I have read, more than once, about the importance of giant planet, like Jupiter, in order for intelligent life to develope on Earth. The reasoning is that Jupiter can act like a vacuum and pull in threatening asteroids, comets, etc, and prevent them from striking Earth. I don't buy this. It seems that the size of Jupiter relative to the size of the 'sphere' of its orbit is pretty small. An object approaching Earth from beyond Jupiter would most likely cross Jupiter's orbit when Jupiter is somewhere else. And then there are the objects not even in the plane of the orbit.

Yeah, just look at how many craters are on the moon.

Still, where did you read it? It might have been just a casual comment.

Jupiter doesn't act like a vacuum cleaner for all objects entering the solar system. (The craters on Earth, and on the Moon are a testament to that.) However, Jupiter and the other planets are moving in a dynamic system and are not static. Incoming comets and asteroids are also in motion relative to the sun, and their orbits could be perturbed by the relation of Jupiter's motion in addition to its gravity. They can be deflected, but that doesn't mean they will all crash into Jupiter as Comet SHOEMAKER-LEVY did. I think the idea is that Jupiter's presence lowers the odds. However it is interesting that it is theorized that just after the early formation of the solar system, many more objects were crowding in, and that the planets and moons from Mercury to Mars were bombarded. Probably the gas giants too.

Actually, the Earth is a really tiny, hard-to-hit target. Jupiter is a much larger one (and easier to hit, given its gravity well) but it is also unlikely to be smacked by a 1st time visitor from the extreme outer solar system. It is much more likely that a comet or asteroid with Earth's name on it will be in Solar orbit for millions of years before kissing Terra Firma. That means that it likely crosses Jupiter's orbit many times, and even one near miss will give the object a lot of extra orbital momentum. This can have a result ranging from moving its closest approach to the Sun to outside the Earth's orbit to ejecting it from the solar system altogether!

From the simulations that I have seen, it reduces the chance of the Earth being hit pretty dramatically (60-90+% less hits than otherwise!) Yes, intelligent life could develop without Jupiter's help, but it is more likely that a asteroid or comet strike would wipe a planet's higher life forms than if a Jovian planet were in the system in the appropriate orbit.

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Think of it purely in terms of cross-sections: Jupiter's cross section (or silhouette) is about 100 times the size of the earth's. If you were a random bit of space junk, and you were going to hit something, the odds are 100 to 1 that it would hit Jupiter instead of the earth. So, on that basis alone, Jupiter has taken 100 hits that *might* otherwise have hit us, for every object that *did* hit us...

(That's simplistic, of course; we have the disadvantage of being lower in the sun's gravity well, and thus at a bit more peril... But it's one way of considering things...)

Silas

Too simplistic for my taste. Of those 100 objects, what are the odds that they would hit the Earth? If over the age of the Earth, the expected number of hits from those 100 is even as high as one, that would mean that Jupiter only halved the number of hits on Earth.

Not sure I follow that. Doesn't the distance of Jupiter matter too? If you assume that there is an equal probability of an object coming from any of 360 degrees towards the Earth, it seems to me that the moon would filter out more hits than Jupiter, and it is *much* smaller.
--Tommy
http://www.tommyraz.com

In our early solar system, Jupiter and the other outer planets caused a lot of objects to be expelled from our solar system. Or perturbed them enough that they crashed into the sun. So they don't have to actually hit Jupiter to get pushed out. The Trojans are still in orbit because they are in synch with Jupiter, but many other asteroids got tossed out.

IIRC, they say the oort cloud is mostly kuiper belt objects expelled by Neptune (?)

Just to tie together some points that were made:

Quoting from an astronomy textbook - Horizons, 6th edition, by Michael A. Seeds:

"...most of the craters we see [on moons in the solar system, Mercury, Venus, and Mars] appear to have been formed roughly 4 billion years ago in what is called the heavy bombardment, as the last of the debris in the solar nebula [that formed our solar system] was swept up by the planets."
(Earth's craters from that time have since been covered over by geological activity.)
Other space debris was ejected from the solar system "by close encounters with planets. If a small object such as a planetesimal passes close to a planet, it can gain energy from the planet's gravitational field and be thrown out of the solar system. Ejection is most probable in encounters with massive planets, so the Jovian planets [Jupiter, Saturn, Uranus, and Neptune] were probably very efficient at [this]."

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<font size=-1>[ This Message was edited by: nebularain on 2002-06-26 20:50 ]</font>