Common Questions About Pluto

Here are some questions brought forth by the Live From HST participants.

What are the most interesting things about Pluto? What are the most unique aspects of Pluto?

Some of our interest in Pluto is that of studying an planet of extremes.

  1. Most distant planet (on average).
  2. Smallest planet.
  3. Pluto-Charon binary system are the most nearly equal in mass.
  4. Very cold and thin atmosphere.
  5. Surface covered in exotic frosts, methane (CH4), nitrogen (N2), carbon monoxide (CO).
  6. Pluto's surface may well have some of the brightest and darkest suface areas in the solar system. Bright from frost and ice, dark from organic molecules.
  7. May have an "active" surface from winds and frost migration coupled with large change in its distance from the Sun.
  8. Some models predict that Pluto's atmosphere will disappear completely when it is furthest from the Sun. No other planet or satellite boasts this large of a seasonal change. Only comets beat Pluto in this respect.
  9. Its internal composition provides important clues as to planetary building blocks in the outer solar system. We've studied the inner solar system (mostly the Earth and the Moon) and to fully understand the formation of planets we'd like to know what is inside Pluto.

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  1. What could possibly be gleaned from HST pictures?
  2. What could give students the most satisfaction from this observation?
  3. Is Pluto changing? If so what could we find from the changes?

A limited number of pictures of Pluto have already been taken with HST. These pictures show albedo patterns on the surface with very little computer processing.

The most exiting thing we will get from a set of Pluto pictures is a chance to discover changes in these albedo markings. These markings are the result of the distribution of frost overlying darker regions. If winds scour a region free of frost it would appear darker. If frosts collect on a region it will become brighter. Also, as Pluto moves away from the Sun, the solar energy input begins to diminish leaving the possibility of having some of the atmosphere freeze out on the surface. Not only is Pluto moving away from the sun, it is also moving toward winter in its southern hemisphere. Already, the 15 degrees of latitude closest to the south pole is in permanent night that will last for about 120 years. Without an atmosphere, the south pole could get very, very cold. However, the atmosphere can help transport energy from the lit pole to the unlit pole and would keep the pole from getting a lot colder. The observable effect of the atmospheric flow would be to systematically deposit frosts on the cold pole and deplete the frosts from the lit pole, thus making it darker. The rate at which the surface darkens will tell us about the global atmospheric circulation rates on Pluto.

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Apparantly the HST is going to be of great use to students, what makes Pluto the most worthy of this project?

Of the four projects, this one has the most potential for seeing something no one else has seen (changes on the surface). Will we see it for sure? No one knows. It could look the same as it did 3 years ago when we last looked. It could look totally different (which would be a very big surprise). It could also be very slightly different which would show up as subtle changes from the last pictures.

No matter what, the pictures will show some structure on the surface and this structure can be turned into a map. If we look at Pluto just once, we get only one side. If we look at Pluto twice (opposite hemispheres), we would get most of the surface. With two pictures, portions of the surface will be highly foreshortened on the limb and we won't be able to say much about those areas. Three pictures are the absolute minimum to get a global mosaic of pictures covering the entire surface. More is always better but even our professional efforts 3 years ago netted us just 4 pictures, 90 degrees apart from one another. As always with science, one must trade off different goals. 3 separate orbits on Pluto will yield the best data on Pluto but you can't look at anything else. If you cut back to two (or one) picture on Pluto, you won't get as good of a map. So, everyone needs to decide if a picture of another planet is as important as filling in a complete map of Pluto.

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What has been learned recently about Pluto?

Just about everything we know about Pluto has been learned since 1980. Prior to that time all we really knew was where to look to find it and how long its day is (rotation period). Astronomers have finally been able to figure out quite a few things more recently. Some of these are:

  1. Size of Pluto and Charon: we know these number to within about 20 kilometers or so.
  2. Surface composition of Pluto (methane, nitrogen, CO, probably dark organic compounds).
  3. Surface composition of Charon (water ice).
  4. Bulk densities of Pluto+Charon 2.0 g/cc (we don't quite know the individual densities yet, there's still some debates raging over this).
  5. Atmospheric composition and structure of Pluto. Mostly nitrogen with trace methane. Warm upper atmosphere (100 K) and gets cold (~40 K) very fast within 10 km of the surface.

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  1. Is it too cold to have volcanos? It's not too cold for Jupiter's moon, why not Pluto?
  2. Does it have any clouds or wind or any atmosphere at all?
  3. What kind of land is there? Is there going to be anything interesting on it at all? There probably isn't any life because it's so cold, but it'd be neat if we found something.

Pluto may be cold, but that doesn't mean there couldn't be things happening on the surface. Remember the Voyager spacecraft pictures of Neptune's moon, Triton? A couple of the pictures should very clearly an active geyser or plume rising from the surface. This isn't evidence for volcanos like on the earth or Jupiter's moon, Io. It's more like "Old Faithful" in Yellowstone National Park. Old Faithful is caused by subsurface geothermal heating causing a buildup of steam pressure until it releases and spews out of the ground. In the case of Triton, one theory says that there is nitrogen ice being heated underground and the sublimating gas builds up pressure high enough to break free and causing the plume we see. Could the same thing happen on Pluto? Well, sure. They are similar in size, surface composition, distance from the Sun, just about everything is the same. Is it happening? We don't know. Could we see these plumes from HST? No, not directly. These plumes are very, very small and our images would be very coarse. However, if a plume went off and deposited black gunk on the surface over a very large area, we could see that that area had gotten darker.

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How do the new maps of Pluto compare with the maps made during the mutual events? Are there any discrepencies? - Ted Stryk, 11th grader in Bristol, Virginia (4/96)

The new maps of Pluto agree with the older mutual event maps quite well. There are differences, of course, but most of the differences are understood. I don't expect the match to be perfect for a few reasons. First, the mutual event maps are based on measurements that took 6 years to collect. If Pluto were changing during that time, we would have a hard time seeing the change. The HST map is based on data taken over a 10 day span. We're pretty sure changes don't happen that fast so this map won't be bothered by any changes.

Also, the mutual event maps only provide detailed information on the side of Pluto that always faces Charon. The other side is MUCH more poorly known. When you take these difference into account then the maps agree where I'd expect them to agree and they look different where the mutual events had no say in the matter. Perhaps the biggest dicrepancy is in the placement of the brightest area on Pluto. In the mutual event map there is a very bright area seen at roughly 180 degrees longitude (opposite Charon) and in the south polar region. In the HST map, this bright area is much closer to Pluto's equator. This difference is caused by how the data (and maps) were collected and does NOT indicate that Pluto actually changed.

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By using HST at different wavelenths, would it be possible to make very crude compositional maps of Pluto? - Ted Stryk, 11th grader in Bristol, Virginia. (4/96)

Absolutely! In fact, the work we've already done with HST did just this. We took images in visible light and in ultraviolet light. Visible light was chosen to correspond to a color we've studied in detail over the past 40 years. Ultraviolet light was chosen to be sensitive to the age of the ices we know exist on the surface. Weather of the surface on Pluto will tend to first change the UV brightness so differences between UV and visible light can be traced back to how fresh the ice is on the surface. We managed to collect some tantalizing information on this from the 1994 HST observations. Unfortunately, we didn't have enough time on the telescope to get good enough data in the UV to do this job right. Next time I hope we can do better.

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From what picture did you learn the most about Pluto? - Elizabeth Hathaway, Grade 4, Storer School, Muncie, Indiana USA (4/96)

Studying Pluto is much harder than you might think. What does that mean? Well, it means that we get lots of little clues to the puzzle which is Pluto. After we gather enough little clues, we begin to see what Pluto is all about. Have you ever tried to do a picture puzzle without using the picture on the lid of the box? It's pretty tough. Your question is a little like saying which one of the 1000 pieces told you the most about the picture you were putting together.

That's just what it's like working on Pluto. Those of us that study Pluto try lots of experiments to help understand Pluto. Each of these experiments gives us one little piece of the bigger puzzle. I'd say that we're finally getting to the point of getting a reasonable idea of what Pluto is all about but none of these individual pictures got us there all by itself. Now, if we could send a spacecraft to take some nice pictures up close, then I might be able to answer your question differently. I hope we get to do this someday. I'm sure Pluto will be a spectacular site when we do make it there someday.

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Is it possible to put Pluto in orbit around the Earth? It would be good to have a place nearby to possibly live on. - anonymous grade school student (4/96)

Moving a planet is something we may never learn how to do. Even as small as Pluto is, it would take a tremendous amount of energy to move it even a little bit. Besides, if we did bring Pluto that close to the Sun, it would heat up and all the water ice on its surface would melt. About half of Pluto is made up of water ice and after it melted it would create a planet wide ocean with no place left to stand.

Now, perhaps in the next 1000 years we might learn how to move small asteroids around. Some of these asteroids already come pretty close to the Earth. Those close neighbors would only take a relatively small amount of coaxing to put them in a useful orbit near the Earth. These objects wouldn't be any more habitible than our Moon so perhaps it wouldn't be worth the bother. After all, we've already got the Moon handy, right?

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From the Pluto pictures, what kind of materials may exist on the surface? - Nick Cabot, teacher, Washington.

If all we had to study were the pictures from HST, all we could say is that some stuff on the surface is bright and some stuff is dark. The HST images can tell us where stuff is but not what it's made of. To figure out composition, we need other types of observations. To learn about composition we usually turn to spectroscopy. This is a tool that lets us search for characteristic "fingerprints" that reveal the presence of various molecules on the surface of Pluto.

Since 1976, we've known that there is solid methane (CH4) on Pluto. More recently we've discovered large amounts of solid nitrogen (N2) and solid carbon monoxide (CO). All of these materials would have to be relatively fresh frost which also implies they are bright. So, a reasonable assumption is to say that the bright areas are rich in these frosts. The composition of the dark areas is not as easy to understand since much less light is reflected from those areas. However, we expect that the dark areas contain organic residues that are left behind after solar UV radiation rips apart methane molecules.

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What have we learned about the early formation of Pluto from these pictures we have just gotten back. - Nick Cabot, teacher, Washington.

Just like the question about surface materials, the new HST pictures of Pluto don't give us much information about the formation of Pluto. The pictures will be useful to help us understand other observations which in turn tell us about Pluto's formation.

For example, one important clue to the formation is knowing the bulk composition of Pluto. We know the surface is covered with N2, CH4, and CO but this only tells us what's on the top of the surface. The Earth has a molten iron core but we certainly didn't learn this by picking up and studying the rocks on the surface. By measuring the total mass of Pluto and then combining that with its size, we measure the bulk density. Pluto's density implies that about half of the volume is water ice while the rest is rock. This compositional information has already played a critical role in improving theories of what happens as planets are forming in the outer solar system.

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Is it possible that Pluto is a former moon of Neptune that go hit out of its orbit? If so, how can this be proven? - James Fothergill-Brown, gr.6, St. Michael's School, Low, Quebec, Canada

I find this question fascinating but probably not for the reason you think.

First, to answer your question. No, Pluto is not a former moon of Neptune. A colleague of mine, William McKinnon at Washington University studied this question about 10 years ago. He found getting Pluto away from Neptune and putting it where we see things now would have required a "dark star" to pass by our solar system. It had to be dark (perhaps dead) so that it wouldn't fry all the cold stuff in the solar system. Now it is POSSIBLE a star could pass close to us but Bill found one more thing was required. The star would have had to come by at a time when Neptune was nearer the star and ALL of the other planets would have had to been on the opposite side of our Sun so that they wouldn't get messed up too.

There a principle we use in science, actually it is more like a guideline, called Occam's Razor. This guideline says if you have two (or more) different explanations for the same thing, then you'll usually be right if you pick the simpler explanation. In this case, it COULD happen that Pluto was ripped away from Neptune but it's a really complicated method. We've got another, simpler, explanation for Neptune and Pluto.

How's this? Pluto and Neptune formed on their own in the solar system. This is pretty simple, the rest of the planets did this too. There may have even been lots and lots of Pluto's a long time ago. Then as the solar system grew and evolved, those other Pluto's got either gobbled up by Neptune or flung out of the solar system. One of those Pluto's might have even been captured by Neptune. That might even be Triton --- which could explain why it orbits Neptune backward. What we now know as Pluto is the only one of these objects out there that survived being swallowed up by Neptune. This is pretty likely since we know the path that Pluto travels NEVER gets close to Neptune. Now, I realize that this explanation sounds long-winded and even a little complicated instead of simple. But in applying Occam's Razor, this explanation wins becuase it depends on things that we know happen while the escape theory calls for this odd passage of a star.

I haven't gotten to the really interesting part yet. I find this question interesting not for what it tells us about the solar system but for what it tells us about the human race and how knowledge gets passed around and what gets remembered. This "theory" about Pluto escaping Neptune is the result of a casual comment made by a scientist in the 1950's. Someone asked him (he was just about the only planet expert at the time) where Pluto came from. Without doing any serious thinking about the question, he tossed out the idea that Pluto and Triton got tangled up somehow and Pluto got thrown out and Triton's orbit got reversed. It sounded good but he didn't do the work required to prove this idea. Unfortunately, people that were listening didn't know any better and what ever a planet expert said must be right. So, other people that write textbooks heard of this and began to pass the idea around as if it were fact. You STILL see this in today's textbook and just about everyone thinks they know this is what happened. I've been working 10 years now trying to undo this incorrect knowledge and it hardly seems like I've made a dent. Everyone still knows Pluto is escaped from Neptune and they don't know it's WRONG!

So my question is: what else have we been taught in school (or elsewhere) that is wrong like this? I keep this story in mind all the time when I hear an explanation for something. If it's right, then when you check the story from every possible direction it will still make sense. I think this is the essence of what it means to be a scientist. Unfortunately, everyone needs to have this questioning attitude, not just people that do science.

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Can the Hubble Telescope predict or help you predict what weather patterns are occurring on Pluto? If so, how does it do this? - Erin O'Connor, gr.6 student at St. Michael's School in Low, Quebec, Canada

Pluto's weather is very different from that of the Earth. On the Earth the weather changes minute by minute and we need lots of detailed pictures to keep track of things. Pluto is not like that at all. Things happen very slowly and there aren't big storm systems to look for. Pluto's weather patterns are controlled more by how much sunlight is hitting the atmosphere and surface and this changes very slowly over the course of its 250 year orbit. As Pluto moves away from the Sun, it's atmosphere begins to slowly freeze out on the surface. Where does it freeze out? We don't know. It might be at the south pole which is in permanent night now. It might be in other cooler areas (low or high regions). These changes are probably very slow and small. By taking pictures with HST, we can look for these changes but we'd don't need to do it every day. All we need to do is look once every year or so and see if the bright or dark areas on the surface change in their brightness.

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Does the expected cooling of Pluto to the temperature of the March 14, 1996 (or thereabouts) Pluto images correspond to the freezing point of the known gases on the planet? Is that why you expect a freezing of the atmosphere? - anonymous student (4/96)

The surface and atmosphere of Pluto is pretty complicated. We don't know the exact surface temperature but we have an idea of the range. For the temperature and atmospheric pressure on Pluto, most everything is frozen out. Even at those cold temperatures, some gas can exist above the surface. The amount of gas depends on the temperature of the frost. If you pump energy (heat) into the frost, you will create more gas. The really neat thing about this balance is that as long as you have gas and frost together, the temperature doesn't change as you add heat. The extra heat goes into making more gas, not into warming the surface unless you got rid of all the frost. So, the simplest expectation is for the atmosphere to begin to slowly collapse as Pluto moves away from the Sun, it's only source of warmth.

We now expect the surface of Pluto to be quite a bit more complicated. The amount of heat that Pluto gets from the Sun is very, very small. That means that the rocks and ice below the surface could store enough heat to smooth out the temperature changes during an orbit. Depending on how important this reservoir of heat is, we could see the atmosphere collapse completely, or, we could see a nearly constant atmosphere. It's going to take quite a few year to answer this question, maybe as long as 250 years!

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Is Pluto the 8th planet right now? - Nicole Lindenbaum, 7th grader, Mission Valley Middle Sch., Overland Park, KS

If you count the planets in order of their distance from the Sun, right now, then yes, Pluto would be the 8th closest planet to the Sun. However, out of Pluto's 250 year long orbit, it is the 8th planet only 20 years out of that time. Another way of counting the planets is to look at their average distance from the Sun. On average, Pluto is further from the Sun than Neptune so with that definition we'd still say that Pluto is the 9th planet. My own preference is to call Pluto the 9th planet even now but either way is just fine.

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What year was the bright spot on Pluto discovered? - Judah Leon, 2nd grade

This is a pretty hard question to answer. It's easy to say when we first saw Pluto in pictures taken in 1930. The bright spot on Pluto is more complicated. Back in 1954, Pluto was first seen to change in brightness. This change is caused by Pluto's rotation. When Pluto's is at it's brightest we know that that part of Pluto's surface must be brighter than all the rest. So at that time we knew that there was a bright spot on Pluto's surface. We didn't really see it until the new Hubble Space Telescope pictures were taken of Pluto, but we've known it was there all along.

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Pluto is now the eight planet from the sun. Is it any warmer now that it's closer to the sun? - Abigail Hale, Grade 5, Storer Elementary, Muncie, IN

Pluto may be slightly warmer now that it's closer to the Sun. More importantly, its atmosphere is thicker now that it's closer to the Sun. As Pluto moves away from the Sun over the next 120 years, the atmosphere will get thinner and thinner. If it collapses completely, the surface will get much colder.

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Does Pluto have plates or earthquakes like the earth? - Brynna Jones, Grade 4, Storer Elementary, Muncie, IN

Well, I suppose that might be possible. We don't really know about that many planets and we've got a lot to learn yet. However, if I had to guess based on what we DO know, I'd say that Pluto probably doesn't have crustal plates or earthquakes like we have on the Earth. One of the things a planet needs to have plate tectonics is a hot molten core with the crust floating on top. Pluto's core is probably completely frozen and there would be no way for plates to form and move around.

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Does Pluto have four seasons? - Carly Sottak, 6th grade, Hydesville Elementary School

Any planet whose rotation axis is tipped relative to its orbit will have four seasons. Mercury, Venus, Jupiter don't really have seasons. Earth, Mars, Saturn, Uranus, Neptune, and Pluto all will have four seasons.

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Just how cold is the planet Pluto, and what processes would a person go through because of the cold if he/she were on the planet and in what amount of time? - Mrs. Peterson's 5th-graders in Perris, CA

I have a separate page describing the temperature of Pluto.

Here also is a quick summary. Pluto is about 35-45 K (degrees above absolute zero). On a Farenheit scale that would be -378 to -396 F, very cold indeed. The coldest spot on the Earth was measured in Antarctica at 184 K or -128 F. If you were in Antarctica you'd be very cold but that's very warm compared to Pluto. If you were on Pluto, you'd have to live in a very well insulated shelter or you'd freeze to death almost instantly.

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I would like to know how Pluto compares in size to Earth or landforms of Earth. - Tommy Stewart, fifth grade student in San Jose, CA

The Earth is 12756 km in diameter, Pluto is 2300 km in diameter. If you superposed Pluto over the USA, Pluto would cover from the California coast to the Mississipi River. Check out this picture to see for yourself.

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Is it possible to make homes on Pluto if Earth ever proves unlivable? - Stephen Haley, Crittenden Middle School in Mountain View California

I think people are clever enough to build homes just about anywhere, even Pluto. Right now the hard part seems to be getting people to these other places. Despite that, there isn't any place in our solar system that is as nice a place to live than the Earth. Sure, there'd be spectacular scenery and lots of neat things to explore, but living any where else would be a challenge. Every where else we could go would require a great deal of technology just to survive. Just keeping warm (on Pluto) or cool (on Venus) would require a power source (like electricity) that would NEVER fail. If your power got knocked out for a day, you'd freeze (or fry). We really need to keep on exploring and pushing out beyond Earth but we'd be wise to take care of this planet. It may be a long, long time before we find another one like it.

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Back to my home page -o- Back to my Pluto page -o- Back to my LFHST page

Marc W. Buie, Southwest Research Institute