John Borg, age 10, of Salt Lake City, Utah, for his question:

How many solar systems are there?

Until modern times, experts were sure that ours was the only solar system in the entire universe. But this idea was dropped as we learned more about how our sun and its family of planets were formed. Nowadays, scientists suspect that other starry suns were born more or less as our sun was born, in which case, a lot of other stars have planets and perhaps moons like ours.

At present, we know that two or three other starry suns have at least one planet and so we know that there are other solar systems. The rest is guesswork. But modern scientists suspect that most of the stars may be solar systems, perhaps somewhat like ours. Some of them figure that there may be millions of other solar systems in our Milky Way galaxy. And outside the Milky Way there are thousands of other galaxies. Most likely they all have millions of solar systems also. If this guesswork is correct, the number of solar systems in the universe is too big to count.