For many years scientists have studied our solar system, consisting of the Sun plus 9* planets, as well as dozens of moons, hundreds of known comets, and thousands of known asteroids. But until the last few years, we knew of no other solar systems. This may seem surprising, as the Sun is one of around 200 billion stars 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 really really hard to find. As you may have heard me talk about in a previous month, the 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 we can't pick out a faint planet near a distant star any more than you could make out a firefly 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. You can learn more about this gravitational dance by visiting [here] The Space Place Web site at spaceplace.nasa.gov. Go to Space Science in Action, and try to solve the extraterrestrial riddle.

NASA is working on several 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 about 70 solar systems and are discovering new ones every year. Given how many they have found in this neighborhood of the Milky Way galaxy, scientists estimate that there may be many billions of solar systems in our galaxy. 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, 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.

Thanks again for calling Dr. Marc at The Space Place.

* On August 17, 2006, the International Astronomical Union redefined "planet," so that Pluto no longer qualifies. It is now called a "dwarf planet," and the solar system has only 8 planets.