Concept art of Super-Earth. Image:
Evgeniy Ivanov via Getty Images
Evgeniy Ivanov via Getty Images
A special class of planets could potentially host life for as long as tens of billions of years, according to a new study. Super-Earths, which are rocky planets that are more massive than Earth but smaller than ice giants such as Neptune, are abundant in star systems across the Milky Way; indeed, our own solar system may be somewhat of an outlier in lacking this type of world. Now, scientists led by Marit Mol Lous, a PhD student studying exoplanets at the University of Zürich, have presented new evidence that so-called “cold Super-Earths” that orbit their stars at more than twice the distance between Earth and the Sun, “can maintain temperate surface conditions” for up to give to eight billion years, a timespan that “suggests that the concept of planetary habitability should be revisited and made more inclusive,” according to a study published on Monday in Nature Astronomy.
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In addition, Mol Lous and her colleagues found that some Super-Earths that are kicked out of their home star systems by gravitational perturbations, or other mechanisms, could potentially maintain liquid water habitats for as much as 84 billion years, because these rogue worlds would not be affected by the death of any host star.“Here we argue that it should be considered that habitable planets could be very different from Earth, and that we should remain open-minded when investigating such potentially habitable planets,” Mol Lous said in an email. “Of course, it is also important to remain cautious and not jump into conclusions when considering such 'exotic' habitats as we know very little, and a lot can be left to speculation.”The new study is built from theoretical models of these tantalizing worlds, rather than real observations, because it is challenging to spot these cold Super-Earths with current telescopes. Most exoplanets are detected when they pass in front of their star relative to our perspective on Earth, causing a slight dip in starlight. As a result, all known Super-Earths have relatively short orbits that produce frequent brightness dips, making them simpler for telescopes to pinpoint.However, scientists have suspected for years that Super-Earths in more distant orbits could be compelling targets in the search for extraterrestrial life. Models suggest that these planets could retain their primordial atmospheres, which are dominated by hydrogen and helium gas, for billions of years. These atmospheres are distinct from those surrounding some rocky planets in our own solar system, including Earth, which evolved atmospheres with more complicated compounds, such as oxygen, carbon dioxide, and nitrogen gasses.
“The hypothesis that there could be liquid water on a planet that has a primordial atmosphere has been around for over 20 years and since then more studies have worked on this idea,” Mol Lous said. “We wanted to further investigate the evolutionary aspect, in other words, we calculated how long liquid water could be present and what would be necessary for a planet to have the longest possible duration of liquid water.”Liquid water is the magic ingredient for life as we know it on Earth, which is why scientists prioritize it in search for aliens elsewhere in the universe. To delve into the “potential exotic habitability” of cold Super-Earths with primordial atmospheres, in the words of the study, Mol Lous and her colleagues ran over 1,000 simulations of planets with different masses, atmospheres, and orbital distances.The team discovered that planets between one and ten times the mass of Earth, with atmospheres that are 100 to 1,000 times thicker than Earth’s skies, might occupy a hospitable sweet spot. Worlds that orbit too close to their stars are expected to lose their primordial atmospheres under the harsh stellar glare, but planets that are located at distances beyond the orbit of Mars could hang onto this hydrogen-helium envelope. At this potentially safe distance, these atmospheres could act as greenhouse gasses by absorbing infrared radiation, providing a source of heat that might nurture life in liquid water oceans.
This class of planets could provide habitable conditions for five to eight billion years, but would eventually become inhospitable once their stars began expanding during their dying stages, reports the study. In a mind-boggling twist, the researchers found that rogue planets that are ten times as massive as Earth, with atmospheres that are about one percent the mass of Earth, could be habitable for an astonishing 84 billion years, according to the models. The study suggests that these unbound worlds would probably be too hot for life at this point in the universe’s 13.8-billion-year lifespan, but could become hospitable over the next several billion years.Any speculative aliens on these worlds would have to grapple with very different conditions compared to Earth, including enormous surface pressures and a lack of direct sunlight as a result of thick atmospheres. However, the team notes that extreme lifeforms on Earth can deal with high pressures in deep ocean trenches, while some organisms rely on chemical energy sources instead of drawing fuel from the Sun.The implications of the study are exciting, but Mol Lous and her colleagues cautioned that it will take more research, and hopefully direct observations, to back up these initial findings. “There are three important things to address in the future,” Mol Lous said. “The first is if our results hold when we make our model more realistic. We did a few studies on how robust our results are for changing parameters, but we still make simplifications and that should be improved in future work. For example, we don't really let the water interact with the atmosphere in our model and that could actually be important.”
“The second is to study how likely it is that planets can form with the 'right' conditions for liquid water,” she continued. “The third is to think about observations: what can we measure about such planets to determine if they have liquid water or not?’To that end, the team emphasized that these special exoplanets might be detectable to the next-generation observatories, such as the recently launched James Webb Space Telescope or NASA’s forthcoming Nancy Grace Roman Space Telescope. “There are no accurate predictions on the occurrence of super-Earth-sized planets with these initial conditions, but it is likely enough that these alternatively habitable planets constitute a fraction of the habitable worlds in the galaxy,” the researchers said in the study.We “expect that our understanding of this exoplanetary population and its potential habitability will substantially improve in the near future,” they concluded.