What Is Meant By "Habitable Zone" And How Do We Define It?

The term "habitable zone" is bandied about quite often, but what exactly does it mean? And do they apply to just stars, or are there larger HZs in the cosmos?

In the past decade, the number of planets beyond our Solar System - aka. extra-solar planets (or exoplanets) - has grown exponentially. In fact, as of today, a total of 3,925 exoplanets have been confirmed in 2,926 star systems, with another 3,389 candidates awaiting confirmation.

SEE ALSO: 7 EXOPLANETS THAT ARE WEIRDER THAN MOST SCIENCE FICTION FILMS

Much of the credit for these discoveries go to the Kepler Space Telescope, which was used to confirm the existence of 2338 exoplanets (with another 2423 awaiting confirmation) between when it began service on May 12th, 2009, and when it exhausted the last of its fuel on November 15th, 2018.

Within this large collection of extrasolar planets, just 49 have been designated as "potentially habitable" by astronomers. That's just over 1% of the total data sample, which would seem to suggest that life-bearing planets are extremely rare in our Universe.

But when we take a step back and examine what is meant by "potentially habitable" and what goes into making that determination, we find that a lot of assumptions are involved. For one, we are searching for life based on "Earth-analogs" which could be severely limiting.

Second, there are many unknowns and ill-defined parameters when it comes to how "life as we know it" emerged, not to mention what kinds of conditions it can survive under. When we factor that into our search for life, we find that our estimates might be on the generous side.

The only way to clarify the whole issue is to examine what is meant by habitability, what goes into determining it, and keeping this in mind as we conduct future surveys for extrasolar planets.

Circumstellar Habitable Zone:

Also known as the "Goldilocks Zone", a circumstellar habitable zone (habitable zone, or HZ for short) refers to the distance from star where a planet will experience temperatures between 273 K and 373 K (0 and 100 C; 32 and 212 F) - in other words, the temperature range where water is able to exist in liquid form. The range of this zone depends heavily upon the type of star. 

What Is Meant By "Habitable Zone" And How Do We Define It?

Artist's impression of habitable zones around different types of stars. Credit: NASA/Kepler Mission/Dana Berry

For example, larger, higher-magnitude stars like O, B, A-type stars have wider habitable zones that are located at a relatively long distance. These stars are known as "blue giants," which can be up to 1.4 million times as bright as our Sun and generally range from being about three to a few dozen (or even a few hundred) times the mass of our Sun.

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These classes of stars are relatively rare, accounting for about 1 in 3,000,000 (O-type), 1 in 800 (B-type), and 1 in 160 (A-type) of the stars in our galaxy. F-type stars occupy a sort of middle ground, being blue-white in color and generally only a few times more luminous and massive than our Sun. These stars are more common, making up about 3% (1 in 80) stars in our galaxy.

Then you have stars more akin to our own, which fall into the G and K-type classes (yellow and orange dwarfs). These stars make up around 7.5% (1 in 13) and 12% (1 in 8) of the main-sequence stars in the solar neighborhood and have relatively tight and narrow habitable zones.

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Last, but certainly not least, you have the class of low-mass, low-brightness, cooler stars known as M-type (red dwarf) stars. These stars range from being 7.5 to 60% the size and mass of our Sun and only get about 7% as bright. As a result, their habitable zones are rather narrow and very tight. They are also the most common type of star, accounting for about 85% of the stars in our galaxy.

What Is Meant By "Habitable Zone" And How Do We Define It?
The “Goldilocks” zone around a star is where a planet is neither too hot nor too cold to support liquid water. Credit: Petigura/UC Berkeley, Howard/UH-Manoa, Marcy/UC Berkeley.

Despite these differences, the basic rule is generally the same across the board. If a planet is too close to its respective star, its surface water will evaporate rapidly and collect as vapor in the upper atmosphere, causing a “moist greenhouse” effect.

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On the other hand, if a planet is too far away, the atmosphere will be cold and dry, and CO2 levels will remain high to the point that they would be toxic to Earth animals. This is well-illustrated by the planets of Venus and Mars, which orbit at the inner and outer edge of our Sun's HZ.

Our Solar System's HZ:

Ongoing studies of our planet and its neighbors have revealed some very interesting similarities between Earth and other planets of the Inner Solar System. For instance, Mercury, Venus, and Mars are similar in composition, being terrestrial (i.e., rocky) planets like Earth.

Equally interesting are the telltale indications that Venus and Mars also occupy our Sun's HZ and were actually quite similar to Earth during their early history. To break it down, Earth and the other planets of the Solar System are believed to have formed roughly 4.56 billion years ago from a protoplanetary disk.

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What Is Meant By "Habitable Zone" And How Do We Define It?

Illustration of a star surrounded by a protoplanetary disk. Credit: NASA/JPL-Caltech

Over the next hundreds of millions of years, these planets began to cool, and their internal structures began to differentiate between the core, mantle, and crust. Primordial atmospheres began to form as well, largely composed of either volcanic gases and/or elements left over from the solar nebula that formed our Sun.

In either case, one of the key components in this mix was water. Not only did the formation of our planets involve a large amount of water, but water also began to concentrate on the surfaces of Venus, Earth, and Mars at around the same time (ca. 3.8 billion years ago).

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Around this time, Venus is believed to have had oceans on its surface. This is based in part on data gathered by the Galileo spacecraft as it conducted its flyby of Venus in 1990, which revealed that Venus' highland regions are likely composed of felsic rocks (the formation of which require water).

This ocean would have likely vaporized during Venus' early history as a result of rising temperatures. This is theorized to have contributed to the runaway greenhouse effect that caused Venus' atmosphere to thicken and become the incredibly dense and hot one it is today.

What Is Meant By "Habitable Zone" And How Do We Define It?
Comparison between the habitable zones of the TRAPPIST-1 system and the Solar System. Credit: NASA/JPL-Caltech

On Mars, the situation was quite different. Being at the outer edge of our Sun's HZ, it does not receive enough energy to maintain liquid water on its surface, and its atmosphere has remained one that is primarily composed of carbon dioxide (96%, with argon and nitrogen making up most of the remaining 4%).

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Earth, meanwhile, occupies a space between the two extremes. Because of this, it is able to maintain liquid water on its surface, as well as the all-important hydrological cycle - where water evaporates to because vapor in the atmosphere, then condenses to form clouds and return to the surface in the form of precipitation.

Galactic Habitable Zone:

The Galactic Habitable Zone (GHZ) extends the notion of the circumstellar habitable zone by asking, "are there places in the Milky Way that are more suitable to life than others?" To put it another way, do star systems have a better shot at giving rise to life based on their position within a galaxy?

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Once again, we are reliant on the single example of where life is known to exist - the Solar System. Basically, the Solar System enjoys a cozy spot, nestled into the Orion Arm in the Milky Way. Ongoing research has shown that this spot had a lot to do with the Solar System developing the conditions necessary for life to flourish.

What Is Meant By "Habitable Zone" And How Do We Define It?
The location of the Solar System in the Milky Way galaxy. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) 

In this sense, our Sun occupies a place in the Milky Way's GHZ. The term was coined in the 1990s by Guillermo Gonzalez, who was an Assistant Professor of Astronomy at the University of Washington at the time. As he explained the concept:

“Large, complex organisms are much more sensitive to environmental perturbations than simple life. Our hypothesis deals exclusively with complex life, more specifically, aerobic macroscopic metazoan life. The effects of radiation would damage the ozone layer, as well as increase radiation levels at the surface of a planet from secondary particle cascades in the atmosphere.”

To break it down succinctly, our Milky Way Galaxy is structured much like billions of other spiral galaxies, with a galactic disc containing a lot of dust and gas (interstellar matter), as well as young and intermediate-age stars. The older stars, meanwhile, tend to be located in the bulge around the galactic center.

Many of these older stars are gathered into globular clusters that orbit the nucleus of the galaxy in a region known as the "halo." At the center of it all, as evidenced by strong infrared, x-ray and radio emissions, are clouds of ionized gas rapidly circling around Sagittarius A*.

What Is Meant By "Habitable Zone" And How Do We Define It?
The center of the Milky Way, showing the "bulge" and "halo" where much of its older stars are concentrated. Credit: Stephane Guisard/SETI

One of the key differences between younger and older generations of stars is their metallicity (i.e., how rich in metals they are). This is due to the fact that the earliest stars in our Universe formed from the earliest elements (hydrogen, helium, and lithium).

When these stars reached the end of their lifespan, the nuclear reactions triggered by the supernova created heavier elements ("metals") in the process. These metals became part of the raw material which the second generation of stars formed from, and so on with every successive generation.

Based on studies of extrasolar planets, metal-rich stars (like our Sun) are considered more likely to have planets orbiting around them. One possibility is that a certain amount of metals is needed to form rocky planets, which are predominantly composed of heavy metals like iron, nickel, and silicate minerals.

One reason for this may be that a certain minimum amount of metals is needed to form rocky bodies (including the cores of the gas giant planets). Ergo, a sufficiently metal-rich disk is required for a star system to form that will have a shot at potential habitability.

Another advantage our Solar System has is that it is a good distance away from the center of our galaxy (27,000 ± 1000 light years). This distance protects our Sun from the gravitational perturbations and radiation which become more intense the more one ventures towards the center of the galaxy.

Based on geological and astronomical research, astronomers know that throughout the history of our Solar System, long-period comets have resulted from the interaction between the Oort Cloud and passing stars. Some of these comets have reached the Inner Solar System, impacting with the planets and (on rare occasions) causing extinction level events on Earth.

If our Solar System were closer, the Oort Cloud would be disrupted with far greater frequency. This means that comets would have been sent into the Inner Solar System far more often, resulting in far greater disruption. This would significantly reduce the chance of life emerging on our planet.

In addition, being located farther from the galactic center also means that our system is subject to less in the way of harmful radiation. This includes the gamma rays, x-rays and cosmic rays that are known to be damaging to life as we know it. Were it closer; life would have a much harder time surviving.

What Is Meant By "Habitable Zone" And How Do We Define It?

The galactic habitable zone is shown in green here, though the boundaries are not well-defined. Credit: NASA/Caltech

The Milky Way galaxy is at least 50 percent larger than is commonly estimated, according to new findings that reveal that the galactic disk is contoured into several concentric ripples. Credit: Rensselaer Polytechnic Institute

Read more at: https://phys.org/news/2015-03-corrugated-galaxy-milky-larger-previously.html#jCp

 

Another requirement of the GHZ is to keep out of the way of the galaxy's spiral arms. The density of interstellar matter (dust and gas) in the spiral arms is what allows them to form new stars. However, this also leads to intense radiation and gravitational perturbations.

If our Sun were to cross through these arms too often, it would be threatening to life - in much the same way as if our Sun were too close to the galactic center. Luckily, our Sun has a nearly-circular orbit around the center of the galaxy and revolves at about the same rate as the spiral arm population.

This synchronization prevents our Solar System from crossing a spiral arm too often. In this respect, a star system having a stable orbit at the right distance from the center of our galaxy is similar to a planet having a stable orbit and being at the right distance from a star.

While some scientists have criticized Ramirez's theory for failing to provide well-defined limits on its boundaries, the GHZ is commonly believed to extend from a radius of 4 to 10 kiloparsecs (13,000 to 32,600 light-years) from the Galactic Center.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's conception of the Milky Way galaxy. Credit: Nick Risinger

Others, meanwhile, have conducted simulations that show how stars may change their orbits around the galactic center significantly over time, which casts doubt on the idea of some areas being more habitable than others.

The Trouble with Terminology:

While astronomers generally apply the term "potentially habitable" to any planet that orbits within a star's HZ, a lot of other factors come into play when it comes to being able to support life (at least as we know it). At the same time, there is the term "Earth-like," which is also bandied about when dealing with exoplanets.

What do these mean, essentially? Simply put, potentially-habitable is used to refer to any planet that appears to be in a good position to support life. And that is where the term "Earth-like" comes into play, which describes conditions that are analogous to Earth.

This means that an exoplanet is likely to be terrestrial in nature, which means that it is composed of silicate minerals and metals that are differentiated between a core, a mantle, and a crust. It also implies that it is likely to have active plate-tectonics, which have played a major role in stabilizing the climate on Earth.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of Earth-like planets. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech)

It can also imply that the planet has liquid water on its surface, which is the only known solvent that can support life. And last, it could mean that the planet has a viable atmosphere, which would be composed predominantly of nitrogen and oxygen, with enough CO2 to provide a stable greenhouse effect and ozone to protect from radiation.

As you can probably tell, all of these qualifiers say a lot about how we are searching for life elsewhere in the Universe. When looking to other star systems for signs of life, we are essentially looking for other Earths. This is what is known as the "low-hanging fruit" approach in the search for life, where we search for signs that we can recognize.

Naturally, this approach (while understandable) is really quite limiting. For instance, ongoing research by astronomers, astrophysicists, and astrobiologists have called a lot of our notions about habitability into question. For example, the classical definition of HZ is flawed because it assumes an awful lot.

For example, it assumes that carbon dioxide and water vapor are the key greenhouses gases that ensure that habitable planets stay warm enough to support life. However, recent studies have shown that other greenhouse gases that are minor on Earth - such as methane and hydrogen gas (which could be related to volcanic activity) - could extend the limits of an HZ.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of some of the nearest exoplanets to our Solar System. Credit: PHL/UPR Arecibo

On the other hand, while methane has been linked to global warming on our planet, studies have shown that it would cool the surfaces of habitable zone planets orbiting red dwarf stars. As such, high concentrations of atmospheric methane on these planets could lead to frozen conditions.

Exoplanets that orbit cooler stars (such as red dwarfs) are also likely to have higher levels of carbon monoxide, according to some research. This gas is not only toxic to complex lifeforms on Earth, it is not (on its own) a greenhouse gas and therefore does not stabilize temperatures.

To make matters worse, there are also recent studies shows how red dwarf star systems may not have the necessary raw materials for life to form, and that red dwarf stars might not provide enough photons for photosynthesis to occur. This is all disconcerting considering that rocky planets in HZs are expected to be most common around low-mass red dwarfs.

Second, recent research has been conducted that has shown how atmospheric oxygen does not automatically mean the presence of life. For instance, astronomers have noted the presence of oxygen around exoplanets that appeared to be the result of chemical dissociation.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist’s impression of an Earth-like planet orbiting a red dwarf star. Credit: ESO.

This is a process where exposure to ultraviolet radiation causes water vapor to break down into hydrogen and oxygen gas. While the hydrogen gas (being much lighter) is lost to space, the oxygen gas is retained as part of the atmosphere).

Under these conditions, atmospheric oxygen gas would not be considered an indication of life (aka. a "biosignature") since it was not produced by simple organisms like cyanobacteria. Moreover, the presence of oxygen gas during the early periods of a planet’s evolution could prevent the rise of basic life forms.

Another problem arises from another key biosignature, which is water. While water is essential to life on Earth, scientists have found that too much of it could be inimical to life. For example, studies have shown that rocky planets that orbit red dwarf stars could, in fact, be "water worlds," where up to 50% of their mass is water.

On these planets, the surface would be entirely made up of deep oceans, thus preventing the exchange of carbon dioxide between the atmosphere and mantle (volcanic outgassing and the carbon-silicate cycle). The oceans would also be so deep that they would form an icy layer towards the rocky interior, preventing geothermal exchanges with the oceans.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of exoplanets similar to Earth, including the "water worlds." Credit: NASA

However, other research has indicated that water worlds could still be able to experience enough carbon cycling between its atmosphere and its oceans; to the point that they would be able to maintain a stable climate even over the course of billions of years.

Another key assumption is that habitable planets will need to be geologically active - aka. have active plate tectonics. In Earth's case, this is what allows for the carbonate-silicate cycle, which is what ensures that CO2 levels remain largely consistent over time.

The thing is, of all the exoplanets to be discovered so far, all are thought to be stagnant lid planets - where the crust consists of a single, giant spherical plate floating on the mantle. This would be bad news as far as habitability studies are concerned, but there is research that shows that plate tectonics might not be necessary to maintain CO2 cycling.

So when it comes right down to it, none of the key indicators of habitability - atmospheric gases, water, or plate tectonics - are surefire indications of life. What's more, our efforts to find life in the Universe based on these signatures is invariably restricted by the fact that we are looking for "life as we know it."

What Is Meant By "Habitable Zone" And How Do We Define It?

Artist's conception of Huygens' descent, showing what the surface of Titan looks like when seen from an altitude of 70 kilometers (43 mi). Credit: ESA/NASA/JPL/University of Arizona/René Pascal

But what about life as we don't know it? For example, why aren't we looking for a life that relies on solvents like liquid methane or ammonia instead of water? Why not consider creatures that can breathe gaseous hydrocarbons instead of nitrogen and oxygen, and exhale something other than carbon dioxide?

Where to start? Well, for starters, we don't know that such life actually exists, so how do we go about placing constraints on its behavior? How would we know to look for biosignatures when we aren't even sure what that kind of chemical compounds it needs to survive (or emit as a waste product)?

And since we don't know what kind of stress tolerances this life would have (resistance to heat, cold, radiation, etc.) how can we define "habitable zones" around their respective stars? We can't. The truth is, we have no idea how to look for life as we don't know it because we haven't found it yet!

So really, there's a good reason why we employ the low-hanging fruit approach. When it comes right down to it, we know of only one planet that can support life, and that's Earth. So when we look for signs of life out there in the cosmos, we look for signs that we can recognize because we have no choice.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of an Earth-like exoplanet orbiting within the habitable zone of a binary star. Credit: ESO

Still, it is restrictive, and the only solution to it is to keep looking and hope that we find examples of life out there that either confirm or defy our current framework. If, for example, we find that it is possible for life to form under vastly different conditions - where solvents like methane and ammonia can support life - then we'll know to look things other than water.

On the other hand, we may find many examples of Earth-like planets that truly are like our planet, with oceans, a warm atmosphere, and plenty of complex life on the surface. Only time will tell, and only the discovery of inhabited planets will lead to the validation or revision of our search methods.

Some Potentially Habitable Exoplanets:

Of the many planets scientists have detected and confirmed beyond our Solar System, there are those that have shown promise - at least by the standards we use for determining habitability. And in the coming years, with the deployment of next-generation telescopes, scientists hope to learn more about them through follow-up studies.

Some examples, which are located within 100 light years of our Solar System, include:

Proxima Centauri b:

At a distance of 4.25 light years, Proxima b is the closest known planet to Earth, a status it earned in 2016 when its existence was first announced by scientists from the European Southern Observatory (ESO). The planet was discovered by the Pale Red Dot campaign, a team of astronomers who had been monitoring Proxima Centauri from the ESO's La Silla Observatory in Chile.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of Proxima b, the closest exoplanet to Earth. Credit: ESO

Based on spectrographic information from La Silla's 3.6 meter telescope, as well as other observatories around the world, the team determined that this planet (named Proxima b) had a mass of at least 1.3 times that of Earth, and that it orbited its star at a distance of about 7 million km (4.35 million mi) - roughly 5% of the Earth's distance from the Sun.

While "Earth-like" in this respect, the habitability of the planet has not been established. Essentially, a number of factors cast doubt on the planet's ability to support life. Like most terrestrial planets that orbit M-type stars within their HZs, Proxima b is likely to be tidally-locked with its star (which means that one side of the planet is constantly facing towards its star).

In addition, multiple studies have been conducted that indicates how red dwarf stars (Proxima Centauri in particular) exhibit too much instability and flare activity for orbiting planets to remain habitable. With solar wind pressures roughly 2,000 times as strong as those experienced by Earth, it is doubtful that Proxima b would be able to retain an atmosphere for long.

However, other studies have indicated how this tidally-locked planet could still support life - provided it had a thick enough atmosphere and powerful magnetic field - or with a dayside ocean that would allow for heat transport between the sun-facing side and dark side.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of the surface of Proxima b. The brightest star in the sky is Proxima Centauri while the distant binary is Alpha Centauri A and B. Credit: ESO

Barnard's Star b:

Like Proxima b, this exoplanet was discovered by the Red Dots and CARMENES observation campaigns while observing neighboring red dwarf star system. At 6 light years from Earth, this terrestrial (aka. "Earth-like") planet is the second closest exoplanet to the Solar System.

Based on data gathered from multiple observatories, astronomers estimate that Barnard's Star b is likely to be a "super-Earth", with at least 3.2 times the mass of Earth. They also determined that it orbits its star with a period of 233 days and at a distance of 0.4 AU (0.4 times the distance between the Earth and the Sun).

However, unlike Proxima b, this exoplanet has never been considered a viable candidate for habitability. Despite this relatively tight orbit, the low mass and brightness of Barnard's Star mean that the planet receives only about 2% of the energy that the Earth receives from the Sun.

Combined with the planet’s orbit, this places Barnard’s Star b outside of the system's HZ and closer to its Frost Line - the boundary beyond which volatile compounds (i.e. water, carbon dioxide, ammonia, methane, etc.) condense into solid ice. According to the team’s estimates, the planet would have an average surface temperature of -170 °C, making it inhospitable to life as we know it.

On the other hand, recent research has shown that are still possible scenarios in which life could exist on Barnard's Star b (or rather, inside it). These include the possibility that geological activity could provide sufficient energy so that life could exist beneath the surface.

Luyten b:

Also known as GJ 273 b, this rocky planet was discovered with the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph on the 3.6-meter telescope La Silla Observatory in Chile in 2017. The planet orbits Luyten's Star - located about 12 light years from Earth - which makes it the third closest planet to our Solar System.

Based on data collected by the discovery team, Luyten b is estimated to be around 2.9 times the mass of Earth (a "Super-Earth") and orbits its parent star once every 18.6 days and at a distance of 0.091 AU - much closer than Mercury's orbit to the Sun. But since it orbits a red dwarf star, this places it well within the star's habitable zone.

Combined with the fact that Luyten b receives only about 6% more sunlight than Earth, making it one of the best candidates for habitability. And while it is likely to be tidally-locked with its star, a thick enough atmosphere would allow for sufficient transfer.

Gliese 581 c:

This exoplanet was discovered in 2007, also by astronomers at the La Silla Observatory. It was the second of three planets to be confirmed around this star and attracted interest because it was considered to be the first potentially-habitable Earth-like planet located in the HZ of its star.

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of the planetary system around Gliese 581 C, with a close-up on 581 c - a Super-Earth located within the star's habitable zone. Credit: ESO

This exoplanet remains one of the closest to Earth, just 20.37 light-years away in the direction of the constellation of Libra. Initial findings indicated that the planet is roughly 5.5 times as massive as Earth (another Super-Earth) and that it orbits its star with a period of just under 13 days and at a distance of about 0.072 AU.

However, further research has cast doubt on the planet's habitability because of how it is tidally-locked with its star. Depending on the thickness of the atmosphere or the strength of the planet's magnetic field, life would either be confined to the permanent day-night boundary (i.e. the "Terminator zone") or the planet would be unable to retain an atmosphere and water for long.

Since then, two more exoplanet candidates have been detected in this system (Gliese 581 g and d). The former is believed to be a rocky planet with 2.2 times the mass of Earth and orbits at a distance of 0.13 AU with a period of 32 days. The latter is another Super-Earth (4.4. Earth masses) which orbits at a distance of 0.22 AU with a period of under 67 days.

With its greater distance and lower mass, Gliese 581 g is considered the best candidate for habitability in the system. However, the existence of both g and d remain unconfirmed to this day.

Gliese 667 Cc:

This planet was discovered by the ESO in 2011 in the nearby triple-star system known as Gliese 667 (located 23.6 light years from Earth). The system consists of two K-type main sequence stars (667 A and B) that are similar to our Sun, and one red dwarf (667 C), which this exoplanet orbits.

This planet is also a Super-Earth, with a mean radius 1.54 times greater than Earth's and a mass that is between 3.7 and 4.5 times that of Earth. It orbits 667 C at a distance of 0.125 AU with a period of about 28 days, which puts is squarely in the star's habitable zone.

Another planet, Gliese 667 Cb, was previously discovered by the ESO in 2009. However, estimates on its mass are poorly constrained, indicated that it could be anywhere from 5.94 to about 12 Earth masses (which range from it being a Super-Earth to a mini-Neptune).

Eccentricity analysis of this planet also showed that it is not likely to be a rocky planet, which bolsters the case for it being a small gas giant. It also orbits too close to its star to be a likely candidate for habitability and is tidally-locked with Gliese 667 C. 

What Is Meant By "Habitable Zone" And How Do We Define It?
Artist's impression of a sunset over Gliese 667 Cc, showing its red dwarf sun and the two yellow dwarfs that make up the trinary star system. Credit: ESO

HD 85512 b:

This planet, which was also discovered by the ESO in 2011, also had the honor of being considered the most likely candidate for habitability. Basically, this rocky planet, which is estimated to be at least 3.6 times the mass of Earth, orbits a K-type main sequence star about 36 light years away.

It orbits its Sun-like star at a distance of 0.26 AU (a little more than one-quarter the distance between the Earth and the Sun) and with a period of over 54 days. Because of this, scientists have indicated that the planet is likely to have suffered a runaway greenhouse effect in the past (similar to Venus), making it uninhabitable today.

TRAPPIST-1e:

This planet is one of seven that was discovered around this nearby red dwarf star (39.6 light years from Earth). This began in 2016 when a team of Belgian astronomers announced that they had discovered the three inner planets within the system (TRAPPIST-1 b, c, and d).

This was followed by the announcement of four more by 2017, which were the outermost planets of the system (TRAPPIST-1 e, f, g, and h). What was particularly exciting was the fact that three of the planets - e, f, and g - were all found to orbit within the star's habitable zone.

By November of 2018, researchers determined that planet e was the most likely to be habitable, as well as the most Earth-like planet yet found. Its Earth-like characteristics include it being 91% the radius of Earth, 77% the mass, 102.4% the density (5.65 g/cm3), and 93% the surface gravity. Research has also shown that the seven-planet would be ideal for "life-swapping" (lithopanspermia).

However, multiple studies have cast doubt on the likelihood that the planet is habitable. For one, the planet is tidally locked with its star, which means that one side would be subject to intense radiation and flare activity from TRAPPIST-1, which means that it would have a hard time maintaining an atmosphere and liquid water on its surface.

Other lines of research have shown that the nature of the system is also problematic for life. Since the planets are both close to their star and relatively close together, they would be blasted by stellar winds several orders of magnitude stronger than what Earth experiences.

Even if they had protective magnetic fields, the proximity of the planets would mean that these fields would be connected to each other. This would allow charged particles from TRAPPIST-1 (solar wind) to directly flow onto each planet’s atmosphere, facilitating atmospheric loss.

What Is Meant By "Habitable Zone" And How Do We Define It?

Artist's impression of the TRAPPIST-1 system, with seven Earth-sized planets orbiting a dwarf star. Credit: JPL-Caltech/NASA

HD 40307 g:

This planet was discovered in 2012, again by astronomers at the ESO's La Silla Observatory, orbiting a star located 42 light years away. Based on their measurements, the team estimated that the planet is almost 3 times as large as Earth and 7 times as massive.

It orbits its orange K-type main sequence star at a distance of about 0.6 AU with a period of about 198 days. This makes g the farthest planet within HD 40307's six-planet system and the best candidate for habitability. However, this depends on the planet's composition, which straddles the line between Super-Earth or a mini-Neptune.

LHS 1140 b:

Discovered in 2017, this dense rocky planet orbits a red dwarf star located over 40 light years from Earth. It is over 40% larger than Earth, and also nearly 7 times as massive - putting it in the Super-Earth category. It is also one of the densest exoplanets ever found, with over twice the density of Earth (which allows for an estimated surface gravity of 3.25g).

Orbiting its star at a distance of 0.09 AU (and a period of under 25 days), this planet is squarely in LHS 1140s HZ and gets 41% the incident flux of Earth. Combined with a thick atmosphere, LHS 1140 b is considered one of the best candidates for habitability. It is also considered an excellent candidate for atmospheric studies, due to its proximity to Earth and the fact that it regularly makes transits of its star.

What Is Meant By "Habitable Zone" And How Do We Define It?
An artist’s impression of “rocky super-Earth” LHS 1140b and its red dwarf host star. Credit: M. Weiss/CfA

One other planet has been confirmed in this system (LHS 1140 c) and one unconfirmed candidate (LHS 1140 d). Neither are considered good candidates for habitability, the former being too close and tidally-locked with its star, while the latter is likely to be a gas giant.

Will any of these worlds turn out to be where we find our long-lost cosmic siblings? Who knows? The point is, we intend to find out either way, and we're getting better at looking all the time!

Further Reading:

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