开发环境：Unity 2022.3.5f1c1 + Visual Studio 2022
太阳系相关星体：太阳、八大行星、月球
模拟星系：太阳系、地月系
功能：支持行星以太阳为中心，任意轴进行公转，此处演示同一平面。
a1-a8为公转轴，可以任意修改，InitStarPosition()将会修正星体初始位置；MoveAroundSun脚本挂在SolarSystemGroup对象上（空对象）。
动态演示效果

静态展示图片

核心代码

using UnityEngine;

public class MoveAroundSun : MonoBehaviour
{
    // 星体
    public Transform Sun;
    public Transform Mercury;
    public Transform Venus;
    public Transform Earth;
    public Transform Moon;
    public Transform Mars;
    public Transform Jupiter;
    public Transform Saturn;
    public Transform Uranus;
    public Transform Neptune;

    // 公转轴：Mercury至Neptune八大行星
    Vector3 a1 = new Vector3(0, 1, 0);
    Vector3 a2 = new Vector3(0, 1, 0);
    Vector3 a3 = new Vector3(0, 1, 0);
    Vector3 a4 = new Vector3(0, 1, 0);
    Vector3 a5 = new Vector3(0, 1, 0);
    Vector3 a6 = new Vector3(0, 1, 0);
    Vector3 a7 = new Vector3(0, 1, 0);
    Vector3 a8 = new Vector3(0, 1, 0);

    /// <summary>
    /// 初始化行星位置，使Star位于以Center为中心，rotateAxis为公转轴的法平面上
    /// </summary>
    /// <param name="center">公转中心</param>
    /// <param name="star">公转行星</param>
    /// <param name="rotateAxis">公转轴</param>
    void InitStarPosition(Vector3 center, Transform star, Vector3 rotateAxis)
    {
        // 中心点指向行星的方向向量
        Vector3 centerToStar = star.position - center;
        // 通过方向向量和公转轴向量的叉乘，得到这两个向量所在平面的法向量
        Vector3 normal = Vector3.Cross(rotateAxis, centerToStar);
        // 计算方向向量和公转轴向量的角度
        float angle = Vector3.Angle(rotateAxis, centerToStar);
        // 计算方向向量转动到法向量的角度（转动到法平面上的角度）
        float rotateAngle = Mathf.Abs(90 - angle);
        // 以太阳为中心，法向量为转轴来转动角度
        star.RotateAround(center, normal, rotateAngle);
    }

    // Start is called before the first frame update
    void Start()
    {
        // 为了让任意位置的星体都可以绕任意轴旋转，需要在初始时刻定位星体位置（只挂载一个脚本，所以有重复代码）
        InitStarPosition(Sun.position, Mercury, a1);
        InitStarPosition(Sun.position, Venus, a2);
        InitStarPosition(Sun.position, Earth, a3);
        InitStarPosition(Sun.position, Mars, a4);
        InitStarPosition(Sun.position, Jupiter, a5);
        InitStarPosition(Sun.position, Saturn, a6);
        InitStarPosition(Sun.position, Uranus, a7);
        InitStarPosition(Sun.position, Neptune, a8);
        InitStarPosition(Earth.position, Moon, Earth.transform.up);
    }
    // Update is called once per frame
    void Update()
    {
        Mercury.RotateAround(Sun.position, a1, 20 * Time.deltaTime);
        Mercury.Rotate(Vector3.up * 50 * Time.deltaTime);

        Venus.RotateAround(Sun.position, a2, 10 * Time.deltaTime);
        Venus.Rotate(Vector3.up * 30 * Time.deltaTime);

        Earth.RotateAround(Sun.position, a3, 10 * Time.deltaTime);
        Earth.Rotate(Vector3.up * 30 * Time.deltaTime);
        Moon.transform.RotateAround(Earth.position, Vector3.up, 359 * Time.deltaTime);

        Mars.RotateAround(Sun.position, a4, 8 * Time.deltaTime);
        Mars.Rotate(Vector3.up * 30 * Time.deltaTime);

        Jupiter.RotateAround(Sun.position, a5, 7 * Time.deltaTime);
        Jupiter.Rotate(Vector3.up * 30 * Time.deltaTime);

        Saturn.RotateAround(Sun.position, a6, 6 * Time.deltaTime);
        Saturn.Rotate(Vector3.up * 30 * Time.deltaTime);

        Uranus.RotateAround(Sun.position, a7, 5 * Time.deltaTime);
        Uranus.Rotate(Vector3.up * 30 * Time.deltaTime);

        Neptune.RotateAround(Sun.position, a8, 4 * Time.deltaTime);
        Neptune.Rotate(Vector3.up * 30 * Time.deltaTime);
    }
}