Figure 1: Artist’s impression of a transiting planet.
K2-39b is a transiting giant planet in a close-in orbit around a subgiant star with ~3.88 times the radius and ~1.53 times the mass of the Sun. Transit and radial velocity observations indcate that K2-39b has ~50.3 times the mass and 8.3 ± 1.1 times the radius of Earth. This gives K2-39b a mean density that is only half the density of water. The orbital period of K2-39b around its host star is only 4.6 days.
Currently, K2-39b is the shortest-period planet known orbiting a subgiant star. However, only a handful of evolved stars (i.e. stars larger than 3.5 times the Sun's radius) are known to host short-period (i.e. planets with orbital periods less than 100 days) transiting planets. The existence of a planet like K2-39b that is orbiting so close to its host star suggests that tidal destruction may not be that effective in removing planets in close-in orbits around subgiant stars.
Figure 2: Transit light curve indicating the presence of K2-39b. Van Eylen et al. (2016)
Figure 3: Radial velocity curve indicating the presence of K2-39b. Van Eylen et al. (2016)
Figure 4: K2-39b compared with other confirmed planets - transiting planets (open circles) and non-transiting planets (open stars). Short-period transiting planets orbiting evolved stars are indicated with filled blue circles. This figure shows stellar radius versus semi-major axis, where the dotted line indicates the surface of the host star. Van Eylen et al. (2016)
Van Eylen et al. (2016), "The K2-ESPRINT Project V: a short-period giant planet orbiting a subgiant star", arXiv:1605.09180 [astro-ph.EP