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]