• Grades: 3–5

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

What does Pluto look like?
So far we don't have a very detailed picture of Pluto, because none of our spacecraft have gotten very close to it. But we can tell that it is very icy. If you were standing on Pluto, it would probably look like a vast, bleak expanse of ice and snow. The sun would be very far away and the sky (which has no air) would be black, so it would be rather dark. I hope you brought warm clothes!

How far is Pluto from the sun?
Pluto is a looooong way from the sun. Earth is about 93,000,000 miles from the sun, but Pluto is almost 40 times as far. That's almost four BILLION miles away. The sun must look pretty dim from so far away.

How cold is Pluto?
Pretty cold! About –390° Fahrenheit. You can't get much colder than that!

Why is Pluto so cold and Earth so warm?
Pluto is 40 times farther from the sun than Earth. That makes a BIG difference!

How cold is it beyond Pluto?
You asked how cold it is beyond Pluto. The surface of Pluto is about –390° Fahrenheit. That is really cold! Beyond Pluto, as you get further and further from the sun, it gets even colder! Out in space very far from any star, the temperature can get nearly down to what is called "absolute zero" — the coldest anything can get. That would be –460° Fahrenheit on your thermometer!

What do Pluto's gases look like?
Most of the time Pluto's gases are frozen out as ice (methane ice as well as water ice). When they warm up, they probably look like a very icy fog. Since no one has been to Pluto, I'm guessing here!

When was the first time anyone saw Pluto?
Pluto was discovered in 1930 by an American astronomer named Clyde Tombaugh. He had been searching for a new planet for several years, based on the predictions of Percival Lowell, who also funded the search for what he called "Planet X."

How many miles around is Pluto?
If you could put a belt around the middle of Pluto (like the earth's equator), it would be only 4,400 miles around. It is smaller than Earth — it is even smaller than our moon! So it is not only the planet furthest from the sun, it is also the smallest.

What is the latest size estimate of Pluto?
Thanks to the 1978 discovery of a moon orbiting Pluto, named Charon, we know the size and mass of Pluto. The Hubble Space Telescope has also taken images of them. Pluto's mass is 0.0021 of Earth's, so it is the smallest planet in the solar system. Its diameter is 2,300 km (Charon's is 1,186 km).

How many moons does Pluto have and what are their names?
Pluto has one moon, which is called Charon. It's pretty big compared to Pluto — you could almost call them a double planet!

What is the layer of ice on Pluto composed of?
Nearly everything is frozen on Pluto, so the ice is made up of a lot of stuff. But it is mostly water ice and methane ice.

If Pluto is so far from the sun and so cold, how can there be water ice on it?
Water (usually in the form of ice) is actually found in many places around the solar system. The ice caps on Mars are largely water ice. There are ice crystals in the clouds of Jupiter. There is frozen water ice in comets. And there is water ice on Pluto. It's just REALLY COLD ICE! The earth is pretty unusual because our water is mostly in the liquid form (also water vapor in the air and water ice at the North and South Poles, of course).

Where did Pluto get its name from? Who named it and why?
The planets have traditionally been given names of gods from Roman mythology. For instance, Jupiter, the chief god, is the name given to the biggest planet. Since Pluto is the furthest, darkest, coldest planet, it was given the name of the god of the underworld, where the Romans thought the souls of the dead went after they died. The name also honors Percival Lowell, who was responsible for the search for the new planet. His initials, PL, are the first two letters of the planet's name, Pluto.

Could we see more stars if we were on Pluto?
On Pluto, the sun would be further away and so it would be dimmer. That would make it somewhat easier to see the stars. But I don't think we would see more stars. Once we get away from Earth's atmosphere (say into orbit around Earth), the sky is very dark and we can see the stars pretty well. So that part would be about the same whether we were in orbit around Earth (like the Hubble Space Telescope) or on Pluto. What matters most is what kind of telescope you have — right now the Hubble Space Telescope can see fainter stars (and more stars) than any other telescope.

Can Pluto have a ring that we do not know about?
Maybe. The best pictures we have of Pluto and its moon, Charon, are from the Hubble Space Telescope. If there were some thin rings, we might not be able to see them.

Is Pluto's moon as big as Pluto?
No, it is smaller. Pluto is 1,381 miles in diameter, and its moon, Charon, is 712 miles in diameter. So it is smaller, but not a lot smaller.

Is Pluto a planet?
That is an area of debate recently! As you may know, ever since Pluto was discovered in 1930 we have called it a planet. But recently there have been some small objects found in our solar system that blur the distinction between planet, asteroid, and comet! The first of these objects that was found is called Charon. Its orbit takes it from inside the orbit of Saturn out to near Uranus's orbit. That is further out than the asteroid belt, also it has to be pretty big just to be found way out there! A recent estimate is that it is 200 km (120 miles) across. This was a big surprise to many people. It was even more of a surprise when it developed a fuzzy appearance as if it were a comet! That would make it the biggest comet ever known.

Now we know that there are other such objects out in the solar system. Pluto does have some of the characteristics of these objects. It has an elliptical orbit, it is small (as planets go), and it can develop a fuzzy look when it is nearest the sun when the frozen gases on its surface warm up and evaporate. So is Pluto in this special class of objects? If so, what do we call them? Are they planets, comets, or something else? How do we draw the distinctions between them? For now, I call Pluto a planet, but the debate continues!

Will we ever go to Pluto?
We can send a space probe to Pluto. In fact, some people at NASA are hoping to build a spacecraft just to go to Pluto. I don't think they have the final go-ahead yet, but I think that there is a good chance that they will be able to do it.

Could Neptune and Pluto crash together when their orbits cross?
You would think so, but the answer turns out to be no. They are in special orbits so that the planets both keep away from each other. After all, they have been in these orbits probably for over four billion years, so if they were going to crash into each other they should have already done it!

When Neptune and Pluto were orbiting, did they change places?
No, they didn't change places. Their paths crossed. This is like two kids running, one in a circle, and the other in an oval.

Pluto is orbiting in an oval and for a while has come closer to the sun than Neptune (which is going in a circle). But most of the time Pluto is further away.

Is Pluto one of Neptune's escaped moons rather than an actual planet?
Recently some astronomers have shown that this is probably not so. They calculated where each of the planets was in their orbits in the past. It turns out that their orbits are such that they never were close to each other. So it now seems unlikely that Pluto was ever a moon of Neptune.

Is it true that because of Pluto's orbit, it is no longer the furthest planet from the sun?
That it is true for a few years. Pluto's orbit around the sun is not a nice circle like most of the planets. It is oval. Most of the time it is the furthest planet from the sun, but the inner part of the oval gets just a little closer to the sun than the planet Neptune. It takes Pluto 248 years to orbit the sun, but for 20 of those years it is closer to the sun than Neptune. This is occurring right now, from 1979 to 1999!

  • Subjects:
    Astronomy and Space, Planets, Moons, Solar Systems, Satellites and Probes
  • Skills: