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Pluto Loses its Place

Week of February 8, 1999

On occasion, I will take a few weeks to explore a theme in astronomy on these pages. In 1997 that theme was the solar system. This time, for the next few weeks, I invite you to take a look at the Universe In Motion.

We think of the skies as static, unchanging. The only motions we see easily are the rising and setting of the Sun, Moon and stars, but that is a reflection of our own Earth's rotation. The stars themselves don't seem to move at all among themselves, and it takes a keen eye to discern the motion of the planets, which takes days or weeks to become obvious.

Yet objects in the sky are in constant motion, and some move with an incredible intrinsic velocity. Usually, these objects are so far away that the distance itself shrinks the apparent motion, the way distant mountains hardly seem to move at all even though you may be driving past them at 100 kilometers an hour. Sometimes it does take many years to perceive the movement of heavenly bodies, and sometimes it happens in the blink of an eye. Every Monday, we'll take a look at some of these celestial travelers.


Last week, I mentioned that I couldn't start this Universe in Motion series at the Earth and move outwards. The reason is that I wanted this week's Snack to coincide with the actual event it describes.

Most people like to think of the solar system as relatively unchanging in its ways. The Sun stays at the center and the planets orbit it serenely, each in its place. But it's not really that way. Tiny Pluto, orbiting the Sun an average of nearly 40 times the Earth's orbital radius, has an odd orbit. Most planets orbit the Sun in nearly perfect circles, but Pluto's orbit is noticeably elliptical. Most planets orbit the Sun in the same plane; that is, seen edge on the solar system is very nearly flat. But not Pluto. It's orbit is tilted about 17 degrees from the Earth's. Some people (myself included, to be honest) argue that these characteristics are more like the newly discovered Kuiper Belt objects, giant frozen iceballs that orbit the Sun at tremendous distances. But even these quirks of Pluto are not its worst offense. Pluto's problem is that it doesn't know its place. We call it the Ninth Planet, but for 20 years out of its 247 year orbit it gets uppity. It gets closer to the Sun than Neptune!

Thank its elliptical orbit for that. Although Pluto usually stays about 6 billion kilometers from the Sun, at closest approach (called perihelion) it is only about 4.5 billion kilometers away. That distance is a few million kilometers less than Neptune's average distance from the Sun, so in essence Pluto and Neptune swap positions in the planetary pecking order. Pluto reached perihelion last on September 5, 1989. Since then it's been moving slowly away from the Sun, and on February 11, 1999, at 11:22 Greenwich Time, it will once again be farther from the Sun than Neptune, and once more take the reigns as keeper of the long night.
[Note (Feb 10 1999): I made a mistake in my original snack here: I said on the 11th, Pluto passes Neptune's average distance form the Sun. That's not correct; what I have posted now is the right idea. Thanks to Rick Johnson of the Hyde Memorial Observatory for pointing out my mistake!]

Since the two planets cross orbits, many people erroneously think that in the future, they might collide. However, it cannot happen. The orbits of Pluto and Neptune have a coincidental timing to them: Pluto orbits the Sun twice for every three times Neptune does. Now sometimes we find that when two objects have orbits like this, the smaller of the bodies can be gravitationally moved by the larger (for example, Saturn's moons sweep away gaps in the planet's rings). But not in this case! Because of this coincidence in timing, Pluto always reaches perihelion when Neptune is a quarter of the way around the Sun, much too far for any collision. So Pluto is safe for as long as we can mathematically calculate.
Thanks to Bill Wyatt for correcting an error I had here about the relative positions of Pluto and Neptune when the former is at perihelion.

One other weird thing: Since Neptune and Pluto apparently avoid each other, Pluto's minimum distance to Uranus is actually smaller than its minimum distance from Neptune, even though Neptune is farther from the Sun than Uranus. I'll be honest and admit I was quite surprised to learn that when I did the research for this Snack. We've been observing the solar system since before we started recording history, but it still manages to surprise us.

For more info about Pluto, I suggest trying Marc Buie's excellent page. Also for everything solar system, go to Bill Arnett's The Nine Planets. He has an excellent Pluto page and a very good diagram showing just why Neptune and Pluto can never collide!



©2008 Phil Plait. All Rights Reserved.

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