Earth-Similarity Index: Where Could We Live Besides Earth?

Earth has long been home to lush forests; big, blue bodies of water; an assortment of strange and wonderful creatures; organisms that can survive in the most inhospitable places; and, of course, this is the planet every human who has ever lived has called “home.”

Unfortunately, we haven’t been the greatest stewards of our planet. Climate change is a serious issue that must be addressed immediately. If it’s not, the “green and blue marble” could become simply the blue marble, when the ice caps melt and drive billions of people out of their homes and off their land.

Global warming isn’t the only threat we face: A comet or asteroid could come wipe us out, and there would be little we could do to stop it. Tensions may continue to flair between countries with nuclear weapons, and the planet could be rendered uninhabitable from the fallout.

If we somehow manage to avoid all of that, in a couple of billion years, the Sun will run out of fuel, swell into a red giant, and consume the Earth. Either our species will die out, or hopefully, the scientists of the future can figure out a way to transport what remains of us to another habitable planet.

Thanks to the Kepler Space Telescope (RIP), astronomers have discovered dozens of planets in the so-called “Goldilocks-zone,” aka the planets that are at just the right distance from their star to possibly sustain life.

These planets likely have a surface temperature that could allow liquid water. Of course, that doesn't necessarily mean the planet is automatically habitable for us. Aside from water, humans also need oxygen and food to survive. Any planet we could potentially migrate to in the future would need to meet a few characteristics, which is why astronomers have developed an Earth Similarity Index (ESI).

What is the Earth Similarity Index (ESI)?

Proposed in 2011, this index gives alien worlds a number from 0 (no similarity) to 1 (very similar to Earth) based on a few characteristics. These include the exoplanet's bulk density, mean radius, surface temperature, and its escape velocity (this is essentially the speed at which you must travel to break free of a planet's orbit).

Planets that score above 0.8 are considered to be Earth-like. To give you an idea of how the scale works, Venus scores a 0.44 on the ESI, whereas Mars comes in at 0.64-meaning neither are particularly habitable or Earth-like. Of course, we don't always have all of the information about every exoplanet, as we glean all of these measurements from analyzing, for example, the light exoplanets reflect. So some of the numbers are our best guestimates.

Other factors must also be taken into consideration: Does the planet have a solid surface, and an atmosphere and a magnetosphere to protect it from the harshness of space? Is there some sort of energy source (like plate tectonics)? What chemical compounds make up the planet's atmosphere? Does the planet maintain a stable orbit around its star that keeps it in the Goldilocks zone permanently? Is its host star stable in its luminosity fluctuations? Does it have an extreme tilt on its orbital axis? For many planets that have been detected, much of this information is unknown.

Indeed, there is much to be considered. Currently, astronomers have a couple of good prospects based on the Earth Similarity Index. In fact, in 2020, astronomers discovered 24 planets, known as super habitable planets, that may be even better suited for life than Earth. 

So where would we live besides Earth? 

1. Gliese 832 c

One of the most interesting finds is a purportedly Earth-like world called Gliese 832 c. Located approximately 16 light-years from Earth in the constellation of Grus, Gliese 832 c orbits its red dwarf star in the Goldilocks zone. Its relatively close distance makes it the fifth-closest extrasolar planet that meets our habitability requirements.

According to Abel Mendez Torres, director of the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo, the Earth Similarity Index of this planet is 0.81 There are planets that score higher on the index, which we will explore, but this one is a spectacular candidate given its close proximity to Earth.

On the downside, Gliese 832 c is believed to be a super-Earth, meaning it is much more massive than our planet (it's estimated to be 5 times larger than Earth). This would mean the planet has a strong atmosphere that could render the entire planet uninhabitable. It may well be closer to Venus than Earth.

2. KOI-4878.01

What may prove to be the most Earth-like planet ever discovered, with a potential rating of 0.98 (98 percent), is KOI-4878.01. Some have gone so far as to call it the first "second Earth" ever discovered, although this planets conditions are unconfirmed.

It rotates around an F-type main-sequence star found about 1075 light-years from Earth in the Draco constellation. There, a year lasts 449 Earth days, and the planet orbits a star that is slightly older and less massive than our Sun. The planet, on the other hand, is estimated to be between 0.4 and 3.0 Earth masses. It's believed that it receives a similar amount of energy from its sun, and it is in the habitable zone.

Unfortunately, more observations are needed to actually confirm this planet not only exists, but has all of the properties consistent with an Earth-like planet. Perhaps that dream will become a reality when the James Webb Telescope is finally launched into space.

3. TRAPPIST-1e

The second highest-scoring exoplanet per the Earth Similarity Index is a planet called TRAPPIST-1e (or 2MASS J23062928-0502285 e if you can remember it!). This planet is a great possibility because not only does it score very high on the ESI, but it's also relatively "nearby" in a cosmic sense.

Scoring a .95, TRAPPIST-1e is believed to be similar to Earth in size, mass, density, surface gravity, and temperature. It orbits an "ultra-cool" dwarf star approximately 40 light-years from Earth in the constellation of Aquarius.

Interestingly, the star it orbits has only 8 percent of the Sun's mass, which means it could live for up to 4 or 5 TRILLION years (approximately 400-500 times longer than our own Sun), which adds to its allure should humans ever need to venture to another planet. It may even be one of the last stars to burn out in the universe.

However, given its star's size, TRAPPIST-1e must orbit the star very closely in order to be in the Goldilocks Zone, and thus it may be tidally locked. This means, like the Moon, the planet shows one face to its star at all times. Therefore, the sun-facing side would be very hot, and the opposite hemisphere would remain basked in eternal night. Of course, it may have a so-called "terminator line," which is basically a sliver of land where the temperatures might be conducive to the existence of liquid water. There's also some debate about whether its atmosphere could be thick enough to circulate the heat to both sides of the planet. For now, it remains a viable candidate for exploration.

4. Enceladus

Although there is technically no score for Enceladus on the Earth Similarity Index, we'd be remiss to forget to mention Enceladus-one of Saturn's many moons and possibly the most habitable place in our own solar system. It's important to at least mention one celestial body in our neighborhood, as humanity may never develop the technology to completely leave our solar system.

Enceladus is one of the only places in our solar system known to have liquid water. There's also some evidence that there's a vast ocean of water-ice lurking beneath its icy surface. It often sprays water out into space-jets that can shoot water-ice at 800 miles per hour (400 meters per second), which eventually coaleses to form Saturn's E-ring-and we have been able to determine what chemicals it contains. Through that, we have learned that this little moon has "most of the chemical ingredients needed for life." The geysers and evidence of escaping heat, also indicate that the moon is geologically active.

Whether humans could ever colonize Enceladus remains a mystery. It does have some form of tectonic activity from the gravitational pull of Saturn and a nearby moon, which is a good sign. Additionally, when the Sun swells up and becomes a red giant, this icy little moon, where temperatures remain about 330 degrees Fahrenheit (minus 201 degrees Celsius), might become a warm oasis-assuming it escapes the Sun's wrath.

Astronomers estimate that there are more planets than stars in our galaxy. Our little planet orbits just one of approximately 200 billion stars in the Milky Way, so there might be as many as 3.2 trillion planets in our galaxy alone! An estimated 300 million could be habitable. The technology we currently have has already uncovered over 2,500 other "solar systems," and 4,000 planets, and it's still in its infancy. Who knows what exciting discoveries are just beyond the horizon!