Prior to about thirty years ago, astronomers had no way of being able to detect planets outside our own solar system. Fortunately, with breakthroughs in direct imaging through cutting-edge telescoping technology and novel forms of indirect detection as aided by space-based probes have allowed astronomers to determine the definite existence and traits of distant extra-solar planets. Since then thousands of planets have been detected from all over the galaxy, and of that collection, a few just may have the specific set of qualities to be conducive for human settlement.
Granted, not all, and indeed most likely aren’t, guaranteed to be near perfect copies of our Earth. Among the first criteria which astronomers consider is the most pertinent of all: the possibility of a planet being closest enough to its host star, and thus warm enough to harbor liquid water. Beyond that though, myriad other factors go into determining the habitability of such prospective planets including known or predicted geologic composition, atmospheric conditions, as well as its total mass, density, and radius to produce gravity ideal for human life, or at the very least a gravitational field which can be endured.
Most likely, of the dozens of prospects many of the candidate worlds astronomers have nominated as being future homes for humanity simply aren’t conducive for human settlement. But, there are quite possibly at least a few, both near and far, which just may have the suite of life supporting characters found also on Earth.
15. Proxima Centauri b
We’ll start our list with a planet which is just a hop, skip, and a jump over…at least, on a galactic scale. At just under 4.25 light years away, Proxima Centauri b is the closest potentially habitable exoplanet. So far astronomers have determined this planet to be in the “Goldilocks zone” wherein enough sunlight reaches the planet’s surface for liquid water to exist, and this world may be small enough to have gravity comparable to the Earth. That said, just because it has some of the key criteria to be a prospective new earth doesn’t mean that it’d be a tolerable place to live or even set foot on.
Although some of the specifics of this planets are unknown, it is known that its host star is a small, cool M-class red star. Consequently, Proxima Centauri b is tidally locked, meaning this planet doesn’t rotate and one side always faces the sun so the only location to avoid intense constant heat on the sun-facing side or the frozen dark side, prospective colonists would do best to hang out around the planet’s thin “twilight zone”.
Further, one model suggests that early in this system’s evolution its only known planet was wracked for millions of years by intense solar flares which stripped hydrogen from the planet’s ancient atmosphere. If true, this means that water probably isn’t abundant, and thus the planet is either covered in a Venus-like carbon dioxide-rich atmosphere or one rich in oxygen. On the other hand, this planet may have a composition more akin to Uranus, so don’t pack up your spaceship and set course quite yet.
14. Kepler 438b
One of the most important factors when it comes to determining the potential habitability of a planet is its apparent gravity. Multiple stellar bodies within our local solar system do show other promising signs such as the presence of water-ice, liquid water, hydrocarbons, and other volatiles, but are far too small to possess gravitational fields suitable for healthy human development. By contrast, myriad planets outside our home area may be in the habitable zone of their star and may even have developed along certain planetary trajectories similar to earth but are far too large for human beings to walk around on without being crushed flat.
In fact, of the dozens of candidates, most of the potentially habitable planets are classified as “super-Earths”. Many of these planets have several characteristics similar to ours, save the enormous mass and radius which make them a challenge for prospective colonists to acclimate on, if not untenable for human habitation. However, Kepler 438b seems to be a significant exception, with a radius and mass (and thus gravity) nearly identical to ours, as well as an estimated global temperature similar to the Earth. In fact, this world was once rated highest on the Earth Similarity Index with a score of 0.88 out of 1.00.
That said, some researchers have found that this planet may not be the dream it sounds like since it’s subject to powerful stellar storms from its parent star about every hundred days. Still, every candidate has potential.
13. Sol III aka Mars
Granted, some people may accuse us of cheating for selecting a planet in our own solar system, and they’re kinda right. But still, a wealth of research indicates that Mars certainly was once habitable, and may just have enough of the right conditions for permanent human habitation…assuming you could build a series of pressure domes to account for the lack of appreciable atmosphere or magnetosphere.
On an optimistic note for would-be Martian colonists, Mars does have a twenty-four hour day, almost certainly has harvestable water-ice, or perhaps even liquid water in some regions. Plus, despite the lack of a molten core, this planet actually has usable geothermal energy. Although Mars is fairly cold given its distance from the sun, this planet’s orbit is still well within the liberal habitable zone. Combining that fact with the lack of a dense atmosphere, you’d be guaranteed (relatively) sunny days, all-year round. There is still a lingering question as to how human beings would acclimate to living on a world with gravity only about thirty-eight percent of what it is on Earth.
12. Gliese 581g
About a decade ago, the Gliese 581 system grabbed headlines for possibly harboring a “super-earth” in the coveted habitable zone. Although later studies found that the planet in question Gliese 581c is probably too close and too big to be considered for human habitation but a series of other candidates around Gliese 581. For example, 581g, dubbed Zarmina’s World, is a hypothetical planet which is thought to lie about smack in the middle of the system’s habitable zone.
Although this planet’s Goldilocks orbit is about a month-long year, its parent star is so cool that most of the planet could be covered mostly in ice. Like other planets which closely orbit M-class red dwarf stars, this world is predicted to be tidally locked. If these conditions are true, however, the sun-facing side of the planet likely possesses a wet, tropical open water region, making the planet vaguely resemble a giant eyeball.
Keep in mind, though, there’s the possibility that this planet could also be a statistical ghost, as it’s never been observed to transit its star. However, some of the more recent mathematical models tend to support the existence for this promising world.
11. Kepler 186f
With a mass just a bit higher than Earth’s, planetary models which indicate this planet likely has either a terrestrial or ocean covered surface coupled with a thick atmosphere, Kepler 186f seems to be another fantastic habitability candidate. Plus, while it orbits an M-class star, this planet’s orbit is sufficiently far enough away to not be tidally locked unlike the four nearer worlds in this system. So unlike a number of candidate worlds on this list, you don’t have to worry about choosing between living in an area shrouded in everlasting sunshine, ongoing darkness, or a weird endless twilight zone. However, this also means that this planet gets only about a third of the sunlight that ours does, so would-be settlers should plan accordingly. On the other hand, those who are natural night owls, have light sensitivity conditions, or who are vampire cosplay enthusiasts might find this rock ideal.
10. Sol II aka Venus
Much like one of its sister planets, Mars, Venus used to be habitable in the Sol system’s distant past, and probably even had shallow oceans of water. But as the sun began to expand and become brighter over the last four billion years, this planet was caught up in a disastrous runaway greenhouse effect. As a result, Venus has a choking, thick carbon dioxide atmosphere with an atmospheric pressure which would crush anyone flat. Assuming you could construct an environmental suit which could survive surface pressure ninety times what we experience on Earth you’d still have to contend with frequent volcanic eruptions and a sulfuric surface atmosphere.
Despite these extreme ground conditions, Venus is, quite arguably, actually a better candidate for human habitation than Mars. Unlike Mars and like Earth, Venus possesses a geological composition, radius, and mass similar to Earth, meaning the gravity between both worlds is almost the same. On top that, astronomers suggested last century that certain sections of Venus’s atmosphere are perfect for colonization. Specifically, aerostats, dirigibles, and eventually, massive floating habitats could be constructed about thirty-one miles above the world’s surface. This would be possible because an Earth-style atmospheric mixture of nitrogen and oxygen is a potent lifting gas in Venus’s predominantly carbon dioxide atmosphere. So think the Cloud City on Bespin from Star Wars, but actually grounded in…er, or supported by real-world physics.
9. Kepler 62f
From a variety of features, Kepler 62f is probably the most Earth-like planet as it’s possible that it possesses a moon, has a 267-day orbit around a slightly dimmer but still somewhat sun-like K-class star. Further, unlike other prospective “sister-Earths” which tend to be significantly larger and have slightly more intense gravity, 62f is possibly one of the smallest habitable candidate exoplanets to date and thus possess gravity nearly identical to Earth.
Early planetary formation models predict that this planet possesses an Earth-like rocky composition and surface covered in ocean. However, if 62f lacks significant amounts of greenhouse gases like carbon dioxide, this planet would be more akin to the fictitious planet Hoth than Earth, as this planet’s oceans have long been frozen into vast fields of global ice.
8. Kepler 61b
If it’s not already obvious, most extra-solar planets that are likely to be able to support human life orbit relatively small and cold M-class red dwarf stars. However, a few potentials encircle stars somewhat similar to our own. Kepler 61b, for example, orbits a K-class star which is slightly dimmer (though significantly younger) than our sun. On top of that, 61b skates along the inner edge of the habitable zone and has an average equilibrium temperature the same as earth. However, the sheer size of this planet means that it’s probably more akin to a gaseous planet similar to Uranus but may also hold a habitable satellite. Thus far astronomers aren’t able to definitively show whether an earth-size moon orbits this world or not, but if one were present it is possible it would have volcanic activity, possess a protective magnetic field, and exhibit plate tectonic movement.
Keep in mind though, at over a thousand light-years away, even with a breakthrough in space engine design, no one’s likely to get there anytime soon.
7. Gliese 667 Cc
When discussing the topic of habitable planets, most people, and in fact a lot of astronomers, focus on single star systems. However, there’s now evidence that triple stars systems may have Earth-like worlds as well. Gliese 667 is comprised of three stars: a dim K-class, a sun-like G-class, and a tiny M-class sun. Around the latter sun orbits a relatively small rocky world in the Goldilocks zone. Despite the abundance of nearby stars, 667 Cc’s host red dwarf star is so dim and emits so much infrared rather than visible light, and neighboring suns are still far enough, that 667 Cc would still be weirdly dusky.
Still, this planet is one of the best candidates to be a terrestrial world which holds liquid water. And at a mere twenty-two light-years away it’s basically right next door. Now we’ve just got to get engineers to start working on warp drive.
6. Kepler 452b
Unlike most planets, potentially habitable or otherwise, named in relation to the planet they orbit, Kepler 452b has a few very apt nicknames, like Coruscant, Earth’s Cousin, and Earth 2.0, wit good reason. Is it in the habitable zone and with a similar surface temperature? Yep. Does it happen to circle a G-class, sun-like star? Yep. Does it have a (relatively) small size? Yep. Does it possess a nearly identical orbiting period? At 385-days, yep. Not only is this planet a candidate for human habitation, it’s a particularly good candidate for the development of alien life.
5. 55 Cancri f
This inclusion is kind of a cheat, but way too good to include. For sure, astronomers are convinced that 55 Cancri f is some kind of gaseous planet, analogous in composition either to Saturn or Uranus. Alternatively, it may be a Neptune-sized planet with a hazy atmosphere of water clouds, which, while interesting to look at from orbit, would be toxic to mammals like us. But given the fact that this planet is also definitely within its star’s Goldilocks zone, it’s still worth looking into as a candidate for human settlement…kind of.
55 Cancri f, also dubbed Harriot, may have one (or perhaps even multiple) moons with composition and size similar to the Earth’s. If it does, I move we change Harriot’s common name to Polyphemus.
4. Gliese 1214b
Welcome to a world that Kevin Costner’s 1995 character “the Mariner” would call home…at least maybe. Although not conclusively demonstrated Gliese 1214b is one of the best candidates for a planet that not only can hold liquid water, but may be mostly composed of liquid water, and that includes the planet’s core! On top of that, recent observations of the planet seem to suggest that the atmosphere is composed of water-vapor clouds as well.
It should be noted, though, that just because the planet’s composition is mostly water in some form it may not necessarily be a picturesque ocean world. Depending on pressure levels, water may exist on this planet in a liquid and steam form but also a high-pressure ice, superfluid, and even in a plasma state!
3. Tau Ceti e
The Tau Ceti system has long been of interest to exo-planetary researchers. In fact, entire NASA and SETI research projects have been structured around building orbital probes to study the space around that particular star. After all, at only twelve light-years away and being a G-class star, and having not one but two discs of asteroids, Tau Ceti is a fairly close-by sister star system. What’s more, one of its predicted planets may just be in the habitable zone and be small enough to be comfortable for humans to inhabit.
Bizarrely, though Tau Ceti e is thought to receive only about half the amount of sunlight that Earth does, it would get almost double the amount of total radiation that we do. Consequently, if this planet’s atmosphere is like ours, average temperatures would be one hundred and fifty-four degrees Fahrenheit. For context, that’s twenty degrees higher than the highest recorded temperature of any desert on this planet.
Extreme survivalists, this may be the place for you.
2. Kepler 442b
This world has been determined by experts as possessing one of the highest number of Earth-like qualities to date including temperature, size, and proximity to its star. Unlike planets such as Earth and Mars, though, Kepler 442b is thought to have a nearly perfect circular orbit and no axial tilt, meaning there would be no seasons.
Furthermore, given what astrophysicists know about the development of K-class stars, this planet has likely been rocked by fiery solar storms in its distant past making Kepler 442b unlikely to have endemic life in any form. As a plus, Kepler 442 is thought to be comparatively inactive, making its only known planet a great candidate for human settlement.
1. TRAPPIST 1e, 1f, 1g
Given all of the conditions necessary to find a planet suitable for human life, searching for habitable worlds is a crap shoot. Fortunately, there’s at least one system with several candidates orbiting the same star. Announced by astronomers with great fanfare in 2016, three additional planets have been discovered with the orbital periods, size, equilibrium temperature, and predicted composition similar to Earth’s. Although it’s absurdly unlikely that TRAPPIST 1e-g are all habitable, just one of these three planets may have the special combination of variables to be a new home. *Fingers crossed, fingers crossed*
Sources: skyandtelescope.com, forbes.com, space.com, centauri-dreams.org, guinessworldrecords.com
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