A
planet (from
Greek
, alternative form of "wanderer") is a celestial body that is or
was
orbiting a
star or
stellar remnant
and is massive enough to be rounded by its own
gravity, is not massive enough to cause
thermonuclear fusion, and has
cleared its neighbouring region of
planetesimals.
The term
planet is ancient, with ties to history, science,
mythology, and religion. The planets were originally seen by many
early cultures as divine, or as emissaries of the gods. As
scientific knowledge advanced, human perception of the planets
changed, incorporating a number of disparate objects. In 2006, the
International
Astronomical Union officially adopted a resolution
defining planets within the
Solar System. This definition has been
both praised and criticized, and remains disputed by some
scientists.
The planets were thought by
Ptolemy to orbit
the Earth in
deferent and
epicycle motions. Though the idea that the
planets orbited the Sun had been suggested
many times, it was not until the 17th century that this view was
supported by evidence from the first
telescopic astronomical observations, performed
by
Galileo Galilei. By careful
analysis of the observation data,
Johannes Kepler found the planets' orbits to
be not circular, but elliptical. As observational tools improved,
astronomers saw that, like Earth, the
planets rotated around tilted axes, and some share such features as
ice-caps and seasons. Since the dawn of the
Space Age, close observation by
probes has found that Earth and the other
planets share characteristics such as
volcanism,
hurricanes,
tectonics, and even
hydrology. Since 1992, through the discovery of
hundreds of planets around other stars, called
extrasolar planets, scientists are
beginning to understand that planets throughout the
Milky Way Galaxy share characteristics in common
with our own. As of November 2009, there are 404 known extrasolar
planets, ranging from the size of gas giants to that of terrestrial
planets.
Planets are generally divided into two main types: large,
low-density
gas giants, and smaller, rocky
terrestrials. Under IAU
definitions, there are eight planets in the Solar System. In order
from the
Sun, they are the four terrestrials,
Mercury,
Venus,
Earth, and
Mars, then the four gas giants,
Jupiter,
Saturn,
Uranus, and
Neptune. The Solar
System also contains at least five
dwarf
planets:
Ceres,
Pluto (originally classified as the Solar System's
ninth planet),
Makemake,
Haumea and
Eris. With the exception of Mercury,
Venus, Ceres and Makemake, all of these are orbited by one or more
natural satellites.
History
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Printed rendition of a geocentric
cosmological model from
Cosmographia, Antwerp, 1539
The idea of planets has evolved over its history, from the divine
wandering stars of antiquity to the
earthly objects of the scientific age. The concept has expanded to
include worlds not only in the Solar System, but in hundreds of
other extrasolar systems. The ambiguities inherent in defining
planets have led to much scientific controversy.
In ancient times, astronomers noted how certain lights moved across
the sky in relation to the other stars. Ancient Greeks called these
lights " " ( : wandering stars) or simply " " ( : wanderers), from
which today's word "planet" was derived.
In ancient Greece,
China, Babylon
and indeed
all pre-modern civilisations, it was almost universally believed
that Earth was in the centre of the Universe and that all the "planets" circled the
Earth. The reasons for this perception were that stars and
planets appeared to revolve around the Earth each day, and the
apparently
common sense perception that
the Earth was solid and stable, and that it is not moving but at
rest.
Babylon
The first civilisation known to possess a functional theory of the
planets were the
Babylonians, who lived
in
Mesopotamia in the first and second
millennia BC. The oldest surviving planetary astronomical text is
the Babylonian
Venus tablet
of Ammisaduqa, a 7th century BC copy of a list of observations
of the motions of the planet Venus that probably dates as early as
the second millennium BC. The Babylonians also laid the foundations
of what would eventually become
Western astrology. The
Enuma anu enlil, written during the
Neo-Assyrian period in the 7th century
BC, comprises a list of
omens and their
relationships with various celestial phenomena including the
motions of the planets. The
Sumerians,
predecessors of the Babylonians who are considered as one of the
first civilizations and are
credited with the invention of
writing, had
identified at least Venus by 1500 BC.
Ancient Greece to Medieval Europe
Ptolemy's "planetary spheres"
|
Modern |
Moon |
Mercury |
Venus |
the Sun |
Mars |
Jupiter |
Saturn |
Medieval Europe |
☾ LVNA |
☿ MERCVRIVS |
♀VENVS |
☉ SOL |
♂ MARS |
♃ IVPITER |
♄ SATVRNVS |
The
ancient Greek cosmological system
was taken from that of the
Babylonians,
from whom they began to acquire astronomical learning from around
600 BC, including the
constellations
and the
zodiac. In the 6th century BC, the
Babylonians' astronomical knowledge at the time was far in advance
of the Greeks. The earliest known Greek sources, such as the
Iliad and the
Odyssey, do not mention the planets.
By the first century BC, the Greeks had begun to develop their own
mathematical schemes for predicting the positions of the planets.
These schemes, which were based on geometry rather than the
arithmetic of the Babylonians, would eventually eclipse the
Babylonians' theories in complexity and comprehensiveness, and
account for most of the astronomical movements observed from Earth
with the naked eye. These theories would reach their fullest
expression in the
Almagest written
by
Ptolemy in the 2nd century AD. So
complete was the domination of Ptolemy's model that it superseded
all previous works on astronomy and remained the definitive
astronomical text in the Western world for 13 centuries.
To the Greeks and Romans there were seven known planets, each
presumed to be
circling the Earth
according to the complex laws laid out by Ptolemy. They were, in
increasing order from Earth (in Ptolemy's order): the
Moon, Mercury, Venus, the
Sun, Mars,
Jupiter, and Saturn.
European Renaissance
Renaissance planets
|
Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
The five
naked-eye planets may have
been known since ancient times, and have had a significant impact
on
mythology,
religious cosmology, and ancient
astronomy. As scientific knowledge
progressed, however, understanding of the term "planet" changed
from something that moved across the sky (in relation to the
star field); to a body that orbited the
Earth (or that were believed to do so at the time); and in the 16th
century to something that directly orbited the Sun when the
heliocentric model of
Copernicus,
Galileo and
Kepler gained sway.
Thus the Earth became included in the list of planets, while the
Sun and Moon were excluded. At first, when the first satellites of
Jupiter and Saturn were discovered in the 17th century, the terms
"planet" and "satellite" were used interchangeably – although the
latter would gradually become more prevalent in the following
century. Until the mid-19th century, the number of "planets" rose
rapidly since any newly discovered object directly orbiting the Sun
was listed as a planet by the scientific community.
19th Century
Planets in early 1800s
|
Mercury |
Venus |
Earth |
Mars |
Vesta |
Juno |
Ceres |
Pallas |
Jupiter |
Saturn |
Uranus |
In the 19th century astronomers began to realize that recently
discovered bodies that had been classified as planets for almost
half a century (such as
Ceres,
Pallas, and
Vesta)
were very different from the traditional ones. These bodies shared
the same region of space between Mars and Jupiter (the
Asteroid belt), and had a much smaller mass;
as a result they were reclassified as "
asteroids." In the absence of any formal
definition, a "planet" came to be understood as any "large" body
that orbited the Sun. Since there was a dramatic size gap between
the asteroids and the planets, and the spate of new discoveries
seemed to have ended after the discovery of Neptune in 1846, there
was no apparent need to have a formal definition.
20th Century
Planets from late 1800s to 1930
|
Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
However, in the 20th century,
Pluto was
discovered. After initial observations led to the belief it was
larger than Earth, the object was immediately accepted as the ninth
planet. Further monitoring found the body was actually much
smaller: in 1936,
Raymond
Lyttleton suggested that Pluto may be an escaped satellite of
Neptune, and
Fred Whipple suggested in 1964 that
Pluto may be a comet. However, as it was still larger than all
known asteroids and seemingly did not exist within a larger
population, it kept its status until 2006.
Planets
1930-2006 |
Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
Pluto |
In 1992, astronomers
Aleksander
Wolszczan and
Dale Frail announced
the discovery of planets around a
pulsar,
PSR B1257+12. This discovery is
generally considered to be the first definitive detection of a
planetary system around another star.
Then, on October 6,
1995, Michel Mayor and Didier Queloz of the University of
Geneva
announced the first definitive detection of an
exoplanet orbiting an ordinary main-sequence star (51
Pegasi).
The discovery of extrasolar planets led to another ambiguity in
defining a planet; the point at which a planet becomes a star. Many
known extrasolar planets are many times the mass of Jupiter,
approaching that of stellar objects known as "
brown dwarfs". Brown dwarfs are generally
considered stars due to their ability to fuse
deuterium, a heavier isotope of
hydrogen. While stars more massive than 75 times
that of Jupiter fuse hydrogen, stars of only 13 Jupiter masses can
fuse deuterium. However, deuterium is quite rare, and most brown
dwarfs would have ceased fusing deuterium long before their
discovery, making them effectively indistinguishable from
supermassive planets.
21st Century
Planets 2006- |
Mercury |
Venus |
Earth |
Mars |
Jupiter |
Saturn |
Uranus |
Neptune |
With the discovery during the latter half of the 20th century of
more objects within the Solar System and large objects around other
stars, disputes arose over what should constitute a planet. There
was particular disagreement over whether an object should be
considered a planet if it was part of a distinct population such as
a
belt, or if it was large enough to
generate energy by the
thermonuclear fusion of
deuterium.
A growing number of astronomers argued for Pluto to be declassified
as a planet, since many similar objects approaching its size had
been found in the same region of the Solar System (the
Kuiper belt) during the 1990s and early 2000s.
Pluto was found to be just one small body in a population of
thousands.
Some of them including
Quaoar,
Sedna, and
Eris were heralded in the popular press
as the
tenth planet, failing however to
receive widespread scientific recognition. The discovery of Eris,
an object more massive than Pluto, brought things to a head.
Acknowledging the problem, the IAU set about creating the
definition of planet, and eventually
produced one in 2006. The number of planets dropped to the eight
significantly larger bodies that had
cleared their orbit (Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a
new class of
dwarf planets was created,
initially containing three objects (Ceres, Pluto and Eris).
Extrasolar planet definition
Dwarf Planets
2006- |
Ceres |
Pluto |
Makemake |
Haumea |
Eris |
In 2003, The
International Astronomical
Union (IAU) Working Group on Extrasolar Planets made a position
statement on the definition of a planet that incorporated the
following working definition, mostly focused upon the boundary
between planets and brown dwarves:
- Objects with true masses below the
limiting mass for thermonuclear fusion of deuterium (currently
calculated to be 13 times the mass of Jupiter for objects with the
same isotopic abundance as the
Sun) that orbit stars or stellar remnants are "planets" (no matter
how they formed). The minimum mass and size required for an
extrasolar object to be considered a planet should be the same as
that used in the Solar System.
- Substellar objects with true masses above the limiting mass for
thermonuclear fusion of deuterium are "brown
dwarfs", no matter how they formed or where they are
located.
- Free-floating objects in young star
clusters with masses below the limiting mass for thermonuclear
fusion of deuterium are not "planets", but are "sub-brown dwarfs"
(or whatever name is most appropriate).
This definition has since been widely used by astronomers when
publishing discoveries of exoplanets in
academic journals. Although temporary, it
remains an effective working definition until a more permanent one
is formally adopted. However, it does not address the dispute over
the lower mass limit, and so it steered clear of the controversy
regarding objects within the Solar System.
2006 definition
The matter of the lower limit was addressed during the 2006 meeting
of the
IAU's General Assembly. After much debate and one failed
proposal, the assembly voted to pass a resolution that
defined planets within the Solar
System as:
Under this definition, the Solar System is considered to have eight
planets. Bodies which fulfill the first two conditions but not the
third (such as Pluto, Makemake and Eris) are classified as
dwarf planets, provided they are not also
natural satellites of other
planets. Originally an IAU committee had proposed a definition that
would have included a much larger number of planets as it did not
include (c) as a criterion. After much discussion, it was decided
via a vote that those bodies should instead be classified as dwarf
planets.
This definition is based in theories of planetary formation, in
which planetary embryos initially clear their orbital neighborhood
of other smaller objects. As described by astronomer
Steven Soter:
In the aftermath of the IAU's 2006 vote, there has been controversy
and debate about the definition, and many astronomers have stated
that they will not use it. Part of the dispute centres around the
belief that point (c) (clearing its orbit) should not have been
listed, and that those objects now categorised as dwarf planets
should actually be part of a broader planetary definition. The next
IAU
conference is not until
2009, when modifications could be made to the IAU definition, also
possibly including extrasolar planets.
Beyond the scientific community, Pluto has held a strong cultural
significance for many in the general public considering its
planetary status since its discovery in 1930. The discovery of Eris
was widely reported in the
media as the
tenth planet and therefore the
reclassification of all three objects as dwarf planets has
attracted a lot of media and public attention as well.
Former classifications
The table below lists
Solar
System bodies formerly considered to be planets:
Bodies |
Notes |
Sun, Moon |
Classified as planets in antiquity, in accordance with the definition
then used. |
Io, Europa, Ganymede, and Callisto |
The four largest moons of Jupiter, known as the Galilean moons after their discoverer
Galileo Galilei. He referred to them
as the "Medicean Planets" in honor of his patron, the Medici
family. |
Titan, Iapetus, Rhea,
Tethys, and Dione |
Five of Saturn's
larger moons, discovered
by Christiaan Huygens and
Giovanni Domenico
Cassini. |
Ceres, Pallas, Juno, and Vesta |
The first known asteroids, from their discoveries between 1801 and
1807 until their reclassification as asteroids during the
1850s.Ceres has subsequently been classified as a dwarf planet. |
Astrea, Hebe,
Iris, Flora, Metis, Hygeia, Parthenope, Victoria, Egeria,
Irene, Eunomia |
More asteroids,
discovered between 1845 and 1851. The rapidly expanding list of
planets prompted their reclassification as asteroids by astronomers, and this was widely
accepted by 1854. |
Pluto |
Trans-Neptunian object with a
semi-major axis beyond Neptune. In 2006, Pluto was reclassified as a
dwarf planet. |
Mythology
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220 px
The names for the planets in the Western world are derived from the
naming practices of the Romans, which ultimately derive from those
of the Greeks and the Babylonians. In
ancient Greece, the two great luminaries the
Sun and the Moon were called
Helios
and
Selene; the farthest planet was
called
Phainon, the shiner; followed by
Phaethon,
"bright"; the red planet was known as
Pyroeis, the
"fiery"; the brightest was known as
Phosphoros, the light
bringer; and the fleeting final planet was called
Stilbon,
the gleamer. The Greeks also made each planet sacred to one of
their pantheon of gods, the
Olympians: Helios and Selene were the names
of both planets and gods; Phainon was sacred to
Kronos, the
Titan who fathered the Olympians; Phaethon
was sacred to
Zeús, Kronos's son who
deposed him as king; Pyroeis was given to
Ares, son of Zeus and god of war; Phosphorus was
ruled by
Aphrodite, the goddess
of love; and
Hermes, messenger of
the gods and god of learning and wit, ruled over Stilbon.
The Greek practice of grafting of their gods' names onto the
planets was almost certainly borrowed from the Babylonians. The
Babylonians named Phosphorus after their goddess of love,
Ishtar; Pyroeis after their god of
war,
Nergal, Stilbon after their god
of wisdom
Nabu, and Phaethon after their chief
god,
Marduk. There are too many
concordances between Greek and Babylonian naming conventions for
them to have arisen separately. The translation was not perfect.
For instance, the Babylonian Nergal was a god of war, and thus the
Greeks identified him with Ares. However, unlike Ares, Nergal was
also god of pestilence and the underworld.
Today, most people in the western world know the planets by names
derived from the
Olympian pantheon of
gods. While modern Greeks still use their ancient names for the
planets, other European languages, because of the influence of the
Roman Empire and, later, the
Catholic Church, use the Roman (or Latin)
names rather than the Greek ones. The Romans, who, like the Greeks,
were
Indo-Europeans, shared
with them a
common pantheon under
different names but lacked the rich narrative traditions that Greek
poetic culture had given
their gods.
During the later period of the
Roman
Republic, Roman writers borrowed much of the Greek narratives
and applied them to their own pantheon, to the point where they
became virtually indistinguishable. When the Romans studied Greek
astronomy, they gave the planets their own gods' names:
Mercurius (for Hermes),
Venus (Aphrodite),
Mars (Ares),
Iuppiter (Zeus) and
Saturnus (Kronos). When subsequent
planets were discovered in the 18th and 19th centuries, the naming
practice was retained:
Uranus (
Ouranos) and
Neptūnus (
Poseidon).
Some
Romans, following a belief
possibly originating in
Mesopotamia but
developed in
Hellenistic Egypt,
believed that the seven gods after whom the planets were named took
hourly shifts in looking after affairs on Earth. The order of
shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from
the farthest to the closest planet). Therefore, the first day was
started by Saturn (1st hour), second day by Sun (25th hour),
followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus.
Since each day was named by the god that started it, this is also
the order of the
days of the week in
the
Roman calendar after the
Nundinal cycle was rejected –
and still preserved many modern languages. Sunday, Monday, and
Saturday are straightforward translations of these Roman names. In
English the other days were renamed after
Tiw, (Tuesday)
Wóden (Wednesday),
Thunor (Thursday), and
Fríge (Friday), the
Anglo-Saxon gods considered similar or
equivalent to Mars, Mercury, Jupiter, and Venus respectively.
Since Earth was only generally accepted as a planet in the 17th
century, there is no tradition of naming it after a god (the same
is true, in English at least, of the Sun and the Moon, though they
are no longer considered planets). The name originates from the 8th
century
Anglo-Saxon word
erda, which means ground or soil and was first used in
writing as the name of the sphere of the Earth perhaps around 1300.
It is the only planet whose name in English is not derived from
Greco-
Roman mythology. Many of the
Romance languages retain the old Roman
word
terra (or some
variation of it) that was used with the meaning of "dry land" (as
opposed to "sea"). However, the non-Romance languages use their own
respective native words. The Greeks retain their original name,
Γή (
Ge or
Yi); the
Germanic
languages, including English, use a variation of an ancient
Germanic word
ertho, "ground," as can be seen in the
English
Earth, the German
Erde, the Dutch
Aarde, and the Scandinavian
Jorde.
Non-European cultures use other planetary naming systems.
India
uses a
naming system based on the Navagraha,
which incorporates the seven traditional planets (Surya for the Sun, Chandra for
the Moon, and Budha, Shukra, Mangala, and Shani for the
traditional planets Mercury, Venus, Mars, Jupiter and Saturn) and
the ascending and descending lunar nodes
Rahu and Ketu. China
and the
countries of eastern Asia influenced by
it (such as Japan
, Korea
and Vietnam
) use a
naming system based on the five Chinese
elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and
earth
(Saturn).
Formation
It is not known with certainty how planets are formed. The
prevailing theory is that they are formed during the collapse of a
nebula into a thin disk of gas and dust. A
protostar forms at the core, surrounded by
a rotating
protoplanetary disk.
Through
accretion (a
process of sticky collision) dust particles in the disk steadily
accumulate mass to form ever-larger bodies. Local concentrations of
mass known as
planetesimals form, and
these accelerate the accretion process by drawing in additional
material by their gravitational attraction. These concentrations
become ever denser until they collapse inward under gravity to form
protoplanets. After a planet reaches a
diameter larger than the Earth's moon, it begins to accumulate an
extended atmosphere, greatly increasing the capture rate of the
planetesimals by means of
atmospheric
drag.
When the protostar has grown such that it ignites to form a
star, the surviving disk is removed from the
inside outward by
photoevaporation,
the
solar wind,
Poynting-Robertson drag and other
effects. Thereafter there still may be many protoplanets orbiting
the star or each other, but over time many will collide, either to
form a single larger planet or release material for other larger
protoplanets or planets to absorb. Those objects that have become
massive enough will capture most matter in their orbital
neighbourhoods to become planets. Meanwhile, protoplanets that have
avoided collisions may become
natural
satellites of planets through a process of gravitational
capture, or remain in belts of other objects to become either dwarf
planets or
small Solar System
bodies.
The energetic impacts of the smaller planetesimals (as well as
radioactive decay) will heat up
the growing planet, causing it to at least partially melt. The
interior of the planet begins to differentiate by mass, developing
a denser core. Smaller terrestrial planets lose most of their
atmospheres because of this accretion, but the lost gases can be
replaced by outgassing from the mantle and from the subsequent
impact of
comets. (Smaller planets will lose
any atmosphere they gain through various
escape mechanisms.)
With the discovery and observation of
planetary systems around stars other than
our own, it is becoming possible to elaborate, revise or even
replace this account. The level of
metallicity – an astronomical term describing
the abundance of
chemical elements
with an
atomic number greater than 2
(
helium) – is now believed to determine the
likelihood that a star will have planets. Hence it is thought less
likely that a metal-poor,
population
II star will possess a more substantial planetary system than a
metal-rich
population I
star.
Solar System
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The terrestrial planets: Mercury,
Venus, Earth, Mars
(Sizes to scale)
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The four gas giants against the Sun:
Jupiter, Saturn, Uranus, Neptune
(Sizes to scale)
According to the
IAU's current definitions,
there are eight planets and five dwarf planets in the
Solar System. In increasing distance from the
Sun, the planets are:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
Jupiter is the largest, at 318 Earth masses, while Mercury is
smallest, at 0.055 Earth masses.
The planets of the Solar System can be divided into categories
based on their composition:
- Terrestrial:
Planets that are similar to Earth, with bodies largely composed of
rock: Mercury, Venus, Earth and Mars.
At 0.055 Earth masses, Mercury is the smallest terrestrial planet
(and smallest planet) in the Solar System, while Earth is the
largest terrestrial planet.
- Gas giants (Jovians):
Planets with a composition largely made up of gaseous material and are significantly more massive than
terrestrials: Jupiter, Saturn, Uranus, Neptune. Jupiter, at 318
Earth masses, is the largest planet in the Solar System, while
Saturn is one third as big, at 95 Earth masses. Ice giants, comprising Uranus and Neptune, are a
sub-class of gas giants, distinguished from gas giants by their
significantly lower mass (only 14 and 17 Earth masses), and by
depletion in hydrogen and helium in their atmospheres together with
a significantly higher proportion of rock and ice.
- Dwarf planets:
Before the August 2006
decision, several objects were proposed by astronomers,
including at one stage by the IAU, as planets. However in
2006 several of these objects were reclassified as dwarf planets,
objects distinct from planets. Currently five dwarf planets in the
Solar System are recognized by the IAU: Ceres, Pluto, Haumea,
Makemake and Eris. Several other objects in both the Asteroid belt and the Kuiper belt are under consideration, with as
many as 50 that could eventually qualify. There may be as many as
200 that could be discovered once the Kuiper belt has been fully
explored. Dwarf planets share many of the same characteristics as
planets, although notable differences remain – namely that they are
not dominant in their
orbits. By definition, all dwarf planets are members of larger
populations. Ceres is the largest body in
the asteroid belt, while Pluto,
Haumea, and Makemake are members of the Kuiper belt and Eris is a
member of the scattered disc.
Scientists such as Mike Brown
believe that there may soon be over forty trans-Neptunian objects that qualify
as dwarf planets under the IAU's recent definition.
Planetary attributes
Extrasolar planets
The first
confirmed discovery of an extrasolar planet orbiting an ordinary
main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of
Geneva
announced the detection of an exoplanet around
51 Pegasi. Of the 404 extrasolar
planets discovered by November 2009, most have masses which are
comparable to or larger than Jupiter's, though masses ranging from
just below that of Mercury to many times Jupiter's mass have been
observed. The smallest extrasolar planets found to date have been
discovered orbiting burned-out star remnants called
pulsars, such as
PSR
B1257+12. There have been roughly a dozen extrasolar planets
found of between 10 and 20 Earth masses, such as those orbiting the
stars
Mu Arae,
55
Cancri and
GJ 436. These planets have
been nicknamed "Neptunes" because they roughly approximate that
planet's mass (17 Earths). Another new category are the so-called
"
super-Earths", possibly
terrestrial planets far larger than Earth
but smaller than Neptune or Uranus. To date, six possible
super-Earths have been found:
Gliese 876
d, which is roughly six times Earth's mass,
OGLE-2005-BLG-390Lb and
MOA-2007-BLG-192Lb, frigid icy worlds
discovered through
gravitational microlensing,
COROT-Exo-7b, a planet with a diameter
estimated at around 1.7 times that of Earth, (making it the
smallest
super-Earth yet measured), but
with an orbital distance of only 0.02 AU, which means it probably
has a molten surface at a temperature of 1000–1500
°C, and two planets orbiting the nearby
red dwarf Gliese 581.
Gliese 581 d is roughly 7.7 times
Earth's mass, while
Gliese 581 c is
five times Earth's mass and was initially thought to be the first
terrestrial planet found within a star's
habitable zone. However, more detailed
studies revealed that it was slightly too close to its star to be
habitable, and that the farther planet in the system, Gliese 581 d,
though it is much colder than Earth, could potentially be habitable
if its atmosphere contained sufficient greenhouse gases.
It is far from clear if the newly discovered large planets would
resemble the gas giants in the Solar System or if they are of an
entirely different type as yet unknown, like ammonia giants or
carbon planets. In particular, some of
the newly discovered planets, known as
hot
Jupiters, orbit extremely close to their parent stars, in
nearly circular orbits. They therefore receive much more
stellar radiation than the gas giants in the
Solar System, which makes it questionable whether they are the same
type of planet at all. There may also exist a class of hot
Jupiters, called
Chthonian planets,
that orbit so close to their star that their atmospheres have been
blown away completely by stellar radiation. While many hot Jupiters
have been found in the process of losing their atmospheres, as of
2008, no genuine Chthonian planets have been discovered.
More detailed observation of extrasolar planets will require a new
generation of instruments, including
space telescopes. Currently the
COROT spacecraft is searching for stellar luminosity
variations due to
transiting
planets. Several projects have also been proposed to create an
array of
space telescopes to search
for extrasolar planets with masses comparable to the Earth.
These
include the proposed NASA's Kepler
Mission, Terrestrial
Planet Finder, and Space Interferometry Mission
programs, the ESA
's Darwin, and the
CNES' PEGASE. The
New Worlds Mission is an occulting device
that may work in conjunction with the
James Webb Space Telescope.
However, funding for some of these projects remains uncertain. The
first spectra of extrasolar planets were reported in February 2007
(
HD 209458 b and
HD 189733 b). The frequency of occurrence of
such terrestrial planets is one of the variables in the
Drake equation which estimates the number of
intelligent, communicating
civilizations that exist in our galaxy.
Interstellar "planets"
Several
computer simulations of
stellar and planetary system formation have suggested that some
objects of planetary mass would be ejected
into interstellar
space. Some scientists have
argued that such objects found roaming in deep space should be
classed as "planets," although others have suggested that they
could be low-mass stars. In 2005, astronomers announced the
discovery of
Cha 110913-773444,
the smallest brown dwarf found to date, at only seven times
Jupiter's mass. Since it was not found in orbit around a fusing
star, it is a
sub-brown dwarf
according to the IAU's working definition. For a brief time in
2006, astronomers believed they had found a binary system of such
objects,
Oph 162225-240515, which
the discoverers described as "
planemos", or
"planetary mass objects". However, recent analysis of the objects
has determined that their masses are probably each greater than 13
Jupiter-masses, making the pair
brown
dwarfs.
Attributes
Although each planet has unique physical characteristics, a number
of broad commonalities do exist among them. Some of these
characteristics, such as rings or natural satellites, have only as
yet been observed in planets in the Solar System, whilst others are
also common to extrasolar planets.
Dynamic characteristics
Orbit
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According to current definitions, all planets must revolve around
stars; thus, any potential "
rogue
planets" are excluded. In the Solar System, all the planets
orbit the Sun in the same direction as the Sun rotates
(counter-clockwise as seen from above the Sun's north pole). At
least one extrasolar planet,
WASP-17b, has
been found to orbit in the opposite direction to its star's
rotation. The period of one revolution of a planet's orbit is known
as its
sidereal period or
year. A planet's year depends on its
distance from its star; the farther a planet is from its star, not
only the longer the distance it must travel, but also the slower
its speed, as it is less affected by the star's
gravity. Because no planet's orbit is perfectly
circular, the distance of each varies over the course of its year.
The closest approach to its star is called its
periastron (
perihelion
in the Solar System), while its farthest separation from the star
is called its
apastron (
aphelion). As a planet approaches periastron, its
speed increases as it trades gravitational potential energy for
kinetic energy, just as a falling object on Earth accelerates as it
falls; as the planet reaches apastron, its speed decreases, just as
an object thrown upwards on Earth slows down as it reaches the apex
of its trajectory.
Each planet's orbit is delineated by a set of
elements:
- The eccentricity
of an orbit describes how elongated a planet's orbit is. Planets
with low eccentricities have more circular orbits, while planets
with high eccentricities have more elliptical orbits. The planets
in the Solar System have very low eccentricities, and thus nearly
circular orbits. Comets and Kuiper belt objects (as well as several
extrasolar planets) have very high eccentricities, and thus
exceedingly elliptical orbits.
-
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The semi-major axis is the
distance from a planet to the half-way point along the longest
diameter of its elliptical orbit (see image). This distance is not
the same as its apastron, as no planet's orbit has its star at its
exact centre.
- The inclination of a planet
tells how far above or below an established reference plane its
orbit lies. In the Solar System, the reference plane is the plane
of Earth's orbit, called the ecliptic. For
extrasolar planets, the plane, known as the sky plane or
plane of the sky, is the plane of the observer's line of
sight from Earth. The eight planets of the Solar System all lie
very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it. The points
at which a planet crosses above and below its reference plane are
called its ascending and descending nodes. The longitude of the ascending
node is the angle between the reference plane's 0 longitude and
the planet's ascending node. The argument of periapsis (or perihelion
in the Solar System) is the angle between a planet's ascending node
and its closest approach to its star.
Axial tilt
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Earth's axial tilt is about 23°.
Planets also have varying degrees of
axial
tilt; they lie at an angle to the
plane of their
stars'
equators. This causes the amount of light received by each
hemisphere to vary over the course of its year; when the northern
hemisphere points away from its star, the southern hemisphere
points towards it, and vice versa. Each planet therefore possesses
seasons; changes to the climate over the
course of its year. The time at which each hemisphere points
farthest or nearest from its star is known as its
solstice. Each planet has two in the course of its
orbit; when one hemisphere has its summer solstice, when its day is
longest, the other has its winter solstice, when its day is
shortest. The varying amount of light and heat received by each
hemisphere creates annual changes in weather patterns for each half
of the planet. Jupiter's axial tilt is very small, so its seasonal
variation is minimal; Uranus, on the other hand, has an axial tilt
so extreme it is virtually on its side, which means that its
hemispheres are either perpetually in sunlight or perpetually in
darkness around the time of its solstices. Among extrasolar
planets, axial tilts are not known for certain, though most hot
Jupiters are believed to possess negligible to no axial tilt, as a
result of their proximity to their stars.
Rotation
The planets also rotate around invisible axes through their
centres. A planet's
rotation period
is known as its
day. Most of the planets in the
Solar System rotate in the same direction as they orbit the sun
which is counter-clockwise as seen from above the sun's
north pole,
the exceptions being Venus and Uranus which rotate clockwise,
though Uranus's extreme axial tilt means there are differing
conventions on which of its poles is "north", and therefore whether
it is rotating clockwise or anti-clockwise. However regardless of
which convention is used, Uranus has a
retrograde rotation relative to its
orbit. There is great variation in the length of day between the
planets, with Venus taking 243
Earth days to
rotate, and the gas giants only a few hours. The rotational periods
of extrasolar planets are not known; however their proximity to
their stars means that hot Jupiters are
tidally locked (their orbits are in sync with
their rotations). This means they only ever show one face to their
stars, with one side in perpetual day, the other in perpetual
night.
Orbital clearing
The defining dynamic characteristic of a planet is that it has
cleared its neighborhood.
A planet that has cleared its neighborhood has accumulated enough
mass to gather up or sweep away all the
planetesimals in its orbit. In effect, it
orbits its star in isolation, as opposed to sharing its orbit with
a multitude of similar-sized objects. This characteristic was
mandated as part of the
IAU's official
definition of a planet in August,
2006. This criterion excludes such planetary bodies as
Pluto,
Eris and
Ceres from full-fledged
planethood, making them instead
dwarf
planets. Although to date this criterion only applies to the
Solar System, a number of young extrasolar systems have been found
in which evidence suggests orbital clearing is taking place within
their circumstellar discs.
Physical characteristics
Mass
A planet's defining physical characteristic is that it is massive
enough for the force of its own gravity to dominate over the
electromagnetic forces binding
its physical structure, leading to a state of
hydrostatic equilibrium. This
effectively means that all planets are spherical or spheroidal. Up
to a certain mass, an object can be irregular in shape, but beyond
that point, which varies depending on the chemical makeup of the
object, gravity begins to pull an object towards its own centre of
mass until the object collapses into a sphere.
Mass is also the prime attribute by which planets are distinguished
from
stars. The upper mass limit for planethood
is roughly 13 times Jupiter's mass, beyond which it achieves
conditions suitable for
nuclear
fusion. Other than the Sun, no objects of such mass exist in
the Solar System; however a number of extrasolar planets lie at
that threshold. The
Extrasolar Planets Encyclopedia lists
several planets that are close to this limit:
HD 38529c,
AB Pictorisb,
HD 162020b, and
HD
13189b. A number of objects of higher mass are also listed, but
since they lie above the fusion threshold, they would be better
described as
brown dwarfs.
The smallest known planet, excluding dwarf planets and satellites,
is
PSR B1257+12 a, one of the first
extrasolar planets discovered, which was found in 1992 in orbit
around a
pulsar. Its mass is roughly half
that of the planet Mercury.
Internal differentiation
Illustration of the interior of Jupiter, with a rocky core overlaid
by a deep layer of metallic hydrogen
Every planet began its existence in an entirely fluid state; in
early formation, the denser, heavier materials sank to the centre,
leaving the lighter materials near the surface. Each therefore has
a
differentiated interior
consisting of a dense
planetary core
surrounded by a
mantle which either
is or was a
fluid. The terrestrial planets are
sealed within hard
crusts, but in
the gas giants the mantle simply dissolves into the upper cloud
layers. The terrestrial planets possess cores of magnetic elements
such as
iron and
nickel,
and mantles of
silicates.
Jupiter and
Saturn are
believed to possess cores of rock and metal surrounded by mantles
of
metallic hydrogen.
Uranus and
Neptune, which are
smaller, possess rocky cores surrounded by mantles of
water,
ammonia,
methane and other
ices. The
fluid action within these planets' cores creates a
geodynamo that generates a
magnetic field.
Atmosphere
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All of the Solar System planets have
atmospheres as their large masses mean gravity is
strong enough to keep gaseous particles close to the surface. The
larger gas giants are massive enough to keep large amounts of the
light gases
hydrogen and
helium close by, while the smaller planets lose these
gases into
space. The composition of the
Earth's atmosphere is different from the other planets because the
various
life processes that have transpired on
the planet have introduced free molecular
oxygen. The only solar planet without a substantial
atmosphere is Mercury which had it mostly, although not entirely,
blasted away by the
solar wind.
Planetary atmospheres are affected by the varying degrees of energy
received from either the Sun or their interiors, leading to the
formation of dynamic
weather systems
such as
hurricanes, (on Earth),
planet-wide
dust storms (on Mars), an
Earth-sized
anticyclone on
Jupiter (called the
Great Red Spot),
and
holes in the atmosphere (on
Neptune). At least one extrasolar planet,
HD
189733 b, has been claimed to possess such a weather system,
similar to the Great Red Spot but twice as large.
Hot Jupiters have been shown to be losing their atmospheres into
space due to stellar radiation, much like the tails of comets.
These planets may have vast differences in temperature between
their day and night sides which produce supersonic winds, although
the day and night sides of HD 189733b appear to have very similar
temperatures, indicating that that planet's atmosphere effectively
redistributes the star's energy around the planet.
Magnetosphere
One important characteristic of the planets is their intrinsic
magnetic moments which in turn give
rise to
magnetospheres. The presence
of a magnetic field indicates that the planet is still geologically
alive. In other words, magnetized planets have flows of
electrically conducting material in
their interiors, which generate their magnetic fields. These fields
significantly change the interaction of the planet and solar wind.
A magnetized planet creates a cavity in the solar wind around
itself called
magnetosphere, which the
wind cannot penetrate. The magnetosphere can be much larger than
the planet itself. In contrast, non-magnetized planets have only
small magnetospheres induced by interaction of the
ionosphere with the solar wind, which cannot
effectively protect the planet.
Of the eight planets in the Solar System, only Venus and Mars lack
such a magnetic field. In addition, the moon of Jupiter
Ganymede also has one. Of the magnetized
planets the magnetic field of Mercury is the weakest, and is barely
able to deflect the
solar wind.
Ganymede's magnetic field is several times larger, and Jupiter's is
the strongest in the Solar System (so strong in fact that it poses
a serious health risk to future manned missions to its moons). The
magnetic fields of the other giant planets are roughly similar in
strength to that of Earth, but their magnetic moments are
significantly larger. The magnetic fields of Uranus and Neptune are
strongly tilted relative the rotational
axis and displaced from the centre of the
planet.
In 2004, a team of astronomers in Hawaii observed an extrasolar
planet around the star
HD 179949, which
appeared to be creating a sunspot on the surface of its parent
star. The team hypothesised that the planet's magnetosphere was
transferring energy onto the star's surface, increasing its already
high 14,000 degree temperature by an additional 750 degrees.
Secondary characteristics
Several planets or dwarf planets in the Solar System (such as
Neptune and Pluto) have orbital periods that are in
resonance with each other or with smaller
bodies (this is also common in satellite systems). All except
Mercury and Venus have
natural
satellites, often called "moons." Earth has one, Mars has two,
and the
gas giants have numerous moons in
complex planetary-type systems. Many gas giant moons have similar
features to the terrestrial planets and dwarf planets, and some
have been studied as possible abodes of life (especially
Europa).
The four gas giants are also orbited by
planetary rings of varying size and
complexity. The rings are composed primarily of dust or particulate
matter, but can host tiny '
moonlet' whose gravity shapes and
maintains their structure. Although the origins of planetary rings
is not precisely known, they are believed to be the result of
natural satellites that fell below their parent planet's
Roche limit and were torn apart by
tidal forces.
No secondary characteristics have been observed around extrasolar
planets. However the
sub-brown dwarf
Cha 110913-773444, which has been
described as a
rogue planet, is
believed to be orbited by a tiny
protoplanetary disc.
Related terms
See also
Notes
References
- H. G. Liddell and R. Scott, A Greek–English Lexicon,
ninth edition, (Oxford: Clarendon Press, 1940).
- Cosmographia, Antwerp, 1539 "Celestial Orbs in the
Latin Middle Ages", Isis, Vol. 78, No. 2. (Jun., 1987), pp.
152-173.
- Note: select the Etymology tab
- Note: This journal became the Philosophical Transactions of
the Royal Society of London in 1775. There may just be earlier
publications within the .
- See for example the list of references for:
- (Astrophysics Data System entry)
- http://www.esa.int/esaCP/SEM7G6XPXPF_index_0.html
- Hunten D. M., Shemansky D. E., Morgan T. H. (1988), The
Mercury atmosphere, In: Mercury (A89-43751 19–91). University
of Arizona Press, pp. 562–612
External links