dimitar_d_sasselov's picture
Professor of Astronomy, Harvard University; Director, Harvard Origins of Life Initiative; Author, The Life of Super-Earths
Habitable Zone

The habitable zone defines those distances from a star where a planet similar to Earth would have the surface temperatures for water to be liquid. In the Solar System this zone stretches from in-between the orbits of Venus and Earth out to Mars. Its boundaries are approximate as they are applied to different planetary systems and sometimes the concept is used more broadly, e.g., to galaxies. The habitable zone concept has a venerable history in the search for alien life beyond Earth and most recently it contributed to the spectacular success of the NASA Kepler exoplanet-hunting mission. However, in the post-Kepler era it is a scientific concept that is ready for retirement.

The simple definition of the habitable zone is appealing to use in statistical estimates of habitable planets, because it depends on few parameters that are easy to measure. It is also easy to grasp: not too hot, not too cold—the Goldilocks zone. Simple and robust statistics are crucial to estimating the abundance and distribution of small planets like Earth in the Galaxy and the Kepler space mission excels at that. If our goal now is to search for life, then it is good to know where we should be heading to—habitable exoplanets. Then the "habitable" in the habitable zone is a misnomer, or a gross overstatement at the least. Even in our Solar System we contemplate alien life beyond its confines, e.g. on moons of Jupiter and Saturn. Today we need a concept of what makes an environment habitable—capable of letting life emerge and keep sustaining it over geological timescales, be it on a planet or on a moon. Finding out what makes a planet living and how to recognize a living planet with our telescopes is the big question.

The past year has been historic in the search for alien life. Thanks to Kepler and other exoplanet surveys we now know that Earth-like planets are so common that many close analogs to our home planet should reside in our neighbourhood of the Galaxy. This makes them amenable to remote sensing exploration with existing technology and telescopes under construction. The search for life is set to begin, but we need to understand better what to look for.

In retiring the habitable zone concept, it makes sense to revert to its original name—circa mid-20th century, as the "liquid water belt"—a region very important to the rich geochemistry of rocky planets. Living planets among them will feel like home.