Given the recent discovery of thousands of exoplanets—many of them in the so-called habitable zone of their host star, where liquid water is stable on the planet’s surface—the paradigm has shifted to the point where scientists now believe there are a lot of Earth-like planets in our galaxy. By far, the largest number of known exoplanets orbit dM-stars, or red dwarfs. That’s no coincidence, because 80 percent of the stars in our galaxy are thought to be dM-stars, which have less than half the mass of our Sun.
Smaller size and lower luminosity have generally not been considered a barrier to life developing on planets orbiting red dwarfs. But recent computer modeling by Feng Tian from Tsinghua University in China and Shigeru Ida from Tokyo Institute of Technology in Japan, as reported in Nature Geoscience, raise new doubts.
The researchers included well-known physical properties of dM-stars in their computer model, such as the prolonged decline in luminosity early in the star’s history, and the resulting inward migration of the habitable zone. The results suggest that the evolution of dM-stars favors the extremes: Either you end up with desert planets, with many orders of magnitude less water than on Earth, or you end up with ocean planets. Neither scenario would be conducive to the complex life we see on our planet, because you need both oceans and continents for a diverse and complex biosphere. The rise of technological intelligence is particularly difficult to imagine without continents, where key evolutionary innovations such as the use of fire and electricity arose.
According to the researchers’ simulations, Earth-like continent-ocean planets should be 10 to 100 times less frequent around red dwarf stars than around Sun-like stars. In addition, the land-ocean distribution on an exoplanet is only one factor. Many other ingredients go into making a planet habitable, such as the presence of a suitable atmosphere, magnetic shield, organic chemicals, etc. And even if a planet is habitable, it doesn’t mean that it’s inhabited, because the conditions for the origin of life are likely much more constrained than the conditions necessary for life to persist once it exists.
All of which means that planets like Earth may be rare after all. And these new results may motivate scientists to focus their search for habitable planets on the less common “yellow dwarfs” like our Sun.