Space: the crossword puzzle

has a tail of gas that points toward the Sun
one of the lightest gases in the periodic table
appearing as a streak of light in the sky every few years
agency that is responsible for aerospace research and a space program
means how fast radiation travels in a year
has a dark silhouette and is filled with clouds of dust and gas
seen as the well-ordered whole of the universe
the type of planet that Neptune and Uranus are
a planet that orbits outside our solar system
a rocky or metallic objects that orbits around the Sun