Of the thousands of discovered worlds, there is only one that can definitely support life: the Earth. This has focused the search for habitable planets on worlds that might be like our own.
It is true that an inhabited Earth-like environment will be the easiest to recognise. Yet, it is not necessarily true that this is the only option for supporting life. The Earth may not even be the best location. So what alien landscapes might still support Goldilocks’s perfect breakfast?
Water worlds
The first transiting planet found inside the temperate zone raised hopes of a life-packed land. Kepler-22b seemed too large to be rocky, but too small to be a gas giant. Could it be an intermediate liquid world, blanketed by a deep global ocean? Since we find life wherever there is water on the Earth, this prospect had the splash of potential.
The first question is whether such a water world could truly exist. Without a mass measurement to provide an average density, Kepler-22b was a wild hope. Size alone was not enough to distinguish a new type of ocean planet from a small Neptune or giant terrestrial world. However, stronger evidence for water worlds had already been found.
In 2009, a planet was discovered transiting a red dwarf 42 light years away in the constellation of Ophiuchus, the Serpent Bearer. Even without allowing for an atmosphere, the planet’s surface temperature was estimated at well over 100°C (212 °F), and definitely not within the temperate zone. However, the planet’s close 1.6-day orbit meant that its presence wobbled the star enough to secure a radial velocity measurement of its mass. Combined with the transit data, this gave an average density.
The planet was Gliese 1214b, and was found to have a radius of 2.7 Earths and a mass of 6.6 Earths. The resulting density was 1.87g/cm3, which sits in between the rocky Earth and gaseous Neptune. A possibility that matches this middling value is a composition of 25 per cent rock and 75 per cent water, surrounded by a hydrogen and helium atmosphere. For comparison, our planet has only a teeny 0.1 per cent of its mass in water. The high temperatures on the planet would prevent this huge water reservoir from forming a liquid ocean. Instead, the world would be enveloped in the liquid-like gas of a supercritical fluid. 1
The Hubble Space Telescope attempted to put the watery nature of Gliese 1214b to the test by examining its atmosphere during transit. Unfortunately, it failed. Rather that finding the distinctive fingerprint from water molecules absorbing the starlight, there were no distinctive features at all. The most likely explanation for such a nondescript result is that clouds are obscuring the telescope’s view. Despite this lack of final confirmation, the density of Gliese 1214b makes it a prime candidate for a world made predominately of water. Its discovery secured water worlds as science (very nearly) fact.
To gather so much water, Gliese 1214b would have had to form far from the star. Behind the ice line within the protoplanetary disc, the planet could accumulate its large water fraction in frozen ices. The pull from the gas disc then caused the ice-rich world to migrate closer to the star. If its final stopping place had been in the temperate zone, a liquid ocean water world would have been born.
Water worlds may even not be rare. Gliese 1214b and Kepler-22b are both super Earths; the most common class of exoplanet currently discovered. With sizes a few times larger than the Earth, this planetary group could include two very different types of water world.
Gliese 1214b is an example of a deep-ocean water world. Its middling density implies that a huge fraction of its mass is in water. The planet’s rocky core would be buried beneath oceans tens of thousands of kilometres deep. But the planet does not need to be mainly water to have a global ocean. The stronger gravity on a super-sized rocky planet could flatten the surface topology. Instead of mountainous terrain, the landscape could be compressed to form a featureless ocean bed that could easily become covered with a shallow sea. The difference between these water worlds is like pouring water on to a bowl and a plate. They might both be covered, but one contains far more liquid than the other. A 10 Earth mass rocky planet risks not having exposed continents unless it is very dry, with at least 10 times less water than our own planet. A larger version of our own Earth may therefore always be a water world.
Life on Earth is found wherever there is water. Yet, could an ecosystem really develop on a world without any dry land? If a global ocean renders a planet uninhabitable, this places a size constraint on even rocky planets for developing life.
On a deep-ocean water world, any plants that use light for photosynthesis would need to manage without an anchor. Stems and roots could not bridge the depth of more than 10,000km (6,000mi) between the ocean floor and the starlit surface. Floating plants like algae would need to develop, alongside creatures that could fly or swim. Of course, the Earth offers examples of such life as well as hosting organisms that do not need sunlight at all. Could the ocean-based life on our planet thrive on a deep-ocean water world?
Despite only 0.1 per cent of the Earth’s mass being in water, the bottoms of our oceans are too deep for light to penetrate. Instead, hydrothermal vents spew hot fluid through cracks in the ocean crust. Despite reaching temperatures of well over 100°C (212°F), the towering geysers remain liquid due to the high pressure at the ocean bottom. Hydrothermal vents are surrounded by whole ecosystems that thrive entirely without sunlight. It is this source of energy that might support life on a rogue world, where there is no star to provide light anywhere on the planet. Unfortunately, attempting to apply this system to a deep-ocean water world sends us to a screeching halt.
Switch the Earth’s 0.1 per cent water mass for more than 50 per cent and the bottom of the ocean becomes a very different place. Beneath such a huge reservoir, the pressure becomes high enough to compress the water into thick layers of ice. The silicate rocky core is thus separated from the watery ocean by an icy barrier thousands of kilometres thick. Trapped below the ice, hydrothermal vents cannot form and the potential ecosystem is silenced.
The lack of exposed land also shuts down the carbon-silicate cycle. This is the planet thermostat we met in Chapter 12, which adjusts the surface temperature by varying the amount of carbon dioxide in the atmosphere. Should the planet’s temperature rise, more carbon dioxide is drawn out of the atmosphere by reactions with surface rocks. If the planet cools, this reaction slows and carbon dioxide levels increase to trap more heat. The absence of exposed rocks throttles this planetary temperature controller.
Could this be fixed by reactions with the ocean itself? The Earth’s seas have absorbed 10 times more carbon dioxide than is present in the air. This would also happen on an ocean world, but it turns out that the mechanism is the Devil’s thermostat. Sea absorption of carbon dioxide is most efficient when the temperature is cooler. If the planet’s temperature rises, the sea will draw less carbon dioxide from the atmosphere and allow more heat to be trapped. Should the reverse happen and the planet cools, the sea will increase the removal of carbon dioxide and let more heat escape. Rather than countering a change in the planet’s temperature, the endless oceans will accelerate it.
Without the ability to compensate for variations in temperature, the temperate zone for a deep-ocean water world narrows to a thin strip. Removal of the Earth’s ability to adjust for slightly too much or too little radiation from the star allows the super Earth to retain liquid oceans only if its location is perfect. This does not make it impossible for the planet to be habitable, but is does make the chances of it being in a suitable place much smaller. A silver lining is that a huge ocean changes temperature very slowly. A deep-ocean world on an eccentric orbit that passes through the temperate zone might therefore be able to retain habitable conditions where changing temperatures on an Earth-like world would sterilise the surface.
Should the planet be a shallow-water world then prospects slightly improve. Without the colossally deep oceans, high-pressure ices will not form and separate the rocky ocean bed from the seas. This allows both hydrothermal vents to form and also a weak carbon-silicate cycle. Seawater can recycle carbon dioxide into the rocks in the same process as on the surface. The catch is that the ocean floor does not perfectly reflect the surface temperature of the planet, producing a much poorer thermostat. A shallow-water world may be less able to maintain its environment compared with the Earth, but will probably do better than its deep-ocean counterpart.
While the situation still looks bleak for habitability, an important caveat might rescue shallow-water worlds. The surface of a terrestrial planet is not the only place where the world can store water. The Earth tucks a significant fraction of its seas into the mantle, absorbing the water in the rocky minerals. The surface and mantle reservoirs exchange water during the shuffling of the tectonic plates. As the ocean crust is pushed underground, water is expelled into the mantle. This is then returned to the surface via volcanoes. The stronger gravity on a super Earth will increase the pressure between the two reservoirs and allow more liquid to be pushed into the mantle. If enough can go underground, even the flat sea bed of the super Earth may become partially exposed. With land out of the global bath, we would lose the water world but regain a carbon-silicate cycle.
It is difficult to determine how effective stuffing water below ground would be. This is partly because we do not know how much water is stored in the Earth’s mantle. If it is approximately the same quantity as that of the surface oceans, then a 10 Earth-mass planet with plate tectonics could avoid a water world’s fate. This is a reasonable or even slightly conservative guess, suggesting that larger rocky planets are not out of the habitability stakes.
It is interesting to speculate whether intelligent life could develop on a habitable water world. Without dry land, fire and electricity might never develop and this could prevent the creation of a technologically advanced civilisation. The stronger gravity on a super Earth might also limit the size of any flying life forms. Our own oceans are teeming with massive sea life, but none has reached the intelligence of humans. Is this random chance, or could evolution in the seas not favour cognitive developments?
A gas giant core
Unlike in our own Solar System, gas giant planets do not always keep to the far side of the ice line. Instead, the pull of gas within the protoplanetary disc can cause the young giants to migrate towards the star. Should migration move the planet into the temperate zone, we are left with the obvious question: is there any hope for life?
Our own gas giants do not look promising. It is impossible for a liquid ocean to exist under the colossal atmospheres that roll across these huge planets. The pressure near the solid cores of such worlds is so high that strange forms of matter are thought to exist, including liquid diamond and metallic hydrogen. Any organism attempting to live suspended within the gas would be at the mercy of strong convection currents and continuously slammed between scorching-hot depths and the freezing upper atmosphere. It is certainly an intriguing environment, but not one for a holiday resort.
The story might be different if the migrating planet is small enough to be a mini Neptune. Dragged towards the star, the increase in radiation might be sufficient to strip this intermediate-sized planet of its thick atmosphere. The result would be an exposed solid core. Could such a surface be habitable?
Planets around red dwarf stars are particularly promising for this possibility. The star’s dim light allows the inner disc edge and ice line to sit much closer to the temperate zone boundaries than around a Sun-like star. This increases the chances of a migrating gas planet finding itself stranded in the temperate zone region once the gas disc disperses. Once there, the planet needs to lose its atmosphere and this is where a red dwarf may excel.
Previously, the violence of a young red dwarf was flagged as a problem for life. Planets forming in the temperate zone risk being stripped and sterilised from the strong radiation pouring from the rambunctious protostar. However, a migrating planet with a thick atmosphere can use this radiation to uncover its core.
Such a planet would form beyond the ice line and develop a centre of rock and ice surrounded by a deep hydrogen and helium atmosphere. While the red dwarf was still young, the planet would migrate into the temperate zone. The X-rays and ultraviolet radiation pouring from the protostar would heat the planet’s atmosphere, allowing the gases to escape the gravitational tug of the planet. The ice on the exposed core would melt to leave a world with an ocean.
Whether this mechanism is successful comes down to size and timing. If the core is too massive, then its gravity will resist the radiation bombardment and hold on to the hydrogen and helium atmosphere. A core around the mass of the Earth can be successfully stripped of its inhospitable envelope, while one twice the mass will probably retain the gases. Additionally, if the planet moves into the temperate zone too early, then the young star’s violent days may last long enough to rip away the atmosphere and still evaporate all the water. Conversely, arriving too late risks missing the star’s energetic youth to leave radiation too weak to deal with the gas. But should the planet arrive with the right mass and the right timing, an exposed watery core may result.
An exposed core would offer a very different landscape from a terrestrial planet’s surface. Rather than consisting primarily of rocky silicates, a core would have a comet-like composition with equal parts of ice and rock. If too much of the ice melts, the planet is likely to become a water world.
Having shed its first atmosphere of hydrogen and helium, a habitable exposed core would need a new envelope of gases. The Earth formed its second atmosphere through volcanoes releasing gases trapped in the planet’s interior. Due to the cometary composition of a core, a similar expulsion would eject air rich in ammonia and methane; both greenhouse gases efficient at trapping heat at a planet’s surface. The ideal location for an exposed core might therefore be at the outer edge of the temperate zone, where a boost in surface temperature would not send the planet spiralling into a runaway greenhouse.
The exposed core’s rock and ice mix would also change the tectonics and geology from that on Earth. The result of this is not known, but the core’s new atmosphere would be more resilient to any stellar activity if the planet can still drive a magnetic field.
As planetary migration appears to be common in exoplanet systems, it is worth remembering that rocky planets in the temperate zone may be exposed cores. If these could indeed be inhabited, their life will have developed in an extremely alien land.
The twilight zone
Along with the perils of sterilising radiation, planets in the temperate zone around red dwarf stars risk tidal locking. Orbiting so close to the star, the strong gravitational pull forces one side of the planet to continually face inwards, while the other (quite literally) never sees the light of day.
A guide to how challenging this might be for habitability is to consider what would happen if the Earth became tidally locked to the Sun. We saw in Chapter 12 that without any atmosphere, the Earth’s average temperature would be about 5°C (41°F). In tidal lock, the Sun pounding on one half of our planet would send the day-side temperature soaring to 120°C (248°F). Facing away from the Sun, the night side would be warmed only by the Earth’s internal heat. This paltry energy source would give a surface temperature of -273°C (-459°F). Our present Goldilocks conditions would be replaced by the unappetising options of dying by boiling or freezing.
But this bleak prospect ignores the planet’s atmosphere. While the surface of the planet is locked in place, the enveloping gases can still move around the globe. Is this circulation enough to quench the extremes of day and night into liveable conditions?
The situation does not initially look good. The night side is so cold that gas would condense on to the surface. The loss of atmosphere over the dark hemisphere would cause the pressure to plummet, sending gas from the day side pouring around the globe to fill the void. As fresh gas hit the freezing temperatures, it would also condense until the whole atmosphere was destroyed. This planet would have suffered complete atmospheric collapse.
The catastrophic atmospheric collapse could be avoided if the atmosphere can even out the heat between the hemispheres. If the night side can be kept warm enough to prevent gas from freezing, then the atmosphere stays gaseous. A very thin envelope will not be able to move enough hot gas around the planet to prevent freeze-out, but an approximately Earth-like carbon dioxide or nitrogen atmosphere may succeed.
Whether an Earth-like pervading atmosphere would allow lakes and seas to form is another matter. Success depends on whether the planet’s day side becomes hot enough to boil water. On the night side, water is doomed to be solid ice. With temperatures low enough to risk freezing the atmosphere, not even circulating air will raise conditions sufficiently to keep water liquid in the dark. If heat from the star or a thick greenhouse atmosphere evaporates the water on the day side, the steam will be blown to the night side of the planet as the atmosphere circulates. Now below the freezing point for water, the vapour will condense into snow and fall to the icy surface. The night side will become a cold trap that will eventually hold the planet’s whole water budget as frozen ice. Such a planet would look like a giant eyeball, with a surface covered by ice except where the planet faces the star.
Even at temperatures of less than 100°C (212°F), eyeball worlds are a risk. Any water that evaporates from winds blowing across the seas will be sucked into the cold trap. The planet’s reservoirs will slowly dry unless water can be released from the ice. Fortunately, the glaciers of Greenland and Antarctica demonstrate that there is a way for this to happen.
If water was never released from ice without melting, our own planet would look very different. The water vapour in our atmosphere would freeze at the poles and only be expelled in summer. Instead, gravity pulls piles of ice downhill to form the creeping ice sheets seen in glaciers. Such ice sheets on an eyeball planet could creep the frozen water back towards the day side, where it could melt and be vaporised once again. At the interface between ice and steam, liquid water could flow to form rivers between the dark and light planet hemispheres. This ring would be a twilight zone where life could develop with the star permanently on the horizon in a deep red sunset.
If the planet is cool enough to allow water to exist on its eyeball day side, the desert will be replaced by a sea. This sounds more habitable than a twilight strip, but there is a danger. While land and water absorb much of the radiation that reaches their surfaces, ice is very reflective and turns away the heat. Should the water ever freeze, the resultant ice would reflect heat and become even colder. This could prevent the ice from ever melting back into water.
A cool planet with exposed land might fall into this trap. The focused starlight on the day-side rocks might enhance the carbon-silicate cycle and draw too much carbon dioxide from the air. The reduction in the greenhouse gases could lower the surface temperature to below 0°C (32°F) and freeze the ocean. Now reflecting any warmth, the planet might never emerge from a permanent snowball state.
If the planet is warm enough to avoid this fate and retain liquid water, the best chance for life might be at the icy shore or under the water. This would provide a liquid pool but be out of the direct rays from the star.
A potential eyeball planet is KOI-2626-01. ‘KOI’ stands for Kepler Object of Interest, which labels a transiting planet signature discovered by the Kepler Space Telescope that has not yet been confirmed with additional observations. KOI would-be planets are denoted by numbers, rather than letters, so KOI-2626-01 is the first planet seen around the star KOI-2626. Assuming that the world does exist, KOI-2626-01 is an Earth-sized planet around a red dwarf with an orbital time of 38 days. This probably places it within the star’s temperate zone, but in an eyeball-worthy tidal lock.
Given the alien climate on an eyeball planet, it is worth considering whether we should be discussing the temperate zone at all. To estimate the likelihood of supporting liquid water, the boundaries of the temperate zone assume an Earth-like planet that is uniformly distributing its heat. This is definitely not the case on a tidally locked eyeball world. Interestingly, it is possible that having a frozen and baking interface might help an otherwise Earth-like planet support water beyond the traditional limits. In the previous chapter, Gliese 581c was inside the inner edge of the temperate zone and therefore deemed likely to be a baked Venus-like world. Yet an eyeball planet’s split personality might allow a runaway greenhouse effect on the day side and a cold trap on the night side. The interface would be a melted region where water could flow. This is yet another reminder that global averages do not really apply to any environment.
If the planet’s atmosphere were denser than that of Earth, heat could be evenly redistributed between the day and night sides. With a day lasting longer than a year, Venus is almost in tidal lock with the Sun. Despite this, the surface temperature maintains lead-melting conditions globally over the planet. This is due to the insulation of the thick cloud cover and the strong winds in the Venusian upper atmosphere that balance out the Sun’s heat. The surface of Venus is clearly not habitable, but you would die an identical burning death everywhere on the planet.
Venus’s spin turns out to be an interesting conundrum. Due to its sluggish pace of 243 days per rotation, Venus spins in the opposite direction to the Earth. In the Venusian sky, the Sun rises in the west and sets in the east.
This reverse spin is very surprising. Forming within a common protoplanetary disc, the planets’ orbits and spins should all be in the same direction. An anomalous rotation can often be explained by a strong collision that tilts the planet’s axis. The drunken tilts of Uranus and Neptune are thought to be from major impacts late in their formation. However, the answer to Venus’s reverse spin may instead lie in its atmosphere.
When bathed in sunlight, gas molecules in the Venusian atmosphere increase in speed and boost the local pressure. The pressure difference around the planet drives the hot gas into the colder region to create a patch of high-density gas.
As the gas takes time to heat, the reshuffling of the atmosphere is slightly out of sync with the Sun’s motion. 2 Rather than the gas molecules piling into the region on the exact reverse side of the planet from the Sun, the dense patch of atmosphere ends up at an angle to the Sun’s location. When the Sun’s gravity pulls on this denser region, the result is a torque that turns the atmosphere. As the thick blanket of gases rolls across the planet’s surface, it creates a strong enough drag to rotate the planet in the same direction.
On a shorter orbit than the Earth, Venus risks being in tidal lock with the Sun. The atmospheric drag could be preventing this from occurring, sending the planet slowly rolling in a reverse direction. Intriguingly, this mechanism to break tidal lock may be even more efficient for an Earth-like atmosphere. The thinner air will absorb less of the star’s radiation, allowing more heat to permeate through to the surface-level gas. The gas dragging on the planet will therefore be more strongly affected by the temperature difference created by the star than when buried at the bottom of a thick Venusian atmosphere. This results in a stronger torque where it really matters, close to the planet’s surface.
Without more data from planets orbiting close to their stars, it is not possible to know how many can avoid tidal lock. But should this mechanism be effective, then a planet with an Earth-like atmosphere may avoid an eyeball environment even in the temperate zone of a red dwarf.
Return to Tatooine
The blazing twin suns about Luke Skywalker’s home of Tatooine supported a harsh but habitable desert landscape. But could any liquid water truly exist on a world with a circumbinary P-type orbit around two stars?
In the plane of the orbiting planet, the shape of the temperate zone around a single star is a tidy doughnut. The stellar radiation received by a planet depends on the distance to the star, resulting in a symmetrical ring where the level of heat is able to support water on an Earth-like world. Add a second star to the system and the shape of the temperate zone becomes a complex beast. The radiation a planet now receives comes from two different sources that are continually moving around one another. The temperate zone morphs into a strange, asymmetric structure that changes with time. Even if a planet remained completely stationary, the stars’ motion could cause the temperate zone to move away from the planet’s location, like a carpet pulled from under your feet.
Exactly how strangely shaped the temperate zone can become depends on the two stars. If their mass is very different, the radiation reaching the planet is dominated by the larger star. This can give an approximately doughnut-shaped temperate zone, but with a moving bump that follows the motion of the smaller star. If the two stars are more equally sized, the radiation will depend strongly on each of the stars’ positions relative to the planet. For Sun-like or cooler stars, the temperate zone is sufficiently close to the stars to take the form of a rotating peanut. A planet within this region will feel the separate tugs of the two stars, and risks its orbit becoming unstable due to the stellar pair’s gravity. On an unstable orbit, the dual influence of the stars will eventually scatter the planet into deep space or send it crashing into the binary. Both options bode ill for seas.
Figure 22 The temperate zone around a single star (left) and two binary star systems. The middle system has two different mass stars, while the right-hand system has two identical stars. Dark grey shows the limits of the conservative temperate zone, while the light grey indicates the extensions that allow for surface water only during the planet’s early years.
The issue of stable orbits within a wonky-shaped temperate zone ought to make it difficult to find the potentially habitable Tatooine-like worlds. Strangely, the reverse turns out to be true.
In 2015, 10 planets had been discovered in circumbinary orbits. Eight of these 10 circled their twin stars on paths very close to the limit for orbit stability. Any orbit shorter than this critical threshold would result in the planet being sent rogue or to a fiery doom. Why 80 per cent of the discovered circumbinary planets orbited so close to the stable limit is not clear. It could be a result of migration from the planets being dragged inwards by the protoplanetary disc. Any planet crossing the last stable orbit would be lost, leaving worlds that stopped migrating just inside this safety net to be observed. Alternatively, it could simply be that planets close to their stars are easier to find and these worlds are the closest possible population. Whatever the reason, it has an important consequence: with the limit for stable orbits often lying close to the temperate zone, the chances of finding circumbinary planets even within this awkwardly shaped region is higher than might be expected.
The first-discovered transiting circumbinary planet was introduced in Chapter 10. This was Kepler-16b, a Saturn-sized world that orbited twin stars every 229 days. The binary stars are both smaller than the Sun and of very unequal sizes, at 69 per cent and 20 per cent of our Sun’s mass. This causes the temperate zone to be dominated by the larger star. The shape looks like a badly drawn circle, with a rotating bulge on one side due to the smaller twin.
Despite being nearly symmetrical, the temperate zone bulge makes a difference to the planet. Kepler-16b is on a circular orbit near the temperate zone’s outer edge. The presence of the smaller twin forces the planet to dip in and out of the temperate zone during its year. This causes the planet’s average temperature to fluctuate by about 15°C (59°F) four to five times during the orbit. 3 Of course, the local temperature away from the Equator on Earth can change by more than this during our seasons. Our summers and winters are due to the planet’s tilt, tipping the northern and southern points towards or away from the Sun during the year. However, the seasonal cycle is just once per year and our global average temperature remains the same. Kepler-16b will experience fives times as many annual temperature changes that affect the whole planet.
In contrast to Kepler-16b, Kepler-453b orbits entirely within the temperate zone. The 10th discovered circumbinary world was announced in 2014 and has a radius 60 per cent larger than Neptune. Its sibling stars are also of unequal mass, with one being similar to our Sun, and the other a much smaller red dwarf with just 20 per cent of the mass. The large contrast between the stars creates a more uniformly doughnut-shaped temperate zone, dominated by the larger star. This makes it easier for Kepler-453b to orbit within its boundaries. While the planet’s size ensures that this is another gaseous world, it could host a rocky moon. Such a location would have Tatooine’s dual stars and the rolling atmosphere of a giant gas planet in its sky.
As a small side note, film footage of the twin stars in the sky about Tatooine suggests that the temperate zone would be very lopsided. While Tatooine is portrayed as a tough environment, it is more likely that the planet would be outside the temperate zone and its surface would be uninhabitable. Sorry, Luke.
If a planet orbits just one star in a binary pair, the challenges change. In a circumstellar S-type orbit, the planet circles a single star that is moving around a stellar sibling. So how does the radiation from the sibling star affect the temperate zone of the planet’s host star?
It turns out that the heat from a stellar sibling does not typically change the temperate zone boundaries. As we saw in Chapter 9, the planet usually orbits the bigger and brighter primary star in the pair. If the sibling star is distant enough that the planet’s orbit is stable, then the radiation within the temperate zone is strongly dominated by the primary star.
This might imply that planets on circumstellar binary orbits are just as likely (or unlikely) to be in the temperate zone as worlds orbiting single stars. Sadly, the stellar sibling refuses to be left out of the picture. While the second source of radiation has little effect on the planet, the additional gravitational pull is a different story. Rather than allowing a planet to orbit peacefully on a circular path tucked within the temperate zone boundaries, the stellar sibling will probably pull the world on to an elongated eccentric path. With the temperate zone remaining a doughnut shape around the primary star, an eccentric orbit risks whisking the planet in and out of the most clement locations in the system.
Can a planet on an eccentric orbit ever be habitable? If the eccentricity is small and the temperate zone is wide enough, then an Earth-like planet may be able to stay within its borders. The seasons on such a world will become more extreme as the distance from the star changes, but surface water could stay liquid. If the elongated path extends beyond the temperate zone, then conditions get more tricky, but not necessarily doom-laden. A planet on an eccentric orbit moves fastest when closest to the star. The scorching summer therefore occurs during a small fraction of the year. If the hot period is sufficiently short, the planet might avoid evaporating the majority of its water before returning to cooler climes. This annual flash heating might prevent the water from completely freezing as the planet moves to its furthest point from the star. Climate calculations are heinously tricky, but have suggested that if the average radiation over the planet’s year is similar to that within the temperate zone, this hot and cold balancing act on an Earth-like world could successfully retain liquid water.
Organisms developing on an eccentrically orbiting planet could adapt by hibernating during the extreme hot and cold spells of the year. This would be easiest for ocean-dwelling creatures, since a large body of water is slower to change temperature than the land. Life forms would adapt to be active when the planet was passing through the temperate zone, and would lower their metabolic rates to allow prolonged periods of inactivity and hiding when conditions were brutal.
Such protective behaviour has been exhibited by life forms on Earth. Bacteria can persevere for about a week in outer space, while microorganisms can survive in a falling meteorite if they are buried within a few centimetres of shielding rock. These experiments have only been found to work on very small creatures, but the Earth sits in the temperate zone year round. If it did not, life would have an incentive to adapt to more extreme seasons. If the variety of creatures on Earth proves anything, it is that the limits of life are very hard to guess.
The best of all possible worlds?
Our search for the ultimate habitable planet has seen us hunting the skies for a twin to the Earth. While there is no denying that the Earth is highly suited to life, is our planet truly the best possible option? Could there be planets even more likely to have developed life; the so-called super-habitable worlds?
The concept of super-habitability ironically begins with a land you would never want to visit. The ground is ruptured by continuous volcanic activity that fills the air with sulphur dioxide and methane. Red and yellow sulphur particles rain from the sky to strike a ground pounded by ultraviolet radiation due to the lack of a protective ozone layer. The seas run red with iron, beneath which the microbial life on the planet exists. Welcome to the Earth 2.3 billion years ago. This is the brink of the greatest extinction in our history.
About 200 million years before this moment, a blue-green bacterium appeared in the Earth’s oceans. This microorganism did something never before seen on our planet; it used sunlight to take carbon dioxide and water and produce sugars and oxygen. It was the start of photosynthesis.
These tiny photosynthesising machines are known as cyanobacteria. The oxygen they generated initially reacted with volcanic gases to re-form carbon dioxide and water vapour, or with the iron in the water causing it to rust. But as the cyanobacteria flourished, these sinks were unable to completely absorb the output of oxygen. Oxygen flooded the atmosphere in a juncture referred to as the Great Oxygenation Event.
Unfortunately, most of the populations on the early Earth were anaerobic bacteria that have a toxic response to oxygen. They died in droves, wiping out a huge chunk of life on the planet. Meanwhile, the oxygen in the atmosphere reacted with methane to produce more carbon dioxide and water vapour. While both these products are greenhouse gases, neither is as effective at trapping heat as methane. The removal of methane therefore caused the temperature on the Earth to plummet, producing the massive Huronian glaciation that is our oldest known ice age. During this time, our planet may have become almost entirely frozen, creating a snowball Earth.
While this does not seem like an auspicious beginning for habitability, the oxidising of our atmosphere was a change key in our development. Aerobic organisms began to evolve to use the new oxygen abundance, altering the atmosphere into the air we currently breathe today. The take-home message is that time allows planets to make major modifications to their environments that can promote habitability. If we want to find a planet that probably hosts life, it is therefore possible that a super-habitable candidate will be an older world.
That said, an older world does present another challenge: the star. About 3.5 billion years ago, the Earth was in the centre of our Sun’s temperate zone. As the Sun aged and brightened, the temperate zone moved outwards to leave our planet near its inner edge. Another 1.75 billion years from now and the Earth will not be in the temperate zone at all. We will have joined Venus as a lifeless desert with conditions too hot to retain surface water. A planet much older than the Earth that currently resides in the temperate zone may therefore have spent much of its youth beyond the outer boundary and not been able to develop surface life. However, this problem could be reduced if we could improve our star.
Less massive stars are slower burners than their weightier cousins, resulting in a longer life expectancy. The temperate zone around such slow-agers remains in a steady location for a longer period of time than for our Sun. However, the small and dim red dwarfs have additional problems that threaten the habitability of their planets. We have seen that worlds around these small stars risk tidal locking and being swamped in sterilising radiation, which offset any benefits from a slow-evolving temperate zone. A compromise could be an orange dwarf, a star larger than the red dwarfs but smaller than our Sun. An orange dwarf could shine for roughly double the time of the Sun, giving a significantly longer epoch for a planet to age and develop an environment suitable for life.
The star is not alone in the need to age gracefully. To maintain surface conditions, the planet must keep its geology active. In addition to radiation from the star, planets are warmed internally by the heat left over from their formation, and radioactive elements in the mantle and crust. This heat drives volcanoes and plate tectonics on the Earth, providing the rock shuffling needed for our magnetic field and carbon-silicate cycle. When our internal fires go cold, carbon dioxide will no longer be returned to the air by volcanic eruptions. Greenhouse gases will decrease and the Earth will freeze. The molten iron will also solidify in the Earth’s core, and our shielding magnetic field will drop to leave our atmosphere open to stripping from the solar wind and flares.
Larger planets have a larger reservoir of internal heat to eat through, prolonging their geological life expectancy. However, this is a careful balance. If the planet’s mass is too large, then the planet risks becoming a mini Neptune or at least holding on to its primitive atmosphere of hydrogen and helium. Such a thick envelope of gases would preclude habitability. Even the gravity of a massive rocky world might be too much for plate tectonics, increasing the pressure on the rocks to make movement more difficult. This would affect volcanic activity and risk our magnetic field. To maximise the chances of keeping our successful geology, the planet probably needs to be less than twice the mass of the Earth. Such a world would have a size 25 per cent larger than the Earth’s with over 50 per cent more surface area.
Even with this modest increase in mass, the planet’s topology would change under the stronger gravity. Our super-habitable super Earth would have a thicker atmosphere to combine with the boost in gravity and erode mountains into a flatter vista. This could change the oceans into shallow seas with long coastlines and small islands like those of the Earth’s archipelagos. Such locations on our planet are rich with biodiversity, so this scenario might be excellent at fostering life.
The thicker atmosphere might also permit a different gas composition. Since all multicellular organisms need oxygen, a boost in oxygen levels on a super-habitable planet could increase the options for life. But yet again, this needs care since if our current 21 per cent oxygen content rose to 35 per cent, we would suffer from runaway wildfires. So a lift in oxygen might help life, but too much would roast it.
While a slightly different star and planet mass might lead to a super-habitable world, it at first seems obvious that we would wish to keep Earth’s orbit. On a nearly circular path, the Earth avoids extreme climate fluctuations from variations in the received stellar radiation. But is this truly best for life? Life on Earth has developed assuming a constantly temperate climate. This makes the evolved creatures very sensitive to small changes. Due to the gravity of the Sun, Moon, Jupiter and Saturn, the Earth does experience subtle shifts in its orbit over tens of thousands of years. These are known as the Milankovitch Cycles and are like extremely long seasons. Although the change to the Earth’s orbit is only a few per cent, the Milankovitch Cycle is thought to have been a major trigger for ice ages in the last few million years. If a super-habitable world were permanently on a slightly eccentric orbit, life could develop that would be capable of handling such variations more easily. This would make longer-term perturbations due to sibling planets less deadly.
Our best of all possible worlds might therefore be an old super Earth looping an orange dwarf on a slightly elliptical path. Such a planet could be our best hope for detecting Earth-like surface life. However, what if a planet develops life that does not exist on the surface? We might not be able to find signs of such life forms on a distant world, but that does not mean they do not exist.
Notes
1. We met this bizarre state of matter in Chapter 7, as a possibility for the composition of 55 Cancri e.
2. This is also why the hottest part of the day on Earth is at about 2 p.m. rather than at noon, after the ground has had time to warm up.
3. This is the temperature at the top of the atmosphere from the stars’ heat. Below any gas, the temperature becomes much harder to estimate. Of course, since Kepler-16b is a gaseous world, there is no proper surface to consider.