Observing the Earth as an exoplanet with LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth

The detections of small, rocky exoplanets have surged in recent years and will likely continue to do so. To know whether a rocky exoplanet is habitable, we have to characterise its atmosphere and surface. A promising characterisation method for rocky exoplanets is direct detection using spectropolarimetry. This method will be based on single pixel signals, because spatially resolving exoplanets is impossible with current and near-future instruments. Well-tested retrieval algorithms are essential to interpret these single pixel signals in terms of atmospheric composition, cloud and surface coverage. Observations of Earth itself provide the obvious benchmark data for testing such algorithms. The observations should provide signals that are integrated over the Earth's disk, that capture day and night variations, and all phase angles. The Moon is a unique platform from where the Earth can be observed as an exoplanet, undisturbed, all of the time. Here, we present LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth, a small and robust spectropolarimeter to observe our Earth as an exoplanet.Comment: 14 pages, 3 figures, submitted in special Issue of Planetary and Space Science on Scientific Preparations for Lunar Exploratio

LOUPE: Observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets

LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter with a mission to observe the Earth as an exoplanet. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterising such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarisation data of sunlight reflected by the Earth, the only planet known to harbor life. This data will be used to test numerical codes to predict signals of Earth-like exoplanets, to test algorithms that retrieve planet properties, and to fine-tune the design and observational strategies of future space observatories. From the Moon, LOUPE will continuously see the entire Earth, enabling it to monitor the signal changes due to the planet's daily rotation, weather patterns, and seasons, across all phase angles. Here, we present both the science case and the technology behind LOUPE's instrumental and mission design.Comment: 13 pages, 5 figures. Accepted for publication in Royal Society Philosophical Transactions A. Corrected typos in v

Earth-as-an-exoplanet: comparing earthshine observations to models of an exo-Earth

Traditional methods of exoplanet characterization that only make use of emitted or reflected flux lack the ability to fully distinguish between different physical features of the target, such as cloud layers, hazes, or surface features. Polarimetry, however, is a powerful, more sensitive technique that has this ability, as it measures light as a vector (by the orientation of the electric field) rather than a scalar intensity. It is therefore extremely sensitive to the composition and structure of the planetary atmosphere and surface, being affected by properties such as the mixing ratios of atmospheric absorbing gases, cloud optical thickness, cloud top pressure, cloud particle size, and surface albedo. Various groups have theoretically studied the optical linear polarimetric signals of Earth-like exoplanets as functions of both orbital phase and wavelength. With this project we assess the accuracy of these theoretical models against observations of the Earthshine, the only known observations of an Earth-like planet thus far. Using data of the atmosphere and surface taken by the MODIS instrument aboard the Terra and Aqua satellites, as well as surface reflectance spectra from the JPL EcoStress Spectral Library, we created a gridded model of the Earth. Then, using this model data as input for three separate radiative transfer algorithms, we generate the flux and linear polarization spectra for the model exoplanet-Earth across the optical to near-infrared wavelengths. We compare the results from all three codes to each other and to the observational linear spectropolarimetric data of the Earthshine obtained by a member of our group. We identify similarities and potential pitfalls between the codes, and make necessary adjustments to them, in an effort to improve our future characterizations of terrestrial exoplanets.Stars and planetary system

Comparing models of an exoplanet-earth to earthshine observations

Polarimetry is widely becoming recognized as a powerful technique for enhancing the contrast between a star and an exoplanet, and thus improving upon the direct detection of exoplanets. The real power of polarimetry, however, is in its ability to characterize the physical properties of these worlds. This is because the state of the polarization of the light from the planet is very sensitive to the composition and structure of the planetary atmosphere and surface, being affected by properties such as the mixing ratios of atmospheric absorbing gases, cloud optical thickness, cloud top pressure, cloud particle size, and surface albedo. Various groups have theoretically studied the optical linear polarimetric signals of Earth-like exoplanets as functions of both orbital phase and wavelength. This project aims to validate the accuracy of these theoretical models against the only known observations of an Earth-like planet thus far: Earthshine. Using atmospheric and surface data taken by the MODIS instrument aboard the Terra and Aqua satellites, as well as surface albedo spectra from the EcoStress Spectral Library, we created a detailed model of the Earth. Then, using this model data as input for three separate radiative transfer algorithms, we generate the flux and linear polarization spectra for the model exoplanet-Earth from the optical to near-infrared wavelengths. We compare the results from all three codes to each other and to observational linear spectropolarimetric data of the Earthshine obtained by a member of our group. We identify similarities and potential pitfalls between these codes in an effort to improve our future characterizations of Earth-like exoplanets.Stars and planetary system

LOUPE: observing the Earth from the Moon to prepare for detecting life on Earth-like exoplanets

Instrumentatio

A snapshot full-Stokes spectropolarimeter for detecting life on Earth

Instrumentatio