Latin: Cerēs), also designated
1 Ceres or (1) Ceres (See
Minor Planet Names), is the smallest
planet in the
System and the only one located in the main
asteroid belt. Its name is derived from the
Ceres — the goddess of growing plants and of motherly love. It was
January 1, 1801,
Giuseppe Piazzi. With a diameter of about 950 km, Ceres is by far the
largest and most massive body in the asteroid belt, and contains
approximately a third of the belt's total mass. Recent observations have
revealed that it is spherical, unlike the irregular shapes of smaller asteroids
with less gravity.
Piazzi originally suggested the name Ceres Ferdinandea (Italian,
Cerere Ferdinandea) for this body, after both the mythological figure
goddess of plants) and
King Ferdinand III of Sicily.
"Ferdinandea" was not acceptable to other nations of the world and was thus
dropped. Ceres was also called
Hera for a
short time in
Germany. In Greece, it is called Δήμητρα (Demeter),
after the goddess Ceres' Greek equivalent; in English usage, Demeter is the name
of a different asteroid (1108
Due to the rarity of the usage, there is no consensus as to the proper
adjectival form of the name, although the
Cerian and Cerean have been used in fiction. Grammatically, the
form Cererean would be correct (cf. its
Piazzi was searching for a star listed by Francis Wollaston as Mayer 87
because it was not in Mayer's zodiacal catalogue in the position given. Instead,
Piazzi found a moving star-like object, which he thought at first was a
Piazzi observed Ceres a total of 24 times, the final time on
11, when illness interrupted. On
announced his discovery in letters to fellow astronomers, among them his fellow
Barnaba Oriani of
Milan. He reported it as a comet but "since its movement is so slow and
rather uniform, it has occurred to me several times that it might be something
better than a comet".
In April, Piazzi sent his complete observations to Oriani,
Lalande in Paris.
They were shortly thereafter published in the September, 1801 issue of the
||A899 OF; 1943 XB
|Minor planet category:
November 26, 2005
||2.987 AU (446.818 Gm)
||2.544 AU (380.612 Gm)
AU (413.715 Gm)
||1679.819 d (4.599
Longitude of ascending node:
Argument of perihelion:
|Sidereal rotation period:
||0.3781 d (9.074 h)
To recover Ceres,
Carl Friedrich Gauss, then only 24 years old, developed a method of orbit
determination from three observations. In only a few weeks, he predicted its
path, and sent his results to
Franz Xaver, Baron von Zach, the editor of the Monatliche Correspondenz.
December 31, 1801,
von Zach and
Heinrich W. M. Olbers unambiguously confirmed the recovery of Ceres.
The classification of Ceres has changed more than once. At the time of its
discovery it was considered a
planet; it was
classified as an
for over 150 years; and was first classified a dwarf planet in 2006.
Johann Elert Bode believed Ceres to be the "missing
Johann Daniel Titius had proposed to exist between
Jupiter, at a
distance of 419 million km (2.8 AU) from the Sun. Ceres was assigned a planetary
symbol, and remained listed as a planet in astronomy books and tables (along
with 2 Pallas,
3 Juno and
4 Vesta) for
about half a century until further asteroids were discovered. However as further
objects were discovered in the area it was realised that it represented the
first of a class of many similar bodies. Sir
William Herschel coined in 1802 the term
("star-like") for such bodies,
writing, "they resemble small stars so much as hardly to be distinguished from
them, even by very good telescopes".
As the first such body to be discovered, it was given the designation 1 Ceres
under the modern system of
The 2006 debate surrounding
Pluto and what
constitutes a 'planet' led to Ceres being considered for reclassification as a
planet. An unsuccessful proposal before the
International Astronomical Union for the
definition of a planet would have defined a planet as "a celestial body that
(a) has sufficient mass for its self-gravity to overcome rigid body forces so
that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in
orbit around a star, and is neither a star nor a satellite of a planet".
Had this resolution been adopted, this would have made Ceres the fifth planet in
order from the Sun.
This draft definition was not accepted, and in its place an alternate
definition of "planet" came into effect as of
2006. Under this
definition, a 'planet' is "a celestial body that is in orbit around the sun, has
sufficient mass for its self-gravity to overcome rigid body forces so that it
assumes a ... nearly round shape, and has cleared the neighborhood around its
orbit." By this definition, Ceres (along with Pluto) cannot be classified as a
planet, and both are now classified as "dwarf
planets", although it remains unclear as to whether or not it is also
classified as an asteroid.
Diagram showing internal structure of Ceres
Ceres follows an orbit between Mars and Jupiter, within the main
asteroid belt, with a period of 4.6 years. The orbit is moderately inclined
(i=10.6° to be compared with 7° for Mercury and 17° for Pluto) and
eccentric (e=0.08 to compare with 0.09 for Mars).
The diagram illustrates the orbits of Ceres (blue) and several planets
(white/grey). The segments of orbits below the ecliptic are plotted in darker
colours, and the orange plus sign is the Sun's location. The top left diagram is
a polar view that shows the location of Ceres in the gap between Mars and
Jupiter. The top right is a close-up demonstrating the locations of the
aphelia (Q) of Ceres and Mars. Interestingly, the perihelia of Ceres (as
well as those of several other of the largest main belt asteroids) and Mars are
on the opposite sides of the Sun. The bottom diagram is a perspective view
showing the inclination of the orbit of Ceres compared to the orbits of Mars and
Ceres is the largest object in the
asteroid belt, which mostly lies between
Jupiter. However, it is not the largest object besides the Sun, planets and
their satellites, in the solar system: the
belt is known to contain larger objects, including
At certain points in its orbit, Ceres can reach a magnitude of 7.0. This is
generally regarded as being just barely too dim to be seen with the naked eye,
but under exceptional viewing conditions a very sharp-sighted person may be able
to see the asteroid with the naked eye. The only other asteroid that can be seen
with the naked eye is
Ceres' size and mass are sufficient to give it a nearly spherical shape. That
is, it is close to
hydrostatic equilibrium. Other large asteroids such as
3 Juno, and
4 Vesta are
known to be quite irregular, while lightcurve analysis of
indicates it is oblong although it appears spheroidal in low-resolution images
(presumably due to viewing angle).
With a mass of 9.5 ×1020 kg, Ceres comprises about a third of the
estimated total 3.0 ± 0.2 ×1021 kg mass of all the asteroids in the
(note how all these amount to only about 4% of the mass of the
There are some indications that the surface of Ceres is relatively warm and
that it may have a tenuous
frost. The maximum temperature with the
Sun overhead was
estimated from measurements to be 235
K (about -38°C)
on May 5,
Taking into account also the
heliocentric distance at the time, this gives an estimated maximum of ~239 K
A study led by Peter Thomas of
Cornell University suggests that Ceres has a differentiated interior:
observations coupled with computer models suggest the presence of a rocky core
overlain with an icy
mantle. This mantle of thickness from 120 to 60 km could contain 200 million
cubic kilometres of water, which is more than the amount of
water on the Earth.
There has been some ambiguity regarding surface features on Ceres. Low
Hubble Space Telescope images taken in 1995 showed a dark spot on its
surface which was nicknamed "Piazzi" in honour of the discoverer of Ceres. This
was thought to be a crater. Later images with a higher resolution taken over a
whole rotation with the
Keck telescope using
adaptive optics showed no sign of "Piazzi". However, two dark features were
seen to move with the asteroid's rotation, one with a bright central region.
These are presumably craters. More recent visible light
Hubble Space Telescope images of a full rotation taken in 2003 and 2004 show
an enigmatic white spot, the nature of which is currently unknown.
The dark albedo features seen with Keck are, however, not immediately
recognizable in these images.
These last observations also determined that Ceres' north pole points (give
or take about 5°) in the direction of
right ascension 19 h 24 min,
declination +59°, in the
Draco. This means that Ceres'
is very small (about 4±5°).
Ceres was long thought to be the parent body of the "Ceres
asteroid family". However, that grouping is now defunct because Ceres has
been shown to be an interloper in its "own" family, and physically unrelated.
The bulk of that asteroid group is now called the
Some notable observation milestones for Ceres include:
occultation of a
star by Ceres was observed in
Features on Ceres' surface have been telescopically imaged several times in
Hubble Space Telescope images with 50 km resolution taken in 1995.
- Visible images with 60 km resolution taken with the
Keck telescope in 2002 using
images with 30 km resolution also taken with the
Keck telescope in 2002 using
- The best resolution to date (30 km) visible light images using
Hubble again in 2003 and 2004.
Radio signals from spacecraft in orbit around
Mars and on its
surface have been used to estimate the mass of Ceres from the perturbations
induced by it onto the motion of Mars.
Exploration of Ceres
To date no space probes have visited Ceres. However,
NASA is currently
Mission, with a projected launch in 2007. According to the current mission
profile, Dawn is expected to explore the asteroid
4 Vesta in
2011 before arriving at Ceres in 2015.
- The chemical element
number 58) was discovered in 1803 by
Klaproth, working independently. Berzelius named the element after Ceres.
William Hyde Wollaston discovered
(atomic number 46) as early as 1802 and at first called it Ceresium. By
the time he openly published his discovery in 1805, the name was already taken
(by Berzelius) and he switched it to palladium in honour of