The large extent of the Earth's exosphere (>30RE) was not foreseen and needs investigation. The Moon orbit provides a vantage point for this study. The OUL is a lunar orbiter designed to enable the investigation of the roles of the lunar gravity and the solar wind on exospheric physics.

Observations of the Earth's exosphere carried out by the NASA missions IMAGE/GEO (2003) and TWINS (2010) discovered that the Earth is surrounded by an extended envelope of hydrogen reaching further than 10 Earth radii, not previously foreseen, and composed of atoms orbiting around the Earth. Later the instrument LAICA on board the JAXA PROCYON probe, obtained a Lyman-alpha (Lya) image that showed that the exosphere extends further than 30 Earth radii, probably reaching the Roche lobe. This large envelope increases significantly the opacity of the Earth to Lya photons coming from the Sun, to the point of making feasible the detection of the Earth's transit signature from nearby stars. Understanding the physics behind the formation and stability of the Earth's exosphere, its interaction with the solar wind and the impact of the lunar gravitational field on it, it is crucial for geophysics but also planetology and the search of exoplanets. The Moon provides a unique, vantage point to study this huge structure. In ~28 days, a full orbit is completed and an unbiased 3D view of the exosphere is obtained.

In this contribution, I will describe the current status of our understanding of the Earth's exosphere, its implications for exoplanetary research and the activities being carried by the consortium I lead to set a lunar probe to study the exosphere of the Earth, primarily and other planets (Mars, Jupiter) from lunar orbit. In particular, I will describe the Observatorio Ultravioleta Lunar (OUL). The OUL is a wide field imager designed as a small, additional payload susceptible to be attached to any lunar mission. The instrument has a field of view of 20◦ x 20◦ and provides images with angular resolution 3 arcmin in several far ultraviolet bands, including Lya, He II at 164nm and several continuum bands that will enable measuring the degree of hydration of near Earth bodies.