The term "circumstellar habitable zone," also called the "Habitable zone (HZ)" or "Goldilocks Zone" has been thrown around a lot lately in the astronomical community. This is not surprising since it invariably comes up in the context of extrasolar planet discoveries.
And in recent years, thousands of exoplanets have been discovered, many of which just happen to orbit within their star's respective habitable zone.
This term has the tendency to elicit excitement since it implies that scientists could be a step closer to finding evidence of life beyond Earth. However, the term is somewhat problematic for this very same reason.
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Just as the term "Earth-like," the phrase "habitable zone" is loaded with significance and carries with it a certain amount of assumption and guess-work. So if we're to understand what this term means, what it implies, and what the implications of it being applied means, we need to do a few things.
For starters, we need a little refresher on what the actual definition of this term is. Second, we need to look at all the research (especially the current stuff) to see what goes into deciding where it applies.
Basically, HZ refers to the region around a star where a planet would receive enough light and heat to ensure surface temperatures that could maintain water in liquid form. As a reference point, consider Venus, Earth, and Mars, all of which reside in (or straddle) our Sun's HZ.
Our Sun is a main-sequence G-type yellow dwarf, which is relatively moderate in size and mass and experience surface temperatures of about 5,800 K (5,500 °C; 10,000 °F). This type of star accounts for roughly 7% of the stars in our galaxy.
Earth orbits the Sun at an average distance of 1 Astronomical Unit (AU), which works out to 150 million km (93 million mi), or well within our Sun's HZ. However, Earth's axis is tilted 23.4° towards the Sun, which means temperatures vary from season to season.
In fact, temperatures as low as −89 °C (−128.5 °F) have been recorded during a cold night in Vostok, Antarctica, and as high as 71°C (159°F) in the Lut Desert of Iran during the summer.
Nevertheless, this works out to an average surface temperature of about 15 °C (58 °F), which means that the surface of the Earth (the majority of which is covered in oceans) is able to maintain water in liquid form, which is essential to life as we know it.
By contrast, Venus straddles the HZ's inner edge, orbiting the Sun at an average distance of 0.72 AU (108.2 million km; 67.2 million mi). This change in distance means Venus receives roughly twice the amount of solar radiation as Earth.
Combined with the composition of its atmosphere (which leads to a runaway greenhouse effect) this results in Venus being the hottest planet in the Solar System: 737 K (462 °C; 864 °F). With surface temperatures hot enough to melt lead, and well beyond temperatures used for sterilization, Venus is uninhabitable.
At the other end of things, there's Mars, which orbits our Sun at an average distance of about 1.5 AU (227.9 million km; 141.6 million mi). This places it at the outer edge of our Sun's HZ and it has an average surface temperature of 210 K (−63 °C; −82 °F).
Given that Mars' axis is tilted like Earth's (25.19° towards the Sun), Mars also experiences seasonal temperature variations. All told, surface temperatures range from a low of −143 °C (−226 °F) at the poles during winter to a high of 35 °C (95 °F) at the equator during the summer at midday.
For this reason, Mars is a very dry and desiccated place. All known sources of water are either frozen in the polar ice caps or in the soil around the polar regions as permafrost. Anything else would have to be located beneath the surface, most likely in the form of brines.
Using just our Solar System as an example, one can see the meaning behind the term "Goldilocks Zone."
Whereas a planet like Venus is too close to the Sun (and therefore too hot) and a planet like Mars is too far (too cold), Earth sits roughly in the center and is just right.
Not just a question of orbit
Unfortunately, determining if a planet is habitable is not just a matter of plotting its orbit. And there's a lot of evolution that goes into making a planet hospitable to "life as we know it." This was certainly the case with Earth.
A very long process that involved billions of years of geological evolution, changes in our Sun, and primitive life forms were needed in order to make Earth the type of planet that we know and love today.
At the same time, Venus and Mars were not always the way they are today. In fact, scientists believe that both planets once had liquid water on their surfaces and atmospheres that were much more conducive to life. But owing to a series of events (which also took billions of years) they became worlds that hostile to life as we know it.
In the case of Venus, the predominant theory is that a "wet greenhouse" effect occurred millions of years ago that triggered runaway global warming.
As Dr. Michael J. Way, an information technology specialist at NASA Goddard Institute for Space Flight Studies, explained via email, this process is believed to have begun 750 million years ago as a result of a near-global resurfacing event:
"In this scenario most of the carbon would have been locked up as it is on earth – in carbonate rocks in the crust/lithosphere. Then something happened inside the planet that caused massive resurfacing. As a part of this the surface temperature increased, the surface carbon stores were released and it was dumped into the atmosphere where it remains today."
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Mars also experienced severe changes in its climate as a result of its geological evolution. To put it succinctly, Mars today has a very thin atmosphere because (unlike Earth) it has no protective magnetosphere that prevents solar wind from stripping its atmosphere away.
However, roughly 4.3 billion years ago, scientists theorize that Mars had a magnetosphere which (like Earth) was powered by convection in the core. Given that Mars is smaller and less massive than Earth, the planet's interior cooled faster than Earth's, causing the outer part of the core to solidify.
As a result, Mars lost its magnetosphere, its atmosphere began to be slowly stripped away, and the planet began experiencing some drastic changes to its climate. By about 3.7 billion years ago, the surface of Mars had become the very cold, dry, and inhospitable place it is today.
Using these Solar analogs, it becomes clear that habitability does not come down to orbit alone. There are also a number of factors to consider, such as atmospheric composition, geological history, and a number of other factors that cannot be ascertained by distant surveys.
Once again, using Earth as the example, exoplanet-hunters also look for the signs of specific chemical elements, molecules, or isotopes that are associated with life as we know it (aka. "biosignatures" or "biomarkers").
These include water, which is essential to life as we know it and the only solvent known to us that can host life. Gaseous water is also a greenhouse gas, so as part of a water cycle, it also helps to keep a planet's temperatures stable over time.
There's also oxygen gas, which is not only essential to life as we know it, but also a byproduct of photosynthetic organisms. Hydrogen and carbon are also key indicators since they are the key components of water (H²O), carbon dioxide (CO²), and oxides such as sulfates, silicates, and other minerals in a planet's crust.
Carbon dioxide is a major biomarker, along with carbon compounds and carbonate minerals. For starters, carbon dioxide gas is food for photosynthetic organisms and a byproduct for complex oxygen-breathing life forms. In addition, it is a natural greenhouse gas, which makes it an effective climate stabilizer.
Nitrogen is an important biomarker since it is an important buffer gas in Earth's atmosphere. Minerals like phosphorus and sulfur are also a key part of life on Earth, which makes them possible indicators of life in other systems.
One might get the impression from all of this that finding habitable exoplanets is just a simple matter of looking planets that orbit within their stars HZ and contain all the necessary elements. However, there is considerable research that casts a shadow over this straightforward approach.
This research has indicated that the conditions that give rise to life might be a lot more temperamental than we thought. For starters, there is the role played by greenhouses gases and biomarkers like oxygen gas, which could actually be hostile to life under the right conditions.
For example, the recent explosion in the number of exoplanet discoveries has shown that M-type red dwarf stars are the most likely to have terrestrial planets orbiting within their HZs. For starters, these stars have very tight HZs compared to brighter and more massive stars.
As a result, any planet orbiting close enough to have liquid water on its surface would be tidally-locked with its star (i.e. with one side constantly facing the star). This means that one side would be constantly exposed to solar radiation, which could be hazardous to any lifeforms there.
It also increases the likelihood that the dayside would not be able to maintain liquid water on its surface. Because of all the UV radiation bombarding the surface, chemical dissociation would be likely to occur. In this process, water is broken down into hydrogen gas that is lost to space and oxygen gas that remains in the atmosphere.
While this process would ensure an atmosphere that contains oxygen gas (a key biomarker), it would not guarantee life. In fact, recent research has shown that it could work against it. On Earth, oxygen has was the result of photosynthetic organisms metabolizing CO² gas.
However, an oxygen atmosphere that is the result of chemical dissociation would be toxic to such lifeforms. What's worse, planets that orbit cooler stars are believed to have higher concentrations of carbon monoxide (CO) in their atmospheres, which would be toxic to basic and complex organisms alike.
In the past, scientists have also argued that some planets which straddle the outer edge of their HZs could still be habitable if they had high enough concentrations of CO² in their atmosphere, thus ensuring enough of a greenhouse effect. However, too much CO² would be bad for life as we know it.
A good example of this is Kepler-62f, a super-Earth that orbits a star that slightly smaller and dimmer than our Sun about 990 light years from Earth. When it was discovered in 2013, this planet was thought to be a good candidate for extraterrestrial life, assuming the presence of a sufficient greenhouse effect.
However, subsequent calculations by researchers at the NASA Astrobiology Institute showed that it would take 1,000 times more carbon dioxide (300 to 500 kilopascals) than what existed on Earth when complex lifeforms were first evolving (ca. 1.85 billion years ago) - which would be toxic for most complex lifeforms here on Earth.
Once these physiological constraints are factored in, it is estimated that the habitable zone for complex life must be significantly narrower than previously estimated – roughly a quarter of what we thought.
Water, water everywhere!
Another big concern has to do with the prevalence of water on extrasolar planets. To put it simply, many of these planets may have too much water, which would actually be bad for life. As with most things, too much of a good thing can kill you!
Based on data from the Kepler Space Telescope and Gaia mission, scientists have managed to precisely measure the radii of the over 4000 exoplanets that have been discovered so far, along with their orbital periods and other parameters.
These exoplanet candidates can be divided into two size categories: those that have 1.5 times the radius of Earth, and those that average around 2.5 Earth radii. Whereas planets that fall into the former category are believed to be rocky, the latter ones are generally thought to range from super-Earths to Neptune-sized gas giants.
According to compositional models of these planets, it is estimated that many of the exoplanets that are between two to four times the size of Earth are in fact "water worlds." These are planets where roughly 50% of the mass consists of water (whereas it makes up just 0.2% of the Earth’s mass).
Combined with their orbital parameters, surface temperatures on these planets are likely to be quite high, leading to a water-vapor-dominated atmosphere. Beneath the surface, the oceans are likely to have a layer of high-pressure ice around a rocky core.
None of this is particularly good for life. Beyond the extreme heat and the lack of access to sufficient sunlight, there is also the issue there being no land masses. According to multiple lines of research, planets need continents and oceans for complex life to emerge.
A thick layer of ice between the core and ocean would also mean that hydrothermal activity would not be taking place on the ocean floor, which may also be essential to life. This is based on the fact that on Earth, the earliest fossilized evidence of life (roughly 3.77 billion years old) has been found on the seafloor around hydrothermal vents.
Such deep oceans would also be an impediment to carbon cycling. One of the reasons Earth has been able to maintain stable temperatures over long timescales is thanks to regular exchanges of CO² between the atmosphere and crust.
This is what is known as the carbon cycle, where tectonic activity turns atmospheric CO² into carbonate minerals (which leads to global cooling) and then releases it again via volcanoes (leading to global warming).
Such a process would not be possible on water worlds, where the entire surface is covered in very deep oceans. On these worlds, water would prevent the absorption of carbon dioxide by rocks and suppress volcanic activity - though it is possible that the oceans themselves could cycle enough CO2.
Geologically "stagnant" worlds
Last, but not least, there is the issue of the tectonic activity itself. On Earth, the crust and mantle (aka. lithosphere) is made up of a series of plates that are in constant motion. When two plates collide, the result is subduction, where one plate is pushed beneath the other and deeper into the subsurface.
This subduction causes the dense mantle to melt and form buoyant magma that then rises through the crust to the Earth’s surface to creates volcanoes. As noted already, this process is central to the carbon cycle since it pushes CO² into the mantle and backs out into the atmosphere.
In this respect, plate tectonics and volcanic activity were central to the emergence of life here on Earth by ensuring that surface temperatures remained stable. However, on "stagnant lid" planets where tectonic activity does not exist, the situation would be quite different.
These are planets where the crust consists of a single, giant spherical plate floating on mantle, rather than in separate pieces. So far, no extrasolar planets have been confirmed that show tectonic activity yet, which may indicate that stagnant lid planets are much more common.
Basically, these planets would have a much harder time maintaining a carbon cycle and maintaining temperatures that favor habitability. However, more recent research has indicated that it would still be possible if these planets had enough heat-producing elements when they formed (that is, it's initial heat budget).
Lingering mystery of life
Another issue when it comes to finding worlds that could have life on them has to do with the unresolved question of how life emerges. While scientists certainly know what basic elements are essential to life here on Earth, they are still not sure exactly how it all came to be.
At some point in the distant past, all the inorganic ingredients that are essential to life came together to create organic life (a process known as "abiogenesis"). At present, it is still not clear how that happened, though experiments are getting closer to an answer all the time.
Then again, it is also possible that the earliest prebiotic compounds or even life forms came to Earth via asteroids or meteorites (in accordance with the theory of "panspermia"). If this is true, then the process of turning inorganic elements into life happened somewhere else.
In the end, the best we can do is keep looking. While scientists in the lab continue to study terrestrial lifeforms in the hopes of unlocking how life began on Earth.
Meanwhile, exploratory missions will continue to search the Solar System to see where else life could emerge while astronomers continue to probe the Universe in the hopes of finding more examples of life-bearing planets out there.
While experimental researchers benefit from improved instruments, research and data-sharing methods, exploratory efforts will benefit from the deployment of next-generation telescopes and robotic explorers in the coming years and decades.
In the case of the former, these include theJames Webb Space Telescope (JWST) and the Wide-Field Infrared Space Telescope (WFIRST), as well as ground-based observatories like the Extremely Large Telescope (ELT), the Thirty Meter Telescope, and the Giant Magellan Telescope (GMT).
In the case of the latter, these include the Mars 2020 rover, the Europa Clipper spacecraft, the JUpiter Icy moon Explorer (JUICE), the Dragonfly mission to Titan, and many, many more.
- ESA - What are Exoplanets?
- NASA - Galactic Habitable Zones
- PHL - Habitable Exoplanets Catalog
- ESA - How to Find an Extrasolar planet
- NASA - JWST - Planets & Origins of Life
- NASA - A New Model for Habitable Zones
- Wikipedia - Circumstellar Habitable Zone
- NASA - Looking for Life in All the Right Places
- USM - Planetarium: "Is the Sun getting hotter?"
- NASA - Warm Welcome: Finding Habitable Planets
- NASA - The Outer Edge of a Star's Habitable Zone a Hard Place for Life