Author: Ethan Siegel / Source: Forbes
While there are hundreds of billions of stars in our galaxy, many with Earth-sized planets at the right distance for liquid water on their surface, there are great chances for life all throughout the Milky Way.
At least, that’s what we assume. But isn’t it possible that the conditions we have at our location make us very special as far as life surviving and thriving the way it has here on Earth? That’s what Tayte Taliaferro wants to know, as she asks:[W]hat would happen if our solar system had formed a little farther up the arm of the galaxy? What would happen if we were at the tip of the arm? What if, theoretically, instead of the humongous black hole in the center of our galaxy, our solar system was there? Would there be major climate difference[s]? Would we be able to survive?
Let’s look at how different things would be.
Here in our Solar System, we’re relatively well-informed about how it all broke down over the past 4.5 billion years. A molecular cloud of gas with a certain amount of enrichment — containing about 2% heavy elements by mass, along with ~28% helium and ~70% hydrogen — collapsed, giving rise to new stars. One of those would be our Sun, which formed a protoplanetary disk around it, like practically all stars do.
Over millions to tens-of-millions of years, the hot Sun boiled off the material in the inner disk, while the outer, cooler material then fell in and accreted around the pre-existing cores. The most massive, giant worlds held onto large amounts of the lightest elements (hydrogen and helium), while the smaller, rocky worlds did not. Gravitational interactions did the rest, and determined the Solar System we arrived at today.
There are many properties about Earth and our Solar System that appear special, but they may not be necessary for life. Unlike all the other rocky planets in our Solar System, Earth has a giant moon, which causes the tides and keeps our axial tilt stable. Unlike many other Solar Systems, ours has a large gas giant — Jupiter — just slightly beyond the location where our asteroid belt exists. And unlike the majority of stars in the galaxy, we’re located on the outskirts of a spur of a spiral arm, some 25,000 light years away from the galactic center.
For 4.5 billion years on Earth, life has continued to survive and evolve, developing more complexity, diversity, and with more information encoded in its DNA. We’ve made it through a large number of mass extinction events, most of which have unknown or only speculatively known causes. Although anywhere from 30% to perhaps 70% of the species on our world have been wiped out at various times, most recently from a giant asteroid strike just 65 million years ago, life on Earth has never faltered. As time has continued on, so has the presence of biological activity on our planet.
Of all the traits that Earth has, though, which ones are absolutely necessary for life? And which ones would lead to a planet where the history of life told a different story than ours, but where everything was still possible?
Until we actually find life beyond Earth, in planets that lie beyond our Solar System, questions like this will inevitably be based in speculation. But this isn’t mere guesswork; these are the theoretical statements we…
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