As earth is also a magnet and we know that magnetic property decreases due to heating so why earth magnetic property means gravity does not decreases

Magnetism and gravity are separate properties. Try the sites below.

Q & A: Gravity vs. Magnetism | Department of Physics | University of Illinois at Urbana-Champaign
http://www.physlink.com/education/askexperts/ae472.cfm

Of course maybe you can be the one to answer the theory of everything.
Here's a little bit of observation I got off another site:

What can be observed: Gravity
- Mass has or causes a gravitational field in and around the space it's in.
- It costs no energy to maintain the force field.
- Gravity is a weak force, relative to other forces. It takes the mass of the whole earth to produce 1G on a small stone, while a simple string can carry that stone. The forces that keep molecules together are so much stronger than gravity.
- A planet has no gravity poles (like magnets have a north and south pole)
- The stronger the force of gravity in an area, the slower speed of time goes there.
- The gravitational field penetrates every known material. There is no known way to shield gravity. Although some have claimed a reduction of between 0.5 to 2% in weight, above fast rotating superconductors.
- Gravity can bend the path of light.
- A gravitational field reaches very far. No limit of distance is known.


What can be observed: Magnetism
- A permanent magnet uses no energy to maintain its force field.
- A flow of electrons creats a magnet field.
- A magnet has poles (north & south). A magnetic monopole is not know yet.
- An electro magnet uses energy to maintain its force field, unless it's a superconducting one (?).
- When shutting of the power to an electro magnet, the force field collapses, which produces an electric current while collapsing.
- Even atoms have tiny magnetic fields.
- A permanent magnet can be made by aligning the spin of hot atoms, in a magnet field, and then freezing their positions, by cooling.
- Not all atoms are attracted a magnetic force field.
Part of the energetic system of an atom, are one or more electrons. Electrons can be on their own also. A stream of electrons we call an electric current. When an electric current flows, it creates a magnetic field. This is easy to prove with a compass (that contains a permanent magnet)
As long as the current flows, there is a magnetic field. Less people know that it costs energy to create a magnetic field, and that when turning of the power, the field collapses and produces power.

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