A handful of “aqua planets” that are awash with habitable seawater will be discovered within the next decade, according to the predictions of a new study. The research suggests that temperate ocean worlds that orbit small stars, known as red dwarfs, may be far more common than previously assumed, raising the tantalizing prospect that some of these planets may host alien life.
Red dwarfs, also known as M dwarfs, are relatively cool compared to stars like the Sun, but they are a hot topic in the search for extraterrestrial life. The vast majority of stars in our galaxy are red dwarfs, and their ubiquity has galvanized scientists to grapple with their potential habitability. For years, researchers have debated the possible merits and drawbacks for life in these systems, with some arguing that it is likely rare for planets around red dwarfs to end up with the right amount of water to support aliens.
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Now, a pair of astronomers have presented an updated model that estimates somewhere between 5–10 percent of red dwarf planets that are under 1.3 times the radius of Earth “have appropriate amounts of seawater for habitability,” an occurrence rate that is “high enough to detect potentially habitable planets by ongoing and near-future M dwarf planet survey missions,” according to a study published on Thursday in Nature Astronomy.
“At least, it would be fair to say that we have seen the light for finding habitable planets, unlike the previous theoretical prediction that their existence is hopelessly unlikely,” said authors Tadahiro Kimura and Masahiro Ikoma, astronomers at the University of Tokyo and the Graduate University for Advanced Studies (SOKENDAI), respectively, in an email to Motherboard.
“However, there is still much we do not know about the climate of planets other than the Earth,” the team added. “Therefore, more research on planetary habitability is needed.”
In this way, the new model presented an optimistic view of finding habitable conditions, and perhaps even signs of life, on Earth-sized planets in the habitable zone of these systems, which is the region where liquid water can exist.
This sunny perspective stems from new parameters that Kimura and Ikoma wrote into the updated model, including variations in the evolution of the protoplanetary disc that form these systems, as well the process of water enrichment in the primordial atmospheres of the exoplanets. The novel constraints, which were played across 10,000 different simulations, have implications for the production, abundance, and evolution of liquid water on exoplanets.
“Water delivery to rocky planets has been long discussed in the context of the origin of the Earth’s oceans,” Kimura and Ikoma said. “It is believed that water-laden small bodies delivered Earth’s water. By contrast, we wanted a more general understanding of how rocky planets obtain water.”
“Then, we noticed that hydrogen (one of the two elements for H2O) is available because planets are formed in hydrogen-rich nebulae,” they continued. “We also noticed that all rocks contain oxygen (namely, the other element for H2O). Thus, we considered that the easiest and most general way for rocky planets to get water is to chemically produce H2O through a reaction between hydrogen from the nebula and oxygen from rocks.”
By accounting for these variables in their model, the researchers show that about 1 in 100 habitable zone near-Earth mass planets (HZ-NEMPs) orbiting M dwarfs can end up with a comparable amount of water to Earth oceans, under specific conditions. One percent may not seem like a lot, but considering that hundreds of planets have been found in orbit around red dwarfs, and countless more are expected to be discovered in the coming years, it is a significant number. In this way, the new study hints that many observable planets around red dwarfs might have oceans similar to those on Earth, where life on our planet first emerged.
“Signals from life living on planets closer to us would be easier to detect,” Kimura and Ikoma said. “M dwarfs are much more abundant in the solar neighbourhood than Sun-like stars (or G dwarfs). That is why M dwarf systems are exciting targets in the search for extraterrestrial life.”
While this is an exciting possibility, Kimura and Ikoma caution that there are still many variables that add uncertainties to the predictions. For instance, they point to some of the unique features of red dwarf systems, including the close proximity of the habitable zone to the star compared to, for instance, the much more distant location of Earth around the Sun.
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As a result of these close orbits, an exoplanet orbiting in the habitable zone of a red dwarf has a higher chance of becoming tidally locked, meaning that one face is always pointed toward the star in perpetual daylight while the other is turned away in eternal night. The researchers note that this configuration could prevent temperate climates or cause them to collapse.
Tidal locking is just one of many variables that will need to be considered when assessing the habitability of HZ-NEMPs in the future. Fortunately, sophisticated observatories such as the James Webb Space Telescope, which became operational this summer, and the forthcoming Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) mission, due for launch in 2029, have the power to resolve incredible details about exoplanets, including signs of aliens life.
“The only habitable planet we currently know is the Earth,” Kimura and Ikoma said. “We have no complete idea how even the Earth has maintained habitable environments. Therefore, we need more studies about how climate depends on planetary properties such as seawater amount, gravity and rotation, and host-stellar properties such as temperature and activity.”
“From observational points of view, accurate measurement of the mass and radius of exoplanets will help us identify the existence of oceans and estimate their depths,” the researchers concluded. “In addition, observations of exoplanet atmospheres conducted by JWST and ARIEL will give crucial hints to understanding planetary surface environments.”
