Where Do Aliens Live?
The short answer
Nobody knows yet. But if anything is alive near us right now, our best bets are dark oceans under the ice of two moons: Europa at Jupiter and Enceladus at Saturn. Both are far outside the Sun's habitable zone. Gravity, not sunlight, likely keeps them warm, and Europa may hold twice as much water as all of Earth's oceans.
In 1950, over lunch at Los Alamos, Enrico Fermi asked, "Where is everybody?" We still don't know. Mars may have had life billions of years ago, and planets around other stars are too far to visit. But we now know which doors to knock on.
Why the search left the Sun's zone
The habitable zone is the ring around a star where a planet is warm enough for water to stay liquid on the ground. For a long time it was the obvious place to look. Then, in 1977, scientists in a small sub called Alvin went looking for hot springs on the sea floor near the Galápagos Islands, about 2,500 m (8,250 ft) down. No sunlight reaches that deep. No one on the trip was a biologist.
They found crowds of life in total darkness: tube worms about 46 cm (1.5 ft) tall with blood red tips, white crabs, and clams up to 30 cm (1 ft) long. They had almost nothing to preserve samples in, so they pickled some in Russian vodka.
None of it runs on sunlight. Hot water from inside the Earth carries chemicals, and microbes eat the chemicals. The tube worms don't even have a mouth. Microbes living inside them make their food. So the search now asks for three things, as far as we know: liquid water, heat and chemical food. A place can have all three in the dark.
Two oceans under the ice
Europa, one of Jupiter's moons, is about 90% the size of our Moon. Jupiter is about 5.2 times farther from the Sun than Earth is, and Europa's surface never gets warmer than minus 160 °C (minus 260 °F). Yet it probably hides an ocean. In the 1990s the Galileo probe flew past and felt Europa disturb Jupiter's magnetic field. Salt water carries electricity and ice doesn't, so the best explanation is a layer of salty water under the ice.
The heat likely comes from gravity. Europa circles Jupiter every 3.5 days on a slightly stretched path. Close in, Jupiter's pull stretches the moon. Farther out, it relaxes. Like a paperclip bent back and forth, it warms. This is called tidal heating. The ice on top is tens of kilometers thick. In 2026, NASA's Juno team reported an average of about 29 km (18 miles) for the part of the moon it measured.
Enceladus, one of Saturn's moons, is only about 500 km (310 miles) across. From cracks at its south pole, it sprays its ocean into space. Cassini spent 13 years at Saturn and flew through the spray and through the ring of ice grains the spray leaves behind. It found salt water, tiny grains of rock that most likely formed in hot water, hydrogen gas, carbon rich molecules and phosphorus. That's water, heat and food: three for three. In 2017, low on fuel, Cassini was steered into Saturn on purpose, so it could never crash into Enceladus and leave Earth germs in that ocean.
Why not Mars, or another star?
Mars may once have had life. Perseverance has explored Jezero Crater since 2021. More than 3.5 billion years ago, the crater held a lake. By October 11, 2026, the rover had driven 45.22 km (28.10 miles), passing Opportunity's record of 45.16 km (28.06 miles). In 2024 it found a rock with leopard spots, and in 2025 NASA called it the closest we have ever come to discovering life on Mars. But the same minerals can form without life. The sample is sealed in a tube with no ride home booked, and anything alive on Mars today would most likely be underground, sheltered from radiation.
We know of more than 6,000 planets around other stars. A probe like Voyager 1 would need about 76,000 years to reach the nearest star. So telescopes like Webb read the starlight that shines through a planet's air. In 2025 a team said K2 18b might have dimethyl sulfide in its air, a gas that on Earth comes from living things such as plankton. Other teams checked the same light and found no solid sign of it.
Europa Clipper launched on October 14, 2024. Its solar arrays span more than 30 m (100 ft), and it is the largest spacecraft NASA has built for a planetary mission. It reaches Jupiter in April 2030 and will fly past Europa 49 times to find out whether that ocean could support life. No mission is headed to Enceladus yet.
Key numbers
| What | Number | Source |
|---|---|---|
| Water in Europa's ocean | possibly twice the water in all of Earth's oceans | NASA, Europa facts |
| Europa's warmest surface temperature | minus 160 °C (minus 260 °F) | NRAO, 2018 |
| Europa's ice shell, Juno's average for the area it measured | about 29 km (18 miles) | JPL, Juno measures Europa's ice shell, 2026 |
| Jupiter's distance from the Sun | about 5.2 times Earth's distance | NASA, Europa facts |
| Width of Enceladus | about 500 km (310 miles) | NASA, Enceladus |
| Depth of the 1977 search on the sea floor off the Galápagos | about 2,500 m (8,250 ft) | WHOI, the 1977 discovery |
| Perseverance's total drive by October 11, 2026 | 45.22 km (28.10 miles), past Opportunity's record of 45.16 km (28.06 miles) | NASA, Perseverance path update, 2026 |
| Cassini's time at Saturn | 13 years, ending in 2017 | NASA, Cassini mission |
| Known planets around other stars | more than 6,000 | NASA, exoplanets |
| Voyager 1's travel time to the nearest star | about 76,000 years | Universe Today, travel to the nearest star |
| Europa Clipper's span with solar arrays open | more than 30 m (100 ft) | NASA, Europa Clipper |
| Europa Clipper's planned flybys of Europa | 49, after arriving at Jupiter in April 2030 | NASA, Europa Clipper mission |
What the film simplifies
- The Sun as a chef, Gravity as a cook who kneads moons, and Rocket the pizza courier are cartoon. Nobody chooses to heat a moon: tidal heating happens because the moon flexes as its distance from Jupiter changes.
- Europa's stretched orbit is drawn far more oval than it really is, and the film labels it "shape exaggerated".
- "Tens of kilometers" of ice covers a real debate. Juno's 29 km figure is for the cold outer layer over the part of Europa it measured, and NASA's Europa facts page gives 15 to 25 km (10 to 15 miles).
- Travel times are rounded for the jokes. Rocket says six years to Jupiter, but Clipper's trip runs about five and a half, from October 2024 to April 2030. Perseverance took about 6.7 months to reach Mars (July 30, 2020 to February 18, 2021), which Rocket rounds to seven.
At a glance
Runtime 9 minutes 18 seconds · Topics: Astrobiology, Ocean worlds, Mars, Planets around other stars · Captions on YouTube · Full script below
Test yourself
Why did the 1977 Alvin dive change where scientists look for life?
Answer
It found tube worms, crabs and clams living about 2,500 m down, where sunlight never reaches. Their food comes from microbes that eat chemicals carried by hot water, so life needs water, heat and chemical food, not necessarily sunlight.
Europa's surface never gets warmer than minus 160 °C. What may keep the ocean under its ice liquid?
Answer
Tidal heating. Every 3.5 days Europa swings closer to Jupiter and farther away on a slightly stretched path, so Jupiter's pull stretches and relaxes it, warming it like a paperclip bent back and forth.
No one has seen Europa's ocean. What evidence did the Galileo probe find for it?
Answer
Galileo felt Europa disturb Jupiter's magnetic field. Salt water carries electricity and ice doesn't, so the best explanation is a salty ocean under the ice.
The script
Read the script
Rocket: Pizza for the aliens!
Host: No answer. There never has been.
Host: 1950. Los Alamos, New Mexico. Four physicists walk to lunch, joking about a cartoon of flying saucers stealing trash cans.
Host: Enrico Fermi calls it a very reasonable theory. It explains the flying saucers and the missing trash cans.
Host: Then, over lunch, out of the clear blue, Fermi, who led the team that built the first nuclear reactor, asks...
Enrico Fermi: Where is everybody?
Host: The universe is billions of years old, with more stars than we can count. If aliens are out there, why haven't we heard from a single one?
Host: We still don't know. So let's start with a smaller question. Where could aliens even live?
Rocket: I'll find 'em! Gimme the address!
Rocket: Rocket. Fastest delivery in the Solar System. Thirty minutes or it's free!
Host: He works for the only oven in town. The Sun. And the Sun says every living thing runs on his cooking.
Rocket: Deliver to: the aliens. Address: question mark.
Rocket: No address? I delivered to a black hole once. Still waiting on the tip.
Host: The Sun's delivery zone. The ring around a star where a planet is warm enough for water to stay liquid on the ground. Scientists call it the habitable zone.
Gravity: Wanna bet?
Host: Here's what life needs, as far as we know. Water. Heat. And food, the chemical kind.
Rocket: Three boxes. One rocket. Let's ride!
Rocket: Seven months? Mars is right next door!
Host: Mars. The most famous alien address there is. In 1895, an astronomer published maps of canals on it and said Martians dug them.
Host: They were a trick of the eye.
Rocket: Hello? Pizza?
Host: The only ones home are robots. This one's Perseverance, here since 2021.
Host: It just broke the record for the farthest drive on another world. 45 kilometers.
Rocket: Forty five kilometers? In five years? I do that in a sneeze.
Host: It landed in a crater that held a lake, billions of years ago.
Rocket: Lakefront property! Where's the lake?
Host: Long gone. Mars today is freezing and dry.
Host: But its rocks hold carbon rich molecules, the stuff food is made of.
Host: And in 2024, Perseverance found a rock with leopard spots.
Host: Tiny spots with dark rims. On Earth, marks like these are often left by microbes.
Host: In 2025, NASA called it the closest we've ever come to finding ancient life on Mars.
Rocket: We found aliens!
Host: We found a maybe. An old one. Rocks can make spots like that without any life at all.
Host: To be sure, scientists need that rock in a lab on Earth. So it's sealed in a tube, waiting for a ride home.
Rocket: A pickup order? That's me!
Host: That ride needs a lander, a launch off Mars and a trip back. It isn't booked yet.
Rocket: Put me on the list, buddy.
Host: And if anything is alive on Mars today, the best guess is underground, sheltered from the radiation.
Rocket: A basement apartment. No doorbell. Classic.
Rocket: Inside the zone, it's Earth or nothing.
Gravity: You're looking in the wrong kitchen.
Rocket: Gravity? You crush stars for a living.
Gravity: I squeeze. Different hobby.
Gravity: Ever been to the bottom of your own ocean?
Host: 1977. Two and a half kilometers down, off the Galápagos Islands.
Host: Scientists in a little sub called Alvin went looking for hot springs on the sea floor.
Host: Sunlight never reaches down here. And not one of them was a biologist.
Host: They found a dinner party. Tube worms with blood red tips. White crabs. Clams as big as plates.
Host: They didn't bring enough chemicals to keep samples. So they pickled some in Russian vodka.
Rocket: Party animals.
Host: None of it runs on sunlight. Hot water from inside the Earth carries chemicals, and microbes eat the chemicals.
Host: The tube worms don't even have a mouth. Microbes living inside them make their food.
Rocket: No mouth? How do they order pizza?
Gravity: Life doesn't need the Sun. It needs a kitchen.
Host: That dinner party changed where we look for aliens. Anywhere with water, heat and food. Even in total darkness.
Gravity: And out past the zone, I run a couple of ovens myself.
Rocket: Six years? The pizza's gonna be cold!
Host: Everything's cold out here. Jupiter is five times farther from the Sun than we are.
Host: Meet Europa, one of Jupiter's moons. A bit smaller than ours. Its surface is about minus 160 degrees Celsius.
Rocket: My nose is frozen to the door!
Host: But under that ice, there's an ocean.
Rocket: How do you know? Nobody's been in!
Host: In the 1990s, the Galileo probe flew past and felt Europa disturb Jupiter's magnetic field.
Host: The best explanation is a layer of salty water under the ice. Salt water carries electricity. Ice doesn't.
Host: And it's huge. Europa may hold twice as much water as all of Earth's oceans.
Rocket: An ocean? At minus 160? How?
Gravity: Me.
Host: Every three and a half days, Europa swings around Jupiter on a slightly stretched path.
Host: Close in, Jupiter's pull stretches it. Farther out, it relaxes.
Host: Stretch, relax, stretch. Like bending a paperclip back and forth until it's warm.
Gravity: I knead. It warms. No Sun required.
Host: Scientists call it tidal heating. It may be what keeps Europa's ocean liquid.
Host: Water, yes. Heat, probably. Food? We don't know yet.
Rocket: Great! Somebody let me in!
Host: Through ice tens of kilometers thick? Nobody can drill that. Yet.
Rocket: Seven years to Saturn? I've got a pizza in here!
Host: Saturn has almost 300 moons. Most are just ice and rock.
Host: But one of them is showing off.
Host: Enceladus. Only about 500 kilometers wide. You could drive across it in a day.
Host: And from cracks at its south pole, it sprays its ocean into space.
Gravity: That one's mine too. I squeeze. I let go. It sprays.
Rocket: Somebody's COOKING!
Host: We know what's in that spray, because the Sun's best courier flew through it. Cassini.
Rocket: Best courier? I'm the best courier!
Host: Thirteen years circling Saturn. It flew through the spray, and through the ring of ice dust the spray leaves behind, and tasted both.
Host: Salt water.
Host: Tiny grains of rock that most likely formed in hot water.
Host: Hydrogen gas. At the vents back home, microbes eat exactly that.
Host: Carbon rich molecules. And phosphorus, one of the building blocks of DNA.
Rocket: Water! Heat! Food! Three for three!
Host: It's one of the best kitchens we know of, anywhere outside Earth.
Rocket: So who's in there?
Host: We don't know. When Cassini was built, no one knew the spray existed. It wasn't built to look for life.
Host: In 2017, Cassini was running low on fuel.
Host: Left alone, it might one day crash into Enceladus and leave Earth germs in that ocean.
Host: So its team steered it into Saturn. On purpose.
Rocket: ...He didn't want to track mud in.
Rocket: Fine. Other stars have their own zones, right? Let's go knock!
Host: We know of more than 6,000 planets around other stars.
Rocket: My timer says... NEVER?
Host: Even the nearest would take a probe like Voyager tens of thousands of years. So we don't deliver. We sniff.
Host: Meet the Webb Space Telescope. When a planet passes in front of its star, starlight shines through its air.
Host: Every gas leaves its own mark in that light. Webb reads the marks.
Rocket: So we can smell their kitchens from here!
Host: In 2025 a team said one planet, K2 18b, might have a gas in its air that on Earth comes from living things.
Rocket: Aliens! I knew it! I KNEW it! Somebody get me a selfie stick!
Host: Other teams checked the same light. They found no solid sign of it.
Rocket: I'm gonna need that confetti back.
Host: That's how science works. One team says maybe, and everybody else tries to break it.
Host: So, where could aliens live?
Host: Mars may have had life billions of years ago. But if anything is alive near us right now...
Host: our best bets are dark oceans under the ice. Enceladus and Europa, far from the Sun, kept warm by Gravity.
Gravity: You're welcome.
Rocket: Then what are we waiting for? Enceladus, best kitchen in town, here I come!
Host: No one's headed to Enceladus yet. But Europa has a ride.
Europa Clipper: Um. Excuse me. I'm the ride?
Host: Europa Clipper. NASA's rookie.
Rocket: Kid, you're tiny.
Host: Thirty meters from tip to tip. Wider than a basketball court is long. The biggest spacecraft NASA has ever built for a planetary mission.
Rocket: Okay. Medium kid.
Host: It won't land. It'll fly past Europa 49 times, to find out if that ocean could support life.
Rocket: You've got thirty minutes, kid.
Europa Clipper: Five and a half years. Just to Jupiter.
Rocket: Thirty minutes or... it's free.
Host: A real rocket did exactly that, on October 14, 2024. Clipper reaches Jupiter in April 2030.
Host: Fermi asked where everybody is.
Host: We still don't know. But now we know which doors to knock on.
Gravity: Next week, we go down. My favorite direction.
Sources
- Fermi asked "Where is everybody?" over lunch in the summer of 1950.Jones, Los Alamos report, 1985
- Fermi led the team that built the first nuclear reactor.US DOE, the first nuclear chain reaction
- The universe is billions of years old (13.8 billion).NASA, WMAP overview
- The habitable zone is where a planet is warm enough for liquid water on its surface.NASA, the habitable zone
- Life at hydrothermal vents runs on chemicals, not sunlight.NASA Astrobiology, hydrothermal vents
- Perseverance launched July 30, 2020, landed February 18, 2021 in Jezero Crater, which held a lake more than 3.5 billion years ago.NASA, Mars 2020 Perseverance
- Perseverance drove 45.22 km by October 11, 2026, breaking Opportunity's 45.16 km record.NASA, Perseverance path update, 2026
- In 1895 an astronomer published maps of Martian canals; they were an illusion.OpenStax, a Martian controversy
- Perseverance found a rock with leopard spots, small spots with dark rims.JPL, intriguing Mars rock, 2024
- In 2025 NASA called the rock the closest we have ever come to discovering life on Mars; nonliving processes cannot be ruled out.NASA, potential biosignature, 2025
- No mission is booked to bring the Mars samples home.Semafor, 2026
- Radiation at the surface of Mars.SwRI, radiation on Mars
- The 1977 expedition: searching about 2,500 m (8,250 ft) down, no biologist aboard, tube worms 1.5 ft tall with red tops, white crabs, clams up to 30 cm, samples kept in Russian vodka.WHOI, the 1977 discovery
- Giant tube worms have no mouth; microbes inside them make their food.Animal Diversity Web, Riftia pachyptila
- Europa is about 90% the size of our Moon, orbits Jupiter every 3.5 days and may hold twice the water of Earth's ocean; Jupiter is 5.2 times Earth's distance from the Sun.NASA, Europa facts
- Europa's surface never warms above minus 160 °C (minus 260 °F).NRAO, 2018
- Europa's ocean may hold twice the water of Earth's oceans; Jupiter is 5.2 times Earth's distance from the Sun.NASA, Europa infographic
- Galileo's magnetic measurements point to a salty ocean under Europa's ice.JPL, Galileo evidence for Europa's ocean
- Tidal heating: Jupiter stretches and relaxes Europa along its stretched orbit.NASA, Europa tide movie
- Juno measured Europa's ice shell at about 29 km (18 miles) on average in the area it observed.JPL, Juno measures Europa's ice shell, 2026
- Saturn has almost 300 moons (293 confirmed as of August 2026).NASA, Saturn's moons
- Enceladus is about 500 km (310 miles) across, and its jets are fed by an ocean inside the moon.NASA, Enceladus
- Cassini spent 13 years at Saturn and was steered into Saturn in 2017, low on fuel, to protect Enceladus from contamination.NASA, Cassini mission
- The spray of Enceladus comes from cracks near its south pole and contains salt water.JPL, ocean spray at Enceladus
- Tiny rock grains from Enceladus most likely formed in hot water.JPL, hydrothermal activity at Enceladus
- Cassini found hydrogen gas in the spray of Enceladus.NASA, ocean worlds and hydrogen
- Carbon rich molecules fresh from the ocean of Enceladus.JPL, fresh organics from Enceladus
- Phosphorus found in ice grains from Enceladus in Saturn's E ring.JPL, phosphorus in Enceladus's ocean, 2023
- No mission is headed to Enceladus yet.Astronomy, Target Enceladus
- More than 6,000 planets are known around other stars.NASA, exoplanets
- Voyager 1 would take about 76,000 years to reach the nearest star.Universe Today, travel to the nearest star
- The Webb Space Telescope reads starlight passing through a planet's air.ESA Webb, 2022
- A 2025 team reported a possible sign of a gas made by life on Earth in the air of K2 18b.Madhusudhan et al., 2025
- Other teams found no solid sign of the gas on K2 18b.Space.com, doubts over K2 18b
- Europa Clipper spans more than 30 m (100 ft), is the largest NASA spacecraft for a planetary mission, launched October 14, 2024 and reaches Jupiter in April 2030.NASA, Europa Clipper
- Europa Clipper will make 49 close flybys of Europa to learn whether its ocean could support life.NASA, Europa Clipper mission
- Europa Clipper's timeline from launch to Jupiter.NASA, Europa Clipper timeline