### Do all objects in the universe exert force on all other objects?

All objects in the universe exert a gravitational force on all others. But, the farther away an object is, the smaller is the force it exerts. (The force is inversely proportional to square of the distance from the object: e.g., twice as far away means only 1/4 the amount of force). Furthermore, the more massive the object, the greater the force it will exert on other objects (e.g., twice the mass means twice the force). So nearby, massive objects, dominate the gravitational force on something.

### Do the orbits of any of our planets change, or do the planets always follow the same paths?

In the solar system, the planets follow orbits determined *mainly* by the Sun's gravity --- since the Sun is the *most* massive object in the system (it is about 1000 times as massive as Jupiter, which is about 300 times more massive than Earth). If the Sun and Earth were the only things in the solar system, our planet's orbit would be an ellipse of virtually constant shape and orientation in space. However, because the other planets pull on Earth, our orbit does change slightly over time, in orientation and in shape. Same for the other planets' orbits. The biggest effect is that the orbit of a planet (which is nearly exaclty an ellipse) gradually shifts its orientation over time: the shifting is like how a circular hula-hoop (representing the orbit) shifts in location around the hula-hooper as the person shakes their hips (ask your teacher what a hula-hoop is). But the shifting of a planet's orbit is much slower than for the hula hoop. It takes many, many centuries for the Earth's orbit to shift entirely around once. These changes would be slow and small.

It is an active area of research whether or not the orbiting objects in the solar system can indefinitely orbit the Sun without ever undergoing *drastic* changes in their orbits --- they may actually change dramatically at some point: such changes would be called "chaotic" and the area of research is called "chaos theory". No new forces would be necessary to make this possibility happen; it would simply be the application of gravity once again. There are known examples of chaotic behavior in the solar system, but only involving a few small objects orbiting the outer planets, or in the asteroid belt. Chaotic behavior is defined as happening when very small differences in the initial or current conditions of an experiment, or of the solar system's motions (perhaps so small as to not be easily measured), would lead to drastically different results later in time.

Besides the planets in the solar system, it is even possible (over a much longer time scale) for a passing neighbor star to cause small changes in the planets' orbits. That's actually how comets, which otherwise have large orbits that never take them near the Sun, are caused to change orbit and pass near the Sun, allowing us to see them. If the incoming comet passes near Jupiter, it may be permanently moved into a small orbit that will keep it repeatedly passing near the Sun (once every 75 years, or so, for Halley's comet, for example). An example of chaos: in this case, if Jupiter is in just a slightly different position when the comet passes during its first fall toward the Sun, the comet may not end up in a small orbit due to Jupiter's gravitational pull, but may instead end up hitting the Sun! Or it might be "ejected" from the solar system entirely.

### Are the planets in our solar system likely to be destroyed when the sun finally burns out and will they be destroyed by a nuclear explosion or just drift off without the sun's gravitational pull?

When the Sun runs out of nuclear fuel in its core (center), its core will contract while the outer layers will expand, engulfing the inner planets: Mercury, Venus, Earth, and maybe Mars. The outer planets will likely survive this part, but later the Sun will undergo a rapid, explosive loss of its outer layers, which may destroy the outer planets as well. This is all scheduled to occur roughly 5 billion years from now. Never will the Sun lose enough mass to actually lose its gravitational grip on the planets, or the majority of the material that is in the planets (some of the "explosion" may push out some of the material).

### I looked around on the Internet but could not find a chart showing years and number of sunspots. Do you know where I might find this information?

Sunspot information can be found at the World Data Center-A for Solar-Terrestrial Physics. Click on "Solar and Interplanetary Phenomena" then, if you are specifically interested in sunspot numbers, click on "Sunspot Numbers".