Speaking at a meeting held by the European Geosciences Union in the Azores this week, Alessandro Morbidelli of the University of Nice, France, gave an intriguing talk on the possible habitability of so-called “Super-Earth” planets, which are thought to be like our own world in many respects, but have several times the mass of Earth.
Scientists used to think these planets were simply scaled-up versions of Earth, but that idea has fallen out of favor. Earth grew to its present size relatively slowly, taking about 40 million years to accrete 90 percent of its mass. Most of that mass came from a series of giant impacts after the initial protoplanetary disk of gas surrounding our Sun disappeared.
Super-Earths, on the other hand, grow much faster than Earth-mass planets at a time when gases—mostly hydrogen and helium—in the protoplanetary disks are still around. These light gases then become incorporated into the planets’ atmospheres. Due to their fast growth and high mass, Super-Earths also are subject to orbital migration, meaning that they end up somewhere else in the Solar System from where they originated.
In fact, many of the planets designated as Super-Earths (based on their mass) are now thought to be Neptune-size gas giants, with a lot of water. Morbidelli further suggests that a lot of Super-Earths are ice-rich bodies that migrated inward from the outer reaches of a solar system. He also concludes that terrestrial, rocky Super-Earths (planets that have an iron core, a silicate mantle, and a crust like Earth or Mars) can generally not be more massive than 2 or 3 Earth masses. Super-Earths are also exposed to higher amounts of heating due to their fast growth and presence of radioactive isotopes.
The tendency of planets to migrate early in a solar system’s history—like a planetary billiards game—would preclude the kind of stable environments that life needs to evolve. And a primitive, hydrogen-helium atmosphere would be very challenging for life. Exceptions to this general principle may still exist. For example, a planet’s original atmosphere can be lost due to radiation or impacts, as happened on Earth.
Even so, it seems that Earth’s mass is close to the optimum for a life-hosting planet. Given the new modeling results and insights, it also appears that most Super-Earths are likely not to be habitable. In the end, however, only observations will answer this question for certain. Fortunately, we should able to determine the atmospheric compositions of some Super-Earth planets within a decade.