Comparison of transit paths

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.]