We may not have images of terrestrial planets around another star yet, but many things can be learned about such worlds by computer simulation. A team of British astronomers, for example, has examined known exoplanetary systems in hopes of isolating those in which Earth-like worlds could exist in stable and habitable orbits. This is tricky business, because the massive planets present in almost every exoplanetary system we know about could disrupt such orbits long before life might have a chance to form on any worlds there.
It’s also tricky because to determine which systems could have life-bearing planets requires you to figure out the location of the habitable zone in each. Researchers Barrie Jones, Nick Sleep and David Underwood (Open University, Milton Keynes, UK) here use the classical definition of habitable zone: the distances from a star where water at the surface of an Earth-like planet would be in liquid form. Not surprisingly, they find that the question of planetary migration looms large in their analysis.
If a gas giant orbits well inside the habitable zone around a given star, and if that planet has reached its position by migration through the habitable zone, then Earth-like worlds may be far less common than would otherwise be the case. Here’s a good precis of research on the migration question, as presented in the UK team’s discussion of why habitable planets, depending upon the effects of migration, might be found in a mere 7 percent of the systems surveyed. From the paper:
The decrease to 7% demonstrates the importance of understanding how readily or rarely at least one ‘Earth’ can form in the HZ after a giant planet has migrated through it. This urgent question has received some attention. Formation in 47 Ursae Majoris has been examined by Laughlin et al. (2002). They have shown that Earth-mass planets could form within about 0.7 AU of the star, which is interior to the HZ, and possibly a bit further out in the inner HZ. It is the proximity of the inner giant planet to the HZs that hinders formation, by stirring up the orbits of the planetesimals and planetary embryos. Armitage (2003) concluded that post-migration formation of ‘Earths’ might be unlikely, though he concentrated on the effect of giant migration on planet-forming dust rather than on planetesimals and planetary embryos. On the other hand, Mandell and Sigurdsson (2003) have shown that when the HZ is traversed by a giant planet, a significant fraction of any pre-formed terrestrial planets could survive, eventually returning to circular orbits fairly close to their original
positions. An optimistic outcome has also been obtained by Fogg and Nelson (2005), who have shown that post-migration formation of ‘Earth’ from planetesimals and planetary embryos is fairly likely. Fogg and Nelson’s work is the most comprehensive to date, and gives cause for optimism…
That the migration issue is the hinge of this study is shown in the authors’ summary. They find that of the 152 known exoplanetary systems (as of 18 April 2006), 60 percent offer safe habitable zone orbits for Earth-like planets. A second analysis of 143 of these systems shows that 50 percent would have provided sustainable orbits in the habitable zone for at least a billion years. So the question of how giant planets got closer to their stars than the habitable zone becomes crucial. And if migration through the habitable zone rules out the formation of Earth-like worlds, we are left with that discouraging 7 percent number instead of the much more robust 60 percent.
Centauri Dreams‘ take: The effects of migration will merit much future work, but may become somewhat less pressing if we find that ‘hot Jupiters’ are the exception rather than the rule. Right now the observational bias is built into our methods; massive planets close to their stars are more readily detectible. We have yet to establish any sort of workable ‘norms’ for solar system formation against which to measure such systems, but migration may turn out to be of less interest if we start routinely identifying systems where the gas giants are found well outside the habitable zone.
The paper is Barrie, Sleep and Underwood, “Habitability of known exoplanetary systems based on measured stellar properties,” now accepted for publication in The Astrophysical Journal and available here.
Interesting. It seems the membership system has been dumped in favor of open posting. Is this correct? Did I miss the anouncement?
Anyway, I long for the day when Earth-like worlds are found around (hopefully) nearby stars. What an adventure of discovery we are soon to embark upon! …even if we can’t go there directly… yet.
Let’s hope we all live to see it happen. Then we’ll know… then we’ll know we were right to dream.
Eric, yes, I had to make the switch to open posting because some users found they couldn’t register. It’s a known bug in the WordPress software, and the only way around it seemed to be to open the posting up. So far this has worked.
I certainly share your sentiments about terrestrial worlds!
I know this is mostly a repeat of one of my previous posts, but if we rewrite our definition of habitable zone to include moons that support liquid water because of tidal forces from their gas giant parents, then we don’t really see the chances of extra-solar life diminish quite as dramatically when we consider issues like this.
– A
Yes, a good point, although there would be a significant difference between a gas giant in the habitable zone (with quite interesting moons though perhaps dangerous radiation levels) and a gas giant that migrates quickly through the HZ. But isn’t the idea of life around a gas giant fascinating, and who knows just what we’ll discover as we start firming up these parameters..
It’s my understanding that the magnetosphere around Jupiter is so large that if visible, it’d appear in the sky around the same size as the moon. Are the moons of Jupiter radiation protected by this magnetosphere?
Jupiter’s magnetosphere enhances the flux of high energy particles experienced by the inner Galileans, so it isn’t protective. However Io and Ganymede may have had their fields enhanced or activated by being in Jupiter’s, so it’s not all bad.
Well then, might it be possible then that gas giants in other solar systems might likewise benefit their moons. Wouldn’t this be real intersting to model in some of the recently observed sytems with gas giants in the habitable zone?
arXiv:0708.1771
Date: Mon, 13 Aug 2007 20:42:53 GMT (222kb)
Title: Specific Angular Momentum of Extrasolar Planetary Systems
Authors: John C. Armstrong, Shane L. Larson and Rhett R. Zollinger
Categories: astro-ph
Comments: 7 Pages, 3 Figures, Submitted to ApJ Letters
Angular momentum in our solar system is largely distributed
between the Sun’s rotation and the planetary orbits, with most of it
residing in the orbital angular momentum of Jupiter. By treating
the solar system as a two body central potential between the Sun
and Jupiter, one can show that the orbital specific angular
momentum of the two-body system exceeds the solar rotational
specific angular momentum by nearly two orders of magnitude.
We extend this analysis to the known extrasolar planets available in
the Extrasolar Planet Encyclopedia and estimate the partitioning of each
system’s angular momentum into orbital and rotational components,
ignoring the spin angular momentum of the planets.
We find the range of partitioning of specific angular momentum in
these systems to be large, with some systems near the stellar
rotational limit, and others with orbital specific angular momentum
exceeding this limit by three orders of magnitude. Planets in systems
with high specific angular momentum have masses greater than
two Jupiter masses, while those in systems with low specific
angular momentum are below two Jupiter masses.
This leads to the conclusion that low mass planets lose angular
momentum more efficiently, and are thus more prone to migration,
than larger mass planets.
http://arxiv.org/abs/0708.1771 , 222kb
arXiv:0708.2875
Date: Tue, 21 Aug 2007 17:06:17 GMT (150kb)
Title: Surfing on the Edge: Chaos vs. Near-Integrability in the
System of Jovian Planets
Authors: Wayne B. Hayes
Categories: astro-ph
Comments: 35 pages, 9 figures, 2 tables. Submitted to AJ
We demonstrate that the system of Jovian planets(Sun+Jupiter+ Saturn+Uranus+Neptune), integrated for 200 million years as an
isolated 5-body system using many sets of initial conditions all
within the uncertainty bounds of their currently known positions,
can display both chaos and near-integrability.
The conclusion is consistent across four different integrators,
including several comparisons against integrations utilizing
quadruple precision. We demonstrate that the Wisdom-Holman
symplectic map using simple symplectic correctors as
implemented in Mercury 6.2 (Chambers 1999) gives a reliable characterization of the existence of chaos for a particular
initial condition only with timesteps less than about 10 days,
corresponding to about 400 steps per orbit.
We also integrate the canonical DE405 initial condition out to
5 Gy, and show that it has a Lyapunov Time of 200–400 My,
opening the remote possibility of accurate prediction of the
Jovian planetary positions for 5 Gy.
http://arxiv.org/abs/0708.2875 , 150kb
Numerical simulations of type III planetary migration: III. Outward migration of massive planets
Authors: A. Peplinski (1), P. Artymowicz (2), G. Mellema (1) ((1) Stockholm Observatory, (2) Univ. of Toronto at Scarborough)
(Submitted on 18 Feb 2008)
Abstract: We present a numerical study of rapid, so called type III migration for Jupiter-sized planets embedded in a protoplanetary disc. We limit ourselves to the case of outward migration, and study in detail its evolution and physics, concentrating on the structure of the co-rotation and circumplanetary regions, and processes for stopping migration. We also consider the dependence of the migration behaviour on several key parameters.
We perform this study using global, two-dimensional hydrodynamical simulations with adaptive mesh refinement. We find that the outward directed type III migration can be started if the initial conditions support $Z greater than 1$, that corresponds to initial value $M_\rmn{\Delta} \ga 1.5$. Unlike the inward directed migration, in the outward migration the migration rate increases due to the growing of the volume of the co-orbital region.
We find the migration to be strongly dependent on the rate of the mass accumulation in the circumplanetary disc, leading to two possible regimes of migration, fast and slow. The structure of the co-orbital region and the stopping mechanism differ between these two regimes.
Comments: 18 pages, 13 figures, submitted to MNRAS
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0802.2501v1 [astro-ph]
Submission history
From: Garrelt Mellema [view email]
[v1] Mon, 18 Feb 2008 15:21:27 GMT (2252kb)
http://arxiv.org/abs/0802.2501