The discovery potential of a telescope is hard to measure. However, one key indicator of huge discovery potential is the opening of a new parameter space: looking somewhere that nobody has been able to explore before is very likely to yield new discoveries.  

Many of the most unique discoveries made with telescopes that came before the ELT were unforeseen. The Hubble Space Telescope is perhaps most famous for its spectacular observations of “deep fields” of galaxies, such as the Hubble Deep Field. But observing a small patch of sky and discovering some 3000 objects, mostly young and distant galaxies, was not one of the initial aims of the project. Similarly, ESO’s 3.6-metre telescope at its La Silla Observatory, which is known, amongst other things, for the revolutionary exoplanet discoveries made with its HARPS instrument, was first commissioned in 1977, at a time when exoplanets were totally unheard of. 

By opening new windows into the Universe, the ELT is likely to make discoveries that nobody has ever even thought about. This will trigger theorists to find explanations, observers to verify and find new targets, and engineers to continue pushing the boundaries of technology.  

The ELT will open such new frontiers in at least two ways. First, thanks to its immense collecting power, the telescope will increase the sensitivity of observations by up to a factor of 600. Indeed, the ELT will have more collecting area than all the large 8–10-metre-class telescope combined. Furthermore, it will increase the spatial resolution of images by an order of magnitude, taking even sharper images than future space telescopes.   

These leaps forward in what a telescope can do — coupled with other advances such as unprecedented spectral resolution, and new levels of contrast that allow us to see very faint objects next to very bright objects — mean that the ELT will open up an entire universe of possibilities. Despite the incredible discoveries we expect to make using the ELT, it is in this great unknown that the ultimate excitement lies.