Why is it that the inner planets are all terrestrial, and the outer planets are all gas giants?

Question

There is an obvious pattern- all inner planets are terrestrial, and all outer planets are gas giants. According to the formation of the planets, why is this? And is the fact that there is an asteroid belt in between a coincidence, or is there some reason behind it having to do with the seperation of terrestrial planets from gas giants?

PS: Sorry if I spelled some things wrong, I am a terrible speller, and Yahoo took away the spell check.

Thanks in advance!

Answer 1

There are quite a number of theories of planet formation, as I imagine you are aware, but none of them are really critical to answering your question.  Whether they formed in a coalescing spiral as the sun began, or were ejected, or any other means, the simple fact is that any gas giant forming near the central star would soon not be a gas giant, as the radiation or the star would accelerate the conglomerate gases to escape velocity and literally blow them away with the "solar wind".  Indeed, it is plausible that all of the planets began as "gas giants", but were eventually stripped of their atmospheres and all that remains is the original core and a relatively thin atmosphere, grasped by the core's gravity only because of its proximity to the solid surface.

The asteroid belt is almost certainly the remnant of a planet that broke up as the result of an impact.  None of the gases that once appended to it remain.

There is no reason to believe that our solar system is typical.  Others may well have larger planets nearer the primary, with smaller ones farther out.  However, a simple mathematical model applied to the notion that planets are an accretion of matter in a swirling eddy that swung in the vicinity of the primary as it formed can show that the mass available for planet formation would be minimized as one neared the primary due to the greater attraction of the primary itself, leaving the nearer accretions bereft of mass.  That is, the gravity well of the primary would leave very little for the accumulating mass of the nearer planets to sweep in to their puny gravity wells, while the more distant eddies would influence more greatly the coalescing matter close to them than would the primary at a greater distance, so they could grow larger.  Even farther out, the density of the coalescing cloud would be lighter, resulting in ever smaller planets, though they are larger than those deeper in the well..  This is what we observe in the progression of planetary sizes in our system, more or less.

Indeed, the gravity well of Jupiter is so powerful that it consumes quite a bit of random bits of our solar system, and has been doing so for a very long time.  It is likely that it has grown significantly since its initial formation, and continues to grow more rapidly than any other planet, simply by attracting passing comets and other random stuff.

A more intriguing question arises when one considers the nearly planar aspect or our solar system.  The eddy activity we observe on this planet's surface is pretty much planar, as it occurs parallel to the surface of the planet.  It is flat.  Our solar system has no "flat" point of reference, so the random accretion of dust and gas should have been less planar.  Yet, only one planet in the system strays significantly from the "plane of the ecliptic".    There is poor probability to explain this.  Indeed, probability denies it.  The eddy activity that forms a solar system should not lie in a single plane, nor shout the eddy activity that form a galaxy.  

Reword this response, find the math and crunch the numbers, and you could write an "A" paper for any High School class and many college level courses.  

Hope this helps.

Answer 2

They sometimes call the giant planets the ice giants.  At a certain distance from the Sun, which might very well be near the orbit of Jupiter, water can form a solid and not sublimate to a gas.  Therefore it can stick to the planetesimals that were forming.  This made them grow much faster than those closer to the Sun.  So they got big fast, and quickly got to the size where they could attract gas and gravitationally hang on to it.

The asteroid belt between Mars and Jupiter does have Ceres, which has a surface of water ice, and probably a water ocean.  I'm not sure if it is known if Ceres formed there.  It may have formed farther out and migrated in.  The Dawn mission may tell us something about that in a few years.  But in any case, it is thought that Jupiter's gravity would have disrupted planet formation in this zone, leaving lots of smaller bodies.  Currently, it is thought that there is not enough mass there to form, for example, an Earth sized planet.

Answer 3

spell check is still there  ... -------->

as to the make-up of the Solar System, well, until we get to see a few more examples, we are gonna have to guess.  We had GREAT theories before we knew about extra-Solar planets, but now that we have seen gas giants orbiting at 0.02 AU, well, the old "It's too warm up close for gas giants" argument is rather moot.

one theory that doesn't sound TOO far-fetched is that in some stellar systems there is a LOT of dust, so much dust, in fact, that it causes gas giants to spiral in to close orbits around their primary.

why didn't this happen in our Solar System?  we really don't know, but there are theories, dust got mopped up by asteroids... etc.

personally, I love the old T Tauri theory.  Our Sun was a wild und crazy adolescent and he... er, she erupted in a few flares blowing most of the light stuff out in orbits closer than Mars.

There is still quite a bit of dust floating around, you can even see some of it on a clear night just after dark shining as the Zodiacal Light.

Answer 4

To be honest, it was pure luck on our part that things ended up the way they did. Many of the planets that have been found around other stars are gas giants that orbit very close to the star itself.  

Answer 5

All planets have a terrestrial core, even the gas giants.

Each star began as a spinning disc of superheated gases. As these cooled, the highest boiling elements liquifuied first. This resulted in a molten sphere of heavier elements, surrounded by a gaseous envelope of elements with low boiling points.

NOTE: Two factors affect a planet's ability to hold gaseous elements

           in an atmosphere. First is gravity. Gaseous molecules have

           speeds measured in kilometers per second. If a planet is too

           small, its gravity cannot keep the gaseous molecules from

           escaping into space. Second is temperature. Temperature is a

           measure of molecular motion. The hotter the planet, the

           greater the velocity of the molecules in the atmosphere.

          

In our planetary system the following resulted.

Mercury: Too small and too hot to retain an atmosphere.

Venus: Large enough to hold an atmosphere, but too hot to retain

           hydrogen (so no water). The nitrogen all escaped. The oxygen

           would also have escaped, but it is a very reactive element, and

           formed compounds with other elements, particularly carbon and

           sulfur. As a result, the atmosphere of Venus is mostly carbon

           dioxide and some sulfur dioxide. Carbon dioxide, being a

           greenhouse gas, makes Venus as hot as Mercury, even

           though it is farther from the sun.

Earth:  Being larger and cooler than Venus, it retained its nitrogen and

           oxygen. Nitrogen is fairly inert and mostly remained in its

           elemental state. The oxygen reacted with the hydrogen to form

           water, preventing its escape. It also formed enough carbon

           dioxide to sustain the life cycle. The remainder of the oxygen

           stayed in the atmosphere.

Mars: Being much smaller than Earth, it lost most of its atmosphere,

         but what remained is similar to Earth's. Any water would likely

         exist as ice.

Gas Giants: These planets are far enough away from the Sun to be

         very cold, so even hydrogen could not escape. They have rocky

         cores surrounded by huge envelopes of gas. Their atmospheres

         consist of methane and ammonia, both hydrogen compounds.

         Any water would exist as ice crystals in the interior.

Pluto: Any gaseous elements would be frozen on its surface at -300*

          or so.

The Asteroid Belt: Unknown at this point.

One could expect the same scenario in the planetary systems of all

stars.