Philosophers have been asking “Why are we here?” for thousands of years, kicking off intense debates about the ultimate meaning of life. But it was only in the 1970s that astronomers started asking “Why are we here,” by which they meant “Why is our Solar System in this part of the Milky Way and not somewhere else?”
Astronomers had known for decades that there are habitable zones around stars – regions where planets can get just enough heat from their hosts that liquid water might exist on their surfaces. The 1970s was the first time astronomers pondered the possibility that there are habitable zones in galaxies.
Since then, and especially over the last 20 years as thousands of exoplanets have been discovered throughout the Milky Way, astronomers have tried to figure out if our galactic neighborhood is special. Below are some of the reasons they think our neck of the woods might be particularly well suited for life.
Here on Earth, we’re protected from most of the Sun’s harmful radiation by our atmosphere, but a powerful supernova explosion could rip through those 300 miles of gas like tissue paper.
Supernovae are dramatic explosions caused by either the gravitational collapse of massive stars or the merger of any star with a white dwarf. In just one second, a supernova can produce the same amount of energy as our Sun in its entire 10 billion-year lifetime.
Some of that energy is released in the form of harmful gamma rays, which, if they interact with a planet’s atmosphere, can ionize the molecules and even kill organisms at the base of the food chain.
Scientists estimate that any planet within 50 light-years of a supernova explosion would be rendered mostly uninhabitable.
There’s not too much danger of that happening out here in the suburbs of the Milky Way, where our Sun sits 150 quadrillion miles from the galactic center. In this part of the galaxy, the nearest star massive enough to produce a supernova is more than 100 light-years away. Other parts of the galaxy are much denser, and planets there might not be so lucky as to go hundreds of millions of years without a supernova catastrophe.
The first elements to form after the Big Bang were hydrogen and helium, with some trace amounts of lithium. The earliest stars made of that primordial gas are called Population III stars, and as they evolved, they produced heavier elements in their cores. (Astronomers call all of those heavier elements “metals,” which must drive chemists mad.) After 10 million years or so, the first metals were released into the interstellar medium – the space between stars – and used to form the next generation of stars. This cycle of stars producing heavy elements, then releasing those elements into space to form new, metal-richer stars continues to this day.
The abundance of metals in a star is called its “metallicity,” and this has important implications for habitability. Planets are made of heavy elements like iron and magnesium, and life (at least on Earth) is built around carbon. A solar system without enough of these heavy elements is unlikely to support life. In fact, astronomers have found that high-metallicity stars (stars with a lot of metals) are more likely to host planets than their low-metallicity counterparts.
Different regions of the galaxy do have characteristic metallicities. Generally speaking, spiral galaxies like the Milky Way can be divided into three different regions: the bulge, a giant spherical clump of mostly old stars; the disk, a flat, rotating plane of stars and gas containing our characteristic spiral arms; and the dark matter halo, a cloud of invisible matter enshrouding us all. Other types of galaxies like ellipticals and irregulars don’t have as much structure. Our Sun orbits in the Milky Way’s disk, which has a higher average metallicity than both the bulge and the halo.
It would be a lot easier to study galactic habitability if stars didn’t move, but they do. As a star moves, it can interact with other stars or pass through dense, active regions that diminish the stellar system’s chances of hosting life.
Stars that lie in the Milky Way’s disk like the Sun does move in predictable circular (more or less) orbits around the galactic center, but orbits in the bulge aren’t so simple. Bulge orbits trace out complicated, rarely repeating patterns, overlapping with themselves and those of other stars. This is because mass in the two regions is distributed differently -- the disk is relatively flat, so gravity mostly acts to pull stars toward the galactic center, but the bulge is a messy sphere, so stars are pulled in multiple directions.
If you pair that with the fact that stars are much closer together in the bulge than they are in the disk, you realize that it’s much more likely for stars in the bulge to have close encounters. Not close enough that they actually collide, but enough that their competing gravitational pulls might destabilize any planets that might have formed.
That’s not to say that stars moving in the disk are totally safe. Rotating along with the Milky Way’s disk are its defining features: its spiral arms. Spiral arms are areas of active star formation that contain large, dense clouds of gas and dust, which means stars passing through are at high risk of supernova exposure and gravitational influences.
The Sun hit the jackpot when it comes to galactic motion.
So, is there a galactic habitable zone – a spot in the Milky Way where life is most likely to form? Astronomers don’t know for sure, but a team in Australia looked for places that met all of the criteria above and found that the best estimate for a GHZ is a ring of stars 4-8 billion years old that sit between 7 and 9 kiloparsecs from the galactic center.
Maybe it’s just a coincidence that our Sun falls perfectly within those bounds, or maybe we’re luckier than we ever imagined to find ourselves in the one part of the galaxy that could actually sustain life.
For more from Moiya McTier check out The Supercluster Podcast, where she describes the current state of Exoplanet research.