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.