The Fate of Life on Earth ! Are you ready ?

In the latest articles, we have used our understanding of physics and cosmology to look back through time and watch as structure formed throughout the universe. We have peered into the future and seen the ultimate fate of the universe, from its enormous clusters of galaxies down to its tiniest components. Now we examine our own destiny and contemplate the fate that awaits Earth, humanity, and the star on which all life depends.

Eventually, the Sun Must Die

About 5 billion years from now, the Sun will end its long period of relative stability. Shedding its identity as the passive star that has nurtured life on Earth, the Sun will expand to become a red giant and later an asymptotic giant branch (AGB) star, swelling to hundreds of times its present size. The giant planets, orbiting outside the extended red giant atmosphere, may survive in some form. Even so, they will suffer the blistering radiation from a Sun grown thousands of times more luminous than it is today.

The terrestrial planets will not fare as well. Some perhaps all of the worlds of the inner Solar System will be engulfed by the expanding Sun. Just as an artifi cial satellite is slowed by drag in Earth’s tenuous outer atmosphere and eventually falls to the ground in a dazzling streak of white-hot light, so, too, will a terrestrial planet caught in the Sun’s atmosphere be consumed by the burgeoning star. If this is Earth’s fate, our home world will leave no trace other than a slight increase in the amount of massive elements in the Sun’s atmosphere. As the Sun loses more and more of its atmosphere in an AGB wind, our atoms may be expelled back into the reaches of interstellar space from which they came, perhaps to become incorporated into new generations of stars, planets, and even life itself.

Another planetary fate is possible, however. In this scenario, as the red giant Sun loses mass in a powerful wind, its gravitational grasp on the planets will weaken, and the orbits of both the inner and outer planets will spiral outward. If Earth moves out far enough, it may survive as a seared cinder, orbiting the white dwarf that the Sun will become. Barely larger than Earth and with its nuclear fuel exhausted, the white dwarf Sun will slowly cool, eventually becoming a cold, inert sphere of degenerate carbon, orbited by what remains of its retinue of planets. Thus the ultimate outcome for our Earth consumed in the heart of the Sun or left behind as a frigid, burned rock orbiting a long-dead white dwarf is not yet certain.

The Future of Life on Earth

Life on our planet has even less of a future, for it will not survive long enough to witness the Sun’s departure from the main sequence. Well before that cataclysmic event takes place, the Sun’s luminosity will begin to rise. As solar luminosity increases, so will temperatures on all the planets, including our own.

Eventually Earth’s temperatures will climb so high that all animal and plant life will perish. Even the extremophiles that inhabit the oceanic depths will die as the oceans boil away. Models of the Sun’s evolution are still too imprecise to predict with certainty when that fatal event will occur, but the end of all terrestrial life may be only 1 billion or 2 billion years away. That is, of course, a comfortably distant time from now, but it is well short of the Sun’s departure from the main sequence.

It is far from certain, however, that the descendants of today’s humanity will even be around a billion years from now. Some of the threats that await us come from beyond Earth. For the remainder of the Sun’s life, the terrestrial planets, including Earth, will continue to be bombarded by asteroids and comets. 

Perhaps a hundred or more of these impacts will involve kilometer-sized objects, capable of causing the kind of devastation that eradicated the dinosaurs and most other species 65 million years ago. Although these events may create new surface scars, they will have little effect on the integrity of Earth itself. Earth’s geological record is fi lled with such events, and each time they happen, life manages to recover and reorganize.

"Far-future Earth will be consumed by the Sun or left as an icy cinder."

It seems likely, then, that some form of life will survive to see the Sun begin its march toward instability. However, individual species do not necessarily fare so well when faced with cosmic cataclysm. If the descendants of humankind survive, it will be because we chose to become players in the game by changing the odds of such planetwide biological upheavals. We are rapidly developing technology that could enable us to detect most threatening asteroids and modify their orbits well before they could strike Earth. Comets are more diffi cult to guard against because long-period comets appear from the outer Solar System with little warning. To offer protection, various means of defending ourselves would have to be in place, ready to be used on very short notice. So far, we have been slow to take such threats seriously. 

Although impacts from kilometer-sized objects are infrequent, objects only a few dozen meters in size, carrying the punch of a several-megaton bomb, strike Earth about once every 100 years. We might protect ourselves from the fate of the dinosaurs, but in the long run the descendants of humanity will either leave this world or die out. Planetary systems surround other stars, and all that we know tells us that many other Earth-like planets should exist throughout our galaxy. Colonizing other planets is currently the stuff of science fi ction, but if our descendants are ultimately to survive the death of our home planet, off-Earth colonization must become science fact at some point in the future.

Although humankind may soon be capable of protecting Earth from life-threatening comet and asteroid impacts, in other ways we are our own worst enemy. We are poisoning the atmosphere, the water, and the land that form the habitat for all terrestrial life. As our population grows unchecked, we are occupying more and more of Earth’s land and consuming more and more of its resources while sending thousands of species of plants and animals to their extinction each year. At the same time, human activities are dramatically affecting the balances of atmospheric gases. The climate and ecosystem of Earth constitute a finely balanced, complex system capable of exhibiting chaotic behavior. 

The fossil record shows that Earth has undergone sudden and dramatic climatic changes in response to even minor perturbations. Such drastic changes in the overall balance of nature would certainly have consequences for our own survival. For the first time in human history, we possess the means to unleash nuclear or biological disasters that could threaten the very survival of our species. In the end, the fate of humanity will depend more than anything on whether we accept stewardship of ourselves and of our planet.

Figure 1 is the very famous “pale blue dot” picture, taken by the Voyager 1 spacecraft from beyond the orbit of Neptune at a distance of more than 40 AU. The beams in the picture are sunlight scattered off the spacecraft. The arrow points to a dot, which is Earth the only place in the entire universe where life is confirmed to exist. Compare the size of that dot to the size of the universe. 

Fig 1. This image from the Voyager 1 spacecraft

shows the Earth from a distance of 3.8 billion miles, well

past the orbit of Neptune. The streaks in this image are

scattered sunlight. The “pale blue dot” in the rightmost

streak is Earth.

Compare the history of life on Earth to the history of the universe. Compare Earth’s future with the fate of the universe. Astronomy is humbling. We are so incredibly tiny. And yet we are so incredibly unique, as far as we know. Think for a moment about what that means to you. This may be the most important lesson the universe has to teach us.

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