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Laboratory for Atmospheric and Space Physics

Science

Overview

The 2010 Astrophysical Decadal Survey (New Worlds, New Horizons in Astronomy and Astrophysics) seeks to learn more about the diversity of extrasolar planetary systems. Understanding the physical and chemical evolution of exoplanetary atmospheres is fundamental to questions of the long-term climate and potential habitability on extrasolar planets; CUTE will conduct a survey of mass-loss on short-period, giant planets where atmospheric escape and interactions with the host star have the largest observable effects. CUTE will obtain time-resolved, NUV transit spectroscopy of roughly 12 exoplanetary systems, covering between 5 and 10 transits per system.

Atmospheric Composition and Mass-loss

For a planet to be habitable, our current view is that it must lose its primordial hydrogen atmosphere and acquire/generate (and subsequently retain) a secondary atmosphere. Atmospheric escape is known to have shaped the early atmospheres of Venus, Earth, and Mars, all of which followed different evolutionary paths. However, these planets no longer experience very high mass-loss rates. Thus, we turn to exoplanets to provide laboratories for the study of atmospheric escape.

CUTE’s target list is filled with hot Jupiters and hot Neptunes, planets that are similar in size and density to Jupiter and Neptune (larger gaseous planets), but with a close proximity to their host star, making them extremely hot. CUTE’s primary targets have orbital periods between 1 and 5 days, meaning that the star-planet distance is just a few percent of Earth’s distance to the Sun.  The close distance heats the planets up, and astronomers believe their atmosphere expands and puffs up as a result of the stellar heating.  In some cases, the stellar environment may be extreme enough to blow away the planet’s atmosphere and inducing atmospheric mass loss.

When these planets transit in front of their host star, some of the starlight is blocked by the planet body and some of it is transmitted through the planet’s atmosphere. By comparing how the starlight changes when the planet is transiting to the unblocked starlight, we can learn about the shape, size, and composition of these atmospheres. The mass loss these gassy planets undergo contributes to the atmosphere’s evolution. CUTE will use repeated measurements of several short-period exoplanets in the NUV regime to observe the relevant atomic, molecular, and continuum bands of close-in exoplanets. Continued observations and the buildup of several light curves will allow us to translate planetary transit light curves into atmospheric mass-loss rates.

Exoplanet Magnetic Fields

Exoplanets are a critical component of many planets in our solar system, likely influencing the habitability of rocky planets and the potential habitability of the satellites of gas giant planets. However, there is currently no confirmed magnetic field detection for an exoplanet. Tentative detections of cyclotron maser emission and ultraviolet aurorae from extrasolar giant planets have provided tantalizing suggestions of exoplanetary magnetism, but existing studies have been largely inconclusive.

The space between a planet and its host star is filled with particles carried by the stellar wind and embedded in the stellar magnetic field. For short period planets with magnetic fields, the interaction of the stellar wind and the exoplanet’s magnetosphere may set up a standing bow-shock structure as the planet orbits. This shock structure has been predicted to be observable in the shapes of NUV light curves, and we will search for this structure on CUTE targets.