Better worlds than Earth / Renee Heller

Other planets, largely different from our Earth, may be the best homes for life in the universe 

Are we living in the best of all possible worlds? The German mathematician Gottfried Leibniz thought so, writing in 1710 that our planet, for all its shortcomings, must be the best world imaginable. Leibniz's idea was ridiculed and called unscientific wishful thinking, especially by the French writer Voltaire, in his masterpiece, "Candide". However, Leibniz's idea may receive support from at least one group of scientists: the astronomers. For decades they have seen the earth as a touchstone by which they search for worlds outside our solar system.

A large and rocky "life-rich" world, the holiday around a star smaller than our sun, may be foreign and familiar at the same time. Its landscape will be flatter due to higher gravity on its surface, and the plants there may be a darker shade than the green on Earth, to more effectively absorb the weak starlight.

At the moment, we Earthlings still only know one and only one living world, our world, so it makes some sense to use it as a template by which to look for life elsewhere, such as for example in the regions most similar to Earth on the surface of Mars or on the watery moon of Jupiter, Europe. And now, when planets are discovered that revolve around stars other than our Sun and in principle allow life, they challenge this Earth-centered approach.

During the last twenty years, astronomers have found more than 1,800 planets outside the solar system, called exoplanets, and the statistical calculation shows that our galaxy is home to at least another 100 billion such planets. Of the worlds found so far, only a few are very similar to Earth. In practice, they show a truly tremendous variety of characteristics: they differ to an enormous extent from each other in their orbits, size and composition, and they circle around different and strange stars, including stars that are considerably smaller and fainter than our Sun. The diverse characteristics of these planets point to the possibility, in my opinion and to others, that the Earth may be far from claiming the crown of the most habitable planet. In fact, it is possible that some planets outside the solar system, which are very different from our planet, have much higher chances of creating and maintaining stable life systems. Those worlds, which seem to have an enhanced capacity to support life, may be the most worthy targets for the search for alien life, outside the solar system.

An imperfect planet
It is understood that our planet has several properties that seem, at first glance, ideal for life. The Earth revolved around a middle-aged star, calm and quiet, which has been shining steadily for billions of years and which provided plenty of time for life to form and develop. It has oceans of life-supporting water, mainly because its orbit around the Sun passes within the "habitat zone," a narrow enclosure where the light coming from our star is neither too strong nor too weak. Closer to the Sun, the water on the surface of the planet will evaporate and turn into steam, and further away it will freeze into ice. Earth is also blessed with a life-friendly size: large enough to hold onto a dense enough atmosphere with its gravitational field, but small enough to ensure that gravity doesn't pull in an airtight, suffocating blanket of gas to envelop the planet. The size of the Earth and its rocky composition are an arena suitable for other life-encouraging factors, such as plate tectonics that regulates the climate and a magnetic field that protects life from harmful cosmic radiation.

However, the more we scientists look more closely at how our planet supports life, the less ideal it seems. Nowadays, the Earth's ability to support life varies greatly from region to region, so that large parts of its surface are devoid of life: the arid deserts, the open oceans where nutrients are scarce, and the frozen polar regions. Earth's support for life also changes over time. Consider, for example, that during most of the Paleozoic Coal Age, a period of time that began about 350 million years ago and lasted about 50 million years, the planet's atmosphere was warmer, wetter, and much richer in oxygen than it is today. Crabs, fish and coral reefs thrived in the sea waters, great forests covered the lands, and insects and other land creatures grew to gigantic proportions. Earth as coal may have supported a much larger biomass than exists today, and this means that today's Earth can be seen as a less life-supporting planet than it was in ancient times.

Moreover, we know that the Earth will be much less friendly to life in the future. In about five billion years, our sun will use up almost all of its hydrogen fuel and helium fusion processes, which are much more energetic, will begin to occur in its core. As a result, the sun will swell and become a "red giant" type star that will burn the earth to ashes. However, long before that the end of life on earth will come. The temperature in the core of the sun containing the hydrogen reserves will rise gradually, and will lead to a slow increase in the rate of its total energy emission and its brightness will increase by about 10% every billion years. The meaning of this change is that the inhabited area around the sun is not a fixed area but changes over time. As this region moves further and further away from our ever-brightening Sun, it will eventually leave Earth behind. And that's not all: recent calculations show that the Earth is not in the center of the habitable zone but closer to its inner rim, teetering on the edge of the boundary from which it will overheat.

As a result, in about half a billion years our Sun will be bright enough to create a fiery climate on Earth that will threaten the survival of complex multicellular life. In about 1.75 billion years from now, the steadily increasing brightness of the Sun will make our world so hot that the oceans will evaporate and wipe out any remaining simple life on its surface. In fact, the Earth is long past its prime to support life, and the life on it is rapidly advancing toward its final stretch. The bottom line: It seems plausible to say that the Earth today is a planet whose capacity to carry life is limited.

The search for a more life-friendly world

In 2012 I first began to ponder the question of what worlds more suitable for life might look like. At that time I was researching the capacity of heavy moons, the holidays around gas giants, to support life. The largest moon in our solar system is Jupiter's moon Ganymede, whose mass is only 2.5% of Earth's mass, meaning it is too small to have an atmosphere similar to ours. But it dawned on me that there are plausible ways that moons approaching the mass of Earth could form in other planetary systems. Such moons can orbit giant planets located in their star's habitable zones. Under these conditions they may have atmospheres similar to the one on our planet.

Such heavy "exomoons" could support life better than Earth because they could offer a rich variety of energy sources for a possible biosphere. Unlike the Earth, whose main source of energy is sunlight, the biosphere of such a moon may also draw energy from the light reflected from the nearby giant planet, from the heat emitted from it or even from its strong gravitational field. As a moon orbits a giant planet, tidal forces can cause its crust to bend back and forth, creating friction that heats the moon's interior. Apparently, it is this phenomenon of tidal heating that creates the underground oceans found, so it is assumed, on the moon "Europa" orbiting Jupiter and on the moon Enceladus orbiting Saturn. However, this variety of energies could be a double-edged sword, as a slight disruption in the balance between the overlapping energy sources could easily push such a world into a state where it could not support life.

So far, no extrasolar moons have been identified with certainty, although it is possible that some such moons will be discovered sooner or later in archived data from satellites such as NASA's Kepler Space Telescope. Currently, the existence of such objects and the possibility that they will be found to be capable of carrying life, remain only a hypothesis.

On the other hand, it is possible that in the list of planets outside the solar system whose existence has been confirmed or is a candidate for confirmation, there are some that can apparently support life. The first exoplanets discovered in the mid-90s were all gas giants with a mass similar to that of Jupiter and whose orbits were too close to their stars to support life. However, over the years, with the improvement in techniques, astronomers began to find smaller planets and more and more planets in wider and more convenient orbits. Most of the planets that have been discovered in recent years are of the so-called "super-Earth" type, that is, planets whose mass is up to ten times greater than the Earth's, and whose radius ranges between the radius of the Earth and the radius of Neptune. It turned out that such planets are very common around other stars, but there is not even one such planet around our sun, so our solar system can look somewhat like an unusual anomaly.

In many cases, the radii of planets of this type suggest a thick, inflated atmosphere, so they are more likely to be some sort of "mini-Neptune" rather than extra-large versions of Earth. However, it is possible that some of the smaller worlds, about twice the size of the Earth, do indeed have a composition of iron and rock similar to that of the Earth and may have an abundance of liquid water on their surface, if they orbit within the habitable zone of their star. We now know that some of these planets, slightly heavier than the Earth, which may turn out to be rocky, orbit stars called Nancy-M and K-Nancy, which are smaller than our Sun, dimmer than it and have a much longer lifespan than it. In part because of the extended lifespan of their tiny star, those extra-large Earths are the most compelling candidates today to be worlds with enhanced life-support capabilities, as I showed in a model I recently built with my colleague John Armstrong, a physicist at Weber County State University in Utah, in the state of Bara. "B.

Longevity benefits
In our work, we started from the premise that a life-extending planet is the most fundamental element that gives its planets an upgraded ability to support life. After all, life on a planet's surface is unlikely to survive after its sun goes out. Our Sun is 4.6 billion years old, near the middle of its estimated lifespan of about 10 billion years. But if it were a little smaller, it would be defined as a much longer-lived K-dwarf star. The total amount of nuclear fuel that K-dwarfs can burn is less than the amount available to more massive stars, but they use their fuel more efficiently and therefore have longer lifespans. The middle-aged K-dwarfs we see today are billions of years older than our Sun, and will continue to shine for billions of years after our Sun has passed away. The life assumed based on the planets surrounding it will therefore have much more time to develop and spread.

The light of a K-dwarf would appear slightly redder than sunlight but its spectral range would be able to support photosynthesis on the surface of a planet. M-dwarfs are even smaller and more economical stars and can shine steadily for hundreds of billions of years, but their light is so dim that their habitable zone is very close to them. At such a distance, the planets may be exposed to stellar eruptions and other dangerous phenomena. So it seems that in terms of stellar life-sustaining conditions, K-dwarfs have the winning combination: their lifespans are longer than our Sun's, yet they are not treacherously dim.

Today, some of those long-lived stars may be surrounded by rocky super-Earths that are already several billion years older than our solar system. Life may have originated in those planetary systems long before our sun was born, and thrived and evolved over billions of years even before the first biomolecule emerged from the primordial soup on the young Earth. One of the most fascinating things to me is the possibility that life on ancient worlds might have been able to make adjustments to their global environment to further enhance the habitability of the planet, as life on Earth did. One of the main examples of this is the event known as the "oxygen catastrophe", which occurred approximately 2.4 billion years ago, when significant amounts of oxygen began to accumulate in the Earth's atmosphere for the first time. The oxygen came, apparently, from marine algae and finally led to the evolution of a metabolism that devours more energy, thanks to which creatures could develop a larger, more durable and active body. This progress was a crucial step towards the gradual departure of life from the Earth's oceans to colonize the continents. If similar trends of environmental upgrading are found in alien life systems, we can expect that the capacity of holiday planets around long-lived stars to support life will improve as they age.

But for the holiday planets around small, long-lived stars to support life better than Earth, they would have to be heavier than Earth. The extra mass could herald a cure for the two most likely dangers that threaten aging rocky planets. If our Earth were located in the habitable zone of a small K-dwarf, then the interior of the planet would cool long before the star reached its end. This would disrupt his ability to support life. For example, a planet's internal heat drives volcanic eruptions and plate tectonics, processes that replenish and recycle the greenhouse gas carbon dioxide in the atmosphere. If it weren't for these processes, the amount of atmospheric CO2 on the planet would be constantly decreasing because the rains would wash the gas out of the air and transfer it to the rocks. In the end, the global greenhouse effect, which depends on CO2, would be inhibited, raising the probability that an Earth-like planet would enter a "snowball" state where all the water on its surface would freeze and life would not be able to continue to exist on it.

Besides the possible cancellation of the greenhouse effect that warms the planet, if this aging rocky world cools from the inside, the magnetic field surrounding it may also cancel out and stop protecting it. The Earth has an armor in the form of a magnetic field created by a swirling and swirling core of molten iron, acting like a dynamo. The core remains liquid because of heat left in it from the time when the planet was formed, and because of the decay of radioactive isotopes. As soon as the internal heat bath of a rocky planet is emptied, its core shrinks, the dynamo stops and the magnetic shield falls and no longer prevents cosmic radiation and stellar eruptions from eroding the upper atmosphere and invading the planet's territory. As a result, old Earth-like planets are expected to lose much of their atmosphere to space. And if there is life on them, the increased levels of harmful radiation will damage them.

Rocky super-Earths that are up to twice the size of our planet should age more gracefully than Earth, because their vastly greater mass will help preserve their internal heat for much longer. But planets three to five times the mass of Earth might be too large to have plate tectonics, because the pressures and viscosities in their mantle would be so high that they would prevent the heat from flowing out as much as needed. A rocky planet with only twice the mass of Earth would still have plate tectonics and would be able to maintain its geological cycles and magnetic field for several billion years longer than Earth. Such a planet would also have a diameter about 25% larger than the diameter of the Earth, so that the living space that would be available to any organisms would be about 56% larger than ours.

Life on a super-Earth is full of life
What would such a planet look like? The higher gravity on Earth's surface would tend to give the super-Earth a slightly denser atmosphere than ours, and the mountains on it would drift at a faster rate. In other words, such a planet would have relatively denser air and a flatter surface. If there were oceans there, then the flat earthscape of the planet would cause the water to be contained in a large number of shallow seas where island chains would be scattered, rather than in huge subterranean basins whose continuity is interrupted only by a few large continents [see box on pages 36 and 37]. Just as biodiversity in the Earth's oceans is richer in shallow waters near coastlines, such an "island world" would provide a huge advantage to life. It is also possible that evolution will proceed at a faster rate in isolated island ecosystems, and thus they may be able to provide another boost to biodiversity.

Of course, without large continents, the total area that would be available to terrestrial life in the hypothetical island world would be smaller than in a terrestrial world, and it is possible that such a situation would lower the overall capacity to support life. But not necessarily, especially in view of the fact that the areas in the center of continents tend to become desolate deserts, because they are far from humid and temperate sea air. Moreover, the life-supporting surface area of ​​a planet can be greatly affected by the angle of inclination of its axis of rotation relative to its plane of motion around the star. On Earth, for example, this angle is about 23.4 degrees, and this inclination creates the seasons and moderates the temperature differences between the warmer equatorial regions and the cold polar regions, without which they would be very extreme. Compared to Earth, on an island world with an optimal rotation axis inclination both the equatorial and polar regions could be warm, without glaciers, and thanks to its larger dimensions and greater surface area on it, it could possibly boast a greater amount of life-supporting land than if it had large continents .

If we combine all these reflections regarding the important characteristics of life, they will see that life-rich worlds may be a little larger than the Earth, and that the stars around them are a little smaller and fainter than the Sun. If this conclusion is correct, this is especially exciting news for astronomers, because at interstellar distances, such planets are much easier to identify and study than identical twins of our Earth-Sun system. Currently, the statistics of exoplanet surveys show that such systems of slightly heavier planets orbiting small stars are considerably more common throughout our galaxy than their solar system counterparts. It thus appears that astronomers have many more exciting places to hunt for life than we have so far thought.

One of the most important findings of the Kepler space telescope, the planet Kepler-186f, attracts attention in this context. The diameter of this world, whose identification was announced in April 2014, is 11% larger than the diameter of the Earth, and it is most likely a rocky world orbiting an M-dwarf within its habitable zone. It is probably several billion years old, and maybe even older than the Earth. It is located at a distance of about 500 light years, that is, it is not accessible to observations that can be made today or in the near future and reduce the space of predictions about its capacity to support life, but as far as we know, it could be an island world rich in life.

Closer candidates orbiting nearby small stars could soon be discovered by one of a variety of projects, most notably the European Space Agency's PLATO mission, scheduled for launch in 2024. Such nearby systems could be early targets for the James Webb Space Telescope, scheduled for launch in 2018, which will search for signs of life in the atmospheres of a small number of worlds that may have an enhanced capacity to support life. With enough luck, we may all soon be able to point to some spot in the sky where a more perfect world exists.

in brief
Astronomers are looking for Earth twins orbiting sun-like stars.
Identifying such planets requires us to stretch our technology to the limit.
In contrast, larger planets orbiting smaller stars are easier to detect and may be very common.
Rethinking raises the possibility that such systems, along with massive moons orbiting giant planets, may also encourage the formation of life more than our familiar planet.

Earth vs. Super-Earth

Why heavy earths are good for life
Astronomers looking for life around other stars are increasingly focusing on super-Earths: planets larger than our own planet, which is up to 10 times the mass of Earth, but still smaller than gas giants and therefore possibly rocky. Planets with just twice the mass of Earth are particularly promising targets, as they have certain properties that could make them friendlier to life than our planet.

life on earth
Our planet is successful in many respects. Earth, in the "exactly right" zone of a middle-aged quiescent planet, boasts a global ocean that, despite its depth, is still shallow enough to allow for dry land for life to inhabit. It is large enough to have a decent atmosphere, but small enough to avoid the accumulation of layers of gas that would suffocate life. Thanks to Earth's predominantly rocky composition, it also holds enough internal heat to sustain plate tectonics that stabilizes the climate, and it has a magnetic field that protects the planet.

Life on a super life-friendly earth
A life-rich planet, roughly twice the mass of Earth, would have stronger surface gravity, and therefore a thicker atmosphere, drift-accelerating weather, and flatter topography. The result could be a world of shallow seas dotted with island chains, unlike the more familiar world of deep oceans and large continents. Such geography could be an advantage for life, in light of the fact that island chains scattered on the surface of the earth are among the densest and most biologically diverse areas on the planet. However, the most important features that make such a planet friendly to life are bound up deep within the planet's core.

A heart that never stops

A rocky super-Earth, whose mass is twice that of the Earth, will retain within it a considerable amount of heat, both residual heat from the period of its formation and from the decay of radioactive isotopes. This heat bath could create a molten, swirling core similar to Earth's core, but much longer-lived. This core will create a powerful magnetic field around the planet, protecting the atmosphere and surface from destructive cosmic rays.

Continuous carbon cycle
The more swirling core heat inside a super-Earth could help it maintain volcanism and plate tectonics for longer than Earth. These processes are essential for regulating the planet's carbon cycle, and in any case, for regulating its climate. Volcanoes spew heat-trapping carbon dioxide into the atmosphere, and the falling rains slowly wash it back into the rocks. Plate tectonics moves these rocks into the interior of the planet, where the carbon dioxide "cooks" and finally returns to the air through volcanic activity. Theoretical models show that super-Earths with a mass between three and five times the mass of Earth may be too large to have plate tectonics, so worlds with a mass of only two times the mass of Earth are better candidates for life.

Steady sunlight
Regardless of the qualities of the planet, its ability to support life depends mainly on the planet around which it is orbiting. Stars smaller than the Sun burn their nuclear fuel more efficiently, and can continue to exist for eons upon eons longer than the Sun, so their planets have much more time to develop immune life. Tiny, dim stars known as K-dwarfs can shine for tens of billions of years, in contrast to the estimated lifespan of our Sun, which is about 10 billion years, and so they find the golden path between providing a sufficient amount of light and an extended lifespan. Small super-Earths holidaying in the Nancy-K habitation zone may have the winning combination of living conditions.

Life passes in the sun
As stars age, they increase the heat on their planets
In human standards of time, the habitable zone of a planet appears static. But because stars get brighter as they age, this region drifts out over the ages, eventually leaving the life-bearing worlds behind. Earth is located near the inner limit of the Sun's habitable zone, and will be too hot to support liquid water in about 1.75 billion years. Smaller stars shine more dimly and for a longer period of time than the Sun, so their habitable zone remains almost stationary for tens of billions of years, a situation that may increase the lifespan of their planets.

More on the subject
Habitable Climates: The Influence of Obliquity. David S. Spiegel, Kristen Menou and Caleb A. Scharf in Astrophysical Journal, Vol. 691, no. 1, pages 596-610; January 20, 2009.
http://iopscience.iop.org/0004-637X/691/1/596/article
Exomoon Habitability Constrained by Illumination and Tidal Heating. René Heller and Rory Barnes in Astrobiology, Vol. 13, no. 1, pages 18-46; 2013. http://arxiv.org/abs/1209.5323
Habitable Zone Lifetimes of Exoplanets around Main Sequence Stars. Andrew J. Rushby, Mark W. Claire, Hugh Osborn and Andrew J. Watson in Astrobiology, Vol. 13, no. 9, pages 833-849; September 18, 2013.
Superhabitable Worlds. René Heller and John Armstrong in Astrobiology, Vol. 14, no. 1, pages 50–66; January 16, 2014. http://arxiv.org/abs/1401.2392
The breaking dawn over a distant sky. Michael D. Lemonique, Scientific American Israel, October-November 2013 issue. http://www.sciam.co.il/archives/6999
Post to Twitter

on the notebook
Rene Heller is a postdoctoral fellow at the Origins Institute at McMaster University in Ontario, and a member of the Canadian Astrobiologist Training Program (CATP). His research focuses on the formation of extrasolar moons, their orbital evolution, detections and the possibility of life on them. He holds the unofficial title of German Rice Pudding World Champion.

The article was published with the permission of Scientific American Israel