What are the chances that we’ll find habitable planets around Alpha Centauri A and B? Centauri Dreams has long kept an eye on the work of Greg Laughlin (UC-Santa Cruz) and colleagues, who have been working on the Alpha Centauri question with ever more interesting results. Following their work on Greg’s systemic site has been fascinating, and for those who would like to be quickly brought up to speed, it’s useful to know that Laughlin has made their recent paper summarizing these findings available online. Anyone serious about the study of these closest stars to Earth will want to download and read these promising results.
Laughlin’s group simulated the formation of planetary systems around Centauri B, beginning with a disk populated with 400 to 900 lunar-mass protoplanets, then following its development over 200 million years. To say the results are encouraging would be an understatement. All the simulations lead to multiple-planet systems, with at least one planet of Earth mass or slightly larger within the tantalizing range of 0.5 to 1.5 AU. I’m going to poach Greg’s diagram from the latest systemic article by way of summarizing these findings below; please consider this a pointer as well to add systemic to your regular reading list.
Image: Findings from Laughlin’s team on the kinds of planetary systems we should expect around Alpha Centauri B. The team includes UC-SC graduate student Javiera Guedes, Erica Davis, Eugenio Rivera, Elisa Quintana and Debra Fischer. Credit: Greg Laughlin/systemic.
Note the somewhat larger planetary sizes of the larger planet in each run as compared to the Earth (in the top row). Laughlin notes that Centauri B’s higher metallicity should lead to more massive terrestial planets, as the simulation confirms. The question now arises, with orbits of terrestrial worlds now believed to be theoretically possible around both Centauri A and B, and with simulations showing the possibility of a rocky world in the habitable zone of Centauri B, what can we do to get a confirmed detection? It would be one of the most significant finds in exoplanetary science, one that would fire the imagination and immeasurably boost the case for close-up studies of these stars and their possibilities for life.
We’ve detected almost three hundred exoplanets, some of them (via microlensing) thousands of light years from home. Why haven’t we been able to track down planets around these closest of all stars? The answer lies in the limitations of our radial velocity methods, but as Laughlin and team show, the right kind of survey may be able to surmount them.
But first, what work has been done on Centauri planets? In fact, we have excellent observations of these stars dating back over 150 years because of their proximity and sky-dominating visibility, with radial velocity data for both Centauri A and B tabulated since 1904. A 1999 study was able to determine, for example, that the stars are not orbited by any planet with a mass above ten Jupiter masses. Two years later, a new analysis placed tighter limits on Centauri planets: No planets larger than 2.5 Jupiter masses for Centauri A and 3.5 Jupiter masses for Centauri B could exist.
Laughlin’s team now takes the mass of any planet in a circular orbit within 1 to 3 AU down to somewhere in the neighborhood of 0.3 Jupiter masses around Centauri A and 0.5 Jupiter masses around Centauri B. No gas giants around the Centauri stars! And with consistent findings that Earth-mass planets can form within 2.5 AU of these host stars and remain stable for billions of years, the outlook for small planets is quite promising. Bear in mind, too, that 20 percent of all planets discovered thus far have been found in multiple systems, with three around binaries with an orbital separation similar to that between Centauri A and B.
Finding small planets is no easy task, because it’s much easier to identify a large planet with radial velocity methods; the signature of an Earth-mass planet is vanishingly small. But Laughlin’s team believes it is achievable. From the paper:
A successful detection of terrestrial planets orbiting α Cen B can be made within a few years and with the modest investment of resources required to mount a dedicated radial-velocity campaign with a 1-meter class telescope and high-resolution spectrograph. The plan requires three things to go right. First, the terrestrial planets need to have formed, and they need to have maintained dynamical stability over the past 5 Gyr. Second, the radial velocity technique needs to be pushed (via unprecedentedly high cadence) to a degree where planets inducing radial velocity half-amplitudes of order cm s−1 can be discerned. Third, the parent star must have a negligible degree of red noise on the ultra-low frequency range occupied by the terrestrial planets.
That latter is worth commenting on. While both of the primary Centauri stars are thought to be likely candidates for planets, Centauri B is the easier to investigate because compared to its companion star, Centauri B is exceptionally quiet in terms of the kind of oscillations and stellar activity that can produce ‘noise’ in radial velocity data. Studies of Centauri B in the X-ray spectrum indicate low chromospheric activity of the sort that would be associated with only weak stellar flares. If that assumption holds true, we may be able to push radial velocity methods down to the exceedingly fine scales needed to detect planets of this size.
The key word is ‘dedicated,’ the authors assuming the need to access a telescope capable of continuous observations over lengthy periods. It takes time to accumulate radial velocity data, particularly when working at this level of detail, but a detection of one or more Centauri planets within a few years, possibly within the habitable zone, makes this an exciting prospect. The Centauri stars thus become not just valuable targets in and of themselves, but also serve as a testbed for pushing radial velocity techniques to new levels. Imagine the magnitude of this discovery:
Our simulations and those of Quintana et al. (2007) show that for disks with small inclinations relative to the binary orbit, large terrestrial planets tend to form in or near the habitable zone of α Cen B. Thus, it is possible we may detect a habitable terrestrial planet around at least one of our nearest stellar neighbors.
The paper is Guedes et al., “Formation and Detectability of Terrestrial Planets around α Centauri B,” accepted by The Astrophysical Journal. Now available at the arXiv site.