An astronomical transit occurs when a planet on its orbit moves in front of its star.
All planets orbit their host star. In the Solar System, the eight planets orbit the Sun.
When trying to discover planets around stars other than the Sun, so-called exoplanets, astronomers observe the light of many stars in search of small brightness variations.
When a planet moves in front of their host star, a tiny spec of the star’s disk is obscured, resulting in a very small reduction in observable brightness in the star’s light. The time it takes for the planet to move across the stellar disk is called a transit.
Afterwards, the star’s brightness is back to its full amount. With repeat observations that establish a so-called lightcurve, the entire transit event can be recorded. Its depth and length give clues to the properties of the planet and its orbit around the star. Over the last decade, hundreds of exoplanets have been found this way.
Venus, the second innermost planet in the Solar system, can sometimes be observed from Earth to transit the Sun’s disk.
Photo Credit: NASA
The Planck satellite measured the distribution of the very subtle temperature fluctuations of the photons left over from after the Big Bang (shown in red vs blue patches). The fluctuations reflect tiny density variations from which all structure and galaxies later formed, including the Milky Way.