Is Life On Earth Premature In The Universe?

It is generally thought that our bewitching and bewildering Universe was born about 13.8 billion years ago in the Big Bang, bouncing into existence from a tiny Patch that was as small as an elementary particle, that then expanded exponentially to attain macroscopic size in the merest fraction of a second. That strange, mysterious, and unimaginably tiny Patch was so extremely hot and dense that all that we are, and all that we will ever know, sprung into existence from it in the wild inflation of the Big Bang fireball. Spacetime has been expanding, and cooling off from this initial burst of faster-than-the-speed-of-light inflation ever since. But where did life on Earth come from, and are we alone in this mysterious Universe of ours–a Cosmos that is so undeniably weird that we may not even be able to imagine how genuinely weird it really is? In August 2016, scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, tried to answer one of the most intriguing questions of our existence, and their theoretical work proposes that present-day life on Earth may actually be premature from a Cosmic perspective.

Our Universe is almost 14 billion years old, while Earth formed only about 4.5 billion years ago. Some scientists suggest that this rather large time gap indicates that life on other worlds could be billions of years older than ours. However, Dr. Avi Loeb of the CfA, who is the lead author of the new study, proposes an answer to this very profound question of our very existence another way.

“If you ask, ‘When is life most likely to emerge?’ you might naively say, ‘Now’. But we find that the chance of life grows much higher in the distant future,” Dr. Loeb explained in an August 1, 2016 CfA Press Release.

Life as we know it became potentially possible approximately 30 million years after the Big Bang. This marks the time when the first generation of stars (Population III stars) began to seed the Universe with the necessary heavier atomic elements, such as carbon and oxygen, that paved the way for life to evolve out of non-living substances. Only hydrogen, helium, and small quantities of lithium were manufactured in the Big Bang itself (Big Bang nucleosynthesis). All of the atomic elements heavier than helium–that astronomers call metals–were produced in the searing-hot, roiling nuclear-fusing hearts of the stars in the Cosmos. The stars cooked up increasingly heavier and heavier atomic elements in their seething cores, but met their demise in the tragic and violent blast of supernovae explosions. The heaviest atomic elements of all–such as gold and uranium–were formed in the supernovae explosions that dramatically and furiously brought a massive star to that tragic end of the stellar road. The supernovae that heralded the explosive deaths of massive stars hurled the freshly formed metals into space, where they were incorporated into later generations of stars (Populations I and II). The heavier atomic elements, such as carbon and oxygen, that made life on our own planet possible, were manufactured by the stars. We are star-dust. Life could not have evolved on our Earth, or on other planets hosting life as we know it, if there were no stars to produce the heavier atomic elements.

Life in our Universe will probably come to an end in about 10 trillion years, or so. This will mark the time when the last lingering stars fade away and perish. Dr. Loeb and his team considered the relative probability of life existing between the two boundaries: 30 million years, when the first stars blasted themselves to pieces, seeding the Universe with the necessary newly forged elements enabling life to evolve; and 10 trillion years when the last lingering stars fade and burn out.

The primary determining factor proved to be the lifetime of a star. The greater the star’s mass, the shorter its life on the hydrogen burning main-sequence of the Hertzsprung-Russell Diagram of stellar evolution. Stars possessing masses that are about three times the mass of our Sun will die before they have a chance to evolve.

However, little lower mass stars that weigh-in at less than 10 percent solar-mass “live” much longer than their more massive stellar cousins. The smaller the star’s mass, the longer its life.

Twinkle, Twinkle Little Star

Stars that weigh-in at less than 10 percent of our Sun’s mass light up the Universe with their relatively cool fires for 10 trillion years. These little stars live long enough to give life enough time to emerge on any of the planets that they may host. Because of this, according to the CfA study, the probability of life increases as time goes by.

Our Universe was born barren–with none of the heavy metals that make life possible. The primordial Universe, that existed soon after the Big Bang, did not know oxygen, carbon, nitrogen, iron, and nickel–the atomic elements out of which we, and our entire familiar world, are composed. In the beginning, the neonatal Universe, that knew only the lightest of atomic elements–hydrogen, helium, and a pinch of lithium–was a lifeless expanse. The three lightest, and most ancient of atomic elements, were not exactly the necessary ingredients that could trigger the evolution of life as we know it on our world or on any other.

But, then, a wonderful event occurred–the first generation of stars were born, and these generally very massive stars fused enormous amounts of hydrogen–the lightest and most abundant of atomic elements–into helium, the second lightest of all atomic elements. The first stars then fused helium into oxygen, carbon, and nitrogen. Ultimately, after they had finished consuming their supply of helium, these ancient stars went on to cook up increasingly heavier and heavier atomic elements, creating nickel and silicon, all the way up to iron. The supernovae blasts themselves, that heralded the demise of massive stars, created all of the atomic elements heavier than iron. When these primeval extremely massive stars died, they left a lingering precious gift behind as a memorial to their now vanished existence. The ancient stars blessed the Universe with the ashes of creation. The newly formed heavier atomic elements were eventually recycled into later stellar generations, into the planets that orbited those more youthful stars, into moons circling those planets, and into life wherever it has managed to evolve and flourish–on our own Earth, and on a multitude of other worlds abundantly scattered throughout Space and Time.

Our Universe is a delightful mystery. It presents a profound challenge to all who seek to understand its many well-kept secrets–it is beautiful, complex, and mystifying. As conscious and aware living creatures, formed from the ancient dust of a myriad of fiery stars, we try to understand the weird Cosmos that is our home, and that we are a precious part of. It has been said before that we are the eyes of the Universe observing itself.

Scientists strongly suspect that we are not the only living creatures to dance happily around in our unimaginably vast Cosmos. We do not know how many big and joyful parties, celebrating life, have occurred, are occurring, and will occur in our bewitching swath of Spacetime.

The new multidisciplinary field of astrobiology–that combines such diverse scientific disciplines as astronomy and molecular biology into a single field of study–encourages scientists to try to find answers to the most profound questions of human existence on Earth: Where did we come from? and Are we alone? The Universe has kept its secrets well. As human space explorers start to hunt for life on other, distant worlds, both in our Solar System and beyond, they are only now first beginning to find some of the elusive answers to those haunting, profound mysteries. Geologists and geneticists, who have studied the origin and history of life on our own planet, can now use the valuable tools that they developed for this purpose, to search for possible life beyond Earth.

Is Life On Earth Premature In The Universe?

“So then you may ask, why aren’t we living in the future next to a low-mass star?” Dr. Loeb asked in the August 1, 2016 CfA Press Release. He went on to add that “One possibility is we’re premature. Another possibility is that the environment around a low-mass star is hazardous to life.”

Little low-mass stars, such as red dwarfs, are the most abundant type of true star in our Milky Way Galaxy, at least in the general neighborhood of our Sun. Indeed, according to some calculations, red dwarfs make up three-quarters of all the stars in our Milky Way. These very small stars evolve slowly, and take their time fusing heavier atomic elements from lighter ones. As a result, they can live for trillions of years on the hydrogen-burning main-sequence before they run out of fuel. Because of the comparatively short age of the Cosmos, there are no red dwarfs in existence at advanced stages of stellar evolution.

A large number of red dwarfs host exoplanets. However, giant planets like our own Solar System’s Jupiter are comparatively uncommon. It has been estimated that 40% of red dwarfs are orbited by a super-Earth. Super-Earths are a class of alien planet orbiting in the habitable zone surrounding their star where water can exist in its life-loving liquid state. The habitable zone is that “Goldilocks” region where temperatures are not too hot, not too cold, but just right for liquid water to exist. Life as we know it cannot evolve in the absence of liquid water.

Therefore, even though they are low in mass, red dwarf stars live for a long time. However, they also pose some unique dangers to their planetary offspring. When they were young and active, red dwarfs hurled out strong flares and ultraviolet radiation that could readily strip the atmosphere from any unfortunate rocky planet in its habitable zone.

In order to determine which possibility is the right one–the unique hazard presented by low-mass stars or our premature existence in the Cosmic scheme of things–Dr. Loeb recommends that astronomers study nearby red dwarf stars and their planet-children for hints of habitability. Space missions in the future, such as the Transiting Exoplanet Survey Satellite (TESS) and the James Webb Space Telescope (JWST) should help astronomers answer these important questions.

The paper describing this research is to be published in the Journal of Cosmology and Astroparticle Physics. The co-authors of the paper are Dr. Avi Loeb (CfA), and Dr. Rafael Batista and Dr. David Sloan (University of Oxford, in the UK).

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