On Monday 11th November 2019, viewers around Earth (using the proper safety equipment) were able to see a dark object moving slowly across the disk of the sun. This was the planet Mercury, which orbits closer to the sun than Earth does and can occasionally be seen in silhouette when it passes between our planet and the star. Around the 18th century, astronomers were able to use transits like these to calculate the distance between the Earth and the sun more accurately than their predecessors. Nowadays the transit of Mercury is viewed more as a curiosity but it is still a rare spectacle which can only be seen around 13 times every 100 years; the next one won't happen until 2032! Take a look at the 2016 transit in the following animation.
The 2019 transit occurred between 12:35 and 18:04 UTC and was observed by members of the SP2RC group from the Hicks building at the University of Sheffield. Interested persons were encouraged to join us and view the event through our solar telescopes, or watch our livestream on YouTube. During a transit Mercury can be hard to spot without a telescope, and a solar filter is essential to prevent damage to your eyes. Despite unfavourable weather conditions, the SP2RC group were able to observe the 2019 transit and capture some images. Mercury appears as a small black dot in the video below.
When a planet passes in front of a star it blocks a tiny portion of the light, which we measure as a slight recurring dip in the light curve (a graph of light intensity - see video below). Scientists look for these dips in light intensity to search for exoplanets orbiting distant stars, as the planets are too small and dim to observe directly. NASA's Kepler mission has found more than 1,000 exoplanets by looking for this telltale drop in brightness.
A transit can also shed light on the composition of a planet's atmosphere. Mercury in particular is surrounded by a tenuous layer of gas called an exosphere, similar to the outermost layer of the Earth's atmosphere (unlike Earth, Mercury has no other layers beneath its exosphere). By observing the spectra of the light that passes through an exosphere during a transit, scientists can learn more about exoplanet atmospheres as well as the influence of stellar wind and magnetic fields.