Ok this may be a dumb one, but I noticed the itinerary for the New Horizons probe we went out in Jan 2006.
To my surprise, it got to Mars (when I mention planets I mean just the orbits, but you get whatimeanright) in only 4 months, quicker than I expected; then on to Jupiter in just a year more or so. Quicker too than I expected.
But then to Saturn (June of 2008) took as long as the entire trip to Jupiter took; and then only passed Neptune in 2011. And won’t cross Uranus until 2014.
Ok why are the outer planets so damn far away? It seems weird that they would not be more equidistant.
Don’t say my elementary school teacher lied when that chart showed Pluto a spitting distance from us.
Well, depending on your age, your elementary teacher lied to you about Pluto even being a planet! But that’s another discussion altogether.
There’s no reason for planets to be equidistant to each other. While there’s always a slight gravitational pull between any two objects, the orbits of planets aren’t really influenced by the other planets that happen to be orbiting them. What affects the orbit more than anything is the speed and direction a passing rock (molten or frozen) happened to be in when captured by the sun’s gravitational well.
I’m not a cosmologist, but my guess is that, as the Solar System was forming, any material orbiting around the Sun starting clumping together at intervals according to the Titius–Bode law because clumps naturally attracted each other under gravity unless they were far enough apart.
There are other planets that are supposed to be at those intervals. They’re just inside Jupiter and Saturn. That’s how they got so big in the first place. What I mean is that big planets clear big orbits. There can’t be anything near them or they’d have been sucked in by now, most likely.
ETA: Of course, this isn’t some physical law of the universe or anything. You’d think that a star would clear a pretty big area of the accretion disk, and yet we have hot jupiters all over the place. Sup widdat?
Obviously, the planets are not equidistant… we’ll have to have a talk with that teacher of yours.
It seems to me that the size and placement of planets is largely random. If they’re too close, then they would interfere with each other, but that’s a pretty small limitation on placement. Certainly, our limited studies of extrasolar planets show that large planets can occur at all kinds of distances from a star.
But it’s also worth noting that space travel has a lot variables other than distance between orbits. Gravity can both slow down and speed up probes - the sun is constantly slowing it, and we use slingshot techniques to get a boost from planets like Jupiter. New Horizons has a path that is a pretty straight shot out of the solar system, but planets don’t always line up so nicely; often, the path between two orbits is a very long arc of a spiral rather than a straight line.
There is the rather enigmatic issue of Bode’s Law. This fits all the orbits out as far as Uranus with worrying accuracy. It does suggest that there is some underlying physical mechanism that controls the orbits, at least for those planets that formed from the main accretion disk. It doesn’t work for Pluto and Kuiper Belt objects.
The Titus-Bode law is now considered to be co-incidental.
Over many millions of years the solar system has organised itself. Gravitational interactions between the planets make some configurations unstable. For example, Jupiter’s gravitational influence as it grew prevented the asteroid belt from coalescing into a single large body. It pulled the asteroids into more eccentric orbits, so that collisions betwen them were more energetic, and tended to break them into smaller pieces rather than sticking together. Models of solar system formation suggest that the outer planets initially formed much closer to the sun than their current locations, and an orbital resonance between Jupiter and Saturn pushed Uranus and Neptune to their corrent locations.
I think that Hot Jupiters are pretty rare, and it’s a case of observational bias. It’s much easier to find large exoplanets that are close to their parent star with current detection methods. We don’t know enough about other planetary systems to know if ours is typical or not.
The placement of the largest of a system of planets is random. (Jupiter in the familiar case) The rest of the planets must then occupy orbits such that the average perturbation due to the big dude is zero. If the gravity of the big planet is too strong at the nearer possible locations, the + and - perturbations will be too large to allow a planet to form, even though the average perturbation is near zero. This is why we have an asteroid belt between Mars and Jupiter. If Jupiter were somewhat smaller, the asteroids could agglomerate into another planet.
To the OP’s specific question regarding the outer planets: It will normally work out that each of the outer planets is twice as far from the star as it’s nearest neighbor. Because gravity decreases as the square of distance, the outer planets can be a lot bigger than the inner ones without ripping each other apart.
So far as time of travel to a planet goes, the distance at which it orbits the Sun is only one factor, and may not be the most important, so travel times will not reflect orbital distance from the Sun in any straightforward way. Whereabouts along its orbit a planet is at the relevant time is also important. If you keep going outwards in the plane of the ecliptic, directly away from the Sun, you are not going to reach each of the planets in order one after another (although you would, more or less, cross the orbit of each, one after another). To actually reach a second planet, after you have reached a first, you will almost certainly have to go a long way “sideways” as well as outwards, and just how far “sideways” you will have to go will depend on the actual configuration of the solar system at the relevant time. This, or course, is constantly changing as the planets move around their orbits, but, fortunately, in a highly regular and predictable way.
The answer to the OP has to do with where the planets formed and how they migrated afterwards. The Nice model has the giant planets forming in between 5 and 17 AU and then migrating to their current locations. Possibly Neptune started closer to the sun and swapped positions with Uranus.
For the inner planets, there’s a model which has them all forming between .7 and 1 AU with Jupiter forming about 3.5 AU out and then some subsequent migrations. It’s described here. This model disagrees with the Nice model as to the distance Jupiter formed and how it moved, so there’s obviously going to have to be some modifications of one or both to get them in sync.
What I mean is that big planets clear big orbits. There can’t be anything near them or they’d have been sucked in by now, most likely.
