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rfmcdonaldOver at Facebook, James Nicoll shared the paper by Jieun Choi et al, "Precise Doppler Monitoring of Barnard's Star". Barnard's Star, for those of you unfamiliar with this object (Sol Station and Wikipedia have much more to say), is a dim, ancient red dwarf star about ten billion years old, more than twice the age of our our sun and only a small fraction of a percent as bright. 6 light years away, Barnard's Star is the second-closest star to our own sun after Alpha Centauri and so has been a common target of study for astronomers and a setting for science fiction writers--Robert Forward's Rocheworld is the most famous book set about Barnard's Star, but I can think of others.
Because of its proximity, Barnard's Star has been a target for planet-hunters for decades. Most famously, in the mid-20th century American astronomer Peter Van De Kamp claimed to have detected two gas giant planets orbiting Barnard's Star. As the state of the technology advanced, however, these very early claims have been disproved. At present, using the latest data, it's uncertain if any substantial worlds orbit Barnard's Star.
As the paper notes, this is something of an anomaly, since red dwarfs as a class have been found to commonly host planets, including worlds the size of Neptune or even Earth. Barnard's Star, though, perhaps on account of its age and its very low metallicity, does not share in this trend of stars of its class.
Because of its proximity, Barnard's Star has been a target for planet-hunters for decades. Most famously, in the mid-20th century American astronomer Peter Van De Kamp claimed to have detected two gas giant planets orbiting Barnard's Star. As the state of the technology advanced, however, these very early claims have been disproved. At present, using the latest data, it's uncertain if any substantial worlds orbit Barnard's Star.
We present 248 precise Doppler measurements of Barnard's Star (Gl 699), the second nearest star system to Earth, obtained from Lick and Keck Observatories during 25 years between 1987 and 2012. The early precision was 20 \ms{} but was 2 \ms{} during the last 8 years, constituting the most extensive and sensitive search for Doppler signatures of planets around this stellar neighbor. We carefully analyze the 136 Keck radial velocities spanning 8 years by first applying a periodogram analysis to search for nearly circular orbits. We find no significant periodic Doppler signals with amplitudes above $\sim$2 \ms{}, setting firm upper limits on the minimum mass (\msini) of any planets with orbital periods from 0.1 to 1000 days. Using a Monte Carlo analysis for circular orbits, we determine that planetary companions to Barnard's Star with masses above 2 \mearth{} and periods below 10 days would have been detected. Planets with periods up to 2 years and masses above 10 \mearth{} (0.03 \mjup) are also ruled out. A similar analysis allowing for eccentric orbits yields comparable mass limits. The habitable zone of Barnard's Star appears to be devoid of roughly Earth-mass planets or larger, save for face-on orbits. Previous claims of planets around the star by van de Kamp are strongly refuted. The radial velocity of Barnard's Star increases with time at $4.515\pm0.002$ \msy{}, consistent with the predicted geometrical effect, secular acceleration, that exchanges transverse for radial components of velocity.
As the paper notes, this is something of an anomaly, since red dwarfs as a class have been found to commonly host planets, including worlds the size of Neptune or even Earth. Barnard's Star, though, perhaps on account of its age and its very low metallicity, does not share in this trend of stars of its class.
