What is a transit?

In astronomy, a transit occurs when one celestial object passes in front of another, like a planet in front of a star. In our own Solar System it is possible to view the transit of Mercury or Venus across the face of the Sun. Because of the how planet orbits line up, transits are quite rare, but they allow astronomers to measure distances in our Solar System, and also test techniques for studying planets in exoplanet systems. For example, a transiting planet causes a dip in brightness of its host star, revealing the presence of an exoplanet.
Space missions like ESA’s Cheops will study known transiting exoplanets to determine more about their characteristics.
On 11 November 2019 a transit of Mercury takes place in our own Solar System, the next will be in 2032. While Mercury appears very small on the disc of the Sun, it is possible to view through telescopes – with special solar filters applied. Never look directly at the Sun; always use special solar filters to protect your eyes.

Both Mercury and the Sun are destinations for ESA missions: BepiColombo will arrive at Mercury in 2025, while Solar Orbiter is getting ready for a 2020 launch to study the Sun up close.