Statistical analysis of existing exoplanetary catalogues has shown that somewhere between 2% and 60% of Sun-like stars may harbour planets similar to the Earth (or super-Earth) in their habitable zones. Although we now know planets orbiting other stars are not uncommon, the occurrence rate of Earth-like planets in the habitable zone of Sun-like stars remains a highly debated topic. Today, we know of more than 4000 such worlds, ranging from gas giants more massive than Jupiter to rocky, terrestrial-type planets considered to be candidates for harbouring life. Since the first discoveries of planets orbiting other stars in the 1990s, exoplanetary research has expanded explosively. This article is part of a discussion meeting issue ‘Astronomy from the Moon: the next decades’. Here, we present both the science case and the technology behind LOUPE’s instrumental and mission design. 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. These 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. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour life. Characterizing such planets and searching for traces of life requires the direct detection of their signals. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as an exoplanet.
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