The European Space Agency (ESA) is Europe’s gateway to space. Its mission is to shape the development of Europe’s space capability and ensure that investment in space continues to deliver benefits to the citizens of Europe and the world.
Find out more about space activities in our 23 Member States, and understand how ESA works together with their national agencies, institutions and organisations.
Exploring our Solar System and unlocking the secrets of the Universe
Go to topicProtecting life and infrastructure on Earth and in orbit
Go to topicUsing space to benefit citizens and meet future challenges on Earth
Go to topicMaking space accessible and developing the technologies for the future
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This diagram compares two scenarios for how an Earth-sized exoplanet is passing in front of its host star. The bottom path shows the planet just grazing the star. Studying the light from such a transit could lead to an inaccurate estimate of the planet’s size, making it seem smaller than it really is. The top path shows the optimum geometry, where the planet transits the full disk of the star. Hubble Space Telescope’s accuracy can distinguish between these two scenarios, yielding a precise measurement of the planet’s diameter.
[Image description: A red giant star is in the centre of the image. An exoplanet passing in front of the star (called a transit) is shown in silhouette in a number of steps from left to right. A similar linear trajectory is shown at the bottom of the image. It is called a grazing transit rather than a full transit because it just clips the bottom of the star. This is considered a less accurate observing geometry in estimating the planet’s size. Hubble’s accuracy can distinguish between these two scenarios, yielding a precise measurement of the planet’s diameter.]