Solar System, Galaxy, Universe: What’s the Difference?
A portion of Hubble Extreme Deep Field. Every spot and smudge in this image is a galaxy. Credit: NASA, ESA
Many people are not clear about the difference between our Solar System, our Milky Way Galaxy, and the Universe.
Let’s look at the basics.
Our Solar System consists of our star, the Sun, and its orbiting planets (including Earth), along with numerous moons, asteroids, comet material, rocks, and dust. Our Sun is just one star among the hundreds of billions of stars in our Milky Way Galaxy. If we shrink the Sun down to smaller than a grain of sand, we can imagine our Solar System to be small enough to fit onto the palm of your hand. Pluto would orbit about an inch from the middle of your palm.
On that scale with our Solar System in your hand, the Milky Way Galaxy, with its 200 – 400 billion stars, would span North America (see the illustration on the right). Galaxies come in many sizes. The Milky Way is big, but some galaxies, like our Andromeda Galaxy neighbor, are much larger.
The universe is all of the galaxies – billions of them! NASA’s telescopes allow us to study galaxies beyond our own in exquisite detail, and to explore the most distant reaches of the observable universe. The Hubble Space Telescope made one of the deepest images of the universe, called the Hubble Extreme Deep Field (image at the top of this article). Soon the James Webb Space Telescope will be exploring galaxies forming at the very beginning of the universe.
You are one of the billions of people on our Earth. Our Earth orbits the Sun in our Solar System. Our Sun is one star among the billions in the Milky Way Galaxy. Our Milky Way Galaxy is one among the billions of galaxies in our Universe. You are unique in the Universe!
You can observe objects in our solar system and even see other galaxies at a star party near you-and rest assured that everything you are seeing is a part of the same universe as you! Find out more by using our club and event finder and connect with your local astronomy club.
Discovering the Ultimate Source of Energy on Earth: The Sun! (Photo Credit: Warren Rupp Observatory)
How many solar systems are in our galaxy?
I have a question from our young friends at the Mountain Home Air Force Base Youth Activities Center in Mountain Home, Idaho. They wonder how many solar systems are in our galaxy. Well, I wish I knew the answer to this thought-provoking question, but not only do I not know, no one does. Even so, this question brings up some thrilling ideas.
For many years scientists have studied our own solar system. But until the last few years, we knew of no other solar systems.
This may seem surprising, as the Sun is one of about 200 billion stars (or perhaps more) just in the Milky Way galaxy alone. With all those other stars, why haven’t scientists studied other solar systems, at least enough to know how many are in our galaxy?
Using regular visible-light telescopes, planets are very hard to see in the glare of a star. Using infrared space telescopes, the planets show up much more clearly.
Well, the reason is that planets around other stars are really hard to find. Planets shine only by the light they reflect from the star they orbit, and they don’t reflect much light at that. And the stars, along with any planets under their control, are so far away that picking out a faint planet near a distant star is like spotting a mosquito next to a brilliant searchlight miles away.
Marc Rayman at age 14 meets astronomer Peter van de Kamp (center), who had “discovered” planets outside our solar system. On the right is radio astronomer Grote Reber. (Image from Sky and Telescope, Aug. 1971.)
So although scientists, philosophers, writers, and people like you who have been fascinated by the universe have thought about other solar systems for centuries, they haven’t had any to study. When I was young, this was one of many topics that I spent a great deal of time wondering about. In fact, when I was in the ninth grade, I was lucky enough to meet an astronomer who thought he had detected two planets around Barnard’s Star, one of the closest stars to our solar system. It was quite a thrill for me to meet someone involved in such an exciting work. Alas, later evidence suggested his conclusions were incorrect, but I learned a great deal about the subject, as well as about the scientific method, by studying what this impressive astronomer had accomplished.
Finally, in the middle 1990s, astronomers found strong evidence of planets around other stars. In all cases, they found the planets not by taking pictures of them, but rather by detecting their astonishingly gentle tugs on the stars they orbit. Although the star holds the planet tightly in its gravitational grip, the planet also exerts a gravitational pull back on the star, and that is what astronomers measure. It amounts to seeing the star wobble back and forth very slightly as the planet completes each orbit. Learn more about this gravitational dance as you try to solve the extraterrestrial riddle.
Brightness of star dips as planets passes between it and Earth.
After that, astronomers started detecting planets through several other methods as well. For example, if the orbit of a planet happens to be aligned so that planet occasionally travels in front of the star from our perspective on Earth, it blocks some of the light. Even though the planet is tiny compared to the star, extremely sensitive instruments can measure the tiny change in brightness. NASA’s Kepler mission used this technique to identify hundreds of stars that may have planets. Astronomers are observing these stars more carefully to confirm the presence of the candidate planets.
NASA is working on more space missions that will allow scientists not only to find other solar systems but also to study the planets there in greater detail. Some of the intriguing questions these missions might help answer are how common are other solar systems; is our solar system typical, with giant planets like Jupiter and smaller ones like Earth; how do solar systems form and evolve; are there other planets capable of supporting life; and is there life on other planets?
The artwork shows a steaming hot (with water!) planet discovered in another solar system.
So far, astronomers have found more than 500 solar systems and are discovering new ones every year. Given how many they have found in our own neighborhood of the Milky Way galaxy, scientists estimate that there may be tens of billions of solar systems in our galaxy, perhaps even as many as 100 billion.
Whether this estimate is correct and how similar other solar systems are to ours, remain to be seen. It has only been a few years since the first solar system apart from ours was detected, and they are still extremely difficult to study, so this whole subject is still in its infancy. By the time our friends who asked the questions are adults, we will know a great deal more.
Perhaps someday you will help find the answers. And even if you don’t, you may grow up in a time when humankind has a much clearer idea of how we and our home planet fit into the cosmos.