Ten Things You Don't Know About Pluto

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Ten Things You Don't Know About Pluto
by Phil Plait (Bad Astronomy)




...avarice drags Pluto himself out of the bowels of the earth
-- Karl Marx

Pity poor Pluto.

Sure, it reigned as the last planet in the solar system for more than 70 years, but then it was stripped of that title by the International Astronomical Union in a manner so profoundly dumb that I'm still wondering what they were thinking. I do think that the definition of planet can be debated, and that Pluto plays its part, but the IAU really screwed the pooch with the way they did it.

Whether you call Pluto a planet, an iceball, or an animated dog, it's still a very interesting object. And today, March 13, 2009, marks the 79th anniversary of the announcement of Pluto to the world (and in Illinois it's officially Pluto Day), so what better time to talk about it? We know a lot more about it than we did in 1930... and while you may know some of its features, I just bet that if I listed ten of them, there may be one or two Things You Don't Know About Pluto.

What follows is my list of Plutorrific stuff about that distant iceball, the fifth in my Ten Things You Don't Know series (the others are the Sun, black holes, the Milky Way, and the Earth). The last one I did was the Sun, which keeps us warm and cozy... so let's head to the opposite end of the solar system and journey out into the deep black.

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1) Pluto was actually detected in 1919, but no one noticed.

The sky, to borrow a phrase from Dave Bowman, is full of stars. Looking for something as distant and faint as Pluto is no small task. Nowadays it's a whole lot easier than it used to be -- telescopes can be programmed to scan the skies, and automated software used to look for anything that moves.

But back in the day (and I mean 1930, when Pluto was discovered by Clyde Tombaugh, or earlier) it was tougher. Telescopes had to be guided by hand. Instead of digital detectors, giant glass plates sprayed with photographic emulsion were used. They weren't very sensitive compared to modern instruments, and it would take hours to get good exposures, and even then something as faint as Pluto was near the limit of what could be seen.

A possible ninth planet had been postulated for decades; observations of Neptune in particular indicated that its orbit wasn't quite right, as if some unseen mass were pulling on it (this turned out to be wrong; see #4). In the late 19th and early 20th centuries, astronomers had scoured the skies looking for this mysterious planet, taking many photographs of the area where the purported planet was predicted to be.

The thing is, they found it. They just didn't see it.

Once an object is discovered, astronomers observe it over time to determine its orbit. The longer you observe it, the better you understand its orbit. But once you determine that orbit, you can use mathematics to figure out not just where it will be in the future, but where it was in the past. By backtracking the orbit of Pluto once it was found, astronomers discovered it was actually visible on some photos taken in December 1919 and January 1920 (Tombaugh's discovery images of Pluto were taken on 21, 23 and 29 of January, 1930, and he found the planet in February). Ironically, these older plates were specifically taken to look for the planet, but Pluto was somehow missed by the astronomers at the time.

It's possible Pluto was seen even earlier as well. After all, it's known that Galileo saw Neptune years before it was "discovered"! In one of his drawings, what he labels a star is actually the seventh planet, but he didn't notice it moving. Too bad. Had he announced that, he would've been famous.

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2) Pluto's orbit crosses Neptune, but they'll never collide.

When I was a kid, I knew that for almost exactly 20 years every orbit, Pluto was closer to the Sun than Neptune. But it baffled me that they could never collide -- 248 years may seem like a long time for Pluto to go around the Sun once, but over the lifetime of the solar system it's made about 18,000,000 circuits, plenty of chances to wipe out into Neptune. So why hasn't it?

The problem here is in the way this is usually described: people say Pluto's orbit crosses Neptune's. But the orbits do not cross! That is, they never actually intersect. However, Pluto does get closer to the Sun than Neptune can. How can this be?

It's because Pluto's orbit is tilted with respect to Neptune's. When you look at an image from directly "above" the solar system (I know, there's no up in space, but astronomers call up the direction you go straight up from the Sun's north pole), it looks like the orbits of Neptune and Pluto intersect. But Pluto's orbit is tilted by about 17 degrees compared to the plane of the solar system (defined as the plane going through the Sun's equator-- it all goes back to our friendly star).

If you look at a diagram of the orbits as seen that way, Pluto's orbit never physically crosses Neptune's; it goes above it when Pluto is nearest the Sun. So if you look at things from an oblique view, you can see the two paths never meet.

As usual, things may look one way, but if you take a step back -- or to the side -- you wind up learning something.

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3) Pluto is the biggest survivor of Neptune's wrath.

Still, even if they never physically smack into each other, Neptune's a big planet with a lot of gravity. How come it hasn't drawn Pluto in over all that time?

It's because of an odd fact about Pluto: it orbits the Sun twice for every three times Neptune goes around (this type of thing is called a 2:3 resonance) -- about 248 years for Pluto versus 165 for Neptune. This, together with its tilted orbit, means that it never really gets very close to Neptune. Every time Pluto is at its closest to the Sun, Neptune is 1/4 of the way away around its orbit! A really weird consequence of this is that Pluto actually gets closer to Uranus than it can to Neptune!

But there's another conclusion we can draw too. You might think it's a huge coincidence that Pluto appears to avoid Neptune in its resonant orbit. But it's not coincidence. Imagine the solar system about 4.5 billion years ago. There may have been thousands, millions of icy balls orbiting the Sun near Neptune's orbit. But ones that were just inside or just outside Neptune's orbit would eventually interact with it, and get tossed into deep space, or dropped closer in to the Sun. Essentially, Neptune cleaned out any objects with orbits it didn't like.

But Pluto was safe! Its orbit never took it close enough to Neptune, and it survived the massacre. In fact, there are many objects in orbits similar to Pluto's; these are called plutinos in honor of their largest member, and quite a few have been discovered. Two of them are marked on the image above for your edification. Yes, Pluto is a plutino. I prefer the name Plutoid, but I didn't get a vote.

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4) Pluto is shrinking!

OK, not really. Pluto itself isn't shrinking. But our estimates of its size and mass are. Or did.

One of the easiest things to figure out about a new object is its distance. As I mentioned earlier, the orbit can be found by observing the object over the course of a few nights or weeks, and the distance to that object naturally comes out of the equations. Pluto is a long way off, between roughly 4.4 to 7.5 billion km (2.5 and 5 billion miles) away (its orbit is elliptical).

If you know its distance, you can guess at its size. After all, a bigger object reflects more sunlight and would look brighter, and a smaller one reflects less and would look fainter. This depends on how reflective the object is as well; a mirror reflects almost all light that hits it, while a chunk of charcoal reflects almost none. At the same distance, a small mirror looks just as bright to you as a huge piece of coal.

Astronomers could guess how reflective Pluto was and use that to estimate its size. At first it was thought to be near the size of Earth. This jibed with estimates of its mass, determined from what astronomers thought was its gravitational influence on Neptune (which turned out to be totally wrong, in fact).

But as time went on, more observations and more calculations were made. And every time astronomers used the new data to figure out Pluto's size, it got smaller... prompting one famous astronomer to predict that Pluto would disappear in 1980. Eventually, observations indicated that Pluto was icy, and exceptionally shiny. It reflected about 50% of the sunlight hitting it. That means it was smaller than previously thought, leading to the modern size measurements.

We've finally settled on a mass for Pluto of about 10 quintillion tons (0.2% of the Earth's mass), and a diameter of about 2300 km (1400 miles), or smaller than the Earth's moon. When you think of it that way, it's remarkable we can see it at all! Most objects in the deep solar system are very dark, reflecting less than a tenth the light Pluto does. That's one reason it was found so long before any other deep solar system objects: it's simply brighter!

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5) It's not the biggest object out there in the black

But just because it's brighter doesn't mean it's alone!

Astronomers figured for decades there must be other objects out there near or beyond Pluto, but they are so far away and so faint that they're incredibly difficult to spot. However, automated telescopes and computers make searching a lot easier, and now we know of hundreds of objects in the deep solar system.

As we found more and more, some started to rival Pluto's size. Most folks -- including me -- predicted it was only a matter of time until a larger object was found.

And sure enough, in January 2005 Mike Brown and his team at Caltech found an object eventually named Eris. It's about 2500 km (1500 miles) across, making it comfortably larger than Pluto. A moon (named Dysnomia) orbiting Eris was discovered, and its orbit gave the mass of Eris as being 27% bigger than Pluto as well.

You may think that astronomers in the IAU suddenly decided out of the blue that Pluto was no longer a planet, but in reality Eris is what spurred them into action. If there was one object out there bigger than Pluto, then there must be more. A lot more, since space out there is pretty roomy! They were faced with a choice: call Pluto and Eris planets (not to mention several other objects already known) and wind up with a solar system possessing potentially hundreds or thousands of planets, or simply make up a rule that excludes those objects.

And you know how that turned out.

Again, I won't go into planet definitions here, but I will say that Eris is almost certainly not the biggest object out there either. Calculations cannot rule out a mass as large as the Earth, as long as it's way, way out-- so far that its gravity doesn't appreciably affect Neptune, or else we'd have noticed by now. So there could be some pretty big objects out there. And if there are, some day we'll find them.

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6) It has an atmosphere

OK, if you're a regular reader of this blog you already knew that. But it still surprises me, so maybe it surprises you too. After all, Pluto is a long, long way from the Sun, and it's so cold that's it's hard to imagine anything being able to stay a gas there.

But there is air there, though a little thin for our taste (the atmsopheric pressure at the surface is about 0.00001 times that of the Earth). Pluto's atmosphere is mostly nitrogen, with a whiff of methane tossed in. Methane is a greenhouse gas, so that does help warm the little iceball up a bit. Also, recent observations indicate that when Pluto is nearer the Sun, as it is now, the frozen stuff on its surface turns directly into a gas (the process is called sublimation). This takes energy away from the surface and gives it to the gas, in much the same way that evaporating sweat takes heat away from your skin. That process cools your skin, and on Pluto it cools the surface, so, oddly, Pluto's air is much warmer than the surface. Though, at 170 below 0 Celsius, it ain't exactly Hawaii.

Still, the fact that atmosphere of Pluto is mostly nitrogen is oddly comforting to me. After all, so is the air you're breathing now!

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7) It's a double planet

OK, whether you think Pluto is a planet or not, it's still a double something.

Think of it this way: imagine two kids, each holding on to the end of a rope, and they start to swing around each other. If one kid is really big, and the other really small, then the little kid makes a big circle, and the big kid makes a small circle as they dance. Each kid is circling some point between them (the center of mass, or, in fancy science-speak, the barycenter), and the size of the circle each makes depends on how massive the kid is, and how far apart they are.

Same thing for bigger masses, too, except that instead of a rope, the tie that binds is gravity. For the Earth and Moon system, the center of mass is inside the Earth, about 3/4 of the way from the center to the surface (or about 1700 km down, if you like). That's a long way from the Earth's center, but still inside the Earth.

Pluto though is different. Its big moon Charon is massive enough (about 1/8th of Pluto's mass) and at such a distance that the barycenter of the system is outside Pluto's surface! About 600 km above it, to be close enough. So really Pluto and Charon orbit each other -- which is really true for any two bodies, but really obvious for these two guys. Every time Charon orbits Pluto once, Pluto itself swings around this invisible point in space too.

If there's a takeaway lesson from this, it's that to us, Pluto may seem like a little kid, but to Charon it's a big kid. Just not a huge one.

OK, that's a stupid lesson. But this is a Top Ten list, not a Very Special Episode of Bad Astronomy. If you want a heart-warming lesson, stick with some place closer to the Sun.

[ILLigitt]

8) We have maps of its surface, even though telescopes can barely see any features.

At Pluto's closest distance, it's just barely resolvable (that is, seen as a disk and not a single dot in the sky) even using cameras on board Hubble Space Telescope. So how did we ever get maps of its surface?

By being clever. Imagine I have a ball, and I paint one hemisphere black and the other white. Now I get a friend to hold it a long way off, where to me the ball is so small it looks like a dot. If my friend slowly spins it, I'll see it get brighter when the white side faces me, and darker when the black side does. So I can make a crude map of the ball even though it's unresolved to me!

You can imagine that if you can precisely measure the amount of light you see from the ball, you can make even better maps. You could tell if just one hemisphere is painted, or if it's splotchy, or striped, or lots of other details.

And that's just what astronomers Alan Stern and Marc Buie did back in 1996. They used a camera on Hubble to precisely measure the amount of light detected from Pluto and its moon Charon. They did this over one rotation period -- a "day" -- of Pluto, about 6.4 Earth days. By measuring the fluctuations in the light received, they were able to make the crude map displayed above.

If Stern's name sounds familiar, it should: he's the Principle Investigator for the New Horizons Pluto mission, which will fly by the distant orb in 2015 and give us far better maps. Although the encounter will be brief, I expect we'll learn more about Pluto in a few days than we will have in the 85 years since it was first discovered.

Alan is a friend of mine, and we've had some lively discussions this past year over Pluto's planetary status, and what it means to be a planet. Because he's an expert on Pluto, I asked him to give me his opinion on some things you don't know about Pluto. The last two entries in this tour were his suggestions, so I'm indebted to him. Thanks Alan!

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9) Its pole is tilted more than Uranus's (122 vs. 98 degrees)

When you look at a classroom globe of the Earth, one of the first things you notice is that the Earth is tilted. Its axis of spin (the line through the Earth connecting the north and south poles) is tipped by about 24 degrees to the plane of its orbit. We think this may have been due to the immense grazing impact of a planetary-sized body shortly after the Earth formed. This impact blasted off debris which eventually formed the Moon, but also slammed the Earth off-kilter, giving it the current tilt.

By measuring the rotation of the planets, we've found that all of them are tipped a little bit to the plane of the solar system (except Mercury, which is in the fierce grip of the Sun, so tidal interactions have forced its axis to be perpendicular to its orbital plane). Uranus is tipped by more than 90 degrees, so in a sense it orbits the Sun on its side.

But Pluto outtips Uranus -- its tilt is 122 degrees. Something must have whacked Pluto pretty hard to tilt it that much. What this means is that from our point of view, we sometimes see Pluto looking down on its equator, and other times down on its pole. In fact, back in the 1980s we were seeing Pluto's system of moons edge-on, as it were. Charon would pass in front of and behind the planet, an event called mutual eclipses or mutual occultations. Those were used to make maps of the surface of both objects in the same manner as described above in #8.

So Pluto is slanty, even more than Uranus. But Venus has them both beat: its orbital tilt is 177 degrees-- it's actually almost completely flipped over! In other words, it rotates backwards, east to west. Whatever happened to Venus in its distant past must have been colossal and catastrophic. Flipping a planet all the way over is hard.