Researchers in the Department of Earth, Atmospheric, and Planetary Sciences (EAPS) at MIT describe a technique that analyzes data from NASA’s Kepler space observatory to determine the types of clouds on planets that orbit other stars, known as exoplanets.
The team, led by Kerri Cahoy, an assistant professor of aeronautics and astronautics at MIT, has already used the method to determine the properties of clouds on the exoplanet Kepler-7b. The planet is known as a “hot Jupiter,” as temperatures in its atmosphere hover at around 1,700 kelvins.
NASA’s Kepler spacecraft was designed to search for Earth-like planets orbiting other stars. It was pointed at a fixed patch of space, constantly monitoring the brightness of 145,000 stars. An orbiting exoplanet crossing in front of one of these stars causes a temporary dimming of this brightness, allowing researchers to detect its presence.
Researchers have previously shown that by studying the variations in the amount of light coming from these star systems as a planet transits, or crosses in front or behind them, they can detect the presence of clouds in that planet’s atmosphere. That is because particles within the clouds will scatter different wavelengths of light.
Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet.
We use a planetary albedo model to investigate variations in visible wavelength phase curves of the exoplanet Kepler-7b and a theoretical Jupiter-like exoplanet at 2 AU. Thermal and cloud properties for these exoplanets are derived using one-dimensional radiative-convective and cloud simulations. The presence of clouds on these exoplanets significantly alters their planetary albedo spectra. We show that non-uniform cloud coverage on the dayside of tidally locked exoplanets will manifest as changes to the magnitude and shift of the phase curve. We investigate a test case of our model using a Jupiter-like planet orbiting at 2.0 AU from a solar type star to consider the effect of H2O clouds. The model is then extended to the exoplanet Kepler-7b and considers the effect of Mg2SiO4 clouds. We show that, depending on the observational filter, the shift of the phase curve maximum will be ~2-10° for a Jupiter-like planet and up to ~30° ( ~0.08 in orbital phase) for hot Jupiter exoplanets at visible wavelengths. The model presented in this work can be adapted for a variety of planetary cases at visible wavelengths to include variations in planet-star separation, gravity, metallicity, and source-observer geometry. Finally, we tailor our model for comparison with the recent optical phase-curve observations of Kepler-7b with the Kepler space telescope. We show that models where Kepler-7b has slightly more than half of its dayside covered in Mg2SiO4 clouds provide a good fit to the observed phase-curve magnitude and offset. We also investigate the effect of varying particle sizes and sedimentation efficiencies to explore the ranges that do not fit the currently available data for Kepler-7b and HD189733b. Furthermore; we explore the use of 3D temperature maps/models to more accurately portray the varying albedo across the planet.
SOURCES – MIT, Astrophysics Journal