“Habitable zone” is sometimes used more generally to denote various regions that are considered favorable to life in some way. One prominent example is the Galactic habitable zone’ (the distance from the galactic centre), based on the position of the Earth in the Milky Way. If different kinds of habitable zones are considered, their intersection is the region considered most likely to contain life.
The location of planets and natural satellites (moons) within its parent star’s habitable zone (and a near circular orbit) is but one of many criteria for planetary habitability and it is theoretically possible for habitable planets to exist outside the habitable zone. The term “Goldilocks planet” is used for any planet that is located within the circumstellar habitable zone (CHZ) although when used in the context of planetary habitability the term implies terrestrial planets with conditions roughly comparable to those of Earth (i.e. an Earth analog). The name originates from the story of Goldilocks and the Three Bears, in which a little girl chooses from sets of three items, ignoring the ones that are too extreme (large or small, hot or cold, etc.), and settling on the one in the middle, which is “just right”. Likewise, a planet following this Goldilocks Principle is one neither too close nor too far from a star to rule out liquid water on its surface. While only about a dozen planets have been confirmed in the habitable zone, the Kepler spacecraft has identified a further 54 candidates and current estimates indicate “at least 500 million” such planets in the Milky Way.
Habitable zones, however, are not stable. Over the life of a star, the nature of the zone moves and changes. Astronomical objects located in the zone are typically close in proximity to their parent star and as such more exposed to adverse effects such as damaging tidal forces and solar flares. Combined with galactic habitability, these and many other exclusionary factors reinforce a contrasting theory of interstellar “dead zones” where life cannot exist, supporting the Rare Earth Hypothesis.
Some planetary scientists have suggested habitable zone theory may prove limiting in scope and overly simplistic. There is growing support for equivalent zones around stars where other solvent compounds (such as ammonia and methane) could exist in stable liquid forms. Astrobiologists theorise these environments could be conducive to alternative biochemistry. Additionally there is probably an abundance of potential habitats outside of the habitable zone within subsurface oceans of extraterrestrial liquid water. It may follow for oceans consisting of ammonia or methane.
Edward Dolnick tells an escape story involving God, humanity, and a huge rewrite of cosmic laws. It began in 1665. A plague hit Cambridge University. All of the students were sent home. One of them is a twenty-something Isaac Newton, who spent his forced summer vacation solving “the problem of the moon” and explaining why that heavenly rock will never be free.
Sucks for the moon. But Newton’s mental leap ultimately lead to humanity leaving the confines of planet Earth. And as producer Lynn Levy explains, we’re about to reach yet another new frontier. The Voyager probe (which we talked about in our Space episode) is about to become the first human-made object to leave the solar system. And the information it’s been sending us along the way has upended what we thought we knew about our little corner of the universe. Merav Opher is an astronomy professor at BU and a Voyager guest investigator. Ann Druyan is one of the creators of the 1977 Golden Album traveling on the Voyager probe. Together they describe how Voyager continues to surprise us.