# Planets

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This annotated image shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Credit: Credit: NASA, ESA, and Z. Levay (STScI).

A planet is often thought of as a celestial body moving in an elliptical orbit around a star.

The "annotated image [at right] shows key features of the Fomalhaut system, including the newly discovered planet Fomalhaut b, and the dust ring. Also included are a distance scale and an insert, showing how the planet has moved around its parent star over the course of 21 months. The Fomalhaut system is located approximately 25 light-years from the Earth."[1]

The extrasolar planet is in orbit around Fomalhaut and is estimated "to be no more than three times Jupiter's mass ... In 2004, the coronagraph in the High Resolution Camera on Hubble's Advanced Camera for Surveys produced the first-ever resolved visible-light image of a large dust belt surrounding Fomalhaut. It clearly showed that this structure is in fact a ring of protoplanetary debris approximately 21.5 billion miles across with a sharp inner edge. This large debris disk is similar to the Kuiper Belt, which encircles the solar system and contains a range of icy bodies from dust grains to objects the size of dwarf planets, such as Pluto."[2]

"Observations taken 21 months apart by Hubble's Advanced Camera for Surveys' coronagraph show that the object is moving along a path around the star and therefore is gravitationally bound to it. The planet is 10.7 billion miles from the star, or about 10 times the distance of the planet Saturn from the sun."[2]

"A follow-up image in 2006 showed that one of the objects is moving through space with Fomalhaut but changed position relative to the ring since the 2004 exposure. The amount of displacement between the two exposures corresponds to an 872-year-long orbit as calculated from Kepler's laws of planetary motion."[2]

"The planet mysteriously dimmed by half a stellar magnitude between the 2004 and 2006 observations."[2]

## Plasma objects

A coronal mass ejection occurs in the upper left from the Sun. Credit: Skylab, NASA.

A magnetic cloud is a transient event observed in the solar wind. It was defined in 1981 by Burlaga et al. 1981 as a region of enhanced magnetic field strength, smooth rotation of the magnetic field vector and low proton temperature [3]. Magnetic clouds are a possible manifestation of a Coronal Mass Ejection (CME). The association between CMEs and magnetic clouds was made by Burlaga et al. in 1982 when a magnetic cloud was observed by Helios-1 two days after being observed by [Solar Maximum Mission] SMM[4]. However, because observations near Earth are usually done by a single spacecraft, many CMEs are not seen as being associated with magnetic clouds. The typical structure observed for a fast CME by a satellite such as [Advanced Composition Explorer] ACE is a fast-mode shock wave followed by a dense (and hot) sheath of plasma (the downstream region of the shock) and a magnetic cloud.

## Gaseous objects

"The atmospheric flow [of a hot Jupiter] is characterized by a super-rotating equatorial jet, transonic wind speeds, and eastward advection of heat away from the dayside. [There is] a dynamically induced temperature inversion ("stratosphere") on the planetary dayside and ... temperatures at the planetary limb [which] differ systematically from local radiative equilibrium values, a potential source of bias for transit spectroscopic interpretations."[5]

Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random. However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and they are polarized.[6]

## Aeronomy

The composite shows upper atmospheric lightning and electrical discharge phenomena. Credit: Abestrobi.

Def. "the atmosphere of the Earth and the other planets with reference to their chemical composition, physical properties, relative motion, and responses to radiation from space"[7] is called aeronomy.

Aeronomy is the study of the upper layers of the atmosphere, where dissociation and ionization are important.

Aeronomy is the science of the upper region of the atmosphere, where dissociation and ionization are important.[8]

## Liquid objects

This is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite. Credit: Robert Simmon and Marit Jentoft-Nilsen, NASA.

The image at right is a detailed, photo-like view of Earth based largely on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite.

"Viewed from space, the most striking feature of our planet is the water. In both liquid and frozen form, it covers 75% of the Earth’s surface. It fills the sky with clouds. Water is practically everywhere on Earth, from inside the rocky crust to inside our cells."[9]

## Rocky objects

This full disk is nearly featureless, a uniform grey surface with almost no dark mare. There are many bright overlapping dots of impact craters. Credit: NASA/GSFC/ASU LRO.

Def. a natural object in the sky especially at night is called an astronomical object.

Def.

1. a "naturally occurring aggregate of minerals"[10] or
2. any "natural material with a distinctive composition of minerals"[10]

is called a rock.

Def. full "of, or abounding in, rocks; consisting of rocks... [l]ike a rock"[11] is called rocky.

Def. a rocky, natural object in the sky especially at night is called an astronomical rocky object, or a rocky object.

Rocky objects are astronomical objects with solid surfaces.

The Moon, shown in the image at right, is a rocky object.

## Theoretical planets

In these clouds, and smaller versions of them, stars and planets are formed. One of the primary fields of study of molecular astrophysics then, is star and planet formation. Molecules may be found in many environments, however, from stellar atmospheres to those of planetary satellites.

Def.

1. any "secondary body in the Solar system that is geologically differentiated or in hydrostatic equilibrium and thus has a planet-like geology"[12],
2. any "substantial body in orbit around the Sun or other star"[12], or
3. any "substantial body associated with the development of planets"[12]

is called a planetary object, or planetary body.

Usage note:

"The phrase is often used for an unidentified body known to have complex geology, such as the unknown parent body of a meteorite."[12]

Def. "a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit" is called a planet.[13]

The proposed more general definition for a planet in orbit around another star substitutes "a star" for "the Sun" in part (a), keeps part (b), does not contain part (c), and adds "is neither a star nor a satellite of a planet."[14]

Def. "[a]ll other objects [not a planet or dwarf planet], except satellites, orbiting the Sun" are called collectively Small Solar-System Bodies.[13]

Def. a wanderer that is a moving light in the sky is called a planet.[13] This is the original description meant by the word "planet".[13]

Def. a "planet which exists outside Earth's solar system"[15] is called an exoplanet, exosolar planet, or extrasolar planet.

"More such announcements will likely come in the months to follow, as the first space observatory dedicated to hunting exoplanets, called COROT, begins full operation and researchers complete their calculations."[16]

Def. any "planet of the Solar System whose orbit is located between the asteroid belt and the sun"[17] is called an inner planet.

Def. any "planet of the Solar System whose orbit is located beyond the asteroid belt"[18] is called an outer planet.

Def.

1. "a planet with a Draft:moon large enough to be considered a planet, were the planet not there, having a substantial portion of the system's overall mass"[19],
2. "a pair of planemos in orbit around each other, where the barycenter of their orbits lies outside of both planemos, which in turn is in orbit about a star"[19],
3. "a pair of planemos in orbit around each other, where the barycenter of their orbits lies outside of both planemos"[19],
4. "a planemo with a moon large enough to be considered a planemo, having a substantial portion of the system's overall mass"[19],

is called a double planet.

Def. an "astronomical object, approximately the size of the Draft:Moon, formed from the mutual gravitational attraction of planetesimals"[20] is called a protoplanet.

## Brown dwarfs

This brown dwarf (smaller object) orbits the star Gliese 229, which is located in the constellation Lepus about 19 light years from Earth. The brown dwarf, called Gliese 229B, is about 20 to 50 times the mass of Jupiter. Credit: .

Brown dwarfs are sub-stellar objects that have fully convective surfaces and interiors, with no chemical differentiation by depth. Brown dwarfs occupy the mass range between that of large gas giant planets and the lowest-mass stars; this upper limit is between 75[1] and 80 Jupiter masses (${\displaystyle M_{J}}$).

Astronomers have reported that spectral class T brown dwarves (the ones with the coolest temperatures) are colored magenta because of absorption by sodium and potassium atoms of light in the green portion of the spectrum.[21][22][23]

"2MASS J10475385+2124234 [is] a brown dwarf more than 33 light-years away in the constellation Leo. The dwarf ... has a surface temperature of just ... 900 Kelvin ... Jupiter's lights are linked to its rapid rotation ... Since brown dwarfs are comparable in size to Jupiter, the brown dwarf flare mechanisms might arise similarly. ... [When] first examined using the ... fixed radio dish at Arecibo Observatory in Puerto Rico In several observations, ... flares of radio activity [occurred] ... [Using the] Karl G. Jansky Very Large Array (VLA) of telescopes ... The radio waves emanating ... are about 4.5 times fainter than the previous record ... observing ... LPP 944-20."[24]

Some of the incontrovertible brown dwarf substellar objects are identified by the presence of the 670.8 nm lithium [I] line. The most notable of these objects was Gliese 229B, which was found to have a temperature and luminosity well below the stellar range. Remarkably, its near-infrared spectrum clearly exhibited a methane absorption band at 2 micrometres, a feature that had previously only been observed in gas giant atmospheres and the atmosphere of Draft:Saturn's moon, Draft:Titan. Methane absorption is not expected at the temperatures of main-sequence stars. This discovery helped to establish yet another spectral class even cooler than L dwarfs known as "T dwarfs" for which Gl 229B is the prototype. Lithium is generally present in brown dwarfs and not in low-mass stars. [T]he presence of the lithium line in a candidate brown dwarf's spectrum is a strong indicator that it is indeed substellar. The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the lithium test Some brown dwarfs emit X-rays; and all "warm" dwarfs continue to glow tellingly in the red and infrared spectra until they cool to planetlike temperatures (under 1000 K).

## Sub-brown dwarfs

A sub-brown dwarf is an astronomical object of planetary mass that is not orbiting a star and is not considered to be a brown dwarf because its mass is below the limiting mass of about 13 Jupiter masses).[25] Sub-brown dwarfs are formed in the manner of stars, through the collapse of a gas cloud (perhaps with the help of photo-erosion), and not through accretion or core collapse from a circumstellar disc although not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[26] The smallest mass of gas cloud that could collapse to form a sub-brown dwarf is about 1 MJ.[27] This is because to collapse by gravitational contraction requires radiating away energy as heat and this is limited by the opacity of the gas.[28]

## Jupiter-like planets

This composite image shows an exoplanet (2M1207b, the red spot on the lower left), orbiting the brown dwarf 2M1207 (centre). 2M1207b is a Jupiter-like planet. It orbits the brown dwarf at a distance nearly twice as far as Draft:Neptune is from the Sun. Credit: ESO.

Def. a "planet whose characteristics are similar to Draft:Jupiter, but which orbits its parent star much more closely and thus has higher temperatures"[29] is called a hot Jupiter.

2M1207b is a planetary-mass object orbiting the brown dwarf 2M1207, in the constellation Centaurus, approximately 170 light-years from Earth.[30] Notable as one of the first candidate extrasolar planets to be directly observed (by infrared imaging), it was discovered in April 2004 by the Very Large Telescope (VLT) at the Paranal Observatory in Chile by a team from the European Southern Observatory led by Gaël Chauvin.[31] It is believed to be from 3 to 10 times the mass of Draft:Jupiter and may orbit 2M1207 at a distance roughly as far from the brown dwarf as Draft:Pluto is from the Earth's Sun.[32]

The object is a very hot gas giant; the estimated surface temperature is roughly 1600 K (1300 °C or 2400 °F), mostly due to gravitational contraction.[33] Its mass is well below the calculated limit for deuterium fusion in brown dwarfs, which is 13 Jupiter masses. The projected distance between 2M1207b and its primary is around 40 AU (similar to the mean distance between Draft:Pluto and the Sun).[34] Its infrared spectrum indicates the presence of water molecules in its atmosphere.[35]

UGPS J072227.51-054031.2 (designation often abbreviated to UGPS 0722-05) is a brown dwarf of late T type, located approximately 4.1 parsecs (13 light-years) from Earth.[36] The astronomical object was discovered by Philip Lucas at the University of Hertfordshire and announced in 2010. The discovery image was taken on 28 November 2006 by the UKIRT Infrared Deep Sky Survey (UKIDSS) with a recovery image confirming the object's proper motion on 2 March 2010.[36] The reported distance is derived from the current measured parallax of 246 milliarcseconds. The object was initially reported to be at an even closer distance of 2.9 parsecs, which would have placed it among the ten nearest stars to the Sun[37] but later measurements revealed that the object was in fact located at a greater distance than initially thought, at 4.1+0.6-0.5 parsecs.[36]

In June 2010, the authors updated their arXiv paper, noting that new parallax measurements were significantly different from earlier estimates, and that they had withdrawn their Nature submission. The revised data gives a distance of 4.1+0.6-0.5 parsecs. The latest version of the discovery paper has been submitted to the journal Monthly Notices of the Royal Astronomical Society.[36]

The object is roughly the volume of Draft:Jupiter, but is estimated to have 5–40 Jupiter masses (MJ).[36] This would make it less massive than ε Indi Ba. Planets have a mass of less than about 13 Jupiter masses. Infrared spectra shows the object contains water vapor and methane and has a surface temperature of approximately 480–560 Kelvin.[36]

## Rogue planets

A rogue planet — also known as an interstellar planet, nomad planet, free-floating planet or orphan planet — is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly.[38][39][40] Astronomers agree that either way, the definition of planet should depend on its current observable state and not its origin.

Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs[41] (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf).[42] The closest rogue planet to Earth yet discovered, CFBDSIR 2149-0403, is around 100 light years away.[43]

When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy.[44][45][46] Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.[47]

## Superplanets

The exoplanet (seen here as a bright splotch to the right of its host star, whose light has been blocked out) is a cloudy world several times more massive than Jupiter. Credit: M. Keppler.{{fairuse}}

Def. any planet larger than Jupiter is called a superplanet.

"New infrared telescope observations [such as in the image on the right] reveal an exoplanet [PDS 70b] that appears to be growing inside the disk of gas and dust around a star [PDS 70] about 370 light-years away."[48]

"PDS 70b [is] a giant, cloudy world about as far from its host star as Uranus is from the sun, but there’s still much we don’t know about the planet. It could be as cool as 730° Celsius or as hot as 1330° C, and its mass could be anywhere from twice to 17 times that of Jupiter."[48]

"PDS 70b has blazed a trail through the disk of planet-making material around its star by packing on the gas and dust in its orbital path. PDS 70b’s position inside this empty track, as well as the young age of its star — just 5.4 million years old — indicate that the planet isn’t finished forming."[49]

## Giant planets

Def. a "planet larger than terrestrial planets, composed mostly of gases and astronomical ices"[50] is called a giant planet.

Def. a "large planet composed mostly of gaseous hydrogen and helium, along with methane and ammonia; possibly with a solid core"[51] is called a gas giant.

"In the past two years, NASA’s Kepler Space Telescope has located nearly 3,000 exoplanet candidates ranging from sub-Earth-sized minions to gas giants that dwarf our own Jupiter."[52]

Def. a "giant planet composed mostly of astronomical ices (condensed forms of volatile compounds) including water, methane, oxygen, carbon, nitrogen, and sulfur, and smaller than a gas giant"[53] is called an ice giant.

## Major planets

Def. any "planet that is significantly larger"[54] in mean radius than Earth is called a major planet.

Def. any "extrasolar rocky planet several times larger than Earth, especially such a planet detected as it transits its sun"[55] is called a super-Earth.

The "detection of a transiting planet around π Mensae (HD 39091), [uses] data from the Transiting Exoplanet Survey Satellite (TESS). The solar-type host star is unusually bright (V=5.7) and was already known to host a Jovian planet on a highly eccentric, 5.7-year orbit. The newly discovered planet has a size of 2.14 ± 0.04 ~ R and an orbital period of 6.27 days. Radial-velocity data from the HARPS and AAT/UCLES archives also displays a 6.27-day periodicity, confirming the existence of the planet and leading to a mass determination of 4.82 ± 0.85 ~ M."[56]

## Terrestrial planets

Def. any "planet of the solar system or any exoplanet which is "Draft:Earth-like" in the sense that it is composed primarily of metals and rock, in contrast to a planet which is a gas giant"[57] is called a terrestrial planet.

Def. "a terrestrial planet composed mostly of silicate minerals"[58] is called a silicate planet.

## Dwarf planets

Ceres is seen by the Hubble Space Telescope (ACS). The contrast has been enhanced to reveal surface details. Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park).

Def. "a celestial body that

(a) is in orbit around the Sun [or another star],

(b) has sufficient mass for its self-gravity to overcome rigid forces so that it assumes a hydrostatic equilibrium (nearly round) shape,

(c) has not cleared the neighbourhood around its orbit, and

(d) is not a satellite" is called a dwarf planet.[13]

Ceres is the smallest identified dwarf planet in the solar system.

Ceres is the only dwarf planet in the asteroid belt.[59]

Def. "a planet that is between the size of Draft:Mercury and Draft:Ceres"[60] is called a mesoplanet.

## Minor planets

As NASA's Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA.

Def. an "astronomical object in direct orbit around the Sun [or another star] smaller than a planet and not classified as a comet"[61] is called a minor planet.

In the full image of Vesta at right, the rocky-object appears to have suffered from meteor damage.

Vesta, minor-planet designation 4 Vesta, is one of the largest asteroids in the Solar System. It lost some 1% of its mass less than a billion years ago in a collision that left an enormous crater occupying much of its southern hemisphere. Debris from this event has fallen to Earth as howardite–eucrite–diogenite (HED) meteorites, a rich source of information about the asteroid.[62][63]

Def.

1. an "asteroid of any size"[64],
2. an "asteroid-like body in an orbit beyond the asteroid belt"[64],
3. a "larger, planetary, body in orbit around the Sun"[64], or other star, or
4. a "dwarf planet"[64],

is called a planetoid.

Usage notes: "The term "planetoid" has never been precisely defined. At first, it was a synonym for asteroid; whereas "asteroid" referred to the star-like image seen through a telescope, "planetoid" referred to its planet-like orbit. Though it approached the popularity of "asteroid" ca. 1915, this usage was never dominant, and largely ceased by ca. 1980. Even before then the etymology of the term was reanalyzed as meaning planet-like in form, and started being used for larger asteroids such as Vesta which had planet-like geologies (that is, were planetary bodies). There was an increase in such usage after 2000 with the discovery of planetary bodies in the Kuiper belt and beyond, which many felt were not appropriately called "asteroids" and concomitant with doubts as to the appropriate definition of "planet". Sedna, for example, was called a "planetoid" in its discovery announcement."

"The group of small bodies that circle round the Sun, outside the orbit of Mars, are known under the designation of the planetoids."[65] Bold added.

For a model on the generation of chondrites, "if one can argue for the early existence of a few largely molten planetoids with dimension of kilometers or tens of kilometers, one can quite as easily argue for very large numbers of such planetoids."[66]

"Eris [...] was the upstart planetoid that knocked Pluto off the planetary lists"[67]

Def. any "of many small, solid astronomical objects, that orbit a star and form protoplanets through mutual gravitational attraction"[68] is called a planetesimal.

## Protoplanetary disks

This Hubble image of the Egg Nebula shows one of the best views to date of the brief but dramatic preplanetary, or protoplanetary nebula phase in a star’s life. Credit: ESA/Hubble & NASA.
The Hubble Space Telescope Advanced Camera for Surveys (ACS) image has H-alpha emission of the Red Rectangle shown in blue. Credit: .
The Red Rectangle is a proto-planetary nebula. Here is the Hubble Space Telescope Advanced Camera for Surveys (ACS) image. Broadband red light is shown in red. Credit: .

The presence of a protoplanetary disk may be used to estimate a stars age.

In December 2006, seven papers were published in the scientific journal, Science, discussing initial details of the sample analysis. Among the findings are: a wide range of organic compounds, including two that contain biologically usable nitrogen; indigenous aliphatic hydrocarbons with longer chain lengths than those observed in the diffuse interstellar medium; abundant amorphous silicates in addition to crystalline silicates such as olivine and pyroxene, proving consistency with the mixing of solar system and interstellar matter, previously deduced spectroscopically from ground observations;[69] hydrous silicates and carbonate minerals were found to be absent, suggesting a lack of aqueous processing of the cometary dust; limited pure carbon (CHON) was also found in the samples returned; methylamine and ethylamine was found in the aerogel but was not associated with specific particles.

Infrared astronomy deals with the detection and analysis of infrared radiation (wavelengths longer than red light). Except at wavelengths close to visible light, infrared radiation is heavily absorbed by the atmosphere, and the atmosphere produces significant infrared emission. Consequently, infrared observatories have to be located in high, dry places or in space. The infrared spectrum is useful for studying objects that are too cold to radiate visible light, such as planets and circumstellar disks. Longer infrared wavelengths can also penetrate clouds of dust that block visible light, allowing observation of young stars in molecular clouds and the cores of galaxies.[70] Some molecules radiate strongly in the infrared. This can be used to study chemistry in space; more specifically it can detect water in comets.[71]

"The NASA/ESA Hubble Space Telescope has been at the cutting edge of research into what happens to stars like our Sun at the ends of their lives ... One stage that stars pass through as they run out of nuclear fuel is the preplanetary, or protoplanetary nebula. This Hubble image [at right] of the Egg Nebula shows one of the best views to date of this brief but dramatic phase in a star’s life."[72]

"The preplanetary nebula phase is a short period in the cycle of stellar evolution — over a few thousand years, the hot remains of the star in the centre of the nebula heat it up, excite the gas, and make it glow as a planetary nebula. The short lifespan of preplanetary nebulae means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. The Egg Nebula, the first to be discovered, was first spotted less than 40 years ago, and many aspects of this class of object remain shrouded in mystery."[72]

"At the centre of this image, and hidden in a thick cloud of dust, is the nebula’s central star. While we can’t see the star directly, four searchlight beams of light coming from it shine out through the nebula. It is thought that ring-shaped holes in the thick cocoon of dust, carved by jets coming from the star, let the beams of light emerge through the otherwise opaque cloud. The precise mechanism by which stellar jets produce these holes is not known for certain, but one possible explanation is that a binary star system, rather than a single star, exists at the centre of the nebula."[72]

"The onion-like layered structure of the more diffuse cloud surrounding the central cocoon is caused by periodic bursts of material being ejected from the dying star. The bursts typically occur every few hundred years."[72]

"The distance to the Egg Nebula is only known very approximately, the best guess placing it at around 3000 light-years from Earth. This in turn means that astronomers do not have any accurate figures for the size of the nebula (it may be larger and further away, or smaller but nearer). This image is produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3."[72]

"The ERE was first recognized clearly in the peculiar reflection nebula called the Red Rectangle by Schmidt, Cohen, & Margon (1980)."[73]

The Red Rectangle Nebula, so called because of its red color and unique rectangular shape, is a protoplanetary nebula in the Monoceros constellation. Also known as HD 44179, the nebula was discovered in 1973 during a rocket flight associated with the AFCRL Infrared Sky Survey called Hi Star.

In the Red Rectangle Nebula, "[d]iffraction-limited speckle images of it in visible and near infrared light reveal a highly symmetric, compact bipolar nebula with X-shaped spikes which imply toroidal dispersion of the circumstellar material. The central binary system is completely obscured, providing no direct light.[74]

## Orbits

ISEE-3 is inserted into a "halo" orbit on June 10, 1982. Credit: NASA.
A hyperbolic pass is indicated by the blue line with an eccentricity of 1.3. A parabolic pass is the green line. The elliptical orbit in red has an eccentricity (e) of 0.7. Credit: Stamcose.
Two bodies orbiting around a common barycenter (red cross) with circular orbits. Credit: Zhatt.
Two bodies orbiting around a common barycenter (red cross) with elliptic orbits. Credit: Zhatt.
The diagram illustrates Kepler's three laws using two planetary orbits. Credit: Hankwang.

Def. "[a] circular or elliptical path of one object around another object"[75] is called an orbit.

Historically, the apparent motions of the planets were first understood geometrically (and without regard to gravity) in terms of epicycles, which are the sums of numerous circular motions.[76] Theories of this kind predicted paths of the planets moderately well, until Johannes Kepler was able to show that the motions of planets were in fact (at least approximately) elliptical motions.[77]

In the geocentric model of the solar system, the celestial spheres model was originally used to explain the apparent motion of the planets in the sky in terms of perfect spheres or rings, but after the planets' motions were more accurately measured, theoretical mechanisms such as deferent and epicycles were added. Although it was capable of accurately predicting the planets' position in the sky, more and more epicycles were required over time, and the model became more and more unwieldy.

In theoretical astronomy, whether the Earth moves or not, serving as a fixed point with which to measure movements by objects or entities, or there is a solar system with the Sun near its center, is a matter of simplicity and calculational accuracy.

Copernicus's theory provided a strikingly simple explanation for the apparent retrograde motions of the planets—namely as parallactic displacements resulting from the Earth's motion around the Sun—an important consideration in Johannes Kepler's conviction that the theory was substantially correct.[78]

Kepler knew that the tables constructed from the heliocentric theory were more accurate than those of Ptolemy"[78] with the Earth at the center. Using a computer, this means that for competing programs, one written for each theory, the heliocentric program finishes first (for a mutually specified high degree of accuracy).

1. The orbit of every planet is an ellipse with the Sun at one of the two foci.
2. A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.[79]
3. The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

The diagram at the right illustrates Kepler's three laws of planetary orbits: (1) The orbits are ellipses, with focal points ƒ1 and ƒ2 for the first planet and ƒ1 and ƒ3 for the second planet. The Sun is placed in focal point ƒ1. (2) The two shaded sectors A1 and A2 have the same surface area and the time for planet 1 to cover segment A1 is equal to the time to cover segment A2. (3) The total orbit times for planet 1 and planet 2 have a ratio a13/2 : a23/2.

The simplest description of the paths astronomical objects may take when passing each other includes a hyperbolic and parabolic pass. When capture occurs it usually produces an elliptical orbit.

## Orbital theory

The animated elliptical orbit has an eccentricity of 0.6. Credit: Willow.
The ellipse and some of its mathematical properties is described. Credit: .

Orbits come in many shapes and motions. The simplest forms are a circle or an ellipse.

The foci of an ellipse are two special points F1 and F2 on the ellipse's major axis and are equidistant from the center point. The sum of the distances from any point P on the ellipse to those two foci is constant and equal to the major axis ( PF1 + PF2 = 2a ). Each of these two points is called a focus of the ellipse.

In the gravitational two-body problem, if the two bodies are bound to each other (i.e., the total energy is negative), their orbits are similar ellipses with the common barycenter being one of the foci of each ellipse. The other focus of either ellipse has no known physical significance. Interestingly, the orbit of either body in the reference frame of the other is also an ellipse, with the other body at one focus.

Ideally, the motion of two oppositely charged particles in empty space would also be an ellipse.

A real orbit (and its elements) changes over time due to gravitational perturbations by other objects and the effects of relativity. A Keplerian orbit is merely an idealized, mathematical approximation at a particular time.

## Conic sections

Conics are of three types: parabolas , ellipses, including circles, and hyperbolas. Credit: .
Conic parameters are shown in the case of an ellipse. Credit: .

Def. "[a]ny of the four distinct shapes that are the intersections of a cone with a plane, namely the circle, ellipse, parabola and hyperbola"[80] is called a conic section.

In mathematics, a conic section (or just conic) is a curve obtained as the intersection of a cone (more precisely, a right circular conical surface) with a plane.

Various parameters are associated with a conic section, as shown in the following table. (For the ellipse, the table gives the case of a>b, for which the major axis is horizontal; for the reverse case, interchange the symbols a and b. For the hyperbola the east-west opening case is given. In all cases, a and b are positive.)

conic section equation eccentricity (e) linear eccentricity (c) semi-latus rectum () focal parameter (p)
circle ${\displaystyle x^{2}+y^{2}=a^{2}\,}$ ${\displaystyle 0\,}$ ${\displaystyle 0\,}$ ${\displaystyle a\,}$ ${\displaystyle \infty }$
ellipse ${\displaystyle {\frac {x^{2}}{a^{2}}}+{\frac {y^{2}}{b^{2}}}=1}$ ${\displaystyle {\sqrt {1-{\frac {b^{2}}{a^{2}}}}}}$ ${\displaystyle {\sqrt {a^{2}-b^{2}}}}$ ${\displaystyle {\frac {b^{2}}{a}}}$ ${\displaystyle {\frac {b^{2}}{\sqrt {a^{2}-b^{2}}}}}$
parabola ${\displaystyle y^{2}=4ax\,}$ ${\displaystyle 1\,}$ ${\displaystyle a\,}$ ${\displaystyle 2a\,}$ ${\displaystyle 2a\,}$
hyperbola ${\displaystyle {\frac {x^{2}}{a^{2}}}-{\frac {y^{2}}{b^{2}}}=1}$ ${\displaystyle {\sqrt {1+{\frac {b^{2}}{a^{2}}}}}}$ ${\displaystyle {\sqrt {a^{2}+b^{2}}}}$ ${\displaystyle {\frac {b^{2}}{a}}}$ ${\displaystyle {\frac {b^{2}}{\sqrt {a^{2}+b^{2}}}}}$

## Antapex

Def. "[t]he point to which the Sun appears to be moving with respect to the local stars"[81] is called the solar apex.

An antapex is a point that an astronomical object's total motion is directed away from. It is opposite to the apex.

The local standard of rest or LSR follows the mean motion of material in the Milky Way in the neighborhood of the Sun.[82] The path of this material is not precisely circular.[83] The Sun follows the solar circle (eccentricity e < 0.1 ) at a speed of about 220 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8 kpc about the center of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR.[84] [The Sun's peculiar motion relative to the LSR is 13.4 km/s.[85][86]] The LSR velocity is anywhere from 202–241 km/s.[87]

## Eccentricity

Def. "[t]he ratio, constant for any particular conic section, of the distance of a point from the focus to its distance from the directrix"[88] is called the eccentricity.

"For an ellipse, the eccentricity is the ratio of the distance from the center to a focus divided by the length of the semi-major axis."[88]

"Mercury's orbit eccentricity [e] varies between about 0.11 and 0.24 with the shortest time lapse between the extremes being about 4 x 105 yr".[89] "Smaller amplitude variations occur with about a 105 yr period."[89]

## Inclination

The diagram describes the parameters associated with orbital inclination (i). Credit: Lasunncty.

Def. "[t]he angle of intersection of a reference plane"[90] is called an inclination.

"The orbital inclination [i] [of Mercury] varies between 5° and 10° with a 106 yr period with smaller amplitude variations with a period of about 105 yr."[89]

## Obliquity

Def. "[t]he quality of being oblique in direction, deviating from the horizontal or vertical; or the angle created by such a deviation"[91] is called obliquity.

Axial tilt (also called obliquity) is the angle between an object's rotational axis, and a line perpendicular to its orbital plane. The planet Venus has an axial tilt of 177.3° because it is rotating in retrograde direction, opposite to other planets like Draft:Earth. The planet Uranus is rotating on its side in such a way that its rotational axis, and hence its north pole, is pointed almost in the direction of its orbit around the Sun. Hence the axial tilt of Uranus is 97°.[92]

The obliquity of the Earth's axis has a period of about 41,000 years.[93]

## Precession

Def. "[a]ny of several slow changes in an astronomical body's rotational or orbital parameters ... [such as] [t]he slow gyration of the Draft:Earth’s axis around the pole of the ecliptic"[94] is called a precession.

Def. "[t]he slow westward shift of the equinoxes along the plane of the ecliptic, resulting from precession of [an object's] axis of rotation, and causing the equinoxes to occur earlier each year"[95] is called the precession of the equinoxes.

The equinoxes of Earth precess with a period of about 21,000 years.[93]

## Resonances

The Laplace resonances of Ganymede, Draft:Europa, and Draft:Io is illustrated. Credit: User:Matma Rex.

An orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. The physics principle behind orbital resonance is similar in concept to pushing a child on a swing, where the orbit and the swing both have a natural frequency, and the other body doing the "pushing" will act in periodic repetition to have a cumulative effect on the motion. Orbital resonances greatly enhance the mutual gravitational influence of the bodies, i.e., their ability to alter or constrain each other's orbits. In most cases, this results in an unstable interaction, in which the bodies exchange momentum and shift orbits until the resonance no longer exists. Under some circumstances, a resonant system can be stable and self-correcting, so that the bodies remain in resonance. Examples are the 1:2:4 resonance of Draft:Jupiter's moons Draft:Ganymede, Draft:Europa and Draft:Io, and the 2:3 resonance between Draft:Pluto and Draft:Neptune. Unstable resonances with Draft:Saturn's inner moons give rise to gaps in the rings of Saturn. The special case of 1:1 resonance (between bodies with similar orbital radii) causes large Solar System bodies to eject most other bodies sharing their orbits; this is part of the much more extensive process of clearing the neighbourhood, an effect that is used in the current definition of a planet.

## Orbital poles

This is a snapshot of the planetary orbital poles. Credit: Urhixidur.

An orbital pole is either end of an imaginary line running through the center of an orbit perpendicular to the orbital plane, projected onto the celestial sphere. It is similar in concept to a celestial pole but based on the planet's orbit instead of the planet's rotation.

The north orbital pole of a celestial body is defined by the right-hand rule: If you curve the fingers of your right hand along the direction of orbital motion, with your thumb extended parallel to the orbital axis, the direction your thumb points is defined to be north.

At right is a snapshot of the planetary orbital poles.[96] The field of view is about 30°. The yellow dot in the centre is the Sun's North pole. Off to the side, the orange dot is Jupiter's orbital pole. Clustered around it are the other planets: Mercury in pale blue (closer to the Sun than to Jupiter), Venus in green, the Earth in blue, Mars in red, Saturn in violet, Uranus in grey partly underneath Earth and Neptune in lavender. Dwarf planet Pluto is the dotless cross off in Cepheus.

## Orbital decay

Orbital decay is the process of prolonged reduction in the altitude of a satellite's orbit. This can be due to drag produced by an atmosphere [frequent collisions between the satellite and surrounding air molecules]. The drag experienced by the object is larger in the case of increased solar activity, because it heats and expands the upper atmosphere.

## Planetary astronomy

A true color image of Ganymede is acquired by the Galileo spacecraft on June 26, 1996. Credit: NASA/JPL.

"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."[97] "[I]nterplanetary space ... is a stormy and sometimes very violent environment permeated by energetic particles and radation constantly emanating from the Sun."[97]

Each of the astronomical objects that constitute planetary science emits, reflects, or fluoresces radiation that is observed and analyzed.

"The spectrum of gaseous methane at 77 K in the 1.1-2.6 µm region [is] a benchmark for planetary astronomy".[98]

## Planetary science

An ultraviolet image of the planet Venus is taken on February 26, 1979, by the Pioneer Venus Orbiter. Credit: NASA.

Planetary science (rarely planetology) is the scientific study of planets (including Draft:Earth), moons, and planetary systems, in particular those of the solar system and the processes that form them. It studies objects ranging in size from micrometeoroids to gas giants, aiming to determine their composition, dynamics, formation, interrelations and history. It is a strongly interdisciplinary field, originally growing from astronomy and earth science,[99] but which now incorporates many disciplines, including planetary astronomy, planetary geology (together with geochemistry and geophysics), atmospheric science, Draft:oceanography, Draft:hydrology, theoretical planetary science, glaciology, and the study of extrasolar planets.[99] Allied disciplines include space physics, when concerned with the effects of the Sun on the bodies of the Solar System, and astrobiology.

## Planetary geology

Planetary geologist and NASA astronaut Harrison "Jack" Schmitt collects lunar samples during the Apollo 17 mission. Credit: NASA.

Planetary geology, ... astrogeology or exogeology, is a planetary science ... concerned with the geology of the celestial bodies such as the planets and their Draft:moons, asteroids, comets, and meteorites. ... [It includes] determining the internal structure of the terrestrial planets, ... planetary volcanism and surface processes such as impact craters, fluvial and aeolian processes.

## Planetary physics

Def. a process in which fragments of material (spall) are ejected from a body due to impact or stress is called spallation.

In planetary physics, spallation describes meteoritic impacts on a planetary surface and the effects of a stellar wind on a planetary atmosphere.

From a theoretical planetary physics perspective: "The shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation."[14]

Def. a celestial body "formed by accumulation of a rocky core, on a much longer timescale, ≳ 107 yr, with subsequent acquisition of a gaseous envelope if the circumstances allow this, and with an initially fractionated elemental composition" is called a planet.[100]

## Mercury

The color image shown here at right was generated by combining the mosaics taken through the MESSENGER WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

"MESSENGER's Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS), is equipped with 11 narrow-band color filters. As the spacecraft receded from Mercury after making its closest approach on January 14, 2008, the WAC recorded a 3x3 mosaic covering part of the planet not previously seen by spacecraft. The color image shown here was generated by combining the mosaics taken through the WAC filters that transmit light at wavelengths of 1000 nanometers (infrared), 700 nanometers (far red), and 430 nanometers (violet). These three images were placed in the red, green, and blue channels, respectively, to create the visualization presented here. The human eye is sensitive only across the wavelength range from about 400 to 700 nanometers. Creating a false-color image in this way accentuates color differences on Mercury's surface that cannot be seen in black-and-white (single-color) images."[101]

"Color differences on Mercury are subtle, but they reveal important information about the nature of the planet's surface material. A number of bright spots with a bluish tinge are visible in this image. These are relatively recent impact craters. Some of the bright craters have bright streaks (called "rays" by planetary scientists) emanating from them. Bright features such as these are caused by the presence of freshly crushed rock material that was excavated and deposited during the highly energetic collision of a meteoroid with Mercury to form an impact crater. The large circular light-colored area in the upper right of the image is the interior of the Caloris basin. Mariner 10 viewed only the eastern (right) portion of this enormous impact basin, under lighting conditions that emphasized shadows and elevation differences rather than brightness and color differences. MESSENGER has revealed that Caloris is filled with smooth plains that are brighter than the surrounding terrain, hinting at a compositional contrast between these geologic units. The interior of Caloris also harbors several unusual dark-rimmed craters, which are visible in this image. The MESSENGER science team is working with the 11-color images in order to gain a better understanding of what minerals are present in these rocks of Mercury's crust."[101]

## Venus

This real-color image of Venus is processed from imaging through the clear and blue filters on Mariner 10. Credit: NASA or Ricardo Nunes.

In the image at right the whiter cloud areas have a bluish tint. This is a real-color image of Venus processed through the clear and blue filters onboard Mariner 10.

"The side of Venus' ionosphere that faces away from the sun can billow outward like the tail of a comet, while the side facing the star remains tightly compacted, researchers said. ... "As this significantly reduced solar wind hit Venus, Venus Express saw the planet’s ionosphere balloon outwards on the planet’s ‘downwind’ nightside, much like the shape of the ion tail seen streaming from a comet under similar conditions," ESA officials said in a statement today (Jan. 29). It only takes 30 to 60 minutes for the planet's comet-like tail to form after the solar wind dies down. Researchers observed the ionosphere stretch to at least 7,521 miles (12,104 kilometers) from the planet, said Yong Wei, a scientist at the Max Planck Institute in Katlenburg, Germany who worked on this research."[102]

Because of the lack of a planetary magnetic field, the free hydrogen has been swept into interplanetary space by the solar wind.[103]

The clouds of Venus are capable of producing lightning much like the clouds on Earth.[104] The existence of lightning had been controversial since the first suspected bursts were detected by the Soviet Venera probes. In 2006–07 Venus Express clearly detected whistler mode waves, the signatures of lightning. Their intermittent appearance indicates a pattern associated with weather activity. The lightning rate is at least half of that on Earth.[104] In 2007 the Venus Express probe discovered that a huge double atmospheric vortex exists at the south pole of the planet.[105][106]

Another discovery made by the Venus Express probe in 2011 is that an ozone layer exists high in the atmosphere of Venus.[107]

Venus has an extremely dense atmosphere, which consists mainly of carbon dioxide and a small amount of nitrogen. The atmospheric mass is 93 times that of Earth's atmosphere, while the pressure at the planet's surface is about 92 times that at Earth's surface—a pressure equivalent to that at a depth of nearly 1 kilometer under Earth's oceans. The density at the surface is 65 kg/m³ (6.5% that of water).

## Earth

The Draft:Earth has a blue halo when seen from space. Credit: .
The Earth can have a blue sky and a blue ocean. Credit: Frokor.
This is the famous Blue Marble image of Earth taken by Apollo 17. Credit: NASA. Photo taken by either Harrison Schmitt or Ron Evans (of the Apollo 17 crew).

"Earth is a blue planet"[108].

Atmospheric gases scatter blue light more than other wavelengths, giving the Earth a blue halo when seen from space. This is shown in the image at right.

When light passes through our atmosphere, photons interact with it through scattering. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun. Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering, shorter (blue) wavelengths scatter more easily than longer (red) wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red. Because the Sun is close to the horizon, the Sun's rays pass through more atmosphere than normal to reach your eye. Much of the blue light has been scattered out, leaving the red light in a sunset.

The bluish color of water is a composite of several contributing agents. Prominent contributors include dissolved organic matter and chlorophyll.[109]

The image on the left is the famous Blue Marble image of Earth taken by Apollo 17. The image shows the eastern Southern Atlantic Ocean, the South African portion of the Southern Ocean, and the Western Indian Ocean. The land consists of most of Africa, Madagascar, Saudi Arabia, and portions of Iran, Irag, Turkey, and southern Greece. The gaseous portion consists of water vapor clouds over the southern portion of this hemisphere. Antarctica is completely covered in snow (a water-ice rocky substance).

If this third image is the one chosen to decide whether the Earth is a dwarf gaseous object, a dwarf liquid object, or a dwarf rocky object, the decision becomes difficult. Here, the Earth is primarily a liquid body.

## Mars

Mars is imaged from Hubble Space Telescope on October 28, 2005, with dust storm visible. Credit: NASA, ESA, The Hubble Heritage Team (STScI/AURA), J. Bell (Cornell University) and M. Wolff (Space Science Institute).

La couleur rouge sang caractéristique de Mars lui valut dans l’Antiquité le rapprochement avec le dieu grec de la guerre Arès puis avec son équivalent romain Mars, le rouge évoquant le sang des champs de bataille. Les Babyloniens la nommaient Nirgal ou Nergal, le dieu de la mort, des destructions et du feu. Les Égyptiens la nommaient « Horus rouge » (ḥr Dšr, Hor-desher) et connaissaient son « déplacement à reculons » (actuellement connu sous le nom de mouvement rétrograde). Dans la mythologie hindoue, Mars est nommée Mangala (मंगल) du nom du dieu de la guerre. Mangala Vallis est nommé en son honneur.

## Pallas

This is an ultraviolet image of Pallas showing its flattened shape taken by the Hubble Space Telescope. Credit: NASA.

Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered (after Draft:Ceres), and one of the largest in the Solar System. It is estimated to comprise 7% of the mass of the asteroid belt,[110] and its diameter of 544 kilometres (338 mi) is slightly larger than that of 4 Vesta. It is however 10–30% less massive than Vesta,[111] placing it third among the asteroids.

"Spectrally blue (B-type) asteroids are rare, with the second discovered asteroid, Pallas, being the largest and most famous example."[112]

"[T]he negative optical spectral slope of some B-type asteroids is due to the presence of a broad absorption band centered near 1.0 μm. The 1 μm band can be matched in position and shape using magnetite (Fe3O4), which is an important indicator of past aqueous alteration in the parent body. ... Observations of B-type asteroid (335) Roberta in the 3 μm region reveal an absorption feature centered at 2.9 μm, which is consistent with the absorption due to phyllosilicates (another hydration product) observed in CI chondrites. ... at least some B-type asteroids are likely to have incorporated significant amounts of water ice and to have experienced intensive aqueous alteration."[112]

## Jupiter

Zones, belts and vortices on Jupiter are shown. Credit: NASA/JPL/University of Arizona.

The wide equatorial zone is visible in the center surrounded by two dark equatorial belts (SEB and NEB).

"The large grayish-blue [irregular] "hot spots" at the northern edge of the white Equatorial Zone change over the course of time as they march eastward across the planet."[113]

"The Great Red Spot shows its counterclockwise rotation, and the uneven distribution of its high haze is obvious. To the east (right) of the Red Spot, oval storms, like ball bearings, roll over and pass each other. Horizontal bands adjacent to each other move at different rates. Strings of small storms rotate around northern-hemisphere ovals."[113]

"Small, very bright features appear quickly and randomly in turbulent regions, candidates for lightning storms."[113]

"The smallest visible features at the equator are about 600 kilometers (about 370 miles) across."[113]

"The clip consists of 14 unevenly spaced timesteps, each a true color cylindrical projection of the complete circumference of Jupiter, from 60 degrees south to 60 degrees north. The maps are made by first assembling mosaics of six images taken by Cassini's narrow-angle camera in the same spectral filter over the course of one Jupiter rotation and, consequently, covering the whole planet. Three such global maps -- in red, green and blue filters -- are combined to make one color map showing Jupiter during one Jovian rotation. Fourteen such maps, spanning 24 Jovian rotations at uneven time intervals comprise the movie."[113]

The passage of time is accelerated by a factor of 600,000.

## Saturn

The image shows Saturn's northern hemisphere from the Cassini spacecraft with Mimas in front. Credit: NASA/JPL/Space Science Institute.

In the image at right, "Mimas drifts along in its orbit against the azure backdrop of Saturn's northern latitudes in this true color view from NASA's Cassini spacecraft. The long, dark lines on the atmosphere are shadows cast by the planet's rings."[114]

"Saturn's northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths."[114]

## Uranus

Uranus's southern hemisphere in approximate natural colour (left) and in shorter wavelengths (right), shows its faint cloud bands and atmospheric "hood" as seen by Voyager 2. Credit: NASA.

In 1986 Voyager 2 found that the visible southern hemisphere of Uranus can be subdivided into two regions: a bright polar cap and dark equatorial bands (see figure on the right).[115] Their boundary is located at about -45 degrees of latitude. A narrow band straddling the latitudinal range from -45 to -50 degrees is the brightest large feature on the visible surface of the planet.[115][116] It is called a southern "collar". The cap and collar are thought to be a dense region of methane clouds located within the pressure range of 1.3 to 2 bar (see above).[117] Besides the large-scale banded structure, Voyager 2 observed ten small bright clouds, most lying several degrees to the north from the collar.[115] In all other respects Uranus looked like a dynamically dead planet in 1986. Unfortunately Voyager 2 arrived during the height of the planet's southern summer and could not observe the northern hemisphere. At the beginning of the 21st century, when the northern polar region came into view, the Hubble Space Telescope (HST) and Keck telescope initially observed neither a collar nor a polar cap in the northern hemisphere.[116] So Uranus appeared to be asymmetric: bright near the south pole and uniformly dark in the region north of the southern collar.[116] In 2007, when Uranus passed its equinox, the southern collar almost disappeared, while a faint northern collar emerged near 45 degrees of latitude.[118]

There are some reasons to believe that physical seasonal changes are happening in Uranus. While the planet is known to have a bright south polar region, the north pole is fairly dim, which is incompatible with the model of the seasonal change outlined above.[119] During its previous northern solstice in 1944, Uranus displayed elevated levels of brightness, which suggests that the north pole was not always so dim.[120] This information implies that the visible pole brightens some time before the solstice and darkens after the equinox.[119] Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[119] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[117] while the northern hemisphere demonstrated increasing activity,[121] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[122] This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[118]

## Neptune

This picture from the Voyager 2 sequence shows two of the four cloud features which have been tracked by the Voyager cameras during the past two months. Credit: NASA.

Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia, and methane. Traces of methane in the outermost regions in part account for the planet's blue appearance.[123]

## Interplanetary medium

The Zodiacal Light is over the Faulkes Telescope, Haleakala, Maui. Credit: 808caver.

Def. that part of outer space between the planets of a solar system and its star"[124] is called interplanetary space.

Def. the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move is called an interplanetary medium.

Chemical ions above the Earth's atmosphere, moving at very high speeds and at concentrations up to 100 particles per cm3 (centimeter cubed, a unit of volume) constitute the interplanetary medium.

The Zodiacal light is a faint, roughly triangular, diffuse white glow seen in the night sky that appears to extend up from the vicinity of the Sun along the ecliptic or zodiac.[125] It is best seen just after sunset and before sunrise in spring and autumn when the zodiac is at a steep angle to the horizon. Caused by sunlight scattered by space dust in the zodiacal cloud, it is so faint that either moonlight or light pollution renders it invisible. The zodiacal light decreases in intensity with distance from the Sun, but on very dark nights it has been observed in a band completely around the ecliptic. In fact, the zodiacal light covers the entire sky, being responsible for major part[126] of the total skylight on a moonless night. There is also a very faint, but still slightly increased, oval glow directly opposite the Sun which is known as the gegenschein. The dust forms a thick pancake-shaped cloud in the Solar System collectively known as the zodiacal cloud, which occupies the same plane as the ecliptic. The dust particles are between 10 and 300 micrometres in diameter, with most mass around 150 micrometres.[127]

Interplanetary scintillation refers to random fluctuations in the intensity of radio waves of celestial origin, on the timescale of a few seconds. It is analogous to the twinkling one sees looking at stars in the sky at night, but in the radio part of the electromagnetic spectrum rather than the visible one. Interplanetary scintillation is the result of radio waves traveling through fluctuations in the density of the electron and protons that make up the solar wind.

Scintillation occurs as a result of variations in the refractive index of the medium through which waves are traveling. The solar wind is a plasma, composed primarily of electrons and lone protons, and the variations in the index of refraction are caused by variations in the density of the plasma.[128] Different indices of refraction result in phase changes between waves traveling through different locations, which results in interference. As the waves interfere, both the frequency of the wave and its angular size are broadened, and the intensity varies.[129]

"The Raman spectra of some [interplanetary dust particle] IDPs also show red photoluminescence that is similar to the excess red emission seen in some astronomical objects and that has also been attributed to [polycyclic aromatic hydrocarbons] PAH s and hydrogenated amorphous carbon. Moreover, a part of the carbonaceous phase in IDPs and meteorites contains deuterium to hydrogen ratios that are greater than those for terrestrial samples."[130]

## Interplanetary magnetic field

Notation: let the symbol IMF stand for interplanetary magnetic field.

"[T]he IMF switched from northward to southward while Mariner [10] was in the Mercurian magnetosphere."[131]

## Classical planets

In antiquity the classical planets were the non-fixed objects visible in the sky, known to various ancient cultures. The classical planets were therefore the Sun and Moon and the five non-earth planets of our solar system closest to the sun (and closest to the Earth); all easily visible without a telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn.

Apparently 5102 b2k (before the year 2000.0), -3102 or 3102 BC, is the historical year assigned to a Hindu table of planets that does not include the classical planet Venus.[132] "Vénus seule ne s'y trouvait pas."[132] "Venus alone is not found there."[133]

## History

"Aristarchus also figured out how to measure the distances to the Sun and the Moon and their sizes."[134]

Ancient astronomers were able to differentiate between stars and planets, as stars remain relatively fixed over the centuries while planets will move an appreciable amount during a comparatively short time.

## Technology

Artist concept is of TESS. Credit: NASA.{{free media}}

The Transiting Exoplanet Survey Satellite (TESS) is a space telescope for NASA's Explorers program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission.[135] It was launched on April 18, 2018 atop a Falcon 9 rocket.[136][137][138][139] During its primary mission, it is expected to find more than 20,000 exoplanets,[140] compared to about 3,800 exoplanets known when it launched.[141] The first light image from TESS was taken on August 7, 2018, and released publicly on September 17, 2018.[142][143][144]

TESS records the nearest and brightest main sequence stars hosting transiting exoplanets, which are the most favorable targets for detailed investigations.[145]

TESS orbits Earth twice during the time the Moon orbits once, a 2:1 resonance with the Moon.[146]

TESS is designed to carry out the first spaceborne astronomical all-sky transiting exoplanet survey.[147][148]

Every 13.7 days at its perigee of 108,000 km (67,000 mi), TESS will downlink the data it has collected during the orbit to Earth over a period of approximately three hours.[149]

## Hypotheses

1. For a spheroidal rogue astronomical object to assume a planetary orbit around a star may require a third astronomical object of comparable or larger size.

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