Why the “habitable zone” doesn’t always mean habitable

In our Solar System, there are two planets too hot to host liquid water, five planets too cold for it, and just one in between. This middle ground — a range of distances from the Sun where surface water can flow and life could evolve — is called the “habitable zone”; occasionally, the “Goldilocks zone.” This concept seems simple at first glance, but it actually presents a number of challenges for astronomers who wish to communicate new discoveries to the public.

The gap between a planet which could have liquid surface water and a planet which hosts life is a very broad one. News outlets often fall into it, flattening the complex topic of habitability into a snappy shorthand that generates interest and excitement. But to cross that gap, many criteria must be met. They can be organized into galactic, stellar, and planetary. 

We don’t often consider the role our location in the galaxy plays in life on Earth, but only because we are—more or less—ideally located. The Sun lives about halfway between the Milky Way’s star-dense, radioactive center (the “Bulge”) and its cold, barren edge. We are not bathed in the radiation kicked out by our central supermassive black hole, nor does our star swing close enough to others to perturb its planets. In the disk, there is plenty of gas and dust for a star system like our own to form, unlike our galaxy’s sparse, ancient halo. Scientists sometimes refer to our stellar neighborhood as a galactic habitable zone.

Stars themselves can be — and often are — bad hosts. 73% of all stars belong to a class of small, but cantankerous objects known as “M” stars. Our own Sun produces flares, and occasionally highly energetic ones known as coronal mass ejections (CMEs). CMEs can be one hundred times more intense than flares, and when one hits the Earth with enough power, it can damage our electrical grid and create dramatic aurorae across the globe. But for the first one to ten billion years of an M star’s life, it lashes its planets with flares and CMEs on a weekly-to-monthly basis. This constant barrage can quickly strip a planet’s atmosphere, leaving the surface airless and unprotected from space. 

Not only that, but M stars experience a drastic drop in brightness during their long formation period. This causes the habitable zone to shrink inwards toward the star, like in the figure below. Earth may have always lived in the Sun’s habitable zone since it formed. But a planet which begins its life in an M star’s habitable zone may find itself frozen over as the zone recedes; one born close to the star may have had its water boiled off by the time its habitable days come around.

And there are more problems with M star systems. Here on Earth, we have a 24-hour day, which allows our planet to be evenly heated by the sun. This is, surprisingly, not the case for many planets. Consider our moon. We see the same face, no matter the phase, or the time of year, or where we are on Earth. It’s tidally locked. There are many factors that determine if and when a smaller celestial object will tidally lock to a larger one, but the closer an object is, the faster it will lock. The habitable zone of an M star is far closer in than the habitable zone of our sun, so we anticipate that most of these planets orbit their star the way our moon orbits us: with a permanent dayside and a permanent nightside — perhaps a more challenging environment for life to arise.

We live in a pleasant suburb of the Milky Way and orbit a quiet star, but there are also a number of things about our planet itself that make it so amenable to life. Earth has a rocky surface featuring both ocean and land as well as the slow grind of plate tectonics. Besides rearranging the continents every half-billion years or so, plate tectonics also — very slowly — removes carbon dioxide from our atmosphere by burying surface materials the gas has reacted with into the mantle. Our active geology provides us with other benefits as well: Earth’s moving iron-nickel core generates a protective magnetic field. Without it, even the most well-behaved star could slowly erode away the air we breathe. 

All of this is to say that when we read a news story that proclaims the discovery of a brand-new habitable zone planet, we cannot, and should not, rush to conclusions. “The major misconception is the assumption that the habitable zone is a hard and fast rule for habitability,” said Dr. Adeene Denton, a planetary scientist and former Astronomer in Residence at the Grand Canyon. They compared the concept of the habitable zone to “bumpers on a bowling lane… their role is to guide us to a specific destination.” But, Dr. Denton cautions, even this is an Earth-centric picture. There may very well be life thriving in the methane-ethane lakes of Titan, the subsurface oceans of moons far outside our habitable zone, or in pools of ammonia in other star systems. Without being certain just how unique the Solar System is, it is hard to even judge the utility of the habitable zone. 

The concept of habitable-zone planets risks more than Earth-centrism — it can easily mislead the public. One such finding was TOI-700 d, the first habitable-zone Earth-sized planet located by the Transiting Exoplanet Survey Satellite. “Every time you publish a habitable zone planet result, there’s always some reaction about whether or not it’s actually inhabited,” said Mr. Jonathan Brande, a Ph.D candidate at the University of Kansas who contributed to the discovery. “There's a thin line to tread between getting people excited about astronomy and not accidentally confusing them.” TOI-700 d is one of many planets discovered to orbit an M star, so its habitability is far more up for debate than if it orbited a star like our own. Near-term studies of the planet will reveal whether it has an atmosphere, but we will need to wait for the Habitable Worlds Observatory of the 2040s to study it in depth, added Mr. Brande. 

The narrative given to the public is just as important as the science surrounding habitability. We must remember that the habitable zone is — as they say — the beginning of wisdom, not the end, and that wisdom must be handled responsibly, for the good of the field and the people who love it.