HDST‘s primary goal is to find and characterize dozens of Earth-like exoplanets. A sample of dozens of exoEarths opens up the opportunity to identify truly Earth-like worlds with rocky surfaces and oceans, amidst a complex zoo of other varieties of terrestrial planets. With this large sample, observing telltale signs of life in the planets’ atmospheres becomes possible. If life is rare, HDST will take us from our current complete ignorance of the occurrence rate of inhabited worlds to a first constraint, potentially showing how remarkable our own existence is. If life is common, a large sample of terrestrial worlds with highly unusual atmospheric chemistry will secure our belief that life of some kind exists beyond the Earth, regardless of possible false positives. Whatever the outcome, HDST will change how we see our place in the Universe.

While thousands of exoplanets are already known to exist, none are yet known to be truly Earth-like, even though some have radii similar to Earth’s. Distinguishing habitable worlds like Earth (i.e., those with surface water oceans) from greenhouse planets like Venus, or barren worlds like Mars, requires understanding a planet’s atmosphere. HDST will therefore not just image new worlds, but will also acquire spectra of their atmospheres at visible (and in some cases out to near-infrared) wavelengths to search for signs that indicate a potential planet like our own.

HDST will search for exoEarths around hundreds of stars, but during that quest will revolutionize the study of planetary systems in general. HDST will discover planets of all sizes, and any surrounding debris disks. Such discoveries will not only place detected exoEarths in context within their own planetary systems, but are also interesting in their own right.

While the search for the exoEarths that are waiting to be discovered is compelling, the search is not easy. Not only are these planets intrinsically faint, they also orbit very bright host stars. Viewed from another star, our Earth’s reflected light would be 10 billion times fainter than the Sun itself, with an orbit that separates the Earth from the Sun by a tiny fraction of an arcsecond. These challenges can be overcome if HDST meets three significant goals. First, HDST must have a large primary mirror area both to gather enough photons (exoEarths are as faint as the faintest objects in the Hubble Deep Field) and to cleanly separate the planet and star for hundreds of star systems, many of which are tens of parsecs[2] away. Second, HDST must have exquisite starlight suppression that blocks out the starlight to 1 part in 10 billion for planet-star projected separations of about 35 milliarcseconds (equivalent to the width of a human thumb viewed from a distance of 130 km). Third, HDST must be thermally and dynamically stable for this starlight suppression to perform at the needed level.

Major advances in all areas of astrophysics are possible with HDST. A telescope with HDST’s degree of sensitivity, resolution, and stability will transform current understanding of how galaxies, stars, and planets form and evolve. With its high-definition resolving power, HDST has the amazing ability to take an optical image or spectrum at about 100-parsec spatial resolution or better, for any observable object in the entire Universe, no matter where a galaxy lies within the cosmic horizon. This 100-parsec threshold is the scale of individual star forming regions and dwarf satellites—the constituent building blocks of galaxies.