It is generally accepted that the key factor in determining whether a planet can support life is the distance from its sun. In our solar system, for instance, Mars is too far and Venus is to close to the sun for liquid water to exist, but Earth is just right for life. Scientists call this the 'habitable zone,' or 'Goldilocks zone.'
But astronomers have increasingly wondered what else makes Earth so unique a world. One key feature of our ever-changing planet is plate tectonics, the process of continents drifting and colliding, rock grinding and scraping, mountain ranges and active volcanoes being formed. After all, plate tectonics once saved Earth from turning into a snowball (as well as a Venus-like hothouse) by regulating greenhouse gases. It also constantly shuttles minerals to the surface that life needs to survive.
Many scientists thought that rocky planets were able to "self regulate" their heat after forming via mantle convection—the underground shifting of rocks caused by internal heating and cooling. A planet might start out too cold or too hot, but it would eventually settle into the right temperature. Plate tectonics, it was assumed, is a given on rocky, Earth-like worlds.
A new study in the journal Science Advances questions that idea and the implications could be enormous. Essentially, we could be overlooking another "Goldilocks" factor in our searches for worlds habitable to aliens: a planet's initial temperature.
"If you assemble all kinds of scientific data on how Earth has evolved in the past few billion years and try to make sense out of them, you eventually realize that mantle convection is rather indifferent to the internal temperature," says study author Jun Korenaga, a geophysicist at Yale University. "Self-regulation is unlikely for Earth-like planets and this has enormous implications for planetary habitability," Korenaga said.
"Studies on planetary formation suggest that planets like Earth form by multiple giant impacts, and the outcome of this highly random process is known to be very diverse. Such diversity of size and internal temperature would not hamper planetary evolution if there was self-regulating mantle convection," Korenaga said. "What we take for granted on this planet, such as oceans and continents, would not exist if the internal temperature of Earth had not been in a certain range, and this means that the beginning of Earth's history cannot be too hot or too cold. This would mean that many planets in the 'Goldilocks zone' may not be habitable after all."
Most planets come together by sweeping up gas and dust around a star — a fairly calm and cool process, as far as building worlds is concerned. But Earth's birth was hot and messy. Around 4.5 billion years ago, a Mars-size planet known as Theia collided with a much larger planet, spraying a cloud of debris into space that became the moon. The bigger, molten mass of rock, metal, and radioactive elements left behind coalesced into Earth, trapping the heat of its violent collision inside. Korenaga argues that this starting internal temperature could have a profound impact on its subsequent evolution over a few billion years.
Something needs to kick-start the process required to regulate a planet's surface temperature, that careful balancing act that traps the right amount of cold and pushes up the right amount of heat. Typically, the thinking goes, it’s radioactive elements in the rock that set this process in motion. But Korenaga recently learned that radioactive elements may be warming and stirring up Earth's mantle a lot less than previously thought — so he decided to run advanced computer simulations to account for this new information.
The new models suggest that rocky planets which can regulate their temperature, and thus develop all the geologic support systems life needs to emerge and thrive, are much rarer than we might hope. It worked splendidly on Earth, but smaller rocky planets like Mars and Venus weren't so lucky. In Venus' case, the consequences of having a "stagnant lid" of relatively unbroken crust are clear: Without the ability to bury carbon it built up a choking, heat-trapping atmosphere of carbon dioxide, resulting in surface temperatures of almost 500 °C.
The models also hint that "super-Earths" — rocky worlds more than two times Earth's mass — may be more likely to have stagnant lids, and thus have a harder go at developing a surface that's cozy enough for aliens. Korenaga notes that measuring the internal temperature of rocky worlds from afar, even with future space-based observatories, is not going to be easy. "We can't remote-sense the internal temperature directly, so we will need to rely on the connection between the atmospheric composition and the internal temperature."
However, according to Vinciane Debaille, a geochemist at Université libre de Bruxelles, plate tectonics may not be a necessary condition for life to emerge. "For life appearing, we need liquid water, organic matter and nutriments, and this can be done on a planet without plate tectonics, because the heat needed for keeping the water liquid at the surface is mainly provided by the star." What's more, she said, "Several studies indicate that the Earth was in a stagnant-lid state 3 billions years ago, and yet, life appeared ... so we should still be optimistic."
More information: This team hopes to find "another Earth" around Alpha Centauri
The ability of life to fundamentally alter the climate and chemistry of its home planet presents us with yet another dilemma. Whether or not a planet is habitable could sometimes depend on whether life has already made itself at home there. This is called the inhabitance paradox; the idea that organisms alter their surroundings to maintain a habitable environment. In other words, life could be a requirement for life. This paradox showcases just how complex the hunt for habitable planets has become.
Despite the expectations of huge planet diversity and the implications, there will still be a large number of planets that are habitable no matter what we think is required. There may be many ways to support life — we just don’t know what they are yet. Our imagination is limited to our own experience and if we’re going to observe extraterrestrial life, we're going to see things we never imagined.
Image credit: ESA–C. Carreau