The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

How many miles long is the solar system from end to end?
Well, it depends a bit on how you define the "end." If you choose Pluto, the outermost planet, as the end, then the diameter of the solar system is 80 A.U. A.U. means "astronomical unit"; then the distance between Earth and the sun equals 93,000,000 miles. So that would be 7,440,000,000 (over seven billion) miles. If we say that the end of the solar system is the cloud of comets that surrounds our solar system, then the diameter is roughly 60,000 A.U., which is 5,580,000,000,000 (over five trillion) miles. It's a big universe!

How long would it take for a rocket ship to visit all the planets in our solar system?
Well, it depends on how fast the rocket ship can go. The Voyager 2 spacecraft was launched in August 1977. It reached Jupiter in July 1979, almost two years later. It passed Saturn in August 1981, another two years later. Then it reached Uranus in January 1986, and Neptune in August 1989. So this spacecraft took 12 years to go most of the way across our solar system.

How were the planets formed?
The planets were formed when the sun was formed. Stars such as our sun are formed when a cloud of gas and dust collapses from its own gravity. The center tends to form first, with a disk of material around it. The material in the disk is in orbit and starts clumping up. Small clumps sweep through the disk, becoming larger and larger. The larger a clump gets, the more gravity it has, so it can sweep up more stuff to make the clump larger, which gives it more gravity, and on and on. Meanwhile the center, which has most of the stuff, forms the young sun. The clumps form the planets.

Is it possible to count the number of stars in our solar system?
When we talk about our solar system, we usually mean the sun and planets. But I bet you are thinking about the Milky Way (not the candy bar!), which is full of stars. There are so many that we can't count each one, but we can make a good guess. Here is how we can estimate how many there are: Suppose you would like to count all the trees in a forest, but you can't see the whole forest — because there are too many trees! But you can count how many trees there are in the acre near you. And you can measure how big the forest is, or how many acres it is. So the number of trees in one acre multiplied by the number of acres equals the total number of trees. We do the same thing. We can count the number of stars within, say, 100 light-years of our sun. We can estimate how many light-years it is across our galaxy. That's how we estimate that there are about ten billion stars in our galaxy!

How big is the Milky Way?
Pretty big! Astronomers don't use miles to measure many distances because they are too large. We use other kinds of "yardsticks" like the light-year. A light-year is the distance that light travels in one year, which is equal to 5,676,000,000,000 miles! We think that our galaxy, the Milky Way, is about 100,000 light-years across.

Why are the orbits of celestial bodies not circular?
If gravity were a rope, connecting a planet to the sun, then the planet would have to orbit the sun in a circle. But it isn't. It is a force. It pulls harder when the planet and sun are closer together, and less when they are far apart.

I can't think of a convincing way to explain it or an analogy. But what about an experiment? Suppose you had a strong magnet and some steel balls. You can make the balls "orbit" the magnet in a way similar to planets orbiting the sun. The orbits may be pretty circular or they may be very elliptical. If you have several balls with very elliptical orbits, they will crash into each other. So only if they are in fairly circular orbits can they keep orbiting the magnet for a long time.

Similarly one reason that the planets have fairly circular orbits is so that they don't crash into each other! If they did, they would have been torn apart long ago. But the orbits don't have to be perfect circles. Of course this really goes back to the formation of the solar system, when there were millions of "planetesimals" (hunks of rock and ice) in a disk around the sun. They crashed into each other, sometimes being torn apart but sometimes sticking together to form larger and larger bodies. The large bodies had stronger gravity and so collected more and more rock and ice, growing and growing until they became planets. So the orbits of the planets come from that time when these swarms of rocks and iceballs orbited the sun. The stuff that was in elliptical orbits tended to get torn apart, while the stuff in fairly circular orbits was captured by the proto-planets.

Why is it cold in space, but if you travel too close to the sun, you'll burn and disintegrate?
On Earth, there are two things that keep us warm: one is the warm air; the other is the light. The light of the sun or a fire striking our skin gives off heat. In space, there is no air, warm or cold. The only source of heat is sunlight. A satellite or the shuttle in orbit around Earth is very hot where the sun shines on it and very cold on the side in shadow. For instance, they like to turn the space shuttle around every few hours to warm the other side. In fact in the last shuttle mission, the astronauts were trying out new spacesuits that will help them stay warm when they are working in the cold, shaded side of the shuttle. They had problems staying warm in the old suits when they were in the shade too long. If you get closer to the sun, the light is brighter and the heat is greater. If a satellite (or a comet or an asteroid) gets too close, it can melt away!

Why does outer space appear all black even when the sun is out?
For you to see something, light must be reflected from that something to your eyes. If there is nothing there, the sunlight doesn't reflect and there is nothing to see.

What's it like in space?
I've never been there, but the IUE satellite that I work on is there and its sensors tell us a lot. You know, of course, that there is no air in space. In fact, it is very close to being a vacuum. Because it is a vacuum, there is nothing to protect you against either heat or cold. On our satellite, one side is always in the sun — it is always very hot. The other side is always in the dark — it is very cold. I don't remember the temperatures exactly, but they are something like +100° Fahrenheit on the sunlit side and 100° Fahrenheit on the dark side. This is a big temperature strain that spacecraft engineers have to take into account when building a satellite.

The sky looks very black, because there is no air to scatter sunlight about to make it look blue (or any other color). The stars are bright points of light —smaller yet brighter because there is no obscuring, blurring atmosphere. Our satellite, the IUE, is in synchronous orbit, about 24,000 miles up, so it is outside the earth's magnetic field. That means it is exposed to the radiation (electrons, protons, ions, cosmic rays) that the sun emits. Our cameras are shielded but they are still affected by some of the electrons and protons.

One of the effects that this has on our satellite is that it is pushed around a bit by the "solar wind." The solar wind is the stream of particles, mostly electrons and protons, blown out by the sun. They are not numerous — it is still pretty close to being a vacuum by Earth standards — but they move pretty fast. Since our satellite has big solar panels, the solar wind tends to push on it. If we didn't have a system to keep the pointing of the satellite stable, the solar wind would push it into a slow spinning motion. Also, we are protected by Earth's ozone layer from the sun's ultraviolet light. In space you would not be protected — so one side of you would get quite a sunburn really fast! Of course there is virtually no gravity — I won't say none because if you are in orbit around a planet then there must be a little gravity to keep you in orbit! This is why they now talk about "microgravity" experiments on the shuttle. Finally, in space there is no sound. There is no air to transmit the sound waves. So all those great sounds in the Star Wars and Star Trek movies are not realistic. In space, no one can hear you scream.

Why do some planets have more moons than others?
For planets, the bigger you are, the bigger your gravity is. If you have stronger gravity, then you can grab onto more moons. Jupiter and Saturn are the biggest planets, so they have the most moons.

Which planet has the most moons? Is it still Saturn?
Yes, it is still Saturn. The Voyager spacecrafts discovered many of the moons. These moons are smaller than the ones we could see from Earth. So we probably won't discover anymore until another spacecraft visits Saturn.

What was in space before the solar system?
Before our sun and its planets were formed, there were other stars (and probably planets) in our galaxy. We think that our galaxy has been around for roughly 15 billion years, but the sun is "only" five billion years old.

Which planet was the first to be discovered? Who discovered it? What kind of equipment did they use?
Five planets have been known since ancient times — Mercury, Venus, Mars, Jupiter, and Saturn. The first new planet discovered was Uranus. It was discovered by the English astronomer Sir William Herschel in 1781. Herschel was one of the first modern astronomers. His patron was King George III of England (the same King George from the time of the American Revolution!). Herschel wanted to name the planet after King George, but nobody else liked that so they gave it the name Uranus.

Herschel and his sister Charlotte (who was an astronomer in her own right) used several reflecting telescopes, some of the first based on a design invented by Sir Isaac Newton. The largest was over 40 feet long and had a mirror 48 inches across. It was held up with a framework of wood, and they had to have helpers move it around using ropes and pulleys. It was the largest telescope in the world until over 100 years later.

Is there another planet like ours?
The planet (which we know of) that is most similar to Earth is Mars. As you probably know, Mars is smaller, colder, and has less atmosphere. But it does seem to have some water (mostly ice) and on a really hot day can get to 80° Fahrenheit.

How many known solar systems exist?
Just recently astronomers have found three stars with planets! You may heard about 51 Pegasi a few years ago. They are part of a project to carefully examine stars that are similar to our sun to see if there are any planets. We know there are planets because their gravity affects the motion of the stars. We can't see the small, dim planets directly. For each of the three stars, we have detected a big Jupiter-like planet. We are very excited about this. More stars are being examined, so hopefully more planets will be discovered soon!

Are there any solar systems with two or more stars?
It has been hard for astronomers to find other solar systems — that is, planets around other stars. Only in the last two years have we been able to find some that we really believe exist. So far I think there are about two dozen stars that we believe have planets. To my knowledge none of these systems has more than one star in it, but that may be because they have not looked at double-star systems. So stay tuned — we are still looking!

Have people found other solar systems with life on the planets?
So far no — we have not found life anywhere but on Earth. We are also having a hard time finding planets around other stars. The problems are first, distances are so great. Second, the star is very much brighter than any planet would be.

Will the earth ever collide with another planet?
I think that it is extremely unlikely. All the planets in the solar system are in stable orbits, meaning that they shouldn't change by much. The only orbits that cross are Neptune and Pluto, and even there it has been shown that the two planets move in such a way that they will never collide. It is, of course, possible that a comet or asteroid could hit Earth — it has apparently happened in the distant past — but that is another story.

Describe some recent discoveries in the solar system.
One recent discovery in our solar system that I find interesting is that there are TWO places where comets seem to come from. One is the Oort cloud, a region out beyond the solar system where comets orbit the sun. They are so far away they are too dark to see, but every once in a while one of these comets falls in toward the sun. This is where many of the comets we know have come from.

But recently we have discovered that there is another place. This is known as the Kuiper belt. It is a region between Saturn and Pluto where some comets and other objects are in orbit. The brightest of these new objects is called Chiron. It seems like a comet, but if it is, then it is the biggest comet known.

Do scientists think there might be a tenth planet in our solar system?
For some time, astronomers have tried various ways to search for a tenth planet. But we now think that there are no new planets. One big reason is that the Infrared Astronomy Satellite, known as IRAS, did a very good map of the sky in infrared light. If any new planet were out there, IRAS should have found it. But it didn't. So we think that there are no more planets in our solar system.

We heard on the news the discovery of two new planets. Can you give us any details?
As you may know, astronomers have been trying to find planets around other stars for some time. Only recently has the technology gotten good enough that we seem to be able to find some for sure. Each of the two new planets orbits a star that is pretty similar to our sun. One star is 70 Virginis (in the constellation Virgo) and the other is 47 Ursae Majoris (in Ursa Major, also called the Big Dipper).

The planet circling 70 Vir is about nine times bigger than Jupiter. It orbits its star once every 116 days, which means it is closer to its star than Earth is to our sun. The discoverers calculated that the planet's temperature would be about 185° Fahrenheit — kind of hot!

The planet circling 47 UMa is about three times the size of Jupiter and orbits its star once every 1,100 days. That puts it about twice as far from its star as Earth is from the sun. It would be a bit cold, about 100° Fahrenheit.

Both of these stars can be seen with the naked eye. But even the astronomers have not seen the planets! What we found was a wobble in the star's position that is due to the gravitational pull of the planet. That takes some pretty careful measurements to determine!

Can scientists see the new planets? If not, how do we know they are there?
Actually we can't see the new planets at all right now! We know they are there because of the effects that they have on their stars. Gravity holds a star and a planet together. It is like two kids holding hands and swinging each other around. One kid (the star) is really big, and the other kid (the planet) is small. So when they swing each other, the little kid swings around the most. But the big kid is swung around a little too. We discovered the planets by seeing that the stars are being swung around a little bit. We can even tell how big the planets are and how far away they are by carefully measuring the small movements of the stars. But we can't see the planets because they are too small and dim compared to their stars. Maybe some day we will have telescopes that will allow us to see the planets.

Do any of the new planets have rings?
We don't know whether those new planets have rings, we can't see the planets directly. But all of the big-gas planets in our solar system have rings. So maybe those new planets, which are also pretty big, also have rings. When I was in school, the only planet that we knew had rings was Saturn! We didn't know about the rings around Jupiter, Uranus, and Neptune until the 1980s.

What is an ice volcano?
Sounds crazy, doesn't it? But many of the moons around Jupiter, Saturn, Uranus, and Neptune are made up of icy material. If an area of ice under the crust is heated, it can push up and make a volcano, with slightly melted ice flowing down the sides.

Which characteristic of our solar system do you consider the most interesting and mysterious? Why?
Some people have found new types of objects, like big comets, in orbit beyond Saturn. Do they come from a big cloud surrounding our solar system, as many people believe? I think these small bodies and comets are interesting because stuff like this probably crashed into the young Earth and helped to make our oceans and atmosphere.