You would think Alpha Centauri would be a prime hunting ground for extrasolar planets simply because of its proximity. But the problem for direct imaging is the sheer brightness of Centauri A and B, creating a halo of diffuse light around the pair. Getting through the glare isn’t easy, but a search based on twin techniques — adaptive optics and CCD imaging — covering a wide-field around the Centauri system has just been completed. Results on the CCD work, using European Southern Observatory equipment, have now been made available and they’ve come up short on planetary detections.
As reported by Pierre Kervella (Observatoire de Paris-Meudon) and Frederic Thévenin (Observatoire de la Côte d’Azur), the team found no co-moving companion objects between 100 and 300 AU. And that’s useful information, because it puts some constraints on possible planets around these stars. From the paper:
Within the explored area, this negative result sets an upper mass limit of 15-30 M J to the possible companions orbiting α Cen B or the pair, for separations of 50-300 AU. When combined with existing radial velocity searches…and our adaptive optics results…this mostly excludes the presence of a 20-30 M J companion within 300 AU.
First of all, note what this is not telling us. We can draw no conclusions about possible terrestrial-sized worlds orbiting within 3-4 AU of either Centauri A or B, for the equipment is not sensitive enough to detect planets that small. Thus the scenario that continues to fire the imagination of many of us — habitable planets around one or both Centauri stars — is still viable. We’ve simply learned that we can rule out massive super-Jupiters in wide orbits.
And that gives us further insight into the Alpha Centauri system itself, for some recent work has indicated that the mass of Centauri B could be higher than what earlier models have suggested. Specifically, radial velocity studies have come up with mass estimates that differ by 28 Jupiter masses (plus or minus 9) from the results of long-baseline interferometry. If the missing mass is in the form of an unseen companion, we can now exclude at least one planetary configuration that might have accounted for it.
The paper is Kervella and Thévenin, “Deep imaging survey of the environment of α Centauri,” accepted as a research note by Astronomy & Astrophysics and available as a preprint online. The team’s earlier work using adaptive optics (which feeds directly into the present paper) is Kervella et al., “Deep imaging survey of the environment of α Centauri: I. Adaptive optics imaging of α Cen B with VLT-NACO,” available here. Centauri Dreams‘ earlier story on the latter is also available.