Tuning parameters for cv2.houghcircle() method ?

edit

Not sure that cv2.HoughCircles is the best option here. I recommend to try findContours also. Anyway question is about houghCircles, so example of code that detects all planets correctly:

import cv2
import numpy as np

planets = cv2.imread(path)
gray_img = cv2.cvtColor(planets, cv2.COLOR_BGR2GRAY)
gray_img = cv2.medianBlur(gray_img, 3)
circles = cv2.HoughCircles(gray_img, cv2.HOUGH_GRADIENT, 1, 10,
                    param1=350, param2=10, minRadius=2, maxRadius=13)
if circles is not None:  # circles found
    circles = np.uint16(np.around(circles))

    for i in circles[0,:]:
        cv2.circle(planets,(i[0],i[1]),i[2],(0,255,0),2)
        cv2.circle(planets,(i[0],i[1]),2,(0,0,255),3)

cv2.imshow("HoughCirlces", planets)
cv2.waitKey()
cv2.destroyAllWindows()

What I corrected in your code: set minimum distance between circles centers from 120 to 10, changed min and max radius for circle, changed param1 and param2. Also I adde check for case if no circles were found.

Docs that I used: link

Hope this helps!