Two planets around Kapteyn's star : a cold and a temperate super-Earth
  orbiting the nearest halo red-dwarf
  Exoplanets of a few Earth masses can be now detected around nearby low-mass
stars using Doppler spectroscopy. In this paper, we investigate the radial
velocity variations of Kapteyn's star, which is both a sub-dwarf M-star and the
nearest halo object to the Sun. The observations comprise archival and new
HARPS, HIRES and PFS Doppler measurements. Two Doppler signals are detected at
periods of 48 and 120 days using likelihood periodograms and a Bayesian
analysis of the data. Using the same techniques, the activity indicies and
archival ASAS-3 photometry show evidence for low-level activity periodicities
of the order of several hundred days. However, there are no significant
correlations with the radial velocity variations on the same time-scales. The
inclusion of planetary Keplerian signals in the model results in levels of
correlated and excess white noise that are remarkably low compared to younger
G, K and M dwarfs. We conclude that Kapteyn's star is most probably orbited by
two super-Earth mass planets, one of which is orbiting in its circumstellar
habitable zone, becoming the oldest potentially habitable planet known to date.
The presence and long-term survival of a planetary system seems a remarkable
feat given the peculiar origin and kinematic history of Kapteyn's star. The
detection of super-Earth mass planets around halo stars provides important
insights into planet-formation processes in the early days of the Milky Way.
