If the extraterrestrial life is capable of spreading from planet to planet – a concept known as “ panspermia ” – then we might be able to detect it, even if we don’t know what we’re looking for, new research suggests.
Astronomers are on the hunt for extraterrestrial life. While there are several promising locations within the solar system, the abundance of exoplanets means it’s very likely we’ll find life on a planet orbiting another star. According to NASA, there are more than 5,000 confirmed exoplanets to date and the number is constantly increasing.
These searches for extraterrestrial life currently face a major obstacle: we don’t know exactly what we’re looking for. We know of only one type of planet that is definitely capable of hosting life, and we only know of one type of life: that is, Earth and the carbon-based life found here. But extraterrestrial life, astronomers say, can take a surprising number of forms throughout the galaxy.
While we may be extremely lucky and find an exact replica of Earth with the exact same type of extraterrestrial life, we are more likely to face confusing, unclear, and nuanced situations that will take many years to resolve.
A pair of astronomers recently proposed an alternative to this approach, focusing less on what life would be like and more on what life would do. Specifically, they propose a detection strategy based on the concept of panspermia, the idea that extraterrestrial life can start on one planet and spread to others by hitchhiking on meteorites.
Although panspermia falls outside the mainstream of scientific research, it is not a true pseudo-science either. Martian meteorites have been found on Earth, and scientists are regularly surprised by the resilience of living creatures and the extreme environments in which they can survive.
One of the key characteristics of any life type is its ability to change the natural balance of a planet. On Earth, for example, we have much more oxygen in our atmosphere than we would if life were not present, and distant observers would notice much more green on our landmasses than there would otherwise be.
We don’t know exactly what properties of an exoplanet would change extraterrestrial life, but, if that life were capable of panspermia, then it would attempt to make the same changes on every planet it encountered as it spread from world to world.
Sometimes it would fail, if conditions weren’t right for life to thrive, but sometimes it would succeed, making the new planet similar to its original world. Then that new planet would serve as the starting point for a new cycle of panspermia.
In their new study, the researchers devised a statistical test in which, if we measure enough properties of enough planets, then we can potentially identify a group of nearby planets that share similar characteristics.
Since these planets, each orbiting a distant star, would have no other reason to be similar to each other, this cluster would stand out from the collection of all exoplanets.
Such a discovery would not represent conclusive evidence to identify extraterrestrial life, but it would be an important clue that something strange is happening to those planets – and that the propagation of life between the stars could be the cause.
The researchers acknowledged that their work has limitations. First, it assumes that panspermia is possible, which is an untested hypothesis.
Second, their technique only works if we can collect enough data on a large number of exoplanets. But the advantage of their technique is that it is “agnostic,” meaning it can identify a potential signature of life without any larger assumptions about how that life works.
The team’s research has been published on the preprint database arXiv and has not yet been peer-reviewed.
We’ve all wondered if we’re alone in the Universe, and with potentially habitable planets, strange space signals, and reports of UFOs hitting the headlines, it’s starting to look like we’re on the verge of a major extraterrestrial discovery. But what is the best scientific evidence we have found for the existence of extraterrestrial life?
The sobering reality is that there isn’t any yet. There is no scientific evidence of the presence of aliens in declassified UFO videos, mutilated cows whose injuries are attributed to extraterrestrial activity, or alleged alien bodies. Nor is there any such evidence in formal academic research. Live Science reached out to the experts to make sure.
“There is no evidence at this time,” Professor Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology and author of “The Smallest Lights in the Universe: A Memoir” (Crown, 2020), told LiveScience.
Nikku Madhusudhan, professor of astrophysics and exoplanetary sciences at the University of Cambridge, gave a similar response, saying: “I don’t think we have clear evidence of any kind yet for extraterrestrial life.”
There are, however, good reasons to hope that the evidence will arrive sooner or later, even if it won’t be delivered personally by a little green man.
Madhusudhan said “there are clues here and there” and “there is evidence of habitable conditions.” In other words, there are signs that some planets and moons could harbor extraterrestrial life, but we have yet to find evidence of life in these places.
There may be hundreds of millions of habitable planets in our galaxy alone. Scientists consider planets capable of hosting life if they are in the so-called habitable zone, the distance from a star where it is possible for a rocky planet to have liquid water on the surface, an essential ingredient for life on Earth.
Even planets and moons outside the habitable zone are not necessarily inhospitable to life. For example, Jupiter’s moon Europa is not in the Sun’s habitable zone, but it has a saltwater ocean beneath its icy crust that may be capable of hosting extraterrestrial life.
Researchers are looking for signs of life inside and outside our solar system. Madhusudhan led a study, published in October 2023 in The Astrophysical Journal Letters, that detected traces of an ocean on a planet called K2-18 b, located more than 100 light-years away. Their data also suggested “potential signs” of the presence of dimethyl sulfide, a chemical that, as far as we know, is only produced by life on Earth.
“This is very temporary,” he said. “We don’t know if it’s there or not, but we see it in the data at some level.
That potential sign of extraterrestrial life will be the subject of further study and may be disproven. Regardless, Madhusudhan said he wouldn’t be surprised if we found evidence of extraterrestrial microbial life within the next decade. “I would be very surprised if there was no life of any kind out there,” he added.
Seager believes we will only confirm the existence of extraterrestrial life with sample return missions, collecting samples from another planet or moon and returning them to Earth for study, in situ measurements, or “incredibly futuristic” technology, such as a solar gravitational lens. telescope, a theoretical instrument that would use the sun’s gravity to magnify light from distant planets.
“With all the tools we have now, or could build with enough money, it’s a sample return, because I don’t see anything else that’s definitive,” Seager said.
Seager was part of a 2020 study published in the journal Nature Astronomy that reported the “apparent presence” of phosphine gas – a potential signature of life – on Venus. The research has since been debated in the scientific community.
Researchers will debate whether potential signatures of extraterrestrial life detected on other planets are real, and then whether those signatures are accurate, and then whether those signatures are actually caused by life, which they currently won’t be able to prove without collecting samples, according to Seager.
She is hopeful, however, that scientists will find more signs of water and oceans that could indicate habitability, as well as gases that could be due to life. “I think it’s enough to advance this generation’s long quest,” she said.
Keep in mind that the space is incredibly vast. It would take humans more than a million years to visit K2-18 b with traditional rocket propulsion. Even sending our fastest probe to the nearest known exoplanet, Proxima Centauri b, would take thousands of years. The planets and moons of our solar system are just within reach by comparison, with probe travel times ranging from a few years to a few months.
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