Person:

Zeng, Li

We report the first planet discovery from the two-wheeled Kepler (K2) mission: HIP 116454 b. The host star HIP 116454 is a bright (V = 10.1, K = 8.0) K1 dwarf with high proper motion and a parallax-based distance of 55.2 ± 5.4 pc. Based on high-resolution optical spectroscopy, we find that the host star is metal-poor with [Fe/H] = −0.16±0.08 and has a radius R = 0.716 ± 0.024 R and mass M = 0.775±0.027 M. The star was observed by the Kepler spacecraft during its Two-Wheeled Concept Engineering Test in 2014 February. During the 9 days of observations, K2 observed a single transit event. Using a new K2 photometric analysis technique, we are able to correct small telescope drifts and recover the observed transit at high confidence, corresponding to a planetary radius of Rp = 2.53 ± 0.18 R⊕. Radial velocity observations with the HARPS-N spectrograph reveal a 11.82 ± 1.33 M⊕ planet in a 9.1 day orbit, consistent with the transit depth, duration, and ephemeris. Follow-up photometric measurements from the MOST satellite confirm the transit observed in the K2 photometry and provide a refined ephemeris, making HIP 116454 b amenable for future follow-up observations of this latest addition to the growing population of transiting super-Earths around nearby, bright stars.

Understanding the interior structures and chemistry of Earth-like exoplanets is crucial for us to characterize exoplanets, and to find potentially habitable planets.

First, I provide a model grid of the mass-radius relations for solid planets in between 0.1 and 100 Earth masses. Planets are modeled as consisting of three layers: Fe, MgSiO3 and H2O. This model is made into an interactive tool available online: http://www.astrozeng.com/

Second, I explore the effects of thermal evolution and phase transitions on the interior structures of H2O-rich planets. It is shown that the bulk H2O in such planets may exist in the plasma, superionic, ionic, Ice VII, or Ice X states depending on sizes, ages, and cooling rates. The results suggest that super-Earth sized planets which are not significantly irradiated by parent stars and which are older than approximately 3 billion years, are mostly solid.

Third, I describe a new, semi-empirical mass-radius relation for solid exoplanets. It is based on the recent mass and radius measurements of 5 exoplanets within 1 to 10 Earth masses and an extrapolation of the seismically derived pressure-density relation of the Earth's interior (PREM). The implication of common core mass fractions of 0.2~0.3 among these solid exoplanets is also discussed.

Fourth, I model the elemental abundance patterns of solid exoplanets based on that of their host stars. This model is constructed from the following steps of planet formation: volatile depletion, core formation, and late delivery. This model could provide constraints on the chemical compositions of solid exoplanets in addition to the constraints derived from their masses and radii.

In terms of future directions of this research, I hope to link my chemical model of solid exoplanets with the chemical evolution model of our galaxy, such as the one being developed by the Lars Hernquist group, which may indicate a different mineralogy of solid exoplanets formed at different ages of our galaxy, as well as the implications for the habitability of these planets. I also hope to understand the origins of the volatile contents on the surfaces of solid planets, which are important prerequisites for possible origins of life on them.

We present the characterization of the Kepler-93 exoplanetary system, based on three years of photometry gathered by the Kepler spacecraft. The duration and cadence of the Kepler observations, in tandem with the brightness of the star, enable unusually precise constraints on both the planet and its host. We conduct an asteroseismic analysis of the Kepler photometry and conclude that the star has an average density of 1.652 ± 0.006 g cm–3. Its mass of 0.911 ± 0.033 M ☉ renders it one of the lowest-mass subjects of asteroseismic study. An analysis of the transit signature produced by the planet Kepler-93b, which appears with a period of 4.72673978 ± 9.7 × 10–7 days, returns a consistent but less precise measurement of the stellar density, 1.72$^{+0.02}_{-0.28}$ g cm–3. The agreement of these two values lends credence to the planetary interpretation of the transit signal. The achromatic transit depth, as compared between Kepler and the Spitzer Space Telescope, supports the same conclusion. We observed seven transits of Kepler-93b with Spitzer, three of which we conducted in a new observing mode. The pointing strategy we employed to gather this subset of observations halved our uncertainty on the transit radius ratio RP /R sstarf. We find, after folding together the stellar radius measurement of 0.919 ± 0.011 R ☉ with the transit depth, a best-fit value for the planetary radius of 1.481 ± 0.019 R ⊕. The uncertainty of 120 km on our measurement of the planet's size currently renders it one of the most precisely measured planetary radii outside of the solar system. Together with the radius, the planetary mass of 3.8 ± 1.5 M ⊕ corresponds to a rocky density of 6.3 ± 2.6 g cm–3. After applying a prior on the plausible maximum densities of similarly sized worlds between 1 and 1.5 R ⊕, we find that Kepler-93b possesses an average density within this group.

Understanding the origin of the astonishing diversity in the composition of small planets (R p < 3 Earth radii) is one of the most important issues in exoplanetary science. Discoveries range from low-density sub-Neptunes containing volatile elements 1 to higher density rocky planets with Earth-like 2 or iron-rich 3 (Mercury-like) compositions. The diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet formation process and/or different evolutionary paths that altered the planetary properties after formation 4 . Planet evolution may be especially affected by photo-evaporative mass loss induced by high stellar X-UltraViolet (XUV) flux 5 or giant impacts 6 . While there is some evidence for the former 7,8 , there are no unambiguous findings to date about the occurrence of giant impacts in an exoplanet system. Here we report on the characterization of the compact near-resonant system Kepler-107 9 in which the two innermost planets Kepler-107b and Kepler-107c have orbital periods of 3.2 and 4.9 days, nearly identical radii (R p =1.5-1.6 Earth radii), but different densities (ρ p ~5.3 and 12.6 g cm -3 ). In consequence, Kepler-107c must have a larger iron core fraction than Kepler-107b. This feature cannot be explained by the stellar XUV irradiation but is consistent with a giant impact event on Kepler-107c. Such an impact would have stripped off part of the silicate mantle of Kepler-107c whose radius and mass indeed match the theoretical predictions from collisional mantle stripping 10 . This is the first evidence of planetary bulk density diversity in the same system that has likely been generated by a giant impact event.