The amount of gravity required to keep a planet in orbit depends on the mass of the planet and the radius of its orbit. The greater the mass of the planet, the greater the amount of gravity is needed to keep it in orbit. The greater the radius of the orbit, the less the amount of gravity is required to keep it in orbit.

The formula for calculating the amount of gravity needed to keep a planet in orbit is:

```

F = GMm / r^2

```

Where:

* F is the force of gravity in newtons

* G is the gravitational constant (6.67 x 10^-11 N m^2 kg^-2)

* M is the mass of the planet in kilograms

* m is the mass of the object in kilograms

* r is the radius of the orbit in meters

For example, the amount of gravity needed to keep the Earth in orbit around the Sun is approximately 3.52 x 10^22 N. This is the force of gravity exerted by the Sun on the Earth.