Is there life that exists beyond earth? If so, where?

Aliens may sound like the stuff of science fiction, but the more we learn about space, the more real the possibility of extraterrestrial life seems. Once scientists realize how vast the universe is, a complete absence of life outside Earth becomes almost a statistical impossibility — and even within our own solar system, other planets and moons are showing signs of conditions that could support life.

“Thinking we are alone in the universe is statistical nonsense,” says Nathalie Cabrol, an astrobiologist serving as director of the Carl Sagan Center for Research at the SETI Institute. “Exploration in the solar system has demonstrated that there are so many possible habitable environments. I think the chances are pretty good that there is something out there in the solar system.” 

Scientists used to think there could only be life within a solar system’s “habitable zone” — the region close enough to a star for water to pool up on the surface of a planet or moon — but recently, they’ve discovered ways life might exist even outside habitable zones, says Cabrol. 

When we look outside our own solar system, there are even more possibilities, though whatever organisms are out there probably would be very different from us, as they wouldn’t have a shared origin, Cabrol explains. There are trillions of stars out there, and even more planets and moons — and the elements that make us up, like carbon, oxygen, and phosphorus, appear to be all over the universe. 

However, complex humanlike organisms are going to be rarer than simple microbes. “Nature develops a lot more small things than big things,” says Cabrol. “You have a lot of small lakes, but very rarely big lakes. You have lots of small mountains but not too many Everests, and the same goes for life.” Still, given the universe’s sheer size, we’re probably not the only large beings out there, she adds.

While the presence of life outside Earth has not yet been proven, here are some places that show promise. 

Mars

The legend of the Martian is still alive. In fact, we might be the Martians. “For Mars, we have evidence that conditions were good early on, but they disappeared very rapidly,” says Cabrol. “We know once life is somewhere, it’s hard to get rid of, so it probably got into the surface and started a new ecosystem underneath there. The physical chemistry of (Earth and Mars) was similar early on, and they exchanged a lot of material early on in the history of the formation of those two planets, so it is very possible that life in some form might have made the trip from one planet to the next.”

There have been methane emissions detected on Mars, which on Earth are largely produced by microbes, but they could also just come from volcanic activity, says Cabrol. The Mars 2020 Rover may be able to tell us more about the source of the methane. Coupled with the discovery of ALH84001 (a meteor fragment from Mars), which contained possible fossilized microscopic bacteria, it’s a strong possibility that there is some semblance of life on Mars.

Europa

Europa, a moon of Jupiter, is a compelling candidate for extraterrestrial life because it has an ocean of liquid water underneath a layer of ice, says Cabrol. In addition, it’s being pushed and pulled in a sort of “tug-of-war” with Jupiter, which creates friction that makes the core of this moon hot. 

“The ice is melting, and because of the pressure underneath, it can stay liquid, so what you have is heated water,” says Cabrol. Underneath the ocean, you’d likely find something similar to Earth’s “black smokers,” or hydrothermal vents, which are conducive to the chemical conditions that produced life on Earth. 

Enceladus

Saturn’s moon Enceladus is similar to Europa in that it also has an ocean below an icy crust, with a similar push-pull dynamic with its parent planet, Cabrol explains. It shows even more promise because, since this moon has geysers that send bits of ocean into space, scientists have been able to analyze samples from it and confirm that there is a salty ocean there. While other moons have oceans, Europa’s and Enceladus’ have access to the moons’ surfaces, which means nutrients could reach them.

“On top of that, although both moons are located in an environment that from the standpoint of radiation is absolutely atrocious, once you are protected under a cover of ice within a few meters, these daily radiations are being stopped,” Cabrol adds. 

Titan

This moon, belonging to Saturn, seems to contain liquid water between methane and ethane lakes, which could potentially support life. Titan has a hydrocarbon-based biochemistry, which is very different from Earth’s carbon chemistry, Cabrol explains. Because of this, it opens up intriguing questions about what unfamiliar forms of life might exist outside Earth. “It epitomizes what we think of as alien life,” Cabrol says. “Their metabolism wouldn’t be using the same solvent our cells on Earth are using.” 

Venus

Venus is not among the most likely places for life to exist, but it is an interesting possibility. “Nobody would really think of Venus as a place where you’d be looking for life, but at the same time, Venus was really like the Earth when it started off,” says Cabrol. Though the surface of Venus doesn’t appear hospitable to life, the atmosphere about 60 km above the surface has a temperature, moisture level, and atmospheric pressure that could potentially accommodate life. So, any organisms there would have to be airborne, she says. 

With advances in artificial intelligence that let scientists run simulations of what would happen if life were introduced under different conditions, as well as knowledge of the conditions that exist outside the solar system, the search for extraterrestrial life is becoming more and more feasible. 

Yet we’re still at the beginning of this search — in fact, scientists haven’t even agreed on a definition of life. “This is the irony of it all: We are searching for something we cannot define yet,” says Cabrol. But that’s also the exciting part: Any extraterrestrial life we find may not only expand our knowledge of the universe but also stretch our very notion of what life is. 

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