home Information about Pluto

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I get occasional questions from students along the lines of "please send me information about Pluto". In self-defense, I've decided to post a few paragraphs about this object. Anyone who wishes to use them for any educational purposes is free to do so.

  • What it's like on Pluto
  • Discovery of Pluto
  • Is it really a planet?
  • Is it the farthest planet from the sun?
  • Can Pluto collide with Neptune?
  • Are there more planets past Pluto?
  • A space probe to Pluto
  • What it's like on Pluto: Pluto is such a bizarre, un-earthly place that it's been the setting for hundreds of stories in science fiction. Somebody should make a movie about it. Almost everything you consider "normal" here is different there.

    The first thing you would notice, were you to step out on the surface of Pluto, would be the cold. Pluto is (right now) about thirty times farther from the sun than we are. That means it gets about 1/900 the amount of sunlight, for a given area, that we do. The temperatures range from about -235 to -210 degrees Celsius (-390 to -345 degrees Fahrenheit, or about 35 to 60 degrees Kelvin.) It's so cold that most of Pluto's atmosphere (layer of air) turns to a liquid or freezes solid. And this is true even though, right now, Pluto is at its warmest. At its coldest, the atmosphere freezes completely.

    The sun would be a tiny point in the sky, 1/30 as big and 1/900 as bright as what we see from Earth. Despite being so small, though, it would still be much brighter than a full moon. It would move very slowly across the sky. The earth turns once a day, so you see the sun rise and set once a day. Pluto turns about six times more slowly, so the sun rises and sets about once a week.

    You might also notice some problems in standing up. Pluto is mostly made up of plain, ordinary ice. The ice is frozen far more solidly than anything you see on Earth, and it's mixed in with dirt.

    If you did stay on your feet, you would notice that you felt much lighter than usual. Maybe you've seen the pictures of astronauts on the moon, bouncing up slowly in the low gravity? Pluto has still less gravity. If you weighed 100 pounds on earth, you would weigh about 17 pounds on the moon, but only 6.5 pounds on Pluto. Try to jump, and you would spend several seconds in the air. (During that time, you would probably turn a little and land on the helmet of your spacesuit. Maybe you shouldn't jump after all...)

    If you were on the right half of Pluto, you might see its largest moon, Charon. It would look about seven times as big as our Moon does from Earth, but it would be about the same brightness as our moon. It would be very different from our moon in another way: you would never see it move. If you saw it in one part of the sky, you would always see it in that part of the sky.

    You might also see Pluto's two other moons, only discovered in the last few years: Nix and Hydra. Each would look to be only about a third as big as our Moon does from Earth, and they wouldn't look perfectly round, the way the Moon does. They would be much, much fainter than the moon, only as bright as a bright star. Also, unlike Charon (and more like our own moon), they would move slowly around the sky.

    Discovery of Pluto: Pluto was discovered after years of hard work and a lot of very careful thinking and planning.

    The story of the search begins in the early 1800s. Back then, people knew of seven planets. Mercury, Venus, Earth, Mars, Jupiter, and Saturn had been known for thousands of years. Uranus had been found in 1781.

    But by this point, astronomers had good instruments for measuring the positions of these planets, including Uranus. Each planet "pulls" a little on every other planet, through gravity; the astronomers knew all about this, and could do the math to figure out just how much gravity was involved.

    The measurements of Uranus gave them a problem. Its motion didn't really make much sense. Even after allowing for the way the other planets pulled at it, Uranus insisted on moving in an unexpected way. It was as if some unseen planet was pulling on it.

    A few people decided to take the idea of an unseen planet seriously. They thought that they might be able to figure out where that unseen planet was. (You can think of it this way: if the unseen planet seemed to be pulling Uranus to the "left", then maybe the unseen planet was to the "left" of Uranus. It was really a lot more complicated than that, but that should give you the idea of it.)

    Two people, John Couch Adams and Urbain Jean Joseph Le Verrier, sat down with pen and paper and a long set of measurements of the position of Uranus, and did the math to figure out where the unseen planet ought to be in the sky. Neither one had their own telescope, so they couldn't then go look for the object themselves. Adams then sent his answer off to George Airy, then the leading astronomer in Britain. Le Verrier sent his answer off to Paris Observatory, and then to Berlin Observatory.

    The observer at Berlin Observatory, Johann Galle, got lucky first. On the night of 23 September 1846, he looked at the part of the sky Le Verrier suggested, and within a few minutes, he had found an object not on any star charts. Pretty soon, he knew he had found a new planet: Neptune.

    A few decades later, though, enough was known about Neptune so that astronomers could say that Uranus still did not move in the way that, in theory, it ought to. And Neptune's motion looked odd, too. It seemed as if there was another unknown planet out there, somewhere.

    Again, mathematicians tried to use the motion of Uranus (and now, Neptune too) to figure out where the unknown planet might be. Clyde Tombaugh, an astronomer at Lowell Observatory in Arizona, took on the job of actually searching for the object. He did this by taking two photographs of a given part of the night sky, and then looking at them with a "blink comparator". This device let him "blink" between one photo and the other, to see what had changed between the time the first photograph was taken and the time the second was taken.

    Stars don't move very much, but Tombaugh could be pretty sure that any unknown planet would move slightly over the time between the two photographs, and that motion would make it stand out against the stars. It was still slow, painstaking work. Tombaugh found quite a few interesting things over the years while looking at the photographs, but it wasn't until February 1930 that, on a newly-made pair of photographs, he finally found the new planet.

    Incidentally, it was later realized that Pluto was far too small to have had any real effect on Neptune or Uranus. That caused people to think that there must be yet another planet out there somewhere, and a lot of searching was done for this "Planet X". Eventually, though, the Voyager 2 space probe was able to measure the mass of Neptune precisely. Once that was known, the math could be redone, and the problems with the positions of Uranus and Neptune went away. So it no longer looks as if there is another, unknown, big planet out there.

    Is it really a planet?: The answer to this depends on who you ask, and there has been a lot of arguing about it in the last ten years or so. This eventually led to a decision by the International Astronomical Union (IAU) that re-defined what a "planet" is, but not many people seem to think much of the IAU decision.

    Right now, some people consider Pluto to be a planet. Some would call it an asteroid, or "minor planet". Some call it a comet. Some think it's "something new", not in any of the above classes.

    The problem is that there is no clear dividing line between these classes. It's a bit like the difference between a rock and a boulder. Most people would say that a piece of stone a few inches across would be a rock, and a piece of stone a yard across would be a boulder. But in between, it's harder to say.

    The "planet" defenders point out that we've called Pluto a planet since it was found in 1930. Everybody is used to learning that there are nine planets when they're in grade school, and we shouldn't just rewrite all the textbooks for no good reason. Also, it's a lot bigger than anything we call an asteroid. (Pluto is something like 2400 kilometers across, 2/3 as big as the Moon. The largest asteroid, Ceres, is about 900 km.)

    There is another proposed dividing line, based on "roundness". Smaller asteroids and satellites tend not to look particularly round. But larger objects have stronger gravity, which pulls them into rounder, smoother shapes. (The Earth, for example, is round to within less than one percent.) By that rule, Pluto would definitely be "round enough" to be considered to be a planet. (So would some other objects beyond Pluto, as well as the asteroid Ceres.)

    The "asteroid/minor planet" people point out that, had Pluto been found in the last ten years or so, we would have applied current knowledge and classified it as an asteroid, without much argument over the matter. That's partly because we have recently discovered that there are a fair number of objects in the outer solar system, and Pluto appears to be one of the largest of many. These are the "Trans-Neptunian objects" or "TNOs", or "Kuiper-belt objects"; they've been a hot subject in astronomy for about twenty years now.

    The people with this viewpoint also mention that Pluto is smaller than the other planets, it's in an asteroid-like orbit, and it doesn't even faintly resemble the other planets that far from the sun. They will also cite:

    Don't go to Pluto. It's a Mickey Mouse planet. -- Robin Williams

    You might be surprised to think of an object like Pluto as a comet, since it doesn't have a tail. But the "comet" defenders point out that Pluto appears to be a lump of ice with some dirt mixed into it, which is the basic definition of a comet. (The lack of a tail is really not that important. When comets are as far from the sun as Pluto is, they are too cold to have tails.) The other TNOs seem to be more rock than ice, so they would not really be considered to be comets.

    The "something new" camp says that, given the above, it's obvious that Pluto is in a class by itself. Some have taken to calling Pluto a "dwarf planet", but there is a lot of disagreement as to what that means.

    Is it the farthest planet from the sun?: It depends on when you ask the question.

    Pluto's orbit is a long ellipse (oval), and it takes it about 248 years to complete that orbit. For about twenty of those years, it's closer to the sun than Neptune.

    For example, between 7 February 1979 and 11 February 1999, Pluto was closer to the sun than Neptune. Then it went out on the part of the ellipse that takes it farther away from the sun. It will get slowly farther and farther from the sun for about 120 years. Then it will start coming back toward the sun, and on 5 Apr 2231, it will again be closer to the sun than Neptune.

    (You'll see some people mention dates that are slightly different from these. A gent at JPL (the Jet Propulsion Laboratory) has written a few paragraphs about how that happened; click here for details.)

    But about 90% of the time, Pluto is quite a bit further from the Sun than Neptune.

    Can Pluto collide with Neptune? No.

    A lot of people hear about Pluto coming closer to the Sun than Neptune (this is discussed above) and conclude that, when Neptune and Pluto are at exactly the same distance from the Sun, there must be a risk of collision. There are several reasons why this isn't actually so.

    The first is that it's a big solar system, and when you're out as far as Neptune and Pluto are, it's a really big solar system. As described above, Pluto crosses the orbit of Neptune twice every 248 years, so it doesn't often get the chance to collide; and even when it does cross the orbit of Neptune, the odds would be very good that Neptune would be someplace else.

    The second reason (which is the only one most books get around to mentioning) is that the orbits of the two planets don't intersect. You can think of the orbits of Neptune and Pluto as being like two links on a necklace; if Neptune ran in a circle inside one of the links, and Pluto ran in a circle inside the other link, they would pass each other from time to time, but they wouldn't collide.

    The third reason is a tricky one. Whenever Pluto comes closest to the Sun, Neptune is always ninety degrees away, either ahead of Pluto or behind Pluto. This guarantees that, when that "chance of collision" seems to occur, Neptune will very definitely be in a totally different part of the sky... in fact, it will usually be even farther from Pluto than the Earth is.

    Are there more planets past Pluto?: There are certainly some objects beyond Pluto that are bigger than Pluto. We know this because at least one such object has been found: Eris, also known as asteroid 136199, is at least a little bit bigger than Pluto. It's quite possible that others are out there, waiting to be discovered.

    There are certainly undiscovered objects past Pluto. And, if you go far enough from the sun, even a large object could go undiscovered. Such objects would be faint, not just because they are far from us, but because they are far from the sun.

    A space probe to Pluto: Pluto is the only planet that has not been visited by a spacecraft. We have some images from the Hubble Space Telescope, but these are not especially clear. These are also interesting times on Pluto; it's just past its closest point to the sun, and it has a thin atmosphere. In a few decades, that will probably get cold enough to freeze.

    Therefore, the New Horizons probe was launched in 2006, to fly past Pluto in 2015. That may seem like a long time, but Pluto is very far away; getting there in nine years is actually making pretty good time. The hope is that the probe will be able to to observe Pluto's atmosphere before it completely freezes out.