Planets around Red Giant Stars

Niedzielski et al. (2015) report the detection of planetary-mass companions around three red giant stars by the ongoing Penn State-Torun Planet Search using the 9.2-meter aperture Hobby-Eberly Telescope (HET) at McDonald Observatory in Texas. The red giant stars are identified as BD+49 828, HD 95127 and HD 216536. All three stars are exhausting the nuclear fuel in their cores and are gradually expanding in size. Their planetary-mass companions were detected with the radial velocity method. It involves a high resolution spectrograph which precisely measures the amount of redshift and blueshift in the star’s spectral lines as the star wobbles back and forth due to gravitational tugging from an orbiting planet.

Figure 1: Artist’s impression of a gas-giant planet.

BD+49 828 has 30 times the Sun’s luminosity, 1.5 times the Sun’s mass and 7.6 times the Sun’s radius. Its planetary-mass companion, BD+49 828 b, with the added suffix “b” to indicate its planetary nature, is 1.6 times Jupiter’s mass. The planet orbits its host star in a 2590-day orbit at a mean distance of 4.2 AU with eccentricity 0.35.

HD 95127 has 190 times the Sun’s luminosity, 1.2 times the Sun’s mass and 20 times the Sun’s radius. Its planetary-mass companion, HD 95127 b, is 5.0 times Jupiter’s mass and orbits its host star in a 482-day orbit at a mean distance of 1.3 AU with eccentricity 0.11.

HD 216536 has 63 times the Sun’s luminosity, 1.4 times the Sun’s mass and 13 times the Sun’s radius. Its planetary-mass companion, HD 216536 b, is 1.5 times Jupiter’s mass and orbits its host star in a 149-day orbit at a mean distance of 0.61 AU with eccentricity 0.38.

HD 95127 b and HD 216536 b are both orbiting close enough to their host stars that they will eventually be engulfed as their host stars continue to evolve and expand in size. For BD+49 828 b, its orbit is sufficiently large that it will not be affected by the evolution of its host star. BD+49 828 b is one of the longest period planets found using the radial velocity method. Interestingly, BD+49 828 b may also be located in the habitable zone (HZ) as inferred from the estimated luminosity of its host star. However, this is only transient because as the host star continues to evolve and grow in luminosity, the HZ will move beyond the orbit of BD+49 828 b. In fact, it may already have occurred.

Figure 2: Best-fit radial velocity curve indicating the presence of BD+49 828 b. Niedzielski et al. (2015).

 Figure 3: Best-fit radial velocity curve indicating the presence of HD 95127 b. Niedzielski et al. (2015).

Figure 4: Best-fit radial velocity curve indicating the presence of HD 216536 b. Niedzielski et al. (2015).

Reference:

Niedzielski et al. (2015), “Three red giants with substellar-mass companions”, arXiv:1501.07076 [astro-ph.SR]