Earthlings are lucky to live near a relatively stable Sun, which has enabled life on our planet to emerge and thrive over the past four billion years. While many worlds in our galaxy might contain the right ingredients to support life, though, a lot of them could be stuck with a more volatile star that prevents them from becoming—or remaining—habitable.
To get a better grip on which types of star systems might be most likely to host aliens, a pair of scientists at New York University Abu Dhabi (NYUAD) Center for Space Science have observed space weather around nearly 500 stars, according to a study published on Sunday in the journal Monthly Notices of Royal Astronomical Society: Letters.
The results suggest that planets subjected to occasional but intense flares are probably more hospitable to life than worlds that receive a constant flux of radiation and low-energy flares, which blows their protective atmospheres away.
Planetary habitability “is one of the most important concepts in exoplanet science” and “is defined as the zone around a star in which a planet is able to sustain liquid water on its surface,” said research scientist Dimitra Atri and graduate student Shane Carberry Mogan, both at NYUAD, in the study.
“While this approach is useful to identify potentially hospitable planets around stars, it fails to take into account the damaging aspect of stellar activity on such planets,” the pair added. “The main goal of this paper is to understand how stellar luminosity and flares can lead to atmospheric escape on [habitable zone] planets on long time-scales and how these losses impact planetary habitability.”
To accomplish that goal, the researchers combed through observations of stellar flares and other dangerous space weather events around hundreds of stars imaged by NASA’s Transiting Exoplanet Survey Satellite (TESS). The team used those observations to model the erosion and loss of planetary atmospheres, due to space weather, on timescales of up to a billion years.
“All we know about these exoplanets is their mass, radius, and properties of their orbit around the host star,” explained Atri in an email. “I am hoping that future missions such as [NASA’s James Webb Space Telescope] will be able to observe the atmospheres of these planets.”
"In absence of these observations, theoretical calculations, like the one we have done are extremely important in estimating the likelihood of a planet in the habitable zone having an atmosphere or not,” he added.
Without a protective atmosphere, such as the robust skies we have on Earth, the surface of planets are exposed to harmful stellar radiation that dramatically reduces the odds that life will emerge or survive over the long term.
The new study indicates that a star with a constant luminous flux of extreme ultraviolet light (XUV) is more likely to gradually strip atmospheres off of nearby planets, compared to high-energy flares that occur once in a while. Flares and other transient forms of hostile space weather can still render a planet uninhabitable, but “for most stars, luminosity-induced escape is the main loss mechanism, with only a minor contribution from flares,” according to the study.
Atri and Mogan focus in particular on “M-dwarf” stars, also known as red dwarfs, which are the most abundant type of star in the Milky Way. Exoplanets that orbit tiny red dwarfs—about 14 percent as massive and the Sun, and smaller—have the best odds of retaining their atmospheres over the long term. Red dwarfs with masses between 20 and 60 percent of the Sun experience more steady XUV radiation, and are therefore more likely to lose their atmospheres.
“These results have significant implications for planetary habitability because about 75 per cent of stars in the Milky Way are M-dwarfs and observations suggest that they host twice the number of planets around them compared to other stars,” the team said in the study.
The researchers also noted that exoplanets orbiting these tiny red dwarfs should be easy to spot and characterize, because they are so much bigger, relative to their stars, than planets orbiting Sun-sized stars. As a result, these exoplanets block out a bigger chunk of starlight when they pass in front of their stars from our perspective on Earth, which could enable scientists to capture more details about their masses, atmospheres, and potential habitability.
Atri and Mogan caution that there are a lot of caveats to their findings given that “atmospheric escape is a complex process and we need a better numerical modelling approach to estimate the total loss by including other channels of escape,” according to the study.
Scientists may be able to constrain models of atmospheric escape around these distant exoplanets by observing conditions closer to home—in this case, on Mars. The United Arab Emirates recently launched its first orbiter to the red planet, called Hope, which is due to arrive at Mars in February.
Data from this mission could help unravel the process that drives atmospheric escape, which remains “one of the biggest missing pieces of the puzzle” in assessing conditions for life, Atri said.
Missions like Hope “will help us in better understanding how solar activity impacts the Martian atmosphere and will enable us to better understand how stellar activity impacts exoplanet atmospheres and their potential habitability,” he concluded.
Update: This article has been updated with comments from author Dimitra Atri.