Over the past few decades, humans have discovered thousands of exoplanets, which are worlds that orbit other stars. New observatories, such as the James Webb Space Telescope, are now working to resolve key features of these distant planets, including whether they might have the right conditions to host life.
One of the biggest open questions in this effort is whether aliens could exist on planets that orbit red dwarfs, also known as M-dwarfs, a class of very small and dim stars that are extremely common in the Milky Way.
Now, Rafael Luque, a postdoctoral scholar and astronomer at the University of Chicago, and Enric Pallé, an astronomer at the Institute of Astrophysics of the Canary Islands and the University of La Laguna, present tantalizing new evidence that these systems contain strange water-logged worlds, as well as rocky and gas-rich planets, that “could potentially be habitable if the appropriate conditions are met,” according to a study published on Thursday in Science.
The team reached this conclusion by compiling newly available mass and radius estimates of small worlds that orbit red dwarfs, in a first-of-its-kind approach. For years, scientists were able to estimate the size of these exoplanets by watching them pass in front of their stars from our perspective on Earth, but only recently has it become possible to estimate the mass of these worlds.
With those two parameters, Luque and Pallé were able to analyze the densities of exoplanets in red dwarf systems, a value that can shed light on planetary composition where size alone falls short. For instance, two planets might be both similar to Earth in size, but if one is made of heavier rocky materials and one is largely composed of much lighter ingredients, such as water, they will have very different masses.
The new study is the first to provide a population-level overview of small exoplanets in red dwarf systems, with a sample of 43 of these worlds, instead of the types of individual case-by-case studies that have been published so far.
“We were building a sample large enough, for these types of stars only, to make a demographic study for the first time,” Luque said in a joint call with Pallé.
“Nobody had gathered all the public data together,” Pallé added. “Once you have mass and once you have radius, you can get densities, which are a much more physical property and more informative thing about what these planets are made of, than only radius.”
As the researchers compiled the data, they were surprised to identify a strange type of exoplanet that appeared lighter in mass than previous estimates that were based only on radius measurements. Whereas planets like Earth are mostly rock, these worlds appear to be as much as 50 percent water in mass.
“Just after doing the analysis, suddenly we saw an alignment that was really striking,” Luque said. “We didn’t expect this alignment at all, because we have never seen it. It was just a consequence of refining the sample, and that the sample has been growing.”
“I think no one in the field was thinking in the past five years that these water worlds could exist and be as common as they are,” he added.
Given that water is the key ingredient for life on Earth, this seems like good news in the search for extraterrestrial life. But while it may be exciting to imagine huge oceans filled with the alien whales and squids, that is not what these water-rich worlds are like, according to models.
These exoplanets are probably more akin to big dirty snowballs, Pallé said, which coalesce in the icy outer reaches of red dwarf systems and migrate closer to their stars over time. As a result, any melted water they host is buried in subterranean pockets, perhaps intermixed with magma, which may not be environments conducive to life. Some worlds, such as Jupiter’s moon Europa, are known to contain subsurface oceans that could be habitable, but the water-rich worlds orbiting red dwarfs are completely different because water, rock, and other materials are likely to be much more mixed up within them.
“Their real nature, we don’t know,” Pallé said. “It’s something new.”
While the researchers emphasized that their study does not make any direct claims about the habitability of these worlds, they said the results have implications for future work that might address the odds of life around red dwarfs. For instance, the study revealed an abundance of rocky worlds that appear similar in density and composition to Earth, suggesting analogs of our planet may be common in red dwarf systems. The team also identified a third gas-rich type of exoplanet, known as sub-Neptunes, that may contain habitable regions with water.
Red dwarf systems provide “low-hanging fruit of possible inhabitable environments,” Pallé said, because it’s easier to spot potential signs of life, known as biomarkers, on their exoplanets. Planets orbiting red dwarfs can block out more of their stars’ light when they pass in front of them, compared to planets orbiting larger stars, which produces a better contrast ratio to tease out their features.
“The M-dwarfs sample of small planets is important because it’s the one that will be accessible for studies of the atmospheres, and the search for biomarkers, in the near future—the next two or three decades—because small planets are easy to detect and easy to characterize if the star is also small,” Pallé noted.
Luque also plans to conduct more population-level studies of exoplanets among larger classes of stars, to see if the strange water-rich worlds are common beyond red dwarf systems.
Expanding our sample to different types of stars is something very immediate and a logical consequence, and also to study these planets with the [James Webb Space Telescope],” said Luque.
That is something that the community will be excited to test—what are the properties of these water worlds, the smaller ones and also these sub-Neptunes?” he concluded. “How much water do they have? What are the properties of their atmospheres, if they have any? These are questions that I would say, in the next few years, the field will try to answer.”