Tuesday, October 14, 2014

A Warm Gas-Giant Planet Orbiting a Giant Star

A study by Ortiz et al. (2014) reported on the spectroscopic confirmation of a Jupiter-like gas-giant planet in a close-in, eccentric orbit around a giant star. This planet is identified as KOI-1299b, with the suffix “b” denoting its planetary nature. The host star of KOI-1299b is entering its later stages of stellar evolution and is about to swell into a red giant star. This star has 1.35 ± 0.10 times the Sun’s mass, 4.15 ± 0.12 times the Sun’s radius and an effective surface temperature of 5020 ± 60 K. KOI-1299b is observed by NASA’s Kepler space telescope to transit its host star once every 52.5 days. The size of KOI-1299b was estimated by measuring the drop in the star’s brightness each time the planet transits in front of it. A separate paper by Ciceri et al. (2014) on the discovery of KOI-1299b also reported similar results.

Figure 1: Artist’s impression of a gas-giant planet with a system of planetary rings circling it.

The planetary nature of KOI-1299b was confirmed using high-resolution spectroscopic follow-up observations conducted between June and October 2014 using the Calar Alto Fiber-fed Échelle spectrograph (CAFE) on the 2.2 m telescope of Calar Alto Observatory in Almería, Spain, and the Fibre-fed Échelle Spectrograph (FIES) on the 2.56 m Nordic Optical Telescope of Roque de los Muchachos Observatory in La Palma, Spain. These follow-up observations measured the amount of gravitational tugging KOI-1299b exerts on its host star, allowing the mass of KOI-1299b to be estimated and its planetary nature to be confirmed. Additionally, these follow-up observations also show that KOI-1299b has a highly eccentric orbit around its host star.

Figure 2: Radial velocity measurements of the host star of KOI-1299b. Upper panel: CAFE (blue circles) and FIES (red squares). Lower panel: Residuals. Ortiz et al. (2014)

 Figure 3: Left panel: Eccentricity and semimajor axis of the extrasolar planets discovered around main sequence stars (black dots) and giant stars (magenta circles). Right panel: Orbital period versus stellar mass. The position of KOI-1299b is marked with a green triangle in both panels. Ortiz et al. (2014)

KOI-1299b has 5.86 ± 0.05 times the mass of Jupiter, 1.08 ± 0.03 times the radius of Jupiter and an orbital eccentricity of 0.479 ± 0.004. With the mass and size known, the estimated density of KOI-1299b is 5.7 ± 0.5 g/cm³. The highly eccentric orbit of KOI-1299b brings the planet as close as ~0.16 AU (periastron) from its host star and out as far as ~0.45 AU (apastron). Between periastron and apastron, KOI-1299b receives ~450 to ~56 times as much insolation as Earth receives from the Sun. On average, the estimated equilibrium temperature of KOI-1299b is 942 ± 20 K. However, the planet’s highly eccentric orbit can cause temperatures to vary by ~500 K. KOI-1299b is classed as a warm-Jupiter since is not as hot as typical hot-Jupiters whose temperatures are well over 1000 K. As the host star of KOI-1299b swells into a red giant, tidal interactions between the star and planet will increase, eventually causing KOI-1299b to be engulfed by its host star.

- Ortiz et al. (2014), “Spectroscopic confirmation of KOI-1299b: a massive warm Jupiter in a 52-day eccentric orbit transiting a giant star”, arXiv:1410.3000 [astro-ph.EP]
- Ciceri et al. (2014), “KOI-1299b: a massive planet in a highly eccentric orbit transiting a red giant”, arXiv:1410.2999 [astro-ph.EP]