  We report on the discovery of gamma-ray emission from several globular
clusters (GCs), including Terzan 5, the second known gamma-ray GCs. By now,
more than a dozen GCs are known to emit gamma-rays of energies above 100 MeV,
thus enabling us to carry out the first detailed correlation study with several
cluster properties. We found strong correlations between the observed gamma-ray
luminosities and four cluster parameters: stellar encounter rate, metallicity
[Fe/H], and energy densities of the soft photons at the cluster locations.
These "fundamental planes" of gamma-ray GCs put an intimate relation of the
observed gamma-rays to the underlying millisecond pulsar population and have
important implications on the origin of the gamma-ray emission of GCs.
