You can plot single points for Earth and Mars, and then update each point's position to the current frame coordinates.

import matplotlib.pyplot as plt
import matplotlib.animation as animation

earthx = planetRs[0][0]
earthy = planetRs[0][1]
marsx = planetRs[2][0]
marsy = planetRs[2][1]

fig, ax = plt.subplots()

line1, = ax.plot([], [], 'bo', label='Earth')  # Earth
line2, = ax.plot([], [], 'ro', label='Mars')   # Mars
ax.set(xlim=[-2, 2], ylim=[-2, 2], xlabel='X position', ylabel='Y position')
ax.legend()
ax.grid(False)

def init():
    line1.set_data([], [])
    line2.set_data([], [])
    return line1, line2

def update(frame):
    line1.set_data(earthx[frame], earthy[frame])
    line2.set_data(marsx[frame], marsy[frame])
    return line1, line2

ani = animation.FuncAnimation(
    fig, update,
    frames=len(earthx),
    init_func=init,
    interval=30,
    blit=True,
)
plt.close()
ani