Showalter was born in
Abington, Pennsylvania. He enjoyed playing with science-themed toys while a child, and later mowed lawns as a teenager so that he might purchase a
telescope in high school. He received a
Bachelor of Arts in
physics and
mathematics from
Oberlin College in 1979. He was initially undecided about pursuing a career in
astronomy after his undergraduate education, but made up his mind after seeing the images of Jupiter sent back to Earth by
Voyager 2. Showalter received his
MS in
astronomy from
Cornell University in 1982, and his
PhD from Cornell in 1985. His thesis was on
Jupiter's
ring system, in which he discovered
the gossamer ring of Jupiter. In 1990, using ten-year-old
Voyager data, Showalter discovered
Pan, the eighteenth and innermost
moon of Saturn. It orbits within and keeps open the
Encke Gap in Saturn's rings via shepherding. In 2003, Showalter and
Jack J. Lissauer discovered two new moons of
Uranus (
Mab and
Cupid) in
Hubble Space Telescope images. In 2006, they announced the discovery of two very faint rings, the μ and ν rings, within the same data. In 2010, Showalter discovered that spiral vertical corrugations in Jupiter's rings were caused by the impact of
Comet Shoemaker–Levy 9 in July 1994. A second smaller set of corrugations appear to be consistent with an unknown impact in early 1990. He and co-researchers also found similar spiral patterns in Saturn's D Ring. Showalter has assisted the New Horizons team in determining what hazards the spacecraft would encounter as it flew close to Pluto. A search for faint dust rings of Pluto using the Hubble Space Telescope in 2011 led to the discovery of the fourth moon
Kerberos. On July 15, 2013, a team of astronomers led by Showalter discovered a previously unknown fourteenth moon of Neptune in images taken by the Hubble Space Telescope from 2004 to 2009. Unnamed at that time,
Hippocamp is thought to measure around 34.8 km in diameter. Showalter was the 2021 recipient of the
Harold Masursky Award for Meritorious Service to Planetary Science. The
Mars-crossing asteroid
18499 Showalter is named after Dr. Showalter. ==Personal life==