**Mass and Weight**

The weight of an object is the force with which it is pulled towards the center of the planet or any other celestial body. It is directly proportional to the mass of the object. The formula to calculate weight is:

Weight = Mass × Acceleration due to gravity

Where:
- Weight is measured in Newtons (N)
- Mass is measured in kilograms (kg)
- Acceleration due to gravity is measured in meters per second squared (m/s²)

**Given Data**

- Mass of the body = 500g = 0.5kg
- Initial velocity of the body = 20m/s
- Time taken to reach back to the surface = 20s

**Finding Acceleration**

To find the acceleration, we can use the equation of motion:

v = u + at

Where:
- v is the final velocity
- u is the initial velocity
- a is the acceleration
- t is the time taken

Rearranging the equation to solve for acceleration:

a = (v - u) / t

Substituting the given values:

a = (0 - 20) / 20
a = -20 / 20
a = -1 m/s²

The negative sign indicates that the acceleration is in the opposite direction to the initial velocity.

**Finding Weight**

Now, we can calculate the weight of the body on the planet using the formula:

Weight = Mass × Acceleration due to gravity

The acceleration due to gravity on the planet is not provided in the given data, so we cannot directly calculate the weight. Hence, we would need additional information about the planet's gravity.

Once we have the acceleration due to gravity on the planet, we can substitute it into the formula along with the mass of the body to find the weight.

Note: The calculation of weight depends on the acceleration due to gravity, which varies from planet to planet.