Meet the 9 most habitable worlds in the universe (so far)
On Monday, the eve of Nicolaus Copernicus' 540th Birthday, a newly-reordered list of the most Earth-like planets in the known universe was released by the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. Some planets once thought to be warm and tropical have fallen off the list entirely, while other planets have been promoted to the coveted Goldilocks Zone in-crowd.
Researchers have come up with a refined system for categorizing the potential habitability of exoplanets on the Earth Similarity Index (ESI), releasing their findings just last week. The existing system was badly in need of an overhaul, since the old system — now 20 years old — was created before even one exoplanet had been located.
This new system changes a lot, not only taking into effect the temperatures and radiation levels of the stars exoplanets orbit, but such sciency-sounding things as HITRAN (high-resolution transmission molecular absorption) and HITEMP (high-temperature spectroscopic absorption parameters) relating to the stability of water.
If the Starship Enterpise is ever to become a reality — boldly going where no one has gone before — then the scientists who are right now confirming and refuting the habitability of these planets are the stellar cartographers of that tomorrow. Starship captains of the future will plot courses to these worlds, seeking new life and new civilizations. Where they might be headed, in order of likelihood, are these nine planets.
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9. Gliese 581 d (0.50 ESI)
Right around the galactic corner from Earth is the star Gliese 581. Back in 2011, a French team concluded that the fifth planet from that star, Gliese 581 d, likely had surface water, clouds and warm rain, although those findings have been called into question. The Earth Similarity Index (ESI) now only ranks "d" as 50% similar to Earth. And with gravity likely twice as strong as that on Earth, it just might be rough going for the average away team.
8. Mars (0.64 ESI)
For most planets on this list, the new data has come as a bit of a body blow. Tau Ceti f and HD 85512 b both lost their habitable status, and many others still on the list have seen their ESI take a hit. On the other hand, Earth's little red neighbor has recieved a statistical shot in the arm. Mars now registers a 64% ESI and sits just inside our sun's habitable zone. While Mars may not be the most likely home of extraterrestrial life in the universe, it definitely belongs on the list. With a little bit of terraforming, Mars could potentially get promoted from "potentially habitable" to "Earth-like."
7. HD 40307 g (0.67 ESI)
A newer discovery, HD 40307 g is thought to lie in an orbit around its star that both allows for liquid water and for free rotation — meaning that it likely has a steady night/day cycle. This, in the old system, contributed to a 79% similarity to our own world. The new math shows a slimmer chance for life on this world, but its updated 67% similarity doesn't quite rule it out, either. In fact, HD 40307g lies smack dab in the center of its star's habitable zone. "G" is also tied for largest super-Earth on our list — leading some to question whether the planet can truly be solid and seven times the size of Earth. Astronomer R. Barnes has suggested that HD 40307 g might be a small, warm gas planet — a Neptune rather than an Earth.
6. Gliese 163 c (0.68 ESI)
If Gliese 163 c is Earth-like, it would be the largest habitable super-Earth yet discovered. It might be host to a "balmy" sea that maintains a temperature of 140 degrees Farenheit. Balmy, indeed: That's a bit warm for you or me, but less complex life, microbes and the like, just might love it. And they'd be small enough not to notice the way their world whips around its star every 26 days.
5. Tau Ceti e (0.74 ESI)
Wheeling our way back to Tau Ceti, we find the system's last habitable planet standing: Tau Ceti e. Much like Gliese 163 c, "e" hovers around the inner edge of its star's habitable zone. Tau Ceti is a star very like our own, with "e" in a tighter orbit than Venus. All things being equal, "e" should be a dead rock. Tau Ceti, however, emits only half the light that our sun does, breathing a chance of life into "e." If life can exist there, it has had quite a while to evolve, since Tau Ceti is twice the age of the sun.
4. Kepler-22b (0.75 ESI)
But for its incredible distance from us (600 light years), Kepler-22 b b seems as if it might be Earth's sister planet. We orbit nearly identical stars in very similar orbits. On Kepler-22b one year lasts 290 days — not too far off from our 365. Surface temperatures on both worlds are likely to be nearly the same as well. This all sounds good, but the possibility of life on Kepler-22b comes down to atmosphere and cloud cover. If an atmosphere similar to our own exists there, then so might life, but if Kepler22-b has a low percentage of cloud-cover, any water that might have once existed there would simply evaporate, never to return.
3. Gliese 667C c (0.79 ESI)
The second-most likely home to extraterrestrial life known to humanity, Gliese 667C c orbits an M-type star, and just might be M-class itself, if we were to go by the Star Trek terminology. It is likely also not rotating, but is instead tidally locked. The running theory is that the twilight regions of this world might be the ideal place for life to exist, with surface temperatures hovering right around 85 degrees. Bonus: Gliese 667 is a system with three stars, meaning that Gliese 667C c might sport some spectacular nighttime vistas.
2. Gliese 581 g (0.82 ESI)
The very existence of this planet has been in question since its signature was discovered. At different times since it was detected, Gliese 581 g has ceased to exist, been returned to reality, and even been given an astounding 92% on the Earth-similarity scale. In the Gliese 581 system alphabet, "g" falls between "c" and "d," making it the fourth planet from its star. "G" took a hit when the new data came out, its ESI index falling to 82%. It may be less like Earth than we once thought, but that score is still enough to make "g" the most Earth-like planet out there.
1. Earth (1.00 ESI)
Once thought to be the ideal toward which all other planets strive, Earth is not immune to the new study's numbers. As it turns out, home sweet home also took a bit of a bashing in the study. No longer are we "just right" in relation to our star, but actually just clinging onto to inner edge of our sun's habitable zone. In fact, Earth is a lot like HD 85512 b, which owes its very capability to sustain life (or not) to its atmosphere and cloud cover. Earth is at 1.00 on the ESI because it's the only planet on which we know life exists so far, but studies like this suggest that maybe the Goldilocks Zone doesn't need everything to be just right after all.
