In 1966 Goldreich published a classic paper on the evolution of the Moon's orbit and on the orbits of other moons in the Solar System.[13] He showed that for each planet there is a certain distance such that moons closer to the planet than that distance maintain an almost constant orbital inclination with respect to the planet's equator (with an orbital precession mostly due to the tidal influence of the planet), whereas moons further away maintain an almost constant orbital inclination with respect to the ecliptic (with precession due mostly to the tidal influence of the Sun). The moons in the first category, with the exception of Neptune's moon Triton, orbit near the equatorial plane. He concluded that these moons formed from equatorial accretion disks. But he found that the Moon, although it was once inside the critical distance from the Earth, never had an equatorial orbit as would be expected from various scenarios for its origin. This is called the lunar inclination problem, to which various solutions have since been proposed.[14]
Goldreich and Alar Toomre first described the process of polar wander in a 1969 paper, although evidence of paleomagnetism was not discovered until later.[15] Goldreich collaborated with George Abell to conclude that planetary nebulae evolved from red giant stars, a view that is now widely accepted.[16][17] In 1979 Goldreich, along with Scott Tremaine predicted that Saturn's F ring was maintained by shepherd moons, a prediction that would be confirmed by observations in 1980.[18][19][20][21] They also predicted that Uranus' rings were held in place by similar shepherd moons, a prediction that was confirmed in 1986.[22] Goldreich, along with Tremaine predicted planetary migration in 1980, which would later be invoked to explain hot jupiters.[23][24][25]
In 1969, Goldreich published a paper[26] together with William Julian that is now considered a classic work on pulsar magnetospheres. They provided a simple and compelling model for the structure of magnetic fields anchored in a neutron star and showed that these fields can extract the neutron star rotational energy to power electromagnetic emission. Similar considerations were later used to understand the magnetospheres of rotating black holes.[27]
In 1995, Goldreich received the National Medal of Science for "his profound and lasting contributions to planetary sciences and astrophysics, providing fundamental theoretical insights for understanding the rotation of planets, the dynamics of planetary rings, pulsars, astrophysical masers, the spiral arms of galaxies, and the oscillations of the Sun".[2][1][31][32]
Goldreich received the 2007 Shaw Prize in Astronomy "in recognition of his lifetime achievements in theoretical astrophysics and planetary sciences".[36]
