So you’ve gone and destroyed the planet you called home.
It was inevitable. Climate change reached the tipping point. Famine and disease ravaged humanity. A noxious information ecosystem sent society into a conspiracy-fuelled death cycle. To survive, we gotta get out of Dodge.
To crib from Blade Runner, a new life awaits on off-world colonies. Maybe your fresh start is in Elon Musk’s Mars city. Maybe it is on the Moon with NASA’s lunar base. Or maybe a Zefram Cochrane-like figure has invented faster-than-light travel. Or, well, near-light speed travel if we are really patient. Who needs the Solar System if we don’t have Earth, am I right?
You’ve got one ticket off Earth. Where do you go? There are a few possible planets we can go to and some are definitely better than others. There’s surviving — where we can make it work with a lot of sacrifices, but it won’t be easy — and then there’s thriving: finding a truly new home for humanity instead of a stopgap.
Here’s a guide to going off-world. At the end of the day, it will make you appreciate Earth.
Welcome to FUTURE EARTH, where Inverse forecasts 100 years of possibilities, challenges, and who will lead the way.
We know, we know. Venus. Some might say it is toxic, hot, and inhospitable. And they are right. But Mars is deadly too, albeit in the opposite direction. Just as Venus is extremely hot, Mars is frigid cold. Venus has a thick, poisonous atmosphere, but Mars has a paper-thin one. Mars has all that radiation exposure to boot, which doesn’t sound that nice.
For Empire Strikes Back fans irresistibly drawn to the idea of living in a Cloud City, NASA has a plan. Storm IV Twin-Pod cloud car not included. Meet HAVOC — or the High Altitude Venus Operational Concept. HAVOC is basically a blimp and a fleet of other buoyant vehicles designed to explore Venus with humans on board.
“Venus... can play a role in humanity’s future in space.”
The truly inhospitable region of Venus lies far below, near, and at the surface. About 30 miles above the surface, temperatures are actually Earth-like. At this altitude, atmospheric pressure is also similar to Earth at sea level. There’s also similar radiation protection at that level. The air isn’t breathable, but the other factors mean spacesuits for venturing outside — like on an observation deck — wouldn’t be overly bulky affairs. HAVOC is still on the drawing board, but the preliminary design calls for a craft that would allow two astronauts to live on Venus for 30 days. Or above it, at least.
The paper introducing HAVOC as a concept reveals ambitions for the world. HAVOC phase I is to send robots to Venus’ clouds. Phases II through IV are composed of increasingly ambitious human missions of increasingly long stays. The goal for Phase V? Permanent human habitation of a Venus cloud city.
“Ultimately, the authors conclude that Venus, with its relatively hospitable upper atmosphere, can play a role in humanity’s future in space,” the authors write in the paper.
Who are we to argue?
The Inverse analysis: Steampunk fans may love living on an airship, and Earth-like pressure at high altitudes could make for less clunky spacesuits. But one small failure could spell major doom for would-be Venusians.
Habitable rating: 🎈☣️ ☣️ ☣️
“There are three reasons underlying the case for Mars,” Robert Zubrin tells Inverse. “For the science, for the challenge, and for the future.” Zubrin should know — he is the founder of the Mars Society, which advocates for moving humans to Mars as soon as possible.
To understand Zubrin’s passion for the Red Planet, let’s break those three elements down — the science, the challenge, and the future. The science refers to the question we could soon answer: if there was life on Mars. If there is life on Mars, even. The challenge is a throwback to the space race that led to Apollo, and a sort of knock-on effect that draws more dreamers to the sciences. The future is the idea of humanity staking a permanent future off-Earth.
Zubrin says has all the resources we need, if we can harness them correctly:
- Water: while surface water is only found seasonally and in small amounts laden with toxins, recent discoveries could make potable water possible by looking underground
- Soil: The soil of Mars is laden with iron and nasty salts, but experiments with growing crops in Mars-like soil have found some success.
- Lava tubes: Rather than building expensive structures or risking inflatable habitats, a la The Martian, a series of caves on Mars built by long-dormant volcanoes could create a radiation-shielded environment. Just add oxygen.
Humanity’s transition to Mars is inevitable, as far as Zubrin is concerned. It will happen in four phases, he says: exploration is the first step before we set up a Mars base, which we can springboard into cities somewhere in between Musk and Total Recall, and from there, entire societies can flourish. Zubrin doesn’t see human movement on Mars as monolithic. Like Earth, different groups will settle in, and pursue different values and goals. The planet will bring them new opportunities to practice the forms of governance and beliefs they wish they could on Earth, he says.
Ultimately, Mars offers a chance for renewal and a clean slate. And it is a mere six-month flight away. Deliverance has never been so handy.
“The goal is the stars.”
The resources of Mars could also be a launching pad for further exploration, Zubrin says. Methane and water on the planet can be used to create rocket fuel. After Mars, the next jump is the asteroid belt. And after that?
“The goal is the stars,” Zubrin says. “As humans become spacefarers, we’re going to learn a lot more about the stars, about the planets orbiting the stars.”
What if there’s already life on Mars, and it is still alive? Zubrin says that most of the arguments for current life on Mars are easily refuted, so humanity can rest easy there. One of the biggest criticisms of potential human settlements on Mars is the idea of cross-contamination: Either that humans will incidentally kill off whatever life there is on the Red Planet, or conversely, that that life could threaten us instead.
But one way we know about the conditions on Mars is because chunks of it frequently fall to Earth, products of impacts ejected across interplanetary distances. “The issue of back contamination which is the one that gets the biggest headlines is nonsense,” Zubrin says. “We’ve been getting materials from Earth every year for four billion years.”
Carl Sagan once said, "If there is life on Mars, I believe we should do nothing with Mars. Mars then belongs to the Martians, even if they are only microbes." But Zubrin calls this an “aesthetic issue.” He says, for instance, that on Earth, we happily murder microbes all day, every day — mouthwash, antibiotics, household cleaners — our poisons are as varied as they are effective. And yet, we don’t regard these acts of destruction on our own planet as an ethical issue. “I think that Sagan got carried away in a poetic fit when he made that statement,” Zubrin says.
“The idea that an entire planet — the whole potential of a new biosphere of not only people but of an entire assortment of wonderful plants and animals that could grow in flourish on Mars — should be abluted because there are microbes on Mars is absurd and I would say unethical to say that people should not exist on Mars so that it can be safe for microbes.”
The Inverse analysis: Despite sub-Antarctic temperatures, space radiation, and other hazards at least there’s terra firma (martis ferma?) to stand on and a gateway to the rest of the Solar System and points beyond. Just hope nothing goes wrong.
Habitable rating: 🚀👩🚀👩🚀
3. Proxima Centauri b
When it comes to planets in the solar system to use as humanity’s ark, the pickings are slim. We’ve got Mars. We’ve (sort of) got Venus. Beyond that, the only terrestrial planet left — Mercury — is too hot on one side and absolutely frigid on the other. It’s also notoriously hard to get to thanks to the gravity of the Sun. Jupiter’s out — we would fry in a bath of radiation. The other gas planets aren’t hospitable either.
So maybe we just build a generation ship or pray for a warp drive and punt ourselves a star system over. And where better to aim than Proxima Centauri?
Abel Mendez is the director of the Planetary Habitability Laboratory (PHL) at the University of Puerto Rico Arecibo. Mendez has maintained the Habitable Exoplanets Catalog for 10 years. At the time the catalog began, the number of potentially habitable exoplanets was scant: Back then, it was made up of just Gliese 667 C f, and the contentious and ever-evolving planets in the Gliese 581 system. (Gliese, in this case, refers to a star catalog — while the stars are 20 and 23 light-years away from Earth, respectively, they’re not close to each other.)
Since then, the number has beefed up to 60 potentially habitable worlds if we’re lucky, or 24 if we’re being realistic. Mendez uses a rubric to rate these worlds using a few factors:
- Are they close enough to their star to be warm, but not too hot, or too cold? This would mean water could exist in its liquid state.
- Are they somewhere in between Mars and an Earth-and-a-half in radius? This would probably place them close to Earth-mass, and thus likely rocky.
- If we don’t know the radius, are they less than five times the mass of Earth? This would again make for a planet that, at the upper limit, would be a very heavy but still rocky planet, rather than a gaseous mini-Neptune.
Of the 4,375 known exoplanets (as of April 16, 2021, according to NASA), this rubric means only 24 of them are potentially habitable. But the right size and distance are only two of the things we need for habitability in reality. Beyond these traits, we don’t have much other information to go on.
This is why Mendez says if we had to go somewhere beyond the Solar System, we should go somewhere close.
“Otherwise we cannot tell, but assuming that information there is right … I think the planets I would prefer to visit are the nearby ones,” he says. And there’s no habitable planet closer than Proxima Centauri b.
Alpha Centauri A and B are pretty well known. They’re the closest stars the size of the Sun to Earth. But they aren’t quite the closest star. The two stars orbit each other. But about .12 light-years out — around 7600 times the distance of the Sun to Earth — they have a faint companion. Alpha Centauri A and B have been known since antiquity and known to be two separate stars since 1689. Their companion, Proxima Centauri, is so small and faint that it wasn’t discovered until 1915.
“I think the planets I would prefer to visit are the nearby ones.”
And while the Alphas Centauri are 4.37 light-years from Earth, that smidge of difference in distance puts Proxima Centauri just a fraction closer to Earth.
We’ve looked time and time again for planets around one of the Alphas Centauri, usually coming up short. But in 2016, astronomers working as part of the Pale Red Dot project used decades of data on Proxima Centauri to draw out the existence of a possible planet, Proxima Centauri b. This planet is likely around the mass of Earth, and orbits in a tight 11-day loop around its star.
While such a close orbit sounds like a recipe for disaster — Mercury, after all, orbits the Sun in 88 days — the small size and mass of Proxima Centauri and its lower temperatures mean that the planet could be habitable, under the right conditions. The star Proxima Centauri is only 20 percent larger than Jupiter and is just 12 percent the mass of the Sun. It’s classified as a red dwarf, also called an M-dwarf star. These stars are the least massive and most common types of stars in the universe. Not surprisingly, many of the planets on PHL’s index are M-dwarf stars. Only three are not.
But — and there’s always a but — that doesn’t mean Proxima Centauri b is much like Earth. With that close an orbit, the planet faces the same side of the star at all times, the same way we only see one face of the Moon. This process is called tidal locking, and it is part of why Mercury is so inhospitable.
“I think that most of these planets are probably tidally locked because there are around M-dwarf stars, and even if they have an atmosphere, water will be locked as ice on the dark side, and the dayside will be a desert,” Mendez says.
But in between the two sides is a little sliver of liminal space that’s neither day nor night — a place of perpetual twilight. That’s where liquid water might exist, and life might flourish.
There’s also the problem of atmospheres. Red dwarfs are small stars, but early in their history, they thrash around violently with flares — which can blow away the primordial atmospheres of planets out into the cosmos. So we have to hope that Proxima Centauri b was able to regrow an atmosphere and that it’s something like Earth, and not, say, Venus or Mars.
The Inverse analysis: It’s close and in the right place to have life, but a lack of known details and a potentially hazardous star mean we shouldn’t go quite yet.
Habitable rating: ❓❓🤷🤷🤷
In 2015, Belgian astronomers working on the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) project announced a new star, dubbed (entirely by coincidence and not for promotion reasons, we are sure) TRAPPIST-1.
It’s small — about the size of Jupiter — and less massive even than Proxima Centauri. In fact, at 8.9 percent the mass of the Sun, it’s barely over the threshold of having enough mass to ignite. TRAPPIST-1 wasn’t alone — the astronomers announced three Earth-sized planets in orbit. And all three are potentially habitable.
It was big news at the time. Three worlds a sneeze of a distance apart, all temperate enough to hold water, all around a fairly calm, tiny star. But subsequent studies pulled a real “hold my beer” and hit us with four more planets in orbit around TRAPPIST-1. All of them were close enough to be temperate too. Tiny but mighty, indeed.
Suddenly, we have a very tantalizing system on our hands. The PHL index lists TRAPPIST-1d as the likeliest of all of them to have Earth-like conditions. But it’s not a one-one comparison. The planet is thought to be somewhere in between Mars and Earth in size. It’s got an Earth-like amount of radiation from its star and thus, likely, similar temperatures, too.
So let’s go there!
Or not. It might be just a little too hot, according to a 2018 study. "We are modeling unfamiliar atmospheres, not just assuming that the things we see in the solar system will look the same way around another star," a statement from the authors reads. In short, they say TRAPPIST-1d may be more Venus-like than we might like for habitability.
They also argue:
- TRAPPIST-1b may be far, far too hot — maybe even too hot for an atmosphere at all.
- TRAPPIST-1c and TRAPPIST-1d have thick atmospheres which create intense greenhouse effects.
- TRAPPIST-1f, TRAPPIST-1g, and TRAPPIST-1h might be too icy.
They conclude that TRAPPIST-1e may be the real world we should look to. The answer isn’t definitive though. And we likely won’t know anything until we can get a close look at the system via next-generation telescopes like the James Webb Space Telescope.
“If one planet doesn’t work, you just move to another planet.”
But if we go to TRAPPIST-1 and any one planet isn’t quite right, we could just jump to the next one — most of them are less than a million miles apart. If we don’t like TRAPPIST-1d, then TRAPPIST-1e is a mere 650,000-mile journey. Earth is 238,000 miles from the Moon. It took three days for Apollo astronauts to get to the Moon.
It’s not the six-month trip to Mars humans would experience — it’d be more like an extended business trip.
“You have seven planets, some in the habitable zone,” Mendez says. “You have in one system everything possible, and multiple planets with those conditions, hot or cold.”
The term Goldilocks Zone is sometimes used in place of the habitable zone. Not too hot, not too cold, just right. But TRAPPIST-1 plays with that by making for a Goldilocks system where, in a few days, we can make our way in between the planets. Even the farthest out of the planets is only 1,400,000 miles from its home star.
“If one planet doesn’t work, you just move to another planet,” Mendez says.
And what’s more, it’s only 40 light-years away. Sure, that’s 10 times the distance of Earth to Proxima Centauri. And sure, even going half the speed of light makes for an 80-year one-way trip. But can you really put a price on the future of humanity?
The Inverse analysis: Don’t like one planet? Hop to another. Heck, if they’re all habitable, choose your own adventure. If none are … good luck.
Habitability rating: 🔥❄️😎🌱🌱🌱🌱🌱
But … what if red dwarfs don’t have the right stuff?
Aside from pillaging the PHL list of worlds, you eliminate the potential for robust life in the universe. You have to stick to stars like the Sun. And suddenly, we don’t have a lot of places nearby to go. Two of the remaining planets around non-M-dwarf stars on the PHL index aren’t an easy shot. Kepler-442 b is nearly 1200 light-years away. Kepler-62f is 980+ light-years away. We can’t get there.
1. Tau Ceti f
Tau Ceti f is a hefty world. We haven’t seen it directly, but it’s at a minimum four times the mass of Earth. Gravity there would be high — not a lot of fun for those who like calisthenics. It’s also likely 1.8 times the radius of Earth (though we haven’t seen it pass in front of its star, so that’s a rough estimate). It has a 642-day orbit — one of the longest period known habitable exoplanets.
But it has a lot of things in its favor. Tau Ceti is a Sun-like star, about 78 percent the mass of the Sun, and burns at similar temperatures. (Both are classified as G-stars.) At 12 light-years away, we could get there in 24 years going half the speed of light — just in time for babies leaving Earth to have a quarter-life crisis on arrival.
Its Sun-like qualities have made it a popular destination for science fiction, including Dan Simmon’s Hyperion and Arthur C. Clarke’s Rama series. The star system also makes an appearance in Star Trek, Barbarella, Doctor Who, The Expanse, and … uh … Earth: Final Conflict. It’s the nerd connoisseur's Sun-like star, to all you Alpha Centauri-loving plebes.
It’s not exactly high on the list of the 24 most habitable planets on the list. In fact, it’s 22 on a list of 24. It’s not graduating with honors. It’s only listed as having a .58 Earth Similarity Index (.99 is the highest possible, and Proxima Centauri b has .87 and TRAPPIST-1d .90, sitting at #4 and #6, respectively.)
So maybe Tau Ceti f isn’t quite what we want there, either — especially with the gravity and potential to be more of a small Neptune than a large Earth. There could be moons. We might find an elusive candidate, Tau Ceti PxP-4, that (if it exists) is in the habitable zone too. In other words, it’s still worth the trip — especially if the red dwarf stars are out.
The Inverse analysis: Earth-like temperatures could make this a great place to go, and if the planet isn’t so great, maybe it has an ok moon … or even another planet waiting for us.
Habitability rating: 🤔🤔🤔
Okay, okay. We know. Not a planet — it’s Jupiter's outermost large moon. But hear us out.
Let’s say astronomers realize Proxima Centauri b isn’t habitable. Fine. There are other potentially habitable planets within the same distance as Tau Ceti, including Teegarden’s Star b (the most potentially habitable planet by PHL standards), Ross 128b, and Luyten’s Star, which we’ve already beamed a message to. We could just go there, but being that they’re all M-dwarf planets, the journey seems iffier.
But let’s say Proxima Centauri b is very habitable. So habitable, in fact, that it’s rife with intelligent life. We aren’t going to be welcomed with open arms. We would, after all, be invaders — the kinds of aliens sci-fi has taught us to fear. (If you want to know how first contact can go very wrong, get out of hand if you will, read The Sparrow by Mary Doria Russell.) Then we can’t go there either.
And if Mars and Venus are non-starters, there are still places we can go. They’re just not planets. They’re the water worlds of our Solar System, places like Ceres in the asteroid belt, Europa and Ganymede circling Jupiter, and Enceladus spewing oceanic water out into space. Even Titan looks like a cold version of early Earth — lakes of ethane and methane dotting the surface while an entire ocean of water is trapped deep below. Pluto might have a subsurface ocean.
But we might not want to go down this path. Jupiter, for example, has some of the most deadly radiation in our solar system. When the Pioneer 10 probe passed it in 1974, NASA hadn’t correctly predicted just how much radiation it would emit. Even passing what they thought was a “safe” point, the probe was damaged by the radiation.
And while Europa and Ganymede are watery ocean worlds underneath an icy crust, they’re within those radiation belts. The ice may protect life below but would endanger or flat-out fry humans above.
There has to be a better way.
And maybe there is. In 2003, NASA Langley researchers suggested a solution to the Ganymede / Europa problem. Just go a world over, to Callisto.
Why is Callisto so promising?:
- It’s outside the most dangerous parts of the radiation belts
- It’s the size of the planet Mercury (Ganymede is slightly larger, Callisto slightly smaller)
- Its heavy cratering gives places to land
It’s also made of water ice. This water could be used for drinking after some processing, sure, but it could also be refined into hydroxyzine for rocket fuel.
What’s so tantalizing about Callisto (in the more immediate) is the chance it could be home base while we send robots off to Europa to explore. We could set up shop on the relatively safe far side of the moon and wait for the data to come back.
Maybe it’s our new home, maybe it’s our new research station. Ultimately, the worlds beyond Earth seem pretty inhospitable. While humans are tough, you’re asking all of humanity to participate in the Shackleton Expedition.
Or maybe we should just stay home and stay put, fixing our problems on Earth instead of running away to another planet. (But if we don’t, dibs on a condo on Tau Ceti f.)