We found that the solar panel output current of JAXA's Venus explorer Akatsuki is below values expected from percentage of shadow during penumbras. We also found that a similar phenomenon occurs in JAXA's Van Allen radiation belts explorer Arase during Earth shadow penumbra. However, during a Lunar shadow penumbra, solar panel output current decreased in proportion to percentage of shadow. We consider that sunlight attenuated by the planetary atmosphere caused this decrease in current. As in the transit spectroscopy technique, planetary atmospheres could be characterized by trends in solar panel output current.