Hi again everyone! Sorry this post is a bit late... but better late than never!
Wow, time sure has flown by! I can't believe that this is my last research round of the year! Anyways, I hope you all enjoy as I wrap up my subject regarding life on earth as I study the habitable zones of galaxies, as well as how different types of galaxies might be able to support life.
First, if you haven't read my previous post, I highly suggest going back and checking it out as most if not all of this post's content will be based off of the research that I did earlier.
Now, as we now know, there are many different variations of galaxies, with many different defining qualities that make them incredibly unique. We also know that these galaxies can be identified by their supermassive blackholes, which can determine whether or not they will become an active galaxy. So, seeing as all galaxies are different, isn't it logical to assume that each galaxy has a different probability rate of supporting life? A research team lead by Pratika Dayal of the University of Durham asked this same question, determining that the habitability of a galaxy depends on three primary factors:
- Total number of stars
More stars means more planets!
- Metallicity of the stars
Planets are more likely to form in stellar vicinities with higher metallicities, since planet formation requires elements heavier than iron.
- Likelihood of Type II supernovae nearby
Planets that are located out of range of supernovae have a higher probability of being habitable, since a major dose of cosmic radiation is likely to cause mass extinctions or delay evolution of complex life. Galaxies’ supernova rates can be estimated from their star formation rates (the two are connected via the initial mass function). (1)
These three conditions have been found to be linked via something termed the “fundamental metallicity relation,” which links the total stellar masses, metallicities, and star formation rates of galaxies. By using this relation, researchers are able to make predictions regarding the number of habitable planets in more than 100,000 galaxies in the local universe, which could result in our first real insights into where we should be looking to identify extraterrestrial life. (2)
Based on these predictions, we have found that the galaxies likely to host the largest number of habitable planets are those that have a mass greater than twice that of the Milky Way and star formation rates less than a tenth of that of the Milky Way. These galaxies tend to be giant elliptical galaxies, rather than compact spirals like our own galaxy. Scientists calculate that the most hospitable galaxies can host up to 10,000 times as many Earth-like planets and 1,000,000 times as many gas-giants (which might have habitable moons) as the Milky Way. (3)
But, what specific zones of a galaxy, specifically a galaxy like ours, make life possible? Well, let's first briefly discuss why life in our solar system is possible. First off, our sun has the right quantities and the right kinds of metals, our sun's orbit around the Milky Way's galactic center is such that it keeps us out of the way our galaxy's spiral arms, and we are just far away enough from Sagittarius A (our galactic center) that we are in no danger of being affected by disruptive gravitational forces or unsafe amounts of radiation. All of these factors create what we call a "galactic habitable zone", a term coined by Assistant Professor of Astronomy at the University of Washington Guillermo Gonzalez, which implies that select regions of select galaxies could be capable of supporting life due to very unique and specific conditions. (4)
Our Milky Way Galaxy is structured much like billions of other spiral galaxies. The galactic disk contains a lot of interstellar matter (like dust and gas), as well as young and intermediate-age stars. While young stars can be found scattered throughout the galaxy, the stellar population tends to be older in the bulge around the galactic center. Many of these older stars are gathered together into globular clusters, which orbit the nucleus of the Galaxy in a region known as the galactic “halo.” Strong emissions of infrared radiation and X-rays from the galactic center suggest that there are clouds of ionized gas rapidly moving around some sort of supermassive object, which we now know is a blackhole. (5)
There are billions of stars in the Milky Way Galaxy, and some are more metal-rich than others. Part of this is a condition of age: the older a star, the more metal-poor it tends to be. That’s because the most ancient stars formed from just hydrogen, helium, and lithium, as at that time, shortly after the Big Bang, elements had not yet fused to create the many elements that we know today. These fusions occurred when the most massive of these stars exploded, and when nuclear reactions converted light elements into heavier ones. These heavier “metals” became part of the raw material from which a second generation of stars formed. Each stellar explosion led to a greater abundance of available metals. A metal-rich star, therefore, has material that came from many previous generations of stars.
Our Sun is unusually metal-rich for a star of its age and type, and scientists aren’t sure why. It could be that the Sun formed in a part of the Galaxy that had an abundance of metals, and then migrated to its present position, but it still remains an anomaly among the scientific community.
Based on studies of extrasolar planets, metal-rich stars are more likely to have planets orbiting around them. One reason for this may be that a certain minimum amount of metals is needed to form rocky bodies (including the cores of the gas giant planets). A metal-rich interstellar cloud that collapses to form a star would therefore be more likely to form planets than would a metal-poor cloud. (6)
Besides requiring a metal-rich star, a Galactic Habitable Zone excludes stars too close to the galactic center. Our Sun is a nice distance away from the galactic center, about 28,000 light years.
Being in the outer region of the Galaxy protects our Solar System from the huge gravitational pull of stars clustered near the bulge by the galactic center. If we were closer in, the combined gravity of all those stars would shift the orbit of comets in the Oort cloud. The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next star - between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers). This cloud, which circles the outer perimeter of our Solar System, contains trillions of comets. The gravitational disturbances caused by other stars would send many of those comets in our direction – increasing the rate of comet impacts and endangering – if not eventually wiping out – life on Earth. (7)
Staying away from the galactic center has an additional advantage. The center of the Galaxy is awash in harmful radiation. Solar systems near the center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet.
Keeping out of the way of the Galaxy’s spiral arms is another requirement of the Galactic Habitable Zone. The density of gases and interstellar matter in the spiral arms leads to the formation of new stars. Although these spiral arms are the birthplaces of stars, it would be dangerous for our solar system to cross through one of them. The intense radiation and gravitation of a spiral arm would cause disruptions in our Solar System just as surely as if we were closer to the center of the Galaxy.
Luckily, our Sun revolves at the same rate as the Galaxy’s spiral-arm rotation. This synchronization prevents our Solar System from crossing a spiral arm too often. At our location, our orbital period is very similar to that of the pattern speed of the spiral arms. This means that the time interval between spiral arm crossings will be a maximum, which is a good thing, since spiral arms are dangerous places. Massive star supernovae are concentrated there, and giant molecular clouds can perturb the Oort cloud comets leading to more comets showers in the inner solar system.” (7,8)
The unusually circular orbit of our Sun around the galactic center also tends to keep it clear of the spiral arms. Most stars the same age as our Sun have more elliptical orbits. Thus, thanks to a lot of unusual characteristics of our Sun, our Solar System is lucky enough to lie in a Galactic Habitable Zone. More than 95 percent of stars in the Galaxy wouldn’t be able to support habitable planets simply because their rotation is not synchronized with the rotation of the galaxy’s spiral arms. Add all the other factors involved in keeping a solar system habitable, and it seems that the odds of finding another solar system in a Galactic Habitable Zone are close to impossible. (3,8,9)
So there you go! We've now determined that life on Earth isn't nearly as simple as it appears. I hope that you all learned something from this, and that next time you think about extraterrestrial life, you realize that despite the fact that complex life is rare, it's almost certainly out there!
Thanks for reading!