From Alpha Centauri to the Southern Cross: Image by the European Southern Observatory
While writing my recent round-up of exoplanetary controversies in the immediate Solar neighborhood, I debated whether to include Alpha Centauri Bb. When this object was originally announced last fall, veteran planet hunter Artie Hatzes sounded a note of caution, given the faintness of the radial velocity signal (Hatzes 2012). Then just last month, during a presentation on the Kepler spacecraft, mission scientist Jon Jenkins registered his own doubts about the robustness of the Bb detection. At the same time, however, he affirmed his faith in the rigor and diligence of its discovery team, led by Xavier Dumusque.
Neither of these sotto voce concerns took the shape of a formal challenge in a peer-reviewed study. So they didn’t seem to provide evidence of a true controversy, especially since the Extrasolar Planets Encyclopaedia – the de facto arbiter of extrasolar reality – lists Bb as a confirmed planet, with a mass slightly larger than Earth’s and a scorched orbital period of just 3.24 days. So I decided to omit Alpha Centauri from my discussion of backyard controversies.
In the past two weeks, however, Hatzes has circulated a preprint featuring a detailed analysis of the results of Dumusque and colleagues. Hatzes concludes that he is “not able to confirm the presence of the Earth-mass planet” that they reported. He presents a judicious criterion for accepting or rejecting it: “Only when the signal of Alpha Centauri Bb rises with certainty above the noise level will we be certain of this planet” (Hatzes 2013).
Hatzes notes that the signal attributable to an Earth-mass planet orbiting very close to a K-type star is still smaller than the noise produced by ordinary magnetic activity in the star’s chromosphere. To detect a planetary signal in radial velocity measurements, it is necessary to filter out the stellar noise. So far, astronomers have no reliable, established standards for doing so, and inappropriate filtering can distort the data enough to inspire erroneous conclusions.
So it seems that Alpha Centauri Bb, a candidate Hellworld allegedly orbiting one of the two nearest Sun-like stars, cannot be regarded as a rock-solid detection comparable to, say, GJ 1214 b or 55 Cancri e. With luck, more astronomers will contribute to the discussion over the next year and help to clarify the ontological status of this object.
During my journeys across the Interweb to find Dr. Hatzes' most recent publications, I discovered a new article on two of my favorite topics: Jupiter analogs and the Sun’s back yard. Led by M. Zechmeister and including Hatzes as a co-author, this study not only joins the chorus of voices questioning the reality of Epsilon Eridani b, but also raises previously unsuspected doubts about another backyard exoplanet, HD 102365 b (also designated HR 4523 b). Regarding the proposed gas giant around Epsilon Eridani, they say, “We cannot find any evidence for the long-period planet,” but note that “this planet is not yet fully disproved.” Regarding the proposed gas dwarf around HD 102365, they say, “We cannot confirm the planet” (Zechmeister et al. 2013). If we accept the validity of their negative results, as well as other recent exo-skepticism, the number of exoplanetary systems located within 10 parsecs declines from 15 (as listed in the Extrasolar Planets Encyclopaedia) to 11.
At least some of the results of Zechmeister and colleagues are exoplanet-friendly. They confirm three previously announced planetary systems located between 10 and 20 parsecs: Iota Horologii, HD 10647, and HD 69830. (To my knowledge, none of these have been seriously challenged anyway.) They also suggest that Epsilon Indi, a very nearby K-type star (3.62 parsecs/11.8 light years away), harbors a long-period gas giant potentially analogous to Jupiter or Saturn. Unfortunately, a complete orbit has not yet been observed. Epsilon Indi has already been shown to host a distant brown dwarf binary, making this an especially interesting system on architectural grounds alone.
The well-known overlap between science and science fiction persists……
Dumusque X, Pepe F, Lovis C, Segransan D, Sahlmann J, Benz W, Bouchy F, Mayor M, Queloz D, Santos N, Udry S. (2012) An Earth-mass planet orbiting Alpha Centauri B. Nature 491, 207-211. Abstract: http://adsabs.harvard.edu/abs/2012Natur.491..207D
Hatzes A. (2012) Meet our closest neighbour. Nature 491, 200-201. Abstract: http://adsabs.harvard.edu/abs/2012Natur.491..200H
Hatzes AP. Radial velocity detection of Earth-mass planets in the presence of activity noise: The case of Alpha Centauri Bb. (2013) Preprint: http://adsabs.harvard.edu/abs/2013arXiv1305.4960H
Zechmeister M, Kürster M, Endl M, Lo Curto G, Hartman H, Nilsson H, Henning T, Hatzes AP, Cochran WD. (2013) The planet search programme at the ESO CES and HARPS: IV. The search for Jupiter analogues around solar-like stars. Astronomy & Astrophysics 552, A78. Abstract: http://adsabs.harvard.edu/abs/2013A%26A...552A..78Z