Thursday, December 17, 2015

Detection of a Dense Super-Earth and a Mini-Neptune

Sinukoff et al. (2015) present a catalogue of 10 multi-planet systems from Campaigns 1 and 2 of the K2 mission - a mission using NASA’s repurposed Kepler space telescope. The catalogue shows the sizes and orbits of 24 planets in six 2-planet systems and four 3-planet systems. The K2 mission hunts for planets by looking for the telltale signature when a planet passes in front of its parent star. Measuring how much starlight the planet blocks and how frequent the planet passes in front of its host star allows the size and orbit of the planet to be determined.

For a particular 2-planet system in this catalogue of 10 multi-planet systems, radial velocity measurements were made using the High Resolution Echelle Spectrometer (HIRES) on the Keck Observatory in Hawaii to determine the masses of the two planets in the planetary system. This 2-planet system orbits EPIC 204221263, a Sun-like star with an effective temperature of 5757 ± 60 K, and 1.07 ± 0.05 times the mass and 1.10 ± 0.09 times the radius of the Sun. The two planets are identified as EPIC 204221263b and EPIC 204221263c, hereafter referred to as planet “b” and planet “c”, respectively.

Figure 1: Artist’s impression of a rocky planet. Image credit: Pauline Moss.

 Figure 2: Transit light curves of EPIC 204221263b and EPIC 204221263c. Sinukoff et al. (2015)

 Figure 3: Radial velocity curves indicating the presence of EPIC 204221263b and EPIC 204221263c. Sinukoff et al. (2015)

Planet “b” is a short-period super-Earth with 1.55 ± 0.16 times the radius and 12.0 ± 2.9 times the mass of Earth. The planet has an orbital period of only 4.02 days and an estimated equilibrium temperature of 1184 ± 51 K. The size and mass of planet “b” are consistent with the planet having a rocky or iron-rich composition. With a remarkably high bulk density of 17.5 [-6.2, +8.5] g/cm³, planet “b” could be the densest planet discovered to date. However, addition radial velocity measurements are needed to better constrain the planet’s mass to confirm this. Furthermore, with its high equilibrium temperature, planet “b” may be the dense remnant core of a gas giant planet whose atmosphere was stripped away by photoevaporation. Such a planet is known as a chthonian planet.

Planet “c” has 2.42 ± 0.29 times the radius and 9.9 ± 4.6 times the mass of Earth. The planet has an orbital period of 10.56 days and an estimated equilibrium temperature of 858 ± 37 K. With its low bulk density of 3.6 [-1.9, +2.7] g/cm³, planet “c” is more akin to a mini-Neptune. The planet probably possesses a substantial amount of low-density volatiles and/or a thick hydrogen-helium envelope. Planet “c” could also be a “water world”, with a core rich in water-ice and surrounded by a thick atmosphere of steam. More precise mass measurements and characterisation of its atmosphere are needed to determine the true bulk composition of planet “c”.

Figure 4: Radii and masses of all confirmed planets whose mass and radius are measured to better than 50 percent (2σ) precision (blue triangles). Solar System planets are represented as black squares. Red circles indicate measurements of EPIC 204221263b and EPIC 204221263c (bottom and top points, respectively). Green curves show the expected planet mass-radius curves for pure iron, rock, and water compositions according to models by Zeng & Sasselov (2013). Sinukoff et al. (2015)

- Sinukoff et al. (2015), “Ten Multi-planet Systems from K2 Campaigns 1 & 2 and the Masses of Two Hot Super-Earths”, arXiv:1511.09213 [astro-ph.EP]
- Zeng & Sasselov (2013), “A Detailed Model Grid for Solid Planets from 0.1 through 100 Earth Masses”, arXiv:1301.0818 [astro-ph.EP]