TESS has recently discovered LTT 1445Ab, a small transiting exoplanet with a radius of only slightly larger than Earth. The planet orbits a mid-type M-dwarf star every 5.35 days, giving it a relatively cool equilibrium temperature of 433 K. The planet is very close to Earth, at a distance of only 6.9 pc, which makes it an extremely good target for atmospheric characterization due to the brightness of its host star and relatively large atmospheric signal. The planet is so close that it is likely the closest transiting planet that will ever be found around a mid-type M-dwarf, making it a rare case where a near-Earth size exoplanet can be characterized. Recent discoveries of water in the atmospheres around two similar planets in M-dwarf systems indicate many of these small transiting planets have hydrogen-rich atmospheres detectable with HST. Our program will measure HST/WFC3 spectroscopy of LTT 1445Ab to measure its broadband transmission spectrum from 0.3 to 1.7 microns. We will use the atmospheric information to help distinguish the exoplanet as a super-Earth with a compact secondary atmosphere from a hydrogen-rich sub-Neptune. With all three WFC3 grisms, a continuous transmission spectrum can be measured where Rayleigh scattering, CH4, and H2O can be constrained. The new, extraordinary planet LTT 1445Ab offers a rare chance to study a nearly Earth-sized exoplanet across the full optical to infrared spectrum, pushing these measurements to a largely unexplored regime of smaller, cooler exoplanets.
- Pub Date:
- October 2019