What determines heaviness of objects in Space?

[Observer2000]

TL;DR Summary

Why planets/stars are considered heavy?

Why for gravity some things like planets, stars are heavy enough to have a gravity so they don't fall of the space?

When a small object can free fall in space, why a planet has a gravity, and they are considered heavy to what formula?

 

[Ibix]

All objects, large and small, have mass and have gravity. It's just too weak to have much effect for anything smaller than something like an asteroid.

The formula for calculating the force of gravity is Newton's law of gravity, $F=GMm/r^2$.

 

[Observer2000]

Thank you Ibix, I will check this out.

Cheers!

 

[PeroK]

Everything "in space" is in free fall: satellites, asteroids, comets, planets, stars, black holes, galaxies. They all move according to their combined gravitational attraction.

 

[phinds]

Why planets/stars are considered heavy?

I suggest that you learn the concepts of weight, mass, and density.

Jupiter Saturn is a very large planet with a large mass, but if it were put into a large enough bathtub with Earth level gravity it would float. Does that count has "heavy"?

As @PeroK pointed out, things in space are generally in free fall which means that they have zero weight.

EDIT: corrected: Saturn, not Jupiter, is lighter than water. My thanks to @SammyS for the correction.

 

[jbriggs444]

Jupiter is a very large planet with a large mass, but if it were put into a large enough bathtub with Earth level gravity it would float.

If you put Jupiter into a bathtub large enough to to contain it, the bathtub water would fall into Jupiter.

If you had a bathtub large enough to contain the water, it would take the form of a spherical blob of water with a crumpled up bathtub in the center.

As @PeroK pointed out, things in space are generally in free fall which means that they have zero weight.

"Weight" can be an ambiguous concept. By one definition, it is the force of gravity on an object. So $W = mg$. By another definition, it is the force that would be needed to support an object, keeping it stationary in the chosen frame of reference. So for an earthbound object $W = mg - F_\text{centrifugal}$.

If we use the former definition, things in space have non-zero "weight".

If we use the latter definition, things in space have [near] zero "weight" relative to a reference frame tied to the space craft within which they float.

Some might even define "weight" as the actual downforce currently exerted by an object on its support. So the "weight" of an athlete jumping up and down would vary during the exercise.

 

[sophiecentaur]

Saturn is a very large planet with a large mass, but if it were put into a large enough bathtub with Earth level gravity it would float. Does that count has "heavy"?

That thing abut Saturn is a real hindrance to understanding. N3 always applies and ruins the Saturn example because what would be 'attracting Saturn' for Saturn to have actual weight? It's just another of those things, originally introduced by someone with poor understanding of the consequences of over-simplification.

There aught to be a LAW which forbids use of the word "weight" in all cases that don't actually involve a small object being pulled to an extremely large one, i.e. an interaction between a planet and a small object. But perhaps 'mass' is a harder concept than I think. Weight and Mass is the number one stock joke about Physics teachers. I'm afraid I blame the present status of Weight on the people who insist on keeping to the pound.

(There's no point in telling me about the 'slug' because there are even fewer people who recognise that term.)