It’s one of the biggest questions in science: How common (or rare) is the story of life here on Earth? This question gets at the very heart of answering whether or not we’re alone in the universe. If we’re here to ask this question due to a long string of fortunate coincidences, then we are probably effectively alone in the vast expanse of time and space. If, however, life can find more than one path to sentience and intelligent civilizations can somehow survive technological adolescence on geological timescales, our galaxy might be teeming with life. While estimates suggest that there may be millions of Earth-like planets out there, one astrophysicist created a buzz in 2016 when he suggested Earth might be a 1-in-700-quintillion type of place. We only need to look at Venus in our own solar system to realize that “Earth-sized” doesn’t necessarily mean “Earth-like.” Here are eight reasons that our place in time and space is special.
8. Earth is Just the Right Distance from the Sun
This is the so-called “Goldilocks” scenario: Life as we know it needs water, and Earth exists in a place where it’s not too hot, and not too cold for liquid water to exist on its surface. In fact, Earth is the only world in our solar system where water can exist in liquid form on its surface. Where all of this water came from is still a bit of a mystery. Looking at the neighboring worlds of Mars, Venus and Mercury inside the frost line, we see planets that are bone dry. Astronomers theorize that comets delivered the bulk of the primordial water to the Earth, but even that idea has recently been put to the test by the European Space Agency’s Rosetta mission to Comet 67/P Churyumov-Gerasimenko, which found the ratio of deuterium (a heavy-water isotope) on the comet differs from seawater on Earth. Perhaps, Earth’s life-giving oceans came from another source, or somehow formed in situ with the planet itself.
7. Jupiter, Saturn Protect Earth from Comet Impacts
We also occupy a safe haven in the inner solar system, thanks to some cosmic goaltenders orbiting farther out. Any incoming comet entering our solar system stands a 40 percent chance of having its orbit modified by the large gas giant planet Jupiter, and to a lesser extent, Saturn. Comet Hale-Bopp, for example, entered the solar system on a 4,200-year orbit, but departed on a shorter, 2,380-year path around the Sun. Of course, this deflection method isn’t perfect: one only has to look at the record of cosmic impacts on the Earthward face of the Moon to realize that the Earth does get pummeled periodically. Just ask the dinosaurs (if they were still around) what the result of not having an asteroid detection and space program might be.
On the flip side, occasional extinction-level impacts might be a good thing, as evolution is driven by death and time. The Chicxulub impact 65 million years ago, while catastrophic, gave our field mouse mammalian ancestors a chance.
6. Moon Stabilizes Earth’s Rotation, Started Evolution
What would we do without Earth’s Moon? The Earth-Moon system is an oddball in the solar system. Measuring 27 percent the diameter of the Earth, no other moon in our solar system is as large relative to its primary planet. The Moon is also the result of a large impact with a Mars-sized body, about 4.5 billion years ago, just 60 million years after the formation of the solar system itself. The pull of Earth’s Moon does a great job at stabilizing our rotational axis, which varies from 22.1 to 24.5 degrees (the current value is 23.44 degrees) over 41,000 years. This tilt causes the seasons. Without the Moon’s influence, that variation would be much larger. Mars, which has no large moon, sees its axis vary from 13 degrees to 40 degrees. That kind of variation would be catastrophic to Earth’s climate — and life — over long spans of time.
The tides are also believed to be responsible for helping spur the development of life. The Moon is the dominant driver of the tides, and allowed for selecting out aquatic animals that could live for extended periods on land, stranded in the intertidal zone.
5. Earth’s Magnetic Field Protects From Space Weather
Thank Earth’s swirling molten core for creating our magnetic field. Otherwise, we’d be scorched by solar storms and cosmic rays. This is another feature that Mars and Venus lack. We live, in a sense, inside the outer atmosphere of our host star, and the protective sheath of our planetary magnetosphere makes this possible. Not only is ionizing radiation deadly to life, but it can strip away lighter gasses in a planetary atmosphere as well, as the solar wind has done on Mars. Ancient life also reworked the atmosphere of the Earth, allowing for the protective layer of ozone we enjoy today.
4. Earth May be in Galaxy’s Habitable Zone
The galaxy may have a sort of habitable zone as well, a place that’s not riddled with radiation like the galactic core, or metal poor (in astronomy, a “metal” is any element heavier than hydrogen or helium) like the stars along the outer rim. Also, life needs a nice, placid place to form, an area where supernovae aren’t exploding every few million years or so within the 25 light-year “kill zone,” sterilizing the planet with lethal radiation. The bright star Betelgeuse in Orion is one of the closest suspects for the next nearby supernova and is located 640 light years away. Ironically, there’s good evidence in the isotopic record seen in ancient meteorites to suggest that early supernovae detonations near our solar system may have actually seeded Earth with several crucial life-giving elements.
3. Earth Has Water Now, But the End is Coming
Ironically, hydrogen, the most common element in the universe, is rare on Earth, and in finite supply. Most of the hydrogen is locked up in the planet’s oceans as H2O, and circulates between the sea, atmosphere and the land in the water cycle. Life on Earth needs water, especially to power photosynthesis. The Sun is increasing in luminosity by a factor of about 1% every 100 million years, meaning that we’ll also lose most of this precious hydrogen to space from the upper atmosphere in about a billion years … perhaps, though, hardy bacteria far below the Earth’s surface will get by for another billion years beyond that. What would be amazing to see will be what evolution cooks up in those final days, as organisms adapt to attempt to get by on what little water is still available.
2. The Universe’s ‘Stelliferous Era’ Makes Life Possible
Life arises from complexity. The four basic elements of life on Earth are hydrogen and oxygen (as in water), nitrogen, and most importantly, carbon. Carbon likes to form long molecule chains and is so complex, it has a branch of organic chemistry all its own. Incidentally, the very best way to irritate an organic chemist is to refer to a grocery store item as “organic,” as all food (except salt) is “organic.” That life-giving carbon comes from the hearts of stars. We live in what’s termed the “Stelliferous Era,” a time 13.7 billion years after the Big Bang when stars can fuse hydrogen into heavier elements … but it won’t always be so. Life on Earth arose just over four billion years ago, and our Sun will become a Red Giant star in about 5.4 billion years. We know this from studying other stars at various life-cycle stages. It will spend longer still (several trillion years, longer than the current age of the universe) as a cooling super-dense ember known as a white dwarf. Such is the fate of most stars, and the ultimate heat death of the universe will be hard for life to escape.
1. Humans Dodged Extinction at End of Last Ice Age
If you’re reading this, congratulations: you (and your entire species of modern homo sapiens) have already won the cosmic lottery. Homo habilis, Neanderthals and other hominid relatives weren’t so lucky, and like 99.9 percent of the species of life that ever lived on Earth, went extinct. For whatever reason at the end of the last Ice Age 12,000 years ago, humanity dodged a bullet. There are estimates that suggest the human population may have been as low as 15,000 around the time of the eruption of the Toba super volcano 70,000 years ago, triggering a prehistoric “nuclear winter.” We all carry the evidence for this drama in markers in our DNA. Humans have a relatively low genetic diversity compared to other primates, suggesting that the current population all descended from a small group of common ancestors in the relatively recent past. Those lucky few selected for survival seem to have had the smarts to gain a technological edge then decided, after millions of years of a hard-scrabble life as hunters and gathers, to start building villages and cities and begin farming and domesticating animals.
Were our ancestors just lucky in the evolutionary lottery, or were they all dancing around a monolith like in 2001: A Space Odyssey? We then went from mastering fire to iPhones, Walmarts and radio telescopes in just over 10,000 years, a geological blink of an eye. There’s even a recent suggestion that the modern era should be called the anthropocene or even the technocene, to highlight the human impact on the environment.
Are we truly rare in the cosmos? Perhaps getting single-celled life started is simple, and the jump to complex life is hard. Life on Earth was happy in its single-celled stage for 80 percent of its current 4.1 billion year run. Perhaps, we’re currently the only space-faring civilization on the scene in our Milky Way galaxy, and we are effectively alone. Maybe we’re simply putting the cart before the horse, and as science fiction author Douglas Adams put it, we’re mud simply marveling at how well it fits the puddle of dirt it’s in. But if we really are alone, think of the burden this places on us; we owe it to our ancestors, the ones that didn’t quit and bothered to leave offspring along the way, to continue to fight the good fight against entropy until the very end.