The four astronauts of SpaceX's Crew-11 mission splashed down safely today (Jan. 15) after the first-ever medical evacuation of the International Space Station.
At the beginning of the exoplanet age, the goals were fairly simple. The first was to find as many of them as possible to flesh out our understanding of the exoplanet population. The second was to determine if any were in the habitable zones around their stars. The definition of a habitable zone was necessarily simple in the beginning. Any planet in the right distance range from its star to allow liquid surface water was considered to be in the habitable zone.

