and presents the results of his simulations on same. Bear this in mind: gas giants, it is now thought, must form within the first few million years of the early protoplanetary disk. Whereas terrestrial worlds take tens of millions of years to finally assemble themselves. Obviously, the two engage in a powerful gravitational dance.
Let’s look at outer gas giants first. Raymond’s simulations say that putting them inside 2.5 AU inhibits the growth of small rocky worlds (0.3 Earth-mass and above) in the habitable zone of Sun-like stars and also prevents the appearance of water-rich habitable planets. In some cases, water-planet formation can only occur when the gas giant is beyond 3.5 AU. That leaves us Sol system dwellers in the clear with Jupiter at 5 AU, and at first blush it puts the brakes on all those hot Jupiter scenarios I was imagining. This is just for Jupiter-mass planets, mind you; more massive planets create even stronger perturbations.
Most exoplanets detected are, of course, in close orbits around their star. Raymond finds that only seven out of 153 planetary systems he worked with meet the criteria for small terrestrial worlds, while only two allow for water-planet formation in the habitable zone.
But here’s the good news: inner system gas giants may not totally rule out terrestrial worlds. In earlier work, Raymond determined that low-eccentricity gas giants inside 0.5 AU might permit habitable planets to form outside their tight orbits. The problem is, most of the detected gas giants display significant orbital eccentricity. Those in orbits closest to their stars (within 0.1 AU) have the lowest eccentricity; those beyond 0.15 AU show an average eccentricity of 0.32.
How you juggle eccentricity thus tells the story. As Raymond writes: “…if we arbitrarily assume that habitable planets can form in systems with giant planets interior to 0.5 AU with eccentricities less than 0.1…then the number of known extra solar systems that could harbor habitable planets increases to 45 (29%).” And my imagined view from the terrestrial world of such a system takes on life once again, though that gas giant will practically be hugging its star as it whips around the stellar disk.
And ponder this: Some recent studies suggest that planetary systems with habitable worlds need not contain gas giants at all. On that one, we have all too little information and must await the findings of future missions like Terrestrial Planet Finder, whose budgetary woes have allowed for serious reassessment to determine the best technologies for the job. All too often we’re shooting in the dark and extracting data on a very long thread — we need hardware in space to build our datasets, but making sure it’s the right hardware (and more on this soon) will pay off even if the continuing delays are frustrating.
The odds for habitable planets seem to have worsened with these simulations. Perhaps the key to the Fermi Paradox is that planets worth journeying between the stars for are too few and far between?
Or perhaps, my personal option, is that if habitable planets really are rare then intelligent species will be forced to learn to live in ‘free space’ using asteroids and comets, or burrowing in on ice-moons around the Gas Giants. The star-travelling part of a species might then prefer free space living and avoid and/or preserve habitable planets.
Greg Matloff suggests so and they, the ETIs, might be in our own Solar System, but in the Kuiper Belt. Or, at least, their traces and probes.
As you say we need a lot more data. We’ve not enough information on planet frequency around red dwarfs and the habitability of such. Nor do we yet know how frequent long-period Jovians really are.
Adam