Press Release

New Moons Found Around Neptune

By SpaceRef Editor
January 13, 2003
Filed under ,
New Moons Found Around Neptune

Cambridge, MA — A team of astronomers led by Matthew Holman
(Harvard-Smithsonian Center for Astrophysics) and JJ Kavelaars
(National Research Council of Canada) has discovered three previously
unknown moons of Neptune. This boosts the number of known satellites
of the gas giant to eleven. These moons are the first to be
discovered orbiting Neptune since the Voyager II flyby in 1989, and
the first discovered from a ground-based telescope since 1949.

It now appears that each giant planet’s irregular satellite
population is the result of an ancient collision between a former
moon and a passing comet or asteroid. “These collisional encounters
result in the ejection of parts of the original parent moon and the
production of families of satellites. Those families are exactly what
we’re finding,” said Kavelaars.

The team that discovered these new satellites of Neptune includes
Holman and Kavelaars, graduate student Tommy Grav (University of Oslo
& Harvard-Smithsonian Center for Astrophysics), and undergraduate
students Wesley Fraser and Dan Milisavljevic (McMaster University,
Hamilton, Ontario, Canada).

Needle in a Haystack

The new satellites were a challenge to detect because they are only
about 30-40 kilometers (18-24 miles) in size. Their small size and
distance from the Sun prevent the satellites from shining any
brighter than 25th magnitude, about 100 million times fainter than
can be seen with the unaided eye.

To locate these new moons, Holman and Kavelaars utilized an
innovative technique. Using the 4.0-meter Blanco telescope at the
Cerro Tololo Inter-American Observatory, Chile, and the 3.6-meter
Canada-France-Hawaii Telescope, Hawaii, they took multiple exposures
of the sky surrounding the planet Neptune. After digitally tracking
the motion of the planet as it moved across the sky, they then added
many frames together to boost the signal of any faint objects. Since
they tracked the planet’s motion, stars showed up in the final
combined image as streaks of light, while the moons accompanying the
planet appeared as points of light.

Prior to this find, two irregular satellites and six regular
satellites of Neptune were known. The two irregular satellites were
also the largest: Triton, discovered in 1846 by William Lassell, and
Nereid, discovered in 1949 by Gerard Kuiper. Triton is considered
irregular because it orbits the planet in a direction opposite to the
planet’s rotation, indicating that Triton is likely a captured Kuiper
Belt Object. (The Kuiper Belt is a disk-shaped collection of icy
objects that circle the Sun beyond the orbit of Neptune.) Nereid is
considered irregular because it has a highly elliptical orbit around
Neptune. In fact, its orbit is the most elliptical of any satellite
in the solar system. Many scientists believe that Nereid once was a
regular satellite whose orbit was disrupted when Triton was
gravitationally captured. The six regular satellites were discovered
by the Voyager probe during its encounter with Neptune. The three new
satellites were missed by Voyager II because of their faintness and
great distance from Neptune. According to Holman, “The discovery of
these moons has opened a window through which we can observe the
conditions in the solar system at the time the planets were forming.”

Tracking Faint Blips

The researchers are currently conducting follow-up observations to
better define the orbits of the newfound moons using orbital
predictions supplied by Brian Marsden (Director of the Minor Planet
Center in Cambridge, Mass.) and Robert Jacobson (Jet Propulsion

To follow up the initial find, team members Brett Gladman (University
of British Columbia, Canada); Jean-Marc Petit, Philippe Rousselot,
and Olivier Mousis (Observatoire de Besancon, France); and Philip
Nicholson and Valerio Carruba (Cornell University) conducted
additional observations using the Hale 5-meter telescope on Mount
Palomar and one of the four 8.2-meter telescopes of the European
Southern Observatory’s Very Large Telescope at Paranal Observatory,
Chile. Grav made additional tracking observations using the 2.6-meter
Nordic Optical Telescope on La Palma, Spain.

Holman says, “Tracking these moons is an enormous, international
undertaking involving the efforts of many people. Without teamwork,
such faint objects could be easily lost.”

Based in La Serena, Chile, the Cerro Tololo Inter-American
Observatory is part of the National Optical Astronomy Observatory,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under a cooperative agreement with the National
Science Foundation.

The Canada-France-Hawaii Telescope is operated by the CFHT
Corporation under a joint agreement between the National Research
Council of Canada, the Centre National de la Recherche Scientifique
of France, and the University of Hawaii.

The European Southern Observatory is an intergovernmental, European
organization for astronomical research. It has ten member countries.
ESO operates astronomical observatories in Chile and has its
headquarters in Garching, near Munich, Germany.

Headquartered in Cambridge, Massachusetts, the Harvard-Smithsonian
Center for Astrophysics (CfA) is a joint collaboration between the
Smithsonian Astrophysical Observatory and the Harvard College
Observatory. CfA scientists organized into six research divisions
study the origin, evolution, and ultimate fate of the universe.

Note to editors: An image of one of the three new Neptunian moons is
online at

SpaceRef staff editor.