What Are Minor
Minor planet is the official term for
asteroids and trans-Neptunian objects. They are objects in the solar system that
orbit the Sun like planets, but which are smaller than planets and not counted
among them. If a minor planet produces coma it is called a comet (though
conversely not all comets are classified as minor planets). The largest minor
planets are also called planetoids.
Minor planet groups and families
Minor planets are divided into groups and families based on their orbital
characteristics. It is customary to name a group of asteroids after the first
member of that group to be discovered (generally the largest). Groups are
relatively loose dynamical associations, whereas families are much "tighter" and
result most probably from the catastrophic breakup of a large parent asteroid
sometime in the past. They were first recognised by
Kiyotsugu Hirayama in 1918 and are often
called Hirayama families in his honour. The only prominent families are:
asteroids have a
semi-major axis between 2.99 AU and 3.03 AU, an eccentricity between 0.01
and 0.13, and an inclination between 8° and 12°. Named after 221 Eos,
~480 members known.
Themis asteroids have a semi-major axis between 3.08 AU and 3.24 AU, an
eccentricity between 0.09 and 0.22, and an inclination less than 3°. Named
Themis, ~535 members known.
Koronis asteroids have a semi-major axis between 2.83 AU and 2.91 AU, an
eccentricity less than 0.11, and an inclination less than 3.5°. Named after 158
Koronis, ~310 members known.
Eunomia asteroids (semi-major axis = 2.5 < a < 2.75 AU, eccentricity = 0.1
< e < 0.2, inclination = 13° < i < 20°), named after 15
Eunomia, ~370 members known.
Flora asteroids have a semi-major axis between 2.1 AU and 2.3 AU with an
inclination of less than 11°. Named after 8 Flora,
~590 members known. Also called the
Ariadne asteroids after 43
Nysa asteroids have a semi-major axis between 2.41 AU and 2.5 AU, an
eccentricity between 0.12 and 0.21, and an inclination between 1.5° and 4.3°.
Named after 44
Nysa, ~375 members known. Also called the
Hertha asteroids after 135
Maria asteroids have a semi-major axis between 2.5 AU and 2.706 AU and an
inclination between 12° and 17°. Named after 170 Maria,
~80 members known.
Other families have been identified using a variety of techniques, most
prominently the Hierarchical Clustering Method (HCM) and the Wavelet
Analysis Method (WAM):
- Vesta asteroids are fragments of 4 Vesta's
parent body, ~235 members.
Erigone asteroid, named after 163
Erigone, ~45 members.
- Ceres asteroids, also known as
Minerva asteroids, named after 1 Ceres or 93
Minerva, ~88 members.
Dora asteroids, named after
668 Dora, ~78 members.
Adeona asteroids, named after 145
Adeona, ~65 members.
Merxia asteroids, named after
808 Merxia, ~25-30 members.
Misa asteroids, named after
569 Misa, ~26 members.
Chloris asteroids, named after 410
Chloris, ~24 members.
Rafita asteroids, named after
1644 Rafita, ~22 members.
Astrid asteroids, named after
1128 Astrid, ~10 members.
Meliboea asteroids, named after 137
Meliboea, ~15 members.
Naëma asteroids, named after
845 Naëma, 6 or 7 members.
Massalia asteroids, named after 20
Massalia, ~47 members.
Lydia asteroids, named after 110 Lydia,
Nemesis asteroids or
Concordia asteroids, named after 128
Nemesis or 58
Concordia, ~29 members.
Bower asteroids or
Endymion asteroids, named after
1639 Bower or 342
Endymion, ~10-15 members.
Hygiea asteroids, named after 10 Hygiea,
Veritas asteroids or
Undina asteroids, named after
490 Veritas or 92 Undina,
Brasilia asteroids, named after 293
Brasilia, ~14 members.
Karin asteroids are a sub-family of the Koronis family; they number 39,
the most prominent being
832 Karin. It is a young family which can be traced back to a break-up
occurring 5.8±0.2 million years ago.
Groups out to the orbit of Earth
There are relatively few asteroids that orbit close to the Sun. Several of
these groups are hypothetical at this point in time, with no members having yet
been discovered; as such, the names they have been given are provisional.
Vulcanoid asteroids are hypothetical asteroids with an aphelion
less than 0.4 AU, ie, they orbit entirely within the orbit of
Mercury. A few searches for Vulcanoids have been conducted but there have
been none discovered so far.
Apoheles are asteroids whose aphelion is less than 1 AU, meaning they
orbit entirely within Earth's orbit. "Apohele" is Hawaiian for "orbit". Other
proposed names for this group are Inner-Earth Objects (IEOs) and Anons (as in
"Anonymous"). As of May 2004
there are only two known Apoheles: 2003 CP20
and 2004 JG6.
Arjuna asteroids are somewhat vaguely defined as having orbits similar to
Earth's; ie, with an average orbital radius of around 1 AU and with low
eccentricity and inclination. Due to the vagueness of this definition some
asteroids belonging to the
Aten groups can also be classified as Arjunas. The term was introduced by
Spacewatch and does not refer to an existing asteroid; examples of Arjunas
Earth Trojans are asteroids located in the Earth-Sun
Lagrangian points L4 and L5. Their location in the
sky as observed from Earth's surface would be fixed at about 60 degrees east
and west of the Sun, and as people tend to search for asteroids at much
greater elongations few searches have been done in these locations. No Earth
trojans are currently known.
Groups out to the orbit of Mars
Amor asteroids orbit between Earth and Mars; they are named after 2062 Aten, 1862
Apollo and 1221 Amor,
Mars-crosser asteroids have orbits that cross that of Mars.
Mars Trojans follow or lead Mars on its orbit, at either of the two
Lagrangian points 60° ahead (L4) or behind (L5). The
only one known is 5261
Minor Planet Center has not listed any Mars trojans with confirmed orbits
for controversial reasons.
Groups out to the orbit of Jupiter
A large number of asteroids have orbits between the orbits of Mars
Jupiter, roughly 2 to 4
AU, in a region known as the Main belt.
These couldn't form a planet due to the gravitational influence of Jupiter.
Jupiter's gravitational influence, through
orbital resonance, clears
Kirkwood gaps in the asteroid belt, first recognised by
Daniel Kirkwood in
1874. As a result of these gaps the asteroids in this region are divided
into a large number of groups. They are:
Hungaria asteroids, with a mean orbital radius between 1.78 AU and 2 AU,
an eccentricity less than 0.18, and inclination between 16° and 34°. Named
434 Hungaria, these are just outside Mars orbit, and are possibly
attracted by the 2:9 resonance.
Phocaea asteroids, with a mean orbital radius between 2.25 AU and 2.5 AU,
an eccentricity greater than 0.1, and inclination between 18° and 32°. Some
sources group the Phocaeas asteroids with the Hungarias, but the division
between the two groups is real and caused by the 1:4 resonance with Jupiter.
Named after 25
Main Belt I asteroids have a mean orbital radius between 2.3 AU and 2.5 AU
and an inclination of less than 18°. This group appears to be a catch-all that
includes everything in the inner main belt that doesn't belong to the Nysa or
Flora families, with the division at 2.3 AU apparently an arbitrary one
without physical significance.
Alinda asteroids have a mean orbital radius of 2.5 AU and an eccentricity
between 0.4 and 0.65 (approximately). These objects are held by the 1:3
resonance with Jupiter. Named after
Pallas asteroids have a mean orbital radius between 2.5 AU and 2.82 AU and
an inclination between 33° and 38°. Named after 2 Pallas.
Main Belt II asteroids have a mean orbital radius between 2.5 AU and 2.706
AU and an inclination less than 33°.
Main Belt IIb asteroids have a mean orbital radius between 2.706 AU and
2.82 AU and an inclination less than 33°.
Main Belt IIIa asteroids have a mean orbital radius between 2.82 AU and
3.03 AU, an eccentricity less than .35, and an inclination less than 30°.
Griqua asteroids have an orbital radius between 3.1 AU and 3.27 AU and an
eccentricity greater than 0.35. These asteroids are in stable 2:1 libration
with Jupiter, in high-inclination orbits. There are about 5 to 10 of these
known so far, with
1362 Griqua and
8373 Stephengould the most prominent.
Main Belt IIIb asteroids have a mean orbital radius between 3.03 AU and
3.27 AU, an eccentricity less than .35, and an inclination less than 30°.
Cybele asteroids have a mean orbital radius between 3.27 AU and 3.7 AU, an
eccentricity less than 0.3, and an inclination less than 25°. This group
appears to cluster around the 4:7 resonance with Jupiter. Named after 65 Cybele.
Hilda asteroids have a mean orbital radius between 3.7 AU and 4.2 AU, an
eccentricity greater than 0.07, and an inclination less than 20°. These
asteroids are in a 2:3 resonance with Jupiter. Named after 153 Hilda.
Thule asteroids appear to consist of only one object, 279 Thule,
in a 3:4 resonance with Jupiter.
Trojan asteroids have a mean orbital radius between 5.05 AU and 5.4 AU,
and lie in elongated, curved regions around the two
Lagrangian points 60° ahead and behind of Jupiter. The leading point, L4,
is called the 'Greek' node and the trailing L5 point is called the
'Trojan' node, after the two opposing camps of the legendary Trojan
War; with one exception apiece, objects in each node are named for members
of that side of the conflict.
617 Patroclus in the Trojan node and 624
Hektor in the Greek node are "misplaced" in the enemy camps.
Between the Hildas and the Trojans (roughly 4.05 AU to 5.0 AU), there's a
'forbidden zone'. Aside from 279 Thule
and five objects in unstable-looking orbits, Jupiter's gravity has swept
everything out of this region.
Groups beyond the orbit of Jupiter
Most of the minor planets beyond the orbit Jupiter are believed to be
composed of ices and
Many are similar to
comets, differing only in that the perihelia
of their orbits are too distant from the Sun to produce a significant tail.
Damocloid asteroids, also known as the "Oort cloud group," are named after
5335 Damocles. They are defined to be objects that have "fallen in" from
cloud, so their aphelia are generally still out past
Uranus, but their perihelia are in the inner solar system. They have high
eccentricities and sometimes high inclinations, including
retrograde orbits. The definition of this group is somewhat fuzzy, and may
overlap significantly with comets.
Centaurs have a mean orbital radius roughly between 5.4 AU and 30 AU. They
are currently believed to be
Trans-Neptunian Objects that "fell in" after encounters with gas giants.
The first of these to be discovered was 2060
Neptune Trojans currently consist of only one object, 2001 QR322.
Trans-Neptunian Objects (TNOs) are anything with a mean orbital radius
greater than 30 AU. This classification includes the Kuiper Belt Objects (KBOs)
and the Oort Cloud.
Kuiper Belt Objects extend from roughly 30 AU to 50 AU and are broken
into the following subcategories:
- Plutinos are KBOs in a 2:3 resonance with Neptune, just like
Pluto. The perihelion of such an object tends to be close to Neptune's
orbit (much as happens with Pluto), but when the object comes to
perihelion, Neptune alternates between being 90 degrees ahead of and 90
degrees behind of the object, so there's no chance of a collision. The MPC
defines any object with a mean orbital radius between 39 AU and 40.5 AU to
be a Plutino.
- Cubewanos, also known as "classical KBOs". They are named after
(15760) 1992 QB1 and have a mean orbital radius between
approximately 40.5 AU and 47 AU. Cubewanos are objects in the Kuiper belt
that didn't get scattered and didn't get locked into a resonance with
- Additional groups exist for other orbital resonances with Neptune than
the 2:3 resonance of the Plutinos and the 1:1 resonance of the Neptune
Trojans (such as 2001
QR322), but they have not yet been officially named. There
are several known objects in the 2:1 resonance, unofficially dubbed "Twotinos,"
with a mean orbital radius of roughly 48 AU and an eccentricity of 0.37.
There are several objects in the 2:5 resonance (mean orbital radius of 55
AU), and objects in the 4:5, 4:7, 3:5, and 3:4 resonances.
Scattered Disk Objects (SDOs) generally have very large orbits of up
to a few hundred AU. They are assumed to be objects that encountered
Neptune and were "scattered" into long-period, very elliptical orbits with
perihelia that are still not too far from Neptune's orbit.
Oort Cloud is a hypothetical cloud of comets with a mean orbital radius
between approximately 50,000 AU and 100,000 AU. No Oort Cloud objects have
been detected, the existence of this classification is only inferred from
indirect evidence. Some astronomers have tentatively associated 90377
Sedna with the Oort cloud.
Quasi-satellites and "horseshoe objects"
Some asteroids have unusual "horseshoe orbits" that are co-orbital with the Earth or some
other planet. Examples are 3753
Cruithne and 2002 AA29.
The first instance of this type of orbital arrangement was discovered between
Epimetheus and Janus.
Sometimes these "horseshoe objects" temporarily become
quasi-satellites for a few decades or a few hundred years, before returning
to their prior status. Both Earth and
Venus are known to have quasi-satellites.
Such objects, if associated with Earth or Venus or even hypothetically
Mercury are a special class of Aten
asteroids. However, such objects could be associated with outer planets as