Wednesday, April 30, 2014

HD 141399’s Retinue of Four Giant Planets

A planetary system consisting of 4 giant planets has been detected around HD 141399, a nearby, slightly evolved, K-type star. This discovery was made by Vogt et al. (2014) using precise radial velocity data from the HIRES (High Resolution Echelle Spectrometer) instrument on the Keck-I telescope and the Levy spectrometer on Lick Observatory’s Automated Planet Finder (APF). 91 observations spread over 10.5 years indicate the presence of 4 planets with orbital periods of 94.35, 202.08, 1070.35 and 3717.35 days, and minimum masses of 0.46, 1.36, 1.22 and 0.69 Jupiter-mass. In order of increasing distance from the host star, the 4 planets are dubbed HD 141399 b, c, d and e.

Figure 1: Artist’s impression of a ringed giant planet with a moon in the foreground. Image credit: Nick Stevens.

Figure 2: Artist’s impression of a giant planet.

Compared to the Sun, HD 141399 has 1.59 ± 0.39 times its luminosity, 1.46 ± 0.15 times its radius and 1.14 ± 0.08 times its mass. HD 141399 is a rare example of a star with a planetary system consisting of multiple giant planets. Other examples of such planetary systems include Upsilon Andromedae - a system of 4 giant planets, and 55 Cancri A - a system with one close-in super-Earth and 4 giant planets. An unusual aspect of HD 141399 is that its inner 3 giant planets lie at distances normally associated with the terrestrial planets (i.e. Mercury, Venus, Earth and Mars) in our Solar System. The fourth planet, labelled HD 141399 e, is a Jupiter-like planet with a Jupiter-like orbital period of roughly a decade.

The two inner planets - HD 141399 b and c, have estimated equilibrium temperatures of 500 K and 390 K, respectively. At these temperatures, the atmospheres of both planets are expected to be dominated by water clouds and complex chemistry. Transits by HD 141399 b or c, if they do occur, would be of great scientific value since it would allow their atmospheres to be studied via transmission spectroscopy by the Hubble Space Telescope (HST) or James Webb Space Telescope (JWST). HD 141399 b and c lie in a relatively unexplored temperature regime between the well-studied hot-Jupiters at one extreme and the giant planets (i.e. Jupiter and Saturn) in our Solar System. Nevertheless, the odds that HD 141399 b or c can be observed in transit are 1.3 and 0.8 percent, respectively.

Vogt et al. (2014), “A 4-Planet System Orbiting the K0V Star HD 141399”, arXiv:1404.7462 [astro-ph.EP]