Aldebaran System

In 1993 radial velocity measurements of Aldebaran, Arcturus and Pollux showed that Aldebaran exhibited a long-period radial velocity oscillation, which could be interpreted as a substellar companion. The measurements for Aldebaran implied a companion with a minimum mass 11.4 times that of Jupiter in a 643-day orbit at a separation of 2.0 AU (300 Gm) in a mildly eccentric orbit. However, all three stars surveyed showed similar oscillations yielding similar companion masses, and the authors concluded that the variation was likely to be intrinsic to the star rather than due to the gravitational effect of a companion.[51]

In 2015 a study showed stable long-term evidence for both a planetary companion and stellar activity.[12] An asteroseismic analysis of the residuals to the planet fit has determined that Aldebaran b has a minimum mass of 5.8±0.7 Jupiter masses, and that when the star was on the main sequence it would have given this planet Earth-like levels of illumination and therefore, potentially, temperature.[11] This would place it and any of its moons in the habitable zone. Follow-up study in 2019 have found the evidence for planetary existence inconclusive though.