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?
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.
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 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.
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?
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.