Last June it was reported that the ancient halo star known as Kapteyn's Star (
Sol Station,
Wikipedia), a red dwarf 12.76 light years from our solar system in the direction of Sirius that a couple billion years older than the universe itself, supported two super-Earths in that star's circumstellar habitable zone. This was very surprising, as it was not thought that ancient stars as poor in heavy elements as Kapteyn's Star would be likely to support planets. As
reported by the University of London, which also included a nice short story by science fiction writer Alistair Reynolds, this discovery potentially had sweeping implications for exoplanet frequency and even the potential likelihood of life in the universe. Andrew Lepage's
analyses of the system suggesting that it was considerably more likely that these worlds were mini-Neptunes than super-Earth, but even so, the discovery was evocative.
Universe Today's David Dickinson
"Is Kapteyn B Not to Be?" reports that this discovery might have been spurious, featuring an interview with the author of a paper critical of the idea.
The idea of a planetary system around Kapteyn’s Star, real or not, is an interesting tale of exoplanet science. The original discovery was made using the High Accuracy Radial velocity Planetary research (HARP) instrument at the European Southern Observatory, with supporting observations from the Las Campanas and Keck Observatory. You’d think that would make the discoveries pretty air-tight. The planets discovered orbiting Kapteyn’s Star were discerned using the radial velocity method, looking at the spectra of the star for the characteristic tugging of an unseen companion.
Recent research led by Paul Robertson of Pennsylvania State University suggests that the signal for the discovery of Kapteyn B may in fact be the result of stellar activity. Starspots—think sunspots on our own host star—can mimic the spectral signal of an unseen planet. Analyzing the HARPS data, we know that Kapteyn’s Star rotates once every 143 days. Kapteyn-b’s orbit of 48 days is very close to an integer fraction (143/48= 2.979) making it extremely suspicious.
More study, clearly, is needed.