Why alien life could be thriving on the "terminator line" of exoplanets

In distant galaxies exist stars that are smaller and cooler than our own Sun. These so-called red dwarf or M dwarf stars also live 10 times as long as stars like ours and are the most numerous in our galaxy, outnumbering stars like ours 10 to 1. Because of their longevity and quantity, they have become a prime target for the constantly evolving search for life outside our planet.

However, there are a few things about these solar systems that may make it difficult for water, the primary indicator of habitability, to exist. Some planets in these systems reside very close to red dwarf stars and rotate with them in synchronicity. This casts one side of these exoplanets into perpetual, icy darkness, never seeing the sun's light, while the other side of the globe is continuously exposed to the red dwarf’s scorching heat. From what we know about the habitability of the one planet we know supports life — Earth — balance is key, and it’s unlikely the harsh climates of either side of these planets would give way to the conditions necessary to sustain life. 

However, a study published earlier this year that applied climate simulations to exoplanets orbiting red dwarfs with Earth-like atmospheres found there might be one region that is conducive to habitability. Writing in The Astrophysical Journal, a team from UC Irvine reported that the line running smack dab in the middle of these opposing sides, ironically named the “terminator line,” could theoretically produce the conditions necessary to support life. 

“Overall, the lack of abundant surface water in these simulations could pose a challenge for life to arise under these conditions, but mechanisms, including glacier flow, could allow for sufficient surface water accumulation to sustain locally moist and temperate climates at or near the terminator,” the team wrote.

Since the first exoplanet was detected in 1992, more than 5,500 have been discovered in our galaxy alone. Extrapolating those numbers to the 200 billion galaxies in the universe, the number of exoplanets out there quickly becomes astronomical, increasing the chances that life could exist on any number of them. 

Generally, like on Earth, if the energy a planet receives from its star allows some form of water to exist, it is considered to rest in a habitable zone, said Stephen Kane, a planetary astrophysicist at the University of California, Riverside, who was not involved in the study. However, hundreds of factors influence habitability apart from a planet’s sun. 

Think of Earth, which is affected by the magnetic fields of its moon and neighboring giant planets like Jupiter that both block external objects from flying into Earth’s orbit and launch things inward. The latter is thought to be how Earth gained much of its water not long after the solar system’s formation, Kane explained.

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“I think a big part of the challenge is trying to understand which of all of these factors have the most important effects,” Kane told Salon in a phone interview. “Right now, we have thousands of planets and we're going to be finding many thousands more. How do we decide which planets we're going to examine more closely?”

Another challenge to determining habitability is that astronomers are still debating what exactly qualifies as the building blocks of life. Initially, it was thought the detection of oxygen and an ozone layer on a planet meant photosynthesis was occurring and, therefore, life could exist there, said Ingo Waldmann, an astrophysicist at the University College London, who wasn’t involved in the study either. 

However, as our understanding of the universe expanded, so too did the range of what could be considered habitable. We now know exoplanets could be similar to Jupiter’s moon, Europa, for example, which is blasted by too much radiation to sustain life on the surface but is thought to have underground oceans below ground.

“There are over 100 definitions of what life is in itself,” Waldmann told Salon in a phone interview. “So if we struggle to define what life is on Earth, it's difficult to ascertain what habitability or life might be like on another planet. That doesn't mean we can't find out about it, but we just don't have the data yet.”

While it makes sense to prioritize studying exoplanets orbiting stars like our Sun in the search for planets in this habitable zone, the distance at which planets in these solar systems orbit their stars makes them more difficult to track down when they’re light years away compared to exoplanets orbiting M-dwarfs, which tend to stick relatively close by. 

However, even if exoplanets like those in this study have the conditions necessary to maintain water in some form once their solar systems have matured, M-dwarfs are 100 to 1,000 times more luminous when they’re young. And they can be temperamental, with lots of solar flares and ultraviolet radiation, said Jonathan Fortney, an astrophysicist at UC Santa Cruz who was not involved in the study either.

That might mean that a lot of volatile elements to support life such as water, carbon, oxygen and nitrogen-bearing molecules “might be baked off and sort of lost to space,” Fortney told Salon in a phone interview. Still, that could also just mean that these exoplanets need to begin their lives with a larger inventory of water to maintain their habitability throughout this stage, he added.

The most studied solar system besides our own is the TRAPPIST-1 system 40 light-years away. There, three of seven exoplanets orbiting an M-dwarf lie in the habitable zone where temperatures are just right for water to potentially exist. While some of these exoplanets have conditions that would support liquid water, those further away from the star could have ice and those closer could have atmospheric water vapor.

Astronomers are also interested in finding “super-Earths,” which are similar in composition to our Earth but, as the name hints, larger. One advantage of looking for habitability on these planets is that their size may make them easier to detect. 

Kane is also studying Venus, which was once Earth’s twin but became a scorching, inhospitable planet after its runaway greenhouse effect turned all surface water into vapor. Studying what went “wrong” with Venus can not only help prevent Earth from ending up with the same fate due to climate change but also help astronomers understand what the “right” conditions are for a planet to be habitable.

Although this study used climate models to estimate the habitability of M-dwarf exoplanets, observational evidence of what truly exists on many more exoplanets could be forthcoming as early as 2060 with the construction of telescopes powerful enough to detect them. The 2023 National Science Foundation decadal survey listed the habitability of exoplanets as a top research priority, and more powerful telescopes like the Extremely Large Telescope (ELT) under construction are expected to be completed within a decade.

“Habitability is the Holy Grail,” Waldmann said. “Before we get there, understanding what planets are and how they form is a more tangible and very fundamental piece that I think we will be able to answer in our lifetime.”