Solar Orbiter captures the transit of Mercury by the Sun

20 Feb. () –

Several instruments aboard the ESA/NASA Solar Orbiter mission captured a transit of Mercury in front of the Sun on January 3 from the point of view of the social ship.

The generator Extreme Ultraviolet Imaging (EUI) took a movie of the planet trailer. In particular, he showed Mercury just after it left the disk and was silhouetted against the gaseous structures in the sun’s atmosphere.

On instrument observation Polarimetric and Helioseismic Imager (PHI), Mercury appears as a black circle in the lower right quadrant of the image. It is clearly distinguished from sunspots that can be seen higher up on the solar disk.

The Spectral Imaging of the Coronal Environment (SPICE) instrument splits light from the Sun into its constituent colors to isolate the light from the various atoms in the Sun’s lower atmosphere. These atoms have been chosen to reveal the various layers of the Sun’s atmosphere, that exist at different temperatures. Neon (Ne VIII) is at a temperature of 630,000 K, carbon (C III) at 30,000 K, hydrogen (Ly Beta) at 10,000 K, and oxygen (O VI) at 320,000 K.

“It’s not just about observing Mercury passing in front of the Sun, but passing in front of the different layers of the atmosphere,” he says. it’s a statement Miho Janvier of the Institut d’Astrophysique Spatiale, France, a SPICE Associate Project Scientist currently seconded to ESA.

Astronomers have used planetary transits for various purposes. In centuries past, they were used to calculate the size of our Solar System. Observers located in places very distant from each other timed the transit and compared the results. Since they were watching from different locations, the exact timing of the event would be slightly different. Knowing the distance between the observers, they could use trigonometry to calculate the distance to the Sun.

More recently, transits have become the most efficient way to find planets around other stars. As the planet moves across the face of the star, the bright surface is marginally covered by the planet’s silhouette, thereby dimming somewhat. The regular repetition of this phenomenon makes it possible to calculate the size and orbit of the planet.

ESA uses the transit method to study exoplanets on its current mission Cheops (CHaracterising ExOPlanet Satellite). In the near future, the PLAnetary Transits and Oscillations of stars (PLATO) mission will use transits to search for Earth-sized planets in the habitable zones of up to a million stars. And in 2029, ESA’s Ariel mission (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) it will use transits to study the atmospheres of some 1,000 known exoplanets.

For Solar Orbiter, this particular transit offered a valuable opportunity to calibrate the instruments. “It’s a certified black object that travels through your field of view,” says Daniel Müller, Solar Orbiter Project Scientist at ESA. Therefore, any brightness recorded by the instrument within Mercury’s disk must be due to the way the instrument transmits its light, called the point spread function. The better this is known, the better it can be eliminated. Thus, studying this phenomenon will further improve the quality of the Solar Orbiter data.