Mirror Earth: the best bets for planets that are like Earth but not Earth

Earth, like all planets, is unique. But, as it turns out, our home world might not be as alone in the universe as we once thought.
Christopher McFadden
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As far as we know, our home planet is the only one that harbors life. But, as many scientists believe, there are likely countless other planets out there with conditions "just right" to allow life to develop and thrive.

If this is true, these planets could, conceivably, provide additional potential homes ripe for colonization by our species. Of course, we'd need to develop long-range spaceships to get there — and make sure they were not already inhabited.

But, if the history of our species is anything to go by, that should only be a matter of time.

So, what conditions should an "Earth 2.0" have, and what are the best candidates? Let's take a look.

Are there planets like Earth in other solar systems?

Like so many things in life and the universe, there is no simple answer to this question. It does depend.

If you are referring to an exact copy of the planet Earth somewhere in the universe, then the answer can only be no. Each planet is unique in some form, whether its physical and chemical makeup or the particular local circumstances of where the planet exists.

Mirror Earth: the best bets for planets that are like Earth but not Earth
Are there any Earth-like planets out there?

This might sound like a copout but think about the chances of finding another solar system with precisely the same number of planets, asteroids, other heavenly bodies, and a sun like ours. Just like a human being, no world is an "island." It is first formed, then its history is dictated by its parent sun and the complex interactions of matter in the local vicinity.

If, however, you are referring to a planet that shares enough similar characteristics to our planet Earth that it could be considered a sort of twin, then the chances are incredibly high indeed. The universe is so vast that the chances are as close to guaranteed as you can get.

This is, after all, the foundation of the Fermi Paradox.

Why are we looking for Earth-like planets?

The answer to this boils down to two main camps.

The first is out of purely intellectual interest. Astrobiologists and astronomers, for example, are very interested in this possibility because they think that the more similar a planet is to Earth, the more likely it is that it could support complex life.

The second is for technical and strategic reasoning. For example, space colonization and advocates of the idea that humanity should have another home have been looking for a second or new Earth-like planet for people to live on for a long time.

Mirror Earth: the best bets for planets that are like Earth but not Earth
Earth is our only home, but might not be forever.

Regarding the latter (long-term human survival), some argue that we will need to one day leave Earth and spread out into the galaxy. This will broaden our horizons and lower our "exposure" as a species to potential extinction over the long haul.

One of the most high-profile advocates for this thinking is Elon Musk, who argues that our species must become multi-planetary or, in effect, go extinct.

“I think there is a strong humanitarian argument for making life multi-planetary in order to safeguard the existence of humanity in the event that something catastrophic were to happen, in which case being poor or having a disease would be irrelevant, because humanity would be extinct. It would be like, ‘Good news, the problems of poverty and disease have been solved, but the bad news is there aren’t any humans left’,” explained Musk in an interview with Aeon in 2014 (warning some strong language is used in the piece).

“Not everyone loves humanity. Either explicitly or implicitly, some people seem to think that humans are a blight on the Earth’s surface. They say things like, ‘Nature is so wonderful; things are always better in the countryside where there are no people around.’ They imply that humanity and civilization are less good than their absence. But I’m not in that school. I think we have a duty to maintain the light of consciousness, to make sure it continues into the future,” he added.

Whether you agree or not, these two camps work tirelessly to scan and filter out likely Earth-like planets throughout our field of view of the cosmos. So, you might be asking, what would be the "shopping list" for such a planet?

Mirror Earth: the best bets for planets that are like Earth but not Earth
What would an Earth-like planet look like?

What essential characteristics would an Earth-like planet need?

We've covered some of the essentials above, like a similar planet, star, etc., but there is no catchall set of requirements that can readily be defined. This might sound a bit odd, as things like liquid water and other life-friendly conditions, such as relatively warm temperatures, etc., would likely be a prerequisite for such a planet.

But, sadly, it is a little more complex than that. Primarily because when searching for candidate planets, unless we can put a probe in orbit from a distance, we can only ever make an educated guess at the conditions on the surface.

So, for this reason, we need to know some essential elements, so we can filter out the best planets to explore in more detail (and, of course, at a more significant expense).

Mirror Earth: the best bets for planets that are like Earth but not Earth
How would we know if a planet was Earth-like?

One useful metric is the planet's mass. We know that mass is an essential element in finding worlds like Earth. Some research shows that objects only 1.6 times the size of Earth are more likely to be gaseous, which makes them less like Earth.

However, it is important to note that the consensus is that planets between 0.8 and 1.9 times the size of the Earth (so-called sub-Earth to super-Earth planets) are thought to be about the right size to sustain an Earth-like atmosphere and be "rocky." Also, candidate planets need to be between half and two times the mass of Earth to fall within this category.

So, this might be a helpful disqualifier when scanning for likely Earth twins. But metrics like mass and size can only be used as one of many metrics to "scan" for.

While a planet may fall within this size and mass range, that doesn't necessarily mean it is likely to be habitable. The surface temperature, for example, will vary widely depending on several factors like distance from the parent star, atmospheric composition, etc.

We don't have to look very far to find examples of why this is problematic. Take the examples of Venus and Mars in our Solar System. They are both somewhat similar to Earth in size and composition but lack the "x factor" to be Earth analogs.

This all comes down to a planet's location in a range called a star's "habitable zone." This is the approximate area around a star where liquid water can exist on a planet's surface.

That makes sense, but this is not as easy to calculate as you might think.

As astrophysicist Moiya McTier, who did her doctoral thesis on whether or not exoplanets could support life, told The Planetary Society in an interview, "I think it's a spectrum."

“There are points you can pass to make a planet more Earth-like, and I’d say mass or radius are some of the benchmarks. Being in the habitable zone of a star is also important. The world should be able to host water and have an atmosphere — not as thin as Mars, but not a puffy, Neptune-like atmosphere,” she added.

Mirror Earth: the best bets for planets that are like Earth but not Earth
Defining what an Earth-like planet would look like is not easy.

But this isn't necessarily a cardinal rule; there is some wiggle room. For example, some scientists say that a planet can't be like Earth unless it orbits around a star like our Sun, called a G-type main sequence star.

So, what else do scientists look for when considering candidate Earth-like exoplanets?

Well, one is its surface geology. Such a planet should be terrestrial.

This means that it possesses geological processes similar to Earth in some way. It doesn't have to be an exact analog for Earth (that would be a big ask). However, it should show signs of similar processes that generate similar rock types like igneous, metamorphic, and sedimentary products.

Again looking at our backyard, we have some great examples, like Mars and Titan. These planets have very similar landforms and surface conditions to Earth. Still, the surface temperature and amounts of ice are considerably different enough to Earth to be considered non-habitable to life as we know it (emphasis on the "as we know it").

And that is the key. The geology on Earth is dictated, in large part, by the complex interaction of liquid water.

Many of Earth's surface materials and landforms are formed from interaction with water (like clay and mud formation) or as a byproduct of life (such as limestone or coal), interacting with the atmosphere, volcanic activity, etc.

Mirror Earth: the best bets for planets that are like Earth but not Earth
Candidate Earth-like planets need to meet a few important criteria.

A true Earth analog, therefore, might need to have formed through similar processes, having possessed an atmosphere, volcanic interactions with the surface, past or present liquid water, and life forms.

From a potential new home-away-from-home aspect for our species, the most critical factor is, of course, if we can ever get to it. If it is too far away, the planet might be fascinating from a scientific point of view but useless for us to consider colonizing it in the future.

So, the best candidate planets will need to be able to be reached in an acceptable amount of time. We currently lack the technology to travel at or even close to the speed of light, so the closer the planet, the better it will be.

Quite what would constitute this is up for debate, but if the future of our species is up for debate, traveling for thousands of years might be a small price to pay to guarantee our future. However, given the dangers and uncertainties of a long space journey to reach somewhere, it might be more beneficial to change to a nearer planet to make it habitable rather than investing the resources on a very long journey.

Mirror Earth: the best bets for planets that are like Earth but not Earth
How far is too far for us to colonize an exoplanet?

This is called terraforming, but we won't discuss it any further here. However, you can check out an earlier article we wrote on this topic to find out more.

Figuring out what it means to be "like Earth" raises many scientific and philosophical questions. Even though it's hard to say what this means, some worlds we have found do seem to fit the description, at least in part.

Let's look at some of the best bets so far discovered.

What are the most Earth-like planets we've found so far?

Despite the inherent issues with looking for Earth-like planets, we already have some excellent candidate exoplanets. Some prime examples include, but are not limited to, the following examples.

1. Kepler-186f is a good bet for Earth-like conditions

First identified in 2014, Kepler-186f is probably a good bet if you want to make money at the bookies. While the name might be very inspiring, the fact that it sits comfortably within the habitable zone of its parent sun certainly is.

According to NASA, who made the groundbreaking find, the discovery of Kepler-186f proves that other stars have habitable zones, which contain planets the size of Earth.

Even though planets have been found in the habitable zone before, they are at least 40 percent bigger than Earth and are likely not very "rocky." However, Kepler-186f appears to be very similar to Earth's mass, instantly making it very interesting.

"The discovery of Kepler-186f is a significant step toward finding worlds like our planet Earth," Paul Hertz, NASA's Astrophysics Division director at the agency's headquarters in Washington, said in a NASA press release.

"Future NASA missions, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, will discover the nearest rocky exoplanets and determine their composition and atmospheric conditions, continuing humankind's quest to find truly Earth-like worlds," he added.

But, it is important to note that we are not certain of the planet's composition.

Kepler-186f lies in the Kepler-186 system, about 500 light-years from Earth, and in the Cygnus constellation. There are also four other planets in the system. They all circle a star that is half the size and mass of our Sun. The star is an M dwarf, also called a "red dwarf," and these stars are some of the most common in the universe.

The planet also has a much shorter year than Earth, around 130 days, and sits much closer to its Sun than Earth does to ours. This would mean, as NASA explains, that on the surface of Kepler-186f, the star at noon is only as bright as our Sun is about an hour before it goes down.

What surface conditions on the planet would be like is currently anyone's guess. The fact that it receives just one-third of the energy from its star that Earth gets from the Sun may mean it lacks enough solar energy to support life. At least, life as we know it.

2. Gliese 667 Cc is another candidate for an Earth-like planet

Mirror Earth: the best bets for planets that are like Earth but not Earth
Artist's impression of the planet' surface.

Gliese 667 Cc, also called GJ 667Cc, HR 6426Cc, and HD 156384Cc, is an exoplanet that orbits the red dwarf star Gliese 667 C. Like other great candidates for Earth-like conditions, this planet happens to sit comfortably within the habitable zone.

Gliese 667 C is part of the Gliese 667 triple star system, about 22 light-years (7 parsecs) away in the constellation Scorpius. The exoplanet was found using the radial velocity method. This was done by measuring the radial velocity of the planet's parent star and looking for Doppler shifts in the spectrum.

This exoplanet is about 3.8 times as massive as Earth and completes one orbit around its host star in as little as 28 days. You might think that would rule it out (being so close to its Sun), but that star is a red dwarf and considerably smaller and cooler than our Sun.

For this reason, it means that the exoplanet is thought to lie in the habitable zone.

However, Gliese 667Cc, discovered with the European Southern Observatory's 3.6-meter telescope in Chile, may orbit close enough to be baked by flares from the red dwarf. As the planet is so close to its central star, it is also likely to be tidally locked. So, it could be a case of "close but no cigar."

3. Kepler 452b could be the most Earth-like planet we've found so far

Mirror Earth: the best bets for planets that are like Earth but not Earth
This artist's impression of planet Kepler-452b.

Returning to the Cygnus Constellation, there might be an even better candidate for finding an Earth copy. Discovered in 2015 by NASA, it is believed to be the closest exoplanet to Earth yet found.

According to NASA, Kepler-452b is the smallest planet found so far that orbits in the habitable zone of a G2-type star like our Sun. However, although small for an exoplanet, compared to Earth, it is much bigger.

Kepler-452b has been termed a super-Earth-size planet because it is 60 percent bigger than Earth. Even though its mass and makeup aren't known yet, past research shows that planets about the size of Kepler-452b are likely to be made of rocks.

Even though Kepler-452b is bigger than Earth, its orbit is only 5 percent longer, taking 385 days instead of 384. The planet is 5 percent farther away from its parent star, Kepler-452, than Earth is from the Sun. Kepler-452 is 6 billion and 1.5 billion years older than our Sun. It has the same temperature as our Sun but is 20 percent brighter and 10 percent bigger in diameter.

“We can think of Kepler-452b as an older, bigger cousin to Earth, providing an opportunity to understand and reflect upon Earth’s evolving environment," explained Jon Jenkins, Kepler data analysis lead at NASA's Ames Research Center in Moffett Field, California, who led the team that discovered Kepler-452b in a NASA press release.

"It’s awe-inspiring to consider that this planet has spent 6 billion years in the habitable zone of its star; longer than Earth. That’s a substantial opportunity for life to arise, should all the necessary ingredients and conditions for life exist on this planet,” he added.

4. Kepler 69c is more-or-less the same size as Earth

Mirror Earth: the best bets for planets that are like Earth but not Earth
Artist’s impression of Kepler-69c as a “super-Venus” exoplanet.

Kepler-69c, known by its Kepler Object of Interest designation KOI-172.02, or "Super Venus", is a confirmed super-Earth planet that orbits the Sun-like star Kepler-69. It is likely made of rock and is the outermost of two planets found by NASA's Kepler spacecraft.

The planet lies about 2,430 light-years away from Earth.

Kepler-69c orbits its star at a distance of 0.64 AU (96,000,000 km; 59,000,000 mi), which gives it a year of about 242.46 days. It has a mass of at least 2.14 times that of Earth and a radius of about 1.7 times that of Earth.

Initial research showed that it might be able to support life. However, a recent study indicates that Kepler-69c is outside the habitable zone's inner edge and likely receives an amount of solar radiation similar to that of Venus. This means it is likely to be more like Venus, with temperatures and conditions too hot to support life.

NASA released data from the Kepler spacecraft in April 2013 that announced the exoplanet had been found.

The transit method was used to find the exoplanet. This method measures how much a planet dims its star as it moves in front of it.

5. KOI-456.04 appears to be Earth-like and isn't too far away

Mirror Earth: the best bets for planets that are like Earth but not Earth
KOI-456.04 is pretty close, but might not be a planet.

While most exoplanets found so far orbit red dwarfs, one recently discovered exoplanet, KOI-456.04, orbits the habitable zone of a star, Kepler-160, similar to our Sun. Because of its proximity to its parent star, it receives about 93 percent of the sunlight Earth receives from the Sun.

That is a good start, but the planet is also pretty Earth-like in other ways. According to researchers who found the planet, it is roughly 3,000 light-years away from us.

The planet also appears to be about twice Earth's size and orbits Kepler-160 (its parent star) at about the same distance as Earth from the Sun, completing an orbit in 378 days. As a candidate Earth-analog, it is shaping up to be a pretty close match.

The researchers who discovered the planet have calculated that if the planet’s atmosphere is similar to Earth’s, then the average temperature should be about 41 degrees Fahrenheit (5 degrees Celsius). Of course, there are still too many unknowns to say this with any certainty.

However, there is also a catch. Scientists are not entirely sure that the object identified as KOI-456.04 is a planet. However, the researchers say there is an 85 percent chance that their findings are correct, although there is also a slight chance that an error could have caused the dimming of light in Kepler's instruments or a statistical accident.

More observations are needed to know whether KOI-456.04 is a confirmed exoplanet. For reference, an exoplanet candidate needs to pass a 99 percent threshold.

Until then, missions like NASA’s James Webb Space Telescope mission are one of the few ways KOI-456.04 can be given the official "seal of approval."

6. This system has two potential "super-Earths"

Mirror Earth: the best bets for planets that are like Earth but not Earth
LP 8909 c may, or may not, be an Earth-like planet.

Another exciting discovery was recently made by NASA when exploring a distant red-dwarf star called LP 890-9. This star is around 98 light-years away and, most notably from our point of view, contains two potential Earth-like planets.

The first, LP 890-9 c, was found during follow-up observations of its sister planet, LP 890-9 b, and is likely to be made of terrestrial. According to NASA, the world is 40 percent bigger than Earth and sits in its parent star's habitable zone.

The inner world, LP 890-9 b, is about 30 percent bigger than Earth and probably too hot to live on, with an estimated 253 degrees Fahrenheit (123 degrees Celsius), and was found using the Transiting Exoplanet Survey Satellite (TESS). The outer planet, LP 890-9 c, may be cold by human standards, with an estimated temperature of 30 degrees Fahrenheit (minus 1.1 degrees Celsius).

However, it does lie in its star's habitable zone, the distance from the star where there could be liquid water on the surface. A ground-based telescope survey called Search for habitable Planets EClipsing Ultra-cool Stars (SPECULOOS) was used to find this planet. Not only did the observations confirm the existence of the innermost planet, which was their primary goal, but they also found a second planet in the system, which was a surprise.

There are many things to keep in mind about the temperature estimates. The real planets' temperatures depend on their atmospheres, which we don't know anything about yet. Astronomers believe that the daytime hemisphere of LP 890-9 c receives about 90 percent of the heat and light of Earth.

However, the atmosphere of the farthest planet may have caused a greenhouse effect to go too far, making it much too hot to live on. This would make it more like Venus than Earth. Both planets orbit close to their star. It takes LP 890-9 b only 2.7 days to go around its star once, while it takes LP 890-9 c 8.5 days.

But the star is much smaller and cooler than our Sun. Planet c is close to the inner edge of this star's habitable zone because it orbits close to the star.

7. Kepler 1639c is another potential future home for our species

Mirror Earth: the best bets for planets that are like Earth but not Earth
Artist's rendition of what Kepler-1649c looks like on the surface as it orbits its red dwarf star,

Yet another potential future home for us humans is Kepler 1649 c. Located roughly 300 light-years away, the planet orbits a red dwarf star and appears roughly the same size as our beloved home planet.

Most importantly, it sits comfortably within its parent star's habitable zone, meaning it has all the right conditions for liquid water to exist. While its orbit around its star is only 19.5 Earth days, because its parent star is much weaker than our Sun, it should get roughly the same solar illuminance.

Kepler-1649c is about 1.06 times Earth's size and receives about 75 percent as much energy from its Sun as Earth gets from the Sun. This combination makes the newly discovered world rather unique.

"There are other exoplanets estimated to be closer to Earth in size, such as TRAPPIST-1f and, by some calculations, Teegarden c," NASA officials wrote in an official statement on the discovery.

"Others may be closer to Earth in temperature, such as TRAPPIST-1d and TOI 700d. But there is no other exoplanet that is considered to be closer to Earth in both of these values that also lies in the habitable zone of its system," they said.

And that is your lot for today.

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