Several ground-based Doppler planet searches target sub-giant and giant stars instead of main-sequence stars. The number of planets known to orbit giant stars (~60; Niedzielski et al. 2015) is still small compared to those known to orbit main-sequence stars, but their number has dramatically increased in recent years and is expected to do so in the near future thanks to TESS (Campante et al. 2016), CHEOPS, and Gaia. The discovery and characterisation of planets orbiting sub-giant and giant stars is of particular importance for the following reasons:
- Confirmation of a planet orbiting a giant star is, in many cases, almost impossible based on RVs alone, since the RV signal of an orbiting planet is hard to disentangle from the signature of radial and non-radial pulsations, unless their timescales are very different. Thus, independent confirmation of planets orbiting giant stars is most useful.
- Sub-giants and giant stars are more massive than Sun-like main-sequence stars, so by finding more planets around giant stars we can disentangle the influence of the host star’s mass and its disk on the forming planets and their properties.
- Sub-giants and giant stars have undergone significant stellar evolution, which will have affected planetary orbits (close-in giant planets risk to be engulfed, see Lillo-Box et al. 2016, and references therein). Studying the planet population around sub-giant and giant stars offers the opportunity to investigate the influence of stellar evolution on the properties of the planetary population.
- It is still not clear if evolved giant planet hosts are mostly metal-rich (Pasquini et al. 2007; Mortier et al. 2013). This could be checked using the PLATO planet sample in combination with its well-characterised host stars.
Kepler has detected a small number of planets around giant stars (e.g. Steffen et al. 2012; Huber et al. 2013; Lillo-Box et al. 2016). Owing to the much larger stellar disk of a giant star compared to the stellar disk of a main-sequence star, the transit probability is much higher for a planet orbiting a giant star than for a planet orbiting a main-sequence star. On the other hand, the transit depth is much smaller for the same reason (a much smaller percentage of the stellar disk is blocked by the planet), so that the planet is harder to detect; indeed, the false-alarm rate of Kepler planet candidates (KOIs) around giant stars in Kepler data was recently found to be higher than previously thought (Santerne et al. 2015).
PLATO is in a better position to find planets around these stars. The depth of a transit of a Jupiter-sized planet in front of a giant star with a radius of 10 solar units is 100 ppm, which is within the mission’s detection capabilities. Detection will still be challenging, however, owing to the photometric activity of giant stars, which will have to be well characterised. The planets found by RV surveys around this type of star have typically periods of several hundreds of days. For example, at 400 day orbital periods, the transit probability is 3% and the transit duration almost 4 days.