The news about possible surface water on today’s Mars points out how far we are from characterizing life’s possibilities even in our own Solar System, much less around other stars. It may take boots on the ground on Mars to solve the question once and for all, but life in underground aquifers certainly is a plausible proposition, and the sooner we have proof (and samples to study), the better for astrobiology in general.
Meanwhile, we push on with the very early wave of exoplanet studies, remembering that it’s just over a decade since 51 Peg gave us the first confirmed detection around a main sequence star. I can’t imagine a more fruitful field for a young astronomer to head for, with so many possibilities for study that you begin to wonder whether we’ll have the human resources to keep up with the vast data inflow that’s coming.
Some of the more intriguing recent work concerns binaries and the planets around them. If we’re getting fairly sanguine about the possibility of planets around close binaries like Centauri A and B, the question of what happens when the stars are farther apart stands open. A new paper by Daniel Malmberg (Lund Observatory, Sweden) and colleagues probes this question for two kinds of binary systems. Co-planar systems are those in which the two stars evolved as a binary from the start, drawing on the same primordial materials. Planets around these stars are likely in the same orbital plane. But the paper’s real focus is on stars that acquire a binary partner later in their evolution in a young stellar cluster.
When that happens, the result is a planetary system that’s randomly oriented with respect to the companion star. The authors go to work on such systems using the Kozai mechanism, first applied to asteroids in inclined orbits in our own Solar System as they are affected by Jupiter. The Kozai mechanism causes planets in wide binaries (the authors study 1000 AU separations, for example, as opposed to the 23 AU mean separation of Centauri A and B) to vary their orbital eccentricity. This is one way to explain the high eccentricity of the planet around 16 Cygni B.
Interesting things follow from all this. If you start with our Solar System and introduce a second, widely separated star, Neptune and Uranus will cross in their orbits if the inclination between companion star and the plane of the planets is greater than 43 degrees. Planetary ejections can follow. The Kozai numbers indicate this scenario will be the case in 73 percent of the binary systems formed by stellar encounters. And it could be that some of the observed extrasolar systems we see are the result of this kind of planet-scattering in a system that was once much like our own, with planets orbiting around a single original star.
Intriguing stuff, and it makes good sense, given that stars forming in groups are going to be in a crowded environment during the first 100 million years after their birth. That makes single stars into binaries easily enough, with obvious implications for stripping the system of one or more planets, and greatly affecting the orbits of those that remain.
The paper is Malmberg et al., “The instability of planetary systems in binaries: how the Kozai mechanism leads to strong planet-planet interactions,” submitted to Monthly Notices of the Royal Astronomical Society and available online as a preprint.