An interesting specialty in exoplanetary science is the simulation of planetary orbits. It’s intriguing, for example, to place a hypothetical terrestrial planet into a system with known giant planets to see what happens. After all, we know that many exoplanetary systems contain potentially stable orbits for such planets; in fact, one-fourth of known systems can support a planet in their habitable zone. And while we don’t know yet whether such worlds exist, we can draw useful conclusions from modeling their orbits if they are there.
Such are the premises of a new paper by Sean Raymond (University of Washington, Seattle) and Rory Barnes (Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder). The two simulate terrestrial planets in four systems: 55 Cancri, HD 38529, HD 37124, and HD 74156. And here is a key issue: most planets we’ve found so far are ‘hot Jupiters,’ in tight, close orbits around their primary. For a terrestrial planet to co-exist with such worlds, it must either form quickly and survive the inward migration of the gas giant (which presumably formed farther out in the protoplanetary disk) or it must form from material remaining after the passage of the gas giant as it moves inward. This would imply an early migration of the gas giant toward its star.
The first scenario is dubious in the extreme, with only one to four percent of terrestrial planets remaining in orbits within the habitable zone. But if the gas giant forms quickly enough, a new generation of planetesimals may form after it does. Raymond and Barnes work through such scenarios and make comparisons with earlier simulations, placing moon to Mars-sized planetary ’embryos’ between the giant planets and letting them evolve over time.
The result: Planets of up to 0.6 Earth masses, some with substantial water content, form relatively easy around 55 Cancri, whereas HD 38529 may support an asteroid belt but no terrestrial worlds. And no terrestrial planets form at all around HD 37124 and HD 74156. About 55 Cancri, the authors have this to say:
Our simulations of 55 Cancri suggest that a potentially habitable planet could form in situ. Such a planet would have a small enough eccentricity to remain in the habitable zone throughout its orbit, substantial mass and water content. However, as shown in Paper II, a Saturn-mass planet could exist on a stable orbit in the habitable zone of 55 Cnc. Such a planet may preclude the existence of habitable planet, although there remains the possibility of a habitable satellite of the giant planet.
A key contribution of this work is an illuminating discussion of the so-called ‘packed planetary systems’ (PPS) hypothesis, which suggests that all planetary systems have as many planets as they can support without becoming unstable. In other words, if you can determine that a stable region exists somewhere in a planetary system, then a planet ought to be there. This paper, which is the third in a series on predicting planets, helps the authors to conclude that the PPS idea is robust. From the paper:
It remains to be seen whether the predicted planets exist. We do not predict that stable regions will contain so much mass as to be borderline unstable. Rather, we suggest that any region in a planetary system which can support a massive planet will contain a planet. Future observations of these well-studied planetary systems will test the credence of the PPS hypothesis.
The paper is “Predicting Planets in Known Extra-Solar Planetary Systems III: Forming Terrestrial Planets,” available in abstract and full-text form at the arXiv site.
