Are there any other planets like the Earth? In the Solar System, there is no other planet that has continents and oceans like the Earth, and on which life evolves and thrives. However, there are planets outside the Solar System that are about the same size as the Earth and neither too close nor too far from their star for oceans to exist on their surface. But do these planets have an atmosphere like the Earth and oceans? That question has yet to be answered.

Within the Solar System, it is possible to image planets such as the Earth and Venus and easily determine if they host an ocean. But planets outside the Solar System are so distant that images cannot currently capture their surface features. While it is not possible to see an ocean on an exoplanet directly, it is possible to observe and analyse the spread of the atmosphere above the planet. On planets with an Earth-like atmosphere, the oxygen atoms cause the atmosphere to extend far above the planet. Conversely, the atmospheres of planets similar to Venus do not spread out nearly as far. This provides a way to identify the nature of the planet by observing the how far the atmosphere extends.

In order to study the upper atmosphere of planets outside the Solar System, it is necessary to observe faint ultraviolet rays. For this, a large UV space telescope is needed, with a highly sensitive instrument for observations. By combining the 1.7m World Space Observatory Ultraviolet (WSO-UV) which is under development in Russia, with the UV Spectrometer for Exoplanets (UVSPEX) that is being developed in Japan, we aim to observe the spread of the atmosphere over a planet similar to the Earth for the first time.

UVSPEX is being developed primality at JAXA and Rikkyo University in Japan, and a prototype design is currently being completed. In Russia, researchers at the Institute of Astronomy (INASAN) and the Space Research Institute (IKI) are leading the development of the part that will connect UVSPEX and the WSO-UV main unit. A Japanese research team is additionally leading simulations of the observations of the planetary atmosphere that UVSPEX will obtain, including the selection of observation targets.

Observations of exoplanets are shifting from the discovery of a large number of planets to characterisation of the planet environment through atmospheric observations. UVSPEX is a plan to bring atmospheric observations quickly to the terrestrial worlds.