The Breakthrough — Ann Tufariello

Thirteen-year old Jack is plucked from the clutches of a riptide by his older brother, Michael. When Michael slides into a coma, Jack can't live with the guilt. He wanders to a carnival and sneaks into a hot air balloon basket. Without warning, the balloon lifts into the air, transporting Jack to the fourth dimension where Earth isn't the only inhabited planet. His first stop takes him to Venus, where a young Venusian gives Jack a magic crystal to heal his brother. ...Jack thinks his problems are solved, but they are just beginning! While attempting to find home, Jack crash lands on Pluto--a planet controlled by the tyrant Danko. Danko learns of the crystal and will stop at nothing to seize it, and ultimately control Earth.