Scientists Discover Alien Life May Exist on Planets We Least Expected in Surprise Find

Extraterrestrial life may be more prevalent in an unexpected type of star system, reports a new study. Planets that orbit stars with fewer “metals,” the term for elements heavier than hydrogen and helium, may be more likely to be habitable than worlds hosted by metal-rich stars, according to the research, which focused on the role of ultraviolet (UV) radiation in planetary habitability. 

Many researchers have assumed that planets near metal-rich stars would be prime targets for searching for signs of life, especially since this class includes our Sun. The surprising finding could inform the search for aliens, and may eventually have implications for understanding the overall habitability of the universe across time.

Stars are element-making factories that forge materials in their bellies by squeezing atoms together as part of a process called nucleosynthesis. The first stars in the universe were composed mostly of the lightest elements, hydrogen and helium, but each new generation of stars has concocted heavier elements, such as gold, carbon, and iron. The heavy elements, called metals, are strewn into space when the stars die, and are then incorporated into new stars that arise from these stellar ashes.

Now, scientists led by Anna Shapiro, an astrophysicist at the Max Planck Institute for Solar System Research, have examined the relationship between the metallicities of stars and the odds that their planets might be habitable. In particular, the team are the first to model the effect of stellar metallicity on the composition of planetary atmospheres, especially the production of ozone, which is a molecule that blocks damaging UV radiation.

The results revealed a bizarre paradox: While metal-poor stars emit more UV radiation than their metal-rich counterparts, the planets orbiting them may have more robust ozone layers that block this harmful light from reaching their surfaces. The unexpected findings “imply that planets hosted by stars with low metallicity are the best targets to search for complex life on land,” according to a study published on Tuesday in Nature Communications.

“I think this narrows the habitability zone a little bit,” said Anna Shapiro in a call with Motherboard that also included study co-author Alexander Shapiro, another astrophysicist at the Max Planck Institute for Solar System Research. “Our research shows that finding life around metal-poor stars is more promising from the point of view of the radiative signature.”

That said, both authors emphasized that stellar metallicity and UV surface radiation are only two pieces of a much larger puzzle that faces scientists looking for life on other worlds. While the new study hints that low-metallicity systems may be more conducive to life, that doesn’t mean we should stop looking for aliens on planets orbiting metal-rich stars similar to the Sun.

“I would not say that we should limit our search for habitability to any specific stars,” noted Alexander Shapiro. “We should really keep our eyes open to surprises.”

Indeed, the team’s new discovery is itself a culmination of a series of surprises about the role that UV radiation plays in the habitability of planets. The origins of the study date back to 2020, when a team of researchers, including Alexander Shapiro, found that the Sun is less active than its solar peers in the galaxy. 

As part of that project, Shapiro and his colleagues explored what might happen if the Sun went into a hyperactive state in which it spewed out much more UV radiation. You might expect life on Earth to be irradiated by such an outburst, but the team found that planetary atmospheres can actually become more resistant to harmful light in this situation. 

“The paradox is that the increase of the UV outside the atmosphere leads to the decrease of the UV on the ground,” Alexander Shapiro said.

This is a key finding because UV light can exact an enormous toll on living things; you may have already had a taste of its power if you’ve experienced a bad sunburn. Exposure to this type of radiation can cause a host of painful conditions and deadly diseases, including cancer, cataracts, immune system disorders, and DNA damage. Fortunately, Earth is shielded by a layer of ozone that absorbs a huge amount of UV radiation, which has allowed complex multicellular life to emerge and persist on our world.

With this in mind, Anna Shapiro and her colleagues set out to probe the possible connections between a star’s metallicity and the potential UV exposure to the surfaces of any planets in the system. The team simulated a host of Earth-like planets orbiting stars with surface temperatures about 5,000°C to about 6,000, which are similar levels to our Sun. The modeled planets were located in the habitable zones of their systems, a region where liquid water might exist on their surfaces.

The results showed that planetary landscapes in metal-poor star systems are more protected from UV light than metal-rich systems, despite the fact that these stars generally produce more of this harmful radiation.

The odd discovery emphasizes the importance of the exact type of UV light emitted by this range of stars. A shorter band of the ultraviolet spectrum, called UV-C, is absorbed by the upper atmospheres of planets where it actually bolsters ozone production. In contrast, the UV-B band, which is made of longer ultraviolet wavelengths, destroys ozone and is the main component of the deleterious radiation that reaches the surfaces of planets like Earth. 

While metal-poor stars emit more UV light overall, they produce a higher ratio of UV-C light to UV-B light compared to metal-rich stars, resulting in more robust ozone layers in the skies of worlds in low-metallicity systems.

“In the case of higher [stellar] activity, the UV-C increases stronger than UV-B and therefore the production of ozone increases,” Anna Shapiro explained. “That's why the ozone layer becomes thicker, and the thicker ozone layer can absorb more UV-B.”  

The counterintuitive findings hint at a population of metal-poor systems that contain worlds enveloped by dense ozone layers that ward off UV-B radiation, allowing complex lifeforms to emerge and thrive on the landscapes below. 

Given that metal-poor systems grow rarer with each stellar generation, the results also hint that planets around Sun-like stars, known as G-dwarfs, may become more inhospitable in the future. 

“What our study shows is that for G-dwarfs, indeed, with time, it will get more and more difficult for the planets to be habitable,” Alexander Shapiro said. However, he added that other types of stars, such as smaller red dwarfs, may not fall into this pattern. For this reason, it’s not yet possible to make broader conclusions about how the overall habitability of the universe will play out in the future, though that may change with more research.

To that end, many of these exciting questions are likely to be constrained by more sophisticated exoplanet models as well as images from next-generation observatories, such as the James Webb Space Telescope, which is currently operating, or the  PLAnetary Transits and Oscillations of stars (PLATO) mission, which is due for launch in 2026.  

“I think it's going to be really exciting, because we can then check different processes and how they work,” Anna Shapiro concluded.