I have recently written about this issue. I will say it here again so that it may be of use:

Use the existing [Physics2D][1]. Don't reinvent the wheel. There's a component called [PointEffector2D][2], you're trying to achieve its effects manually, which will end badly if it's not implemented correctly.

Here's [the manual][3] what you want is to have inverse squared force mode with a negative number as the force. You don't need a rigidbody on the planet but you do need a [Collider2D][4] that will be [used by effector][5].


 [1]: https://docs.unity3d.com/ScriptReference/Physics2D.html
 [2]: https://docs.unity3d.com/ScriptReference/PointEffector2D.html
 [3]: https://docs.unity3d.com/Manual/class-PointEffector2D.html
 [4]: https://docs.unity3d.com/ScriptReference/Collider2D.html
 [5]: https://docs.unity3d.com/ScriptReference/Collider2D-usedByEffector.html