The planets

[xud]

Why are the planets in our Solar System all spherical.
There are millions of irregular shaped ateroids and stuff but all the planets are round - why?

Also, Pluto is no longer classed as a planet. For what reason and what has it been classed as now?

Thanks in advance

[matt_r_baker]

planets are spehical (well earth is an oblate spehoid i.e. its got a bulge in the middle) that doesnt matter at the mo...... the planets are round because the sphere is a perfect shape and all objects in the universe gradually decay to that shape..... the funny shape asteroids are newer than the planets so havent decyed yet to this perfect shape.
To see a sphere as the perfect shape just look at a football comared to a rugby ball..... when a football bounces it bounces exactly as is expected (discounting bumps and lumps on the ground) yet a rugby ball bounces in an almost random pattern.



The pluto question refers to not just the size of the planet but also the orbit. To orbit isn't in the same pattern as the other planets. http://upload.wikimedia.org/wikipedia/en/8/82/ Mainkuiperbelt.jpg

(i hope that link works)

[matt_r_baker]

sorry i forgot to explain the pic...... the circles in the middle are the normal planet orbits but the angled one is plutos

[jake-the-peg]

When a body is massive enough it becomes roughly spherical enough due to it's own gravity.

This was one definition of a planet considered, a body orbiting a star, massive enough to be mostly spherical under it's own mass.

However the International Astronomical Union added that it also had to have cleared the neighbourhood of other bodies and as there are a lot of other Kuiper belt objects knocking around there Pluto was demoted.

It may be classified as a dwarf planet in the near future.

There's more on it on good old wikipedia http://en.wikipedia.org/wiki/2006_definition_o f_planet

But I don't think many professional astronomers care very much what it's called there are many more interesting things in astronomy than naming a chunk of rock half the size of Africa

[Clanad]

While I understand the intent of jake's response, I would argue that it's not just the gravitational aspects that causes sphericity. As the bodies gravitation reaches a certain point, as jake implies, it begins attracting dust and other debris, which, as it impacts the young planetoid, raises the temperature sufficintly to produce a plasticity due to the heat generated. The continued spinning producing oblateness and growing gravitational forces of the newly developing planet produce near spherical shapes. By the way, Mars moons, Phobos and Deimos are oddly shaped... seen here http://history.nasa.gov/SP-441/ch9.htm ,probably because they are captured asteroids and haven't the mass neccessary...

[mibn2cweus]

Planets take a spherical shape for the same reason the surface of an undisturbed body of water becomes flat, level and smooth. Higher areas lack support and fall to the level of the surrounding surface.

Solid features resist conformity but nevertheless are subject to the same equalizing forces of gravity and are therefore limited by their mass and their ability to support themselves as to how irregular a surface they can maintain.

The rotation of a planet tends to flatten it somewhat at the axis of rotation as mass in the equatorial zone is subject to inertial forces in proportion to the velocity of rotational momentum accelerating them outward (The pizza crust effect). The poles do not share the inertial momentum of the equator associated with rotation.

[Teddio]

A body will tend to minimise its gravitational potential energy by forming a sphere, irrespective whether it's solid, liquid or gas. Bodies such as the moons of Mars, Phobos and Deimos, have very low mass and the gravitational force is too small to overcome the tensile and compressive forces inherent in the body. They can, therefore, exist as potatoe-shaped moons. Above a certain mass the force of gravity is too strong and the body will assume a spherical form. Since the accreting material which "condenses" to form the planet will have a net angular momentum component, the resulting planet will spin and assume the shape of an oblate spheroid.

[xud]

Thanks for info. One thing puzzles me. The asteroid belt lies between Mars and Jupiter, so age alone can not explain why asteroids are not spherical for they must be that same age as the planets.
It must be because they are simply not massive enough to be crushed/formed as it were, into a sphere. But they orbit the Sun in the same manner as the planets???

[jake-the-peg]

Let's see if I can flesh this out for you a bit more.

Our solar system comes from material distributed in space, much from the debris of an earlier huge star that went supernova (I hate using that as a verb)

This cloud of debris started to contract under gravity. Natural variations in the density this cloud meant that some points grew bigger than others and became centres of attraction. This contracting in increased the spin like an ice skater pulling in her arms.

Where this spin resulted in a stable orbit the resulting planet or asteroid or even grain of rock was able to avoid capture hence the asteroids you refer to although they're not all between Mars and Jupiter

This resulted in various bodies orbiting a central massive body. At some point there was enough mass there to ignite fusion reactions causing the sun to light up.

But all this attaction resulted in an awful lot of impacts and a constant bombardment this meant that the inner planets which are now quite rocky were pretty much molten lumps at this time ( which we refer to as the Hadean era - after Hades). So the planets own gravitational force is able to shape it into a roughly spherical shape.

[wendilla]

Sorry to hi jack your page but this is where I learn .Can any of you put me on the right track to buy a telescope .I am just starting to learn and bought binoculars 12x50 which do show the moon craters but I would like to buy a telescope and just don't know what size to go for .something about �100-�150 something a bit larger that what I already have.Thanks for any info.