Barnard’s Star has always gotten its share of attention, and deservedly so. It was in 1916 that this M-class dwarf in Ophiuchus was measured by the American astronomer Edward Emerson Barnard, who found its proper motion to be the largest of any star relative to the Sun. That meant the star soon to be named for him was close to us, and unless we’re surprised by a hitherto unobserved brown dwarf, Barnard’s Star remains the closest star to our Sun after the Alpha Centauri triple system. Stick around long enough and Barnard’s Star will close to within 3.75 light years, but even if you make it to 10,000 AD or so, the star will still be too faint to be a naked eye object.
Image: Barnard’s Star, with proper motion demonstrated, part of an ongoing project to track the star. This image shows motion between 2004 and 2008. Credit: Paul Mortfield & Stefano Cancelli/The Backyard Astronomer.
Peter van de Kamp, working at Swarthmore College, had been looking for wobbles in the position of Barnard’s Star going all the way back to 1938, and for a time his results indicated at least one Jupiter-class planet there and possibly two. But other astronomers failed to find evidence for planets, and later work raised the likelihood that the changes in the star fields van de Kamp was looking at were caused by issues related to the refractor he was using. Now we have a new paper from Jieun Choi (UC Berkeley), whose team went to work on Doppler monitoring of Barnard’s Star and concluded that van de Kamp’s findings were erroneous.
The paper, however, gives a generous nod to van de Kamp’s work:
The two planets claimed by Peter van de Kamp are extremely unlikely by these 25 years of precise RVs. We frankly pursued this quarter-century program of precise RVs for Barnard’s Star with the goal of examining anew the existence of these historic planets. Indeed, Peter van de Kamp remains one of the most respected astrometrists of all time for his observational care, persistence, and ingenuity. But there can be little doubt now that van de Kamp’s two putative planets do not exist.
The one-planet model fails to fit as well when studied with radial velocity data from both Lick and Keck:
Even van de Kamp’s model of a single-planet having 1.6 MJup orbiting at 4.4 AU (van de Kamp 1963) can be securely ruled out. The RVs from the Lick and Keck Observatories that impose limits on the stellar reflex velocity of only a few meters per second simply leave no possibility of Jupiter-mass planets within 5 AU, save for unlikely face-on orbits.
The paper goes on to drill down to planets of roughly Earth mass, finding no evidence for such worlds. The result is interesting on a number of levels. We’re finding smaller planets with radii 2 to 4 times that of Earth around M-dwarfs regularly in data from the Kepler mission, in an area close in to the star where this new study of Barnard’s Star is most sensitive to Earth-mass planets. The transit data back up radial velocity data on M-dwarfs from the HARPS spectrograph, which have shown numerous planets with mass a few times larger than Earth’s around M-dwarfs. A 2011 study found the occurrence of super-Earths in the habitable zone is in the area of 41 percent for M-dwarfs, leading to what Jieun Choi and colleagues describe as ‘a lovely moment in science.’
After all, our two major planet-hunting techniques — Doppler measurements to detect planets by their effect on the host star, and brightness measurements for transit detection — both indicate that small planets are apparently common around M-dwarfs. By contrast:
…the non-detection of planets above a few Earth masses around Barnard’s Star remains remarkable as the detection limits here are as tight or tighter than was possible for the Kepler and HARPS surveys. The lack of planetary companions around Barnard’s Star is interesting because of its low metallicity. This non-detection of nearly Earth-mass planets around Barnard’s Star is surely unfortunate, as its distance of only 1.8 parsecs would render any Earth-size planets valuable targets for imaging and spectroscopy, as well as compelling destinations for robotic probes by the end of the century.
Let’s not forget that the early work of Peter van de Kamp had energized speculation about missions to Barnard’s Star. The British Interplanetary Society’s Project Daedalus chose it as a destination largely because of its supposed planetary system even though Alpha Centauri was considerably closer (4.3 light years vs. 6). Robert Forward toyed with Barnard’s Star in his fiction, writing Flight of the Dragonfly, later expanded as Rocheworld, to depict both the planetary system there as well as the technology needed to reach it.
Now we have 248 precise Doppler measurements of Barnard’s Star from the Lick and Keck Observatories saying that the habitable zone of this conveniently nearby star appears to be empty of planets of roughly Earth mass or larger. Let’s hope the Alpha Centauri stars yield a better result. The paper is Choi et al., “Precise Doppler Monitoring of Barnard’s Star,” available online.