Gazing at a starlit night sky is the surest way to make yourself feel small but, are you fully aware of what you are looking at? Join us as we explore the heavens to get a clearer understanding of solar systems, galaxies, and the universe. If you thought you felt small before, you have another thing coming.
The main difference is size. The universe is made up of all the galaxies in existence. Galaxies are made up of stars and planetary systems. Solar systems are planetary systems featuring planets, and other non-stellar objects, that orbit around certain stars, such as the Sun.
The fact that, in this space-crazed day and age, some people still use these terms interchangeably is rather surprising. This article will finally put the confusion to rest, in addition to equipping you with some interesting facts and figures that are guaranteed to wow you and your friends.
What defines the solar system?
From an earthling perspective, “the solar system” is the Sun, its associated planets, moons, and other natural objects floating up there. It is one of an incalculable number of “planetary systems” within the Milky Way Galaxy and the universe.
The two terms can, and often are, used interchangeably (i.e. referring to other planetary systems as “solar systems”). However, due to the fact that we live on Earth, we mostly use “THE solar system” to refer to our home planetary system.
The solar system features eight planets (R.I.P. Pluto), each with its own path, or orbit, around the Sun. There are four INNER planets (Mercury, Venus, Mars, and Earth), which are made of dense refractory materials, such as rock and metals. These four are referred to as “terrestrial planets” because they have solid ground surfaces.
The four OUTER planets, also known as the “giant planets” are Jupiter, Saturn, Uranus, and Neptune. The first two are mostly made of hydrogen and helium gases, while the latter two are made of ice. Each of the four features rings and several moons. Fun fact? Jupiter and Saturn, combined, are nearly 400 times heavier than the Earth. Collectively, these four planets make up almost NINETY-NINE PERCENT of all mass that is currently orbiting the Sun.
Like with most planetary systems, our solar system features other objects such as comets, meteors, natural satellites (moons), asteroids, and more. In fact, the inner and outer planet groups are separated by a giant asteroid belt that lies between the orbits of Mars and Jupiter.
Of course, central to our solar system is the Sun. The Sun is the star (yes, the Sun is a star) that facilitates the existence of our solar system and, by extension, all life on Earth. The word “solar” is derived from the Latin word solaris, which translates to “of the Sun”.
The Sun is a giant sphere of hot plasma that gives off electromagnetic radiation, heat, light, and energy. This awesome power is a result of nuclear fusion reactions that have been occurring for the entirety of the 4.6 billion-year-old star’s existence. The Sun contains a mind-blowing 99.86% of all mass in our solar system and it is over 330,000 times heavier than Earth. This massive size allows the Sun to exert a massive gravitational pull that forces the eight planets, and the asteroid belt, into perpetual orbit.
Other planetary systems may revolve around individual stars, like ours, or star systems.
What defines a galaxy?
According to Scientific American, galaxies are defined as “the fundamental units of a structure in the universe where stars form”. Personally, I prefer the definition presented by astronomers Linda Sparke and John Gallagher III in their book Galaxies in the Universe: An Introduction (Also available on Amazon). The two define a galaxy as “a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter.”
The term is a derivative of the Greek word galaxias, which translates to “milky”. This is because, for a long time, astronomers believed our home galaxy, the Milky Way Galaxy (which resembles a milky band of light), was the only galaxy out there. Some were even convinced that the Milky Way was the universe itself.
Sometime after the turn of the 20th century, it was established that our galaxy was, in fact, one of many. Astronomy icon Edwin Hubble, after whom the Hubble Space Telescope is named, was the man who confirmed this in 1924.
There are three categories of galaxies, each defined by appearance. There are spiral (like the Milky Way), irregular, and elliptical galaxies. Galaxies across all of these categories range in size as well. The smallest galaxies are called “dwarfs”, and these tend to have no more than a few hundred million stars. The largest (known) galaxies, known as giants or supergiants, can have hundreds of trillions of stars.
Our galaxy, the Milky Way is home to about 100-400 million stars, including the Sun. It has a visible diameter of 1.9 million light-years. This was historically considered to be a continuous band of light before Galileo Galilei used his telescope to determine that the light was made up of individual stars.
What defines the universe?
The universe, according to Brian Greene’s book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos, is “all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy”. The term is a derivative of univers, an Old French word for “one root”.
The overall size and scale of the universe are not known to us due to our limited technology. Modern telescopes may be highly advanced, but the ability to see the entire universe from space is still a very distant dream. For now, we have to settle for the observable universe. This is a spherical region (with an approximated 93 billion-lightyear diameter) that contains all matter that is observable from Earth, space probes, and telescopes.
The origin of the universe is as much a point of contention as to its physical extent. Cosmology’s best guess is the Big Bang theory, originally formulated by Belgian priest Georges Henri Lemaitre in 1927. The theory, in simple terms, suggests that the universe began as extremely dense and heated particles that expanded and cooled over time (approximately 13.8 billion years) to form our current observable universe.
In 1929, Edwin Hubble (yes, him again) discovered that neighboring galaxies were slowly drifting away from the Milky Way. While not conclusive, these findings went a considerable way towards proving Lemaitre’s expansion theory. The science community figured that if things in space were constantly moving apart, then there must have been a time when they were close together or even part of the same original entity.
What is the difference between a solar system, a galaxy, and the universe?
Now that we have taken a look at each term, we can identify the difference between the three much more easily.
The main difference between solar (or planetary) systems, galaxies, and the universe is the size. Our solar system is already mind-bogglingly vast, and it will be several decades (at least) before we even scratch the surface of discovery (we haven’t even sent humans to Mars yet).
As we established above, however, the solar system is just one planetary system revolving around one star out of several hundred billion in the Milky Way Galaxy. According to NASA, if the Sun was the size of a grain of sand, the Milky Way would be roughly the size of North America! Talk about scale.
If you want to feel smaller still, remember that our galaxy is far from the biggest. One of the biggest galaxies known to man, IC 1101, is at least FIFTY times larger than the Milky Way. The elliptical galaxy is categorized as a supergiant, and it is home to over 100 trillion stars.
IC 1101 is undoubtedly a titan in the heavens but even it is dwarfed by the all-encompassing, and incalculable, universe. The above sand grain and North America analogy is great and all, but the universe’s immeasurability means we can’t even devise a scale based on the wildest of guesses.
I would go as far as to say that if IC 1101 were the size of a grain of sand, then the universe would be the size of…the universe. Crazy, I know, but not as crazy as the fact that there as more galaxies in the universe than there are stars in the Milky Way.
Not only is the universe home to billions upon billions of galaxies. it also includes adjoining space dust and gas clouds known as nebulae. Some nebulae, like the Milky Way’s Orion, also serve as stellar nurseries, which facilitate the formation of new stars.
The best we can do, for now, is to measure the observable universe in relation to the known galaxies. For instance, the observable universe is approximately 620,000 times bigger than our home galaxy.
To the best of humanity’s knowledge, the universe is where space ends. However, there is a multiverse theory that addresses the possibility of multiple universes (or “parallel” universes), that are running concurrently as part of a larger multiverse. That topic is a little too hot for this article to handle today though. Besides, I’m sure you’re feeling tiny enough already.