Our assumptions about terrestrial planets seem pretty straightforward. We’re only now reaching the level where detecting such worlds becomes a possibility, with advances in ground- and space-based telescopes imminent that will begin to give us an idea how common such planets are. Hoping for the best, we assume Earth-sized worlds in relatively comfortable places are common and even extend our search from G and K-type stars to the much dimmer (and more numerous) M-dwarfs.
But what do we mean by a terrestrial planet? Size is an obvious criterion, but so is placement in the kind of habitable zone we would find conducive to our kind of life. That means liquid water at the surface. So far so good, but keep a sharp eye on the wild card in all this: Orbital ecccentricity. It’s a measure of how far the orbit of a planet deviates from a circle, and we need to know more about it. Obviously a highly eccentric orbit could swing a planet through the habitable zone and right back out again, never allowing a stable and benign environment for life to develop.
Many of the planets already discovered show fairly eccentric orbits. A short but intriguing paper by Daniel Malmberg (Lund Observatory, Sweden) and team now asks a provocative question: Is there a mechanism that ensures high values of orbital eccentricity, and if so, what does it tell us about planet formation in other solar systems? The assumption is that because most stars form in clusters, close encounters between young stars are fairly common. And that poses real problems.
For one thing, the orbits of planets in a given system could be profoundly altered by a close stellar pass, with some of them being ejected entirely. The planets remaining would then be left with significantly more eccentric orbits. A major question to ask is whether our Sun has ever had such a close encounter with another star. If not, that could explain the nearly circular orbits we see in our Solar System, and might also have something to do with the placement of the more massive planets far from the Sun, not the scenario in many of the exoplanetary systems we’ve examined.
These considerations mean that planetary systems that were once much like ours have been made into the kind of systems we have often observed, with planets on orbits so eccentric as to make the emergence of life problematic at best. Consider, for example, what can happen when a single star encounters not just one other star but a binary system:
If a single star instead encounters a binary system, it can be exchanged into it. When this occurs, the orientation of the orbital plane of the planets with respect to that of the companion star is completely random. This means that in about 70 per cent of the cases, the inclination between the two will be larger than 40◦. When that happens, the Kozai Mechanism will operate… Given that the binary is not too wide, the Kozai Mechanism will cause the eccentricities of the planets to oscillate. If the planetary system contains multiple planets, this eccentricity pumping can cause strong planet-planet interactions, causing the orbits of the planets to change signiﬁcantly and sometimes also ejecting one or more planets.
If so-called ‘singletons,’ formed singly and with no history of close stellar interactions, are the only places where Solar Systems like our own can form, we have placed a constraint on habitable terrestrial worlds. How much of one? We begin by ruling out a vast range of multiple star systems. As to solitary stars with Sun-like masses, the authors have numerically simulated a range of stellar clusters like those in which our Sun formed. They conclude that five to ten percent of all planetary systems around such stars have been altered by dynamical interactions as well, most likely to the detriment of life’s chances there.
A history of isolation, then, may play a role in the habitability of any terrestrial world around a Sun-like star. But note: The ‘if’ in the above paragraph is called into question by a good deal of recent work on the stability of planetary orbits in binary systems. The paper is Malmberg, Davies et al., “Is our Sun a Singleton?” to be published in the proceedings of IAUS246 “Dynamical Evolution of Dense Stellar Systems” (abstract).