Tuesday, January 7, 2014

CoRoT-27b: A Massive and Dense Planet

Parviainen H. et al. (2014) report the discovery of a massive high-density planet on a close-in 3.58 day orbit around a 4.2 billion year old Sun-like star. The planet is identified as CoRoT-27b. Like Jupiter, CoRoT-27b is a gas-giant planet. Its presence was detected by the CoRoT space telescope as the planet periodically transits its parent star and blocks a small fraction of the star’s light. CoRoT-27b weighs in at 10.39 ± 0.55 Jupiter-masses and has 1.01 ± 0.04 times the radius of Jupiter. This gives CoRoT-27b a mean density of 12.6 times the density of water, which is more than twice the mean density of Earth and almost 10 times the mean density of Jupiter.

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

Like Jupiter, CoRoT-27b is a gaseous planet comprised primarily of hydrogen and helium. The structure and composition of CoRoT-27b can be inferred from two models. For the first model, the planet is assumed to be made of a central rocky core surrounded by an extensive hydrogen-helium envelop. The 1st model is consistant with a heavy element mass fraction of 0.11, representing a core mass of 366 Earth-masses. For the second model, a central rocky core is absent and the heavy elements are present throughout the hydrogen-helium envelop. The 2nd model is consistant with a heavy element mass fraction of 0.07, representing a heavy element mass of 219 Earth-masses.

CoRoT-27b falls within a sparsely populated overlapping mass regime between the most massive planets and brown dwarfs. Given its high mass, gravity on the “surface” of CoRoT-27b is 27 times the surface gravity on Earth. Technically, CoRoT-27b does not have a surface since it is gaseous through, right down to a central rocky core, if one is present. Being so near to its parent star, the equilibrium temperature on CoRoT-27b is estimated to be 1500 ± 130 K. The discovery of CoRoT-27b is an important addition to a scarcely populated class of massive close-in planets.

Figure 2: Radial velocity curve showing how much CoRoT-27b gravitationally tugs at its parent star. This information allows the planet’s mass to be estimated. Parviainen H. et al. (2014).

Figure 3: Transit light curve showing the amount of dimming of the parent star when CoRoT-27b passes in front of it. This information allows the size of the planet to be measured. Parviainen H. et al. (2014).

Figure 4: CoRoT-27b mass, period and density compared with the population of confirmed transiting exoplanets. Parviainen H. et al. (2014).

Parviainen H. et al. (2014), “Transiting exoplanets from the CoRoT space mission XXVII. CoRoT-27b: a massive and dense planet on a short-period orbit”, arXiv:1401.1122 [astro-ph.EP]