Using the Palomar 18-inch telescope, astronomer Eleanor Helin is credited with discovering or co-discovering 516 asteroids in addition to several comets. She initiated the Palomar Planet-Crossing Asteroid Survey in the early 1970s. An advocate of bringing automated methods to the work of conducting astronomical surveys, she later became the principal investigator for NASA’s Near-Earth Asteroid Tracking program. During eleven years of operation, this program found more than 36,000 Solar System objects, 442 of which have been classified as near-Earth objects.
Carolyn Shoemaker was an astronomer who beginning in 1980 used the 18-inch telescope to search of Earth-crossing asteroids. Gene Shoemaker was a geologist and an early advocate for the science of astrogeology. Together with David Levy, they are credited with using the 18-inch telescope to discover Comet Shoemaker-Levy 9 on March 24, 1993. Over a year later in July 1994, this comet broke apart before colliding with Jupiter.
Caltech astronomer Mike Brown used the 48-inch Samuel Oschin Schmidt telescope to discover an object beyond the orbit of Neptune that would eventually be named Eris. It was this discovery on January 8, 2005 and question of whether or not this object ought to classed as a planet that led directly to a protracted controversy over Pluto's planetary status. This debate resulted in the creation of the new category of “dwarf planet” and the demotion of Pluto. Mike Brown styles himself as the man “who killed Pluto.”
MOREHumans have viewed the night sky with wonder for millennia, watching the stars as they seem to pass east to west above us. On human timescales almost all stars in our sky appear static and unchanging. But even the earliest astronomers and astrologers could identify a few bright lights that appeared to move against the static background. Greek astronomers called these lights wandering stars or planets. Nicolaus Copernicus, Tycho Brahe, Johannes Kepler, and Galileo Galilei, established these planets as our Solar System neighbors, co-orbiting the Sun in roughly the same orbital direction and plane as Earth.
Telescopes in the 19th century hinted at a richer Solar System architecture. A handful of asteroids were discovered between the orbits of Mars and Jupiter in the early 1800s, but by the time Palomar 18-inch observations began in 1936 a growing population of asteroids had been found in this so-called asteroid belt. In addition, comets with their spectacular tails were found in highly elliptical and often inclined orbits—unlike the roughly circular orbits of planets and main belt asteroids.
Since the 1940s Palomar has been central to exploring diversity in Solar System architecture. Sky surveys conducted with the Samuel Oschin Telescope and the 18-inch Schmidt telescope have established the existence of additional individual objects and dynamical families such as near-Earth asteroids, Trojans, comets, and Trans-Neptunian objects. Among the most important Palomar contributions to Solar System science have been the discovery of Comet Shoemaker-Levy 9 by Gene and Carolyn Shoemaker and David Levy, which spectacularly impacted the planet Jupiter in 1994; and the discovery of a number of new Trans-Neptunian objects, or TNOs, by Mike Brown and collaborators. Framed against the emerging population of TNOs discovered at Palomar, in 2006 the International Astronomical Union revised the definition of planet, demoting Pluto to dwarf planet status.
Similarly, the Palomar-discovered Sedna is the prototype of a class of remarkable TNOs in highly elliptical orbits that take them hundreds to thousands of astronomical units from the Sun. Such remarkable orbits are hard for Solar System dynamicists to explain, and are one reason some astronomers have proposed the existence of an unknown planet in the outer Solar System. While the putative existence of this “Planet Nine” is still unconfirmed, clearly the Solar System is a much more dynamic and diverse environment than just a handful of planets going around the Sun.
Palomar’s exploration of our Solar System continues today. Astronomers are using the adaptive optics system on the Hale to image and study the dynamics and atmosphere in the Pluto-Charon system. The Zwicky Transient Facility operating on the Oschin Telescope has found new near-Earth asteroids, asteroids inside the orbit of Venus, and additional asteroids in the main belt. While it remains to be seen whether this work will establish a previously unknown family of potentially hazardous objects, Palomar remains at the forefront of our evolving understanding of the Solar System’s rich dynamical and compositional diversity.