Among the 200-plus exoplanets discovered thus far, the system around the red dwarf Gl 876 stands out. For one thing, it contains the closest thing to a terrestrial-sized planet yet found, with a mass of about six times that of Earth and a tight, two-day orbit around its primary. For another, it houses two gas giants, the only planets of this type known to orbit an M-dwarf, and they exhibit a 2:1 mean-motion resonance — one of them orbits the star twice in the same amount of time the other makes a single orbit. Such resonances provide clues in the study of how systems like these formed and changed over time.
These gas giants, one in a 30-day orbit, the other in a 60, are fascinating in their own right, but the detection of the small inner planet by Doppler techniques shows just how far planetary detection methods have come in the past decade. Now a paper slated for publication in The Astrophysical Journal looks at Gl 876 in terms of planetary transits, for to arrive at the true mass of a planet, independent measures of its orbital inclination are needed. The team’s goal was to discover whether the two outer planets around Gl 876 make such transits, and the answer seems to be no.
This is still useful information, for it provides a tune-up to the existing radial velocity data. And it points to the singularly productive nature of further transit studies. If small, terrestrial-style worlds do form commonly around stars like Gl 876, we have reason to believe they are detectable. As the paper points out, an Earth-sized planet transiting such a star will block about 1 percent of the stellar flux from that star. If in a habitable 3.85-day orbit, the planet will make a transit that lasts about forty minutes. These are workable numbers, as the paper says:
A 1% photometric dip is readily detectable. Amateur astronomers who participate in the Transitsearch collaboration routinely achieve detection thresholds of considerably better than 1%, as evidenced by conﬁrming detections of HD 149026b which has a transit depth of just 0.3%. Indeed, capable amateur observers have demonstrated the photometric capability to detect the passage of a Mars-sized body in front of an 11th magnitude 0.1 solar mass red dwarf.
For the properly equipped amateur astronomer, Transitsearch is an intoxicating challenge. What could be more significant than the detection of an Earth-mass planet around another star? Nearby red dwarfs of great interest include Proxima Centauri, Barnard’s Star, Wolf 359, and Ross 154, among others listed in this paper. “We estimate,” the paper continues, “that each one has a ~ 1% chance of harboring a detectible transiting, potentially habitable planet – readily detectible by distributed photometric observation.” Given these numbers, we may be no more than a year or so away from the first detection of a habitable world.
The paper is Shankland, Rivera, Laughlin et al., “On the Search for Transits of the Planets Orbiting Gl 876,” published in the December 10 issue of The Astrophysical Journal (Vol. 653, p. 701) and available online.