Possible Clues:

• Astronomy's ___ cloud
• Jan --, Dutch astronomer
• Dutch astronomer Jan
• Dutch scientist for whom an astronomical 'cloud' is named
• ___ cloud (source of comets)
• Dutch astronomer who lent his name to a cloud
• Dutch scientist with an eponymous 'cloud'
• Astronomer captivated by Pluto or thereabouts
• __ cloud: solar system region beyond Pluto
• ___ cloud (astronomical sighting)