Rocky or Not, Here We Come: Bridging the Fulton Gap With K2-21b+c
Abstract
The provenance of planets 1.5 - 2 times the size of the Earth is one of the biggest unresolved mysteries from the Kepler mission. Determining the nature and origins of these exoplanets relies not only on measuring their radii, but also requires knowledge about their masses, atmospheric compositions, and interior structures. With this information, we can more confidently estimate planet mass distributions from measured radii, distinguish between rocky and non-rocky compositions, and better constrain the occurrence rate of Earth-like planets. K2-21 is a two-transiting-planet system with bodies of 1.6 and 1.9 Earth-radii. These worlds reside on opposing sides of the Fulton Gap (a minimum in the planet occurrence rate near 1.75 Earth radii), thus one is predicted to have a volatile-rich atmosphere while the other is predicted to be rocky. These exoplanets orbit their relatively bright, nearby M dwarf parent star in a near 5:3 resonance and, based on past successful Spitzer observations, exhibit 60-minute transit timing variations (TTVs). Complete knowledge about their interactions will reveal constraints on the planets' masses, which is important because stellar activity makes RV mass measurements impractical. We propose to continue measuring precise transit times of K2-21b and K2-21c with Spitzer and, when combined with existing K2 timing constraints, improve their mass constraints by a factor of two. Understanding the planets' masses is a critical, first step to ultimately determining their atmospheric compositions and internal structures using JWST. Preliminary mass constraints suggest these temperate super-Earth planets are favorable for atmospheric characterization, with predicted one-scale-height signal sizes of 30 - 50 ppm. These planets will provide an excellent test to current statistical arguments that suggest there is a turning point in composition from rocky, true-to-name super-Earths to volatile-rich sub-Neptunes in the range of 1.5 - 2 Earth-radii.
- Publication:
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Spitzer Proposal
- Pub Date:
- March 2019
- Bibcode:
- 2019sptz.prop14244S