Which planets – and planetary moons – could actually have life?

How close could aliens be? These are the prospects that meet the specific conditions for life to be possible

By Matthew Rozsa

Staff Writer

Published February 9, 2023 10:30AM (EST)

Astronauts on the Alien Planet Discover Plant Life. (Getty Images/gorodenkoff)
Astronauts on the Alien Planet Discover Plant Life. (Getty Images/gorodenkoff)

Science fiction is filled with stories of humans traveling to other planets — and, of course, quite often those planets are inhabited. Whether it's explorers having romantic adventures in "Star Trek" or determined scientists trying to save humanity in "Interstellar," people instinctively want to believe that our universe might allow us to casually planet-hop. It is hopeful to believe that we could escape from Earth's problems to a literal new world, and inspiring to think other fascinating life forms might await us either in our solar system or on exoplanets just beyond.

Wolf 1069b is remarkable similar to Earth: It has roughly 1.26 our planet's mass and 1.08 times its size.

The mundane scientific reality, though, is that scientists aren't even sure what alien life would look like. (Some best guesses posit that it'd be microbial.) To even begin speculating, researchers imagine what types of worlds might have conditions similar to Earth's: They need to orbit stars that possess just the right qualities, be a comparable distance from their star so that liquid water might exist (also known as a a "habitable zone"), have a comparable size (roughly 2.5 Earth radii or 10 Earth masses — or, for rockier worlds, 1.3 Earth radii or 3 Earth masses), and so on. Indeed, of the estimated 5,307 exoplanets in 3,910 planetary systems (as of February 2023), only 1.5 percent of them are catalogued as potentially habitable worlds.

Yet even with odds as low as that, there is still hope. Here are five contenders for extraterrestrial life worthy of consideration.

Wolf 1069b
One of the newest additions to the list of potentially habitable worlds, Wolf 1069b's discovery was announced in the scientific journal Astronomy & Astrophysics earlier this month. In terms of its physical dimensions, Wolf 1069b is an Earth-like exoplanet, and is indeed remarkably similar to Earth: It has roughly 1.26 our planet's mass and 1.08 times its size. Wolf 1069b also, as astronomer at Germany's Max Planck Institute for Astronomy Diana Kossakowski explained in a statement, "orbits the star within 15.6 days at a distance equivalent to one-15th of the separation between the Earth and the sun." Given that its host star is much smaller than our own sun, the closer distance improves its prospects of being habitable.

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Teegarden's Star b
Teegarden's Star b might sound like the name of a whimsical bucolic fairy tale, but in reality it is a rocky ball 12 light years from Earth that was discovered in 2019. According to NASA, Teegarden's Star b is "a super Earth exoplanet," but that "super" does not mean it has special powers. Super-Earths are worlds that range anywhere from 30-70% larger than Earth. This means that they are still large enough to potentially host life, and Teegarden's Star b is also in the habitable zone of its star. This means it could potentially support liquid water.
TOI 700 d
TOI 700 is a red dwarf star more than 101 lightyears away from Earth. While there are four exoplanets orbiting this star, only TOI 700 d is considered to fall in the habitable zone. In addition, it is considered to have a rocky surface with a mass roughly 1.69 times that of Earth and a radius roughly 1.19 times that of Earth. Overall this exoplanet receives roughly 86% the same energy from its star as Earth receives from its sun, which combined with the other variables makes it a prime candidate for life.
Europa is not an exoplanet, or even a planet. Located squarely in our solar system, Europa is a moon orbiting the largest planet in our solar system, Jupiter. From a telescope, Europa resembles nothing so much as a scuffed up cue ball; its icy crust is scarred with ridges that may contain salty pools of liquid water. Indeed, the entire planet is believed to have swirling oceans beneath the icy surface, perhaps warmed by a planetary core. NASA scientists are also poring through data from its Galileo orbiter in the hope that it may yield some insights.
"The modeling found that there could be a variety of different salts present on the surface but suggested that infrared spectroscopy alone is generally unable to identify which specific types of salt are present," team leader and University of Leicester School of Physics and Astronomy Ph.D. student Oliver King said in a statement to Space.com about the ongoing search for potentially life-creating materials.
This is the big one. The one that constantly attracts the fascination of fiction writers and big-talking entrepreneurs like Elon Musk. Yet the reality of Mars is much more mundane: Scientists theorize that life might have existed on Mars at one point, and may have even spread to Earth through space debris in a process known as panspermia. That said, if life does exist on Mars today, astrobiologists suspect it is more likely to be microbial than consisting of little green men. There are also studies suggesting that life may have previously existed on Mars before the planet underwent climate change. Another recent study suggests life on Mars would have gone extinct 1.3 billion years ago.

By Matthew Rozsa

Matthew Rozsa is a staff writer at Salon. He received a Master's Degree in History from Rutgers-Newark in 2012 and was awarded a science journalism fellowship from the Metcalf Institute in 2022.

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Aliens Europa Exoplanets Extraterrestrials List Mars Planets