A relatively large, rounded, celestial body that developed around a star and cannot sustain nuclear fusion reactors in its interior. Although humankind has known about the existence of planets in our solar system for centuries, recent discoveries have challenged a simple, conventional definition of planethood. These discoveries in the last few decades include “failed” stars called brown dwarfs, thousands of “exoplanets” beyond our solar system, including “rogue” exoplanets that do not orbit stars, as well as several new, planet-like bodies in the region beyond Pluto. Based on the richness displayed in our solar system and by exoplanets, planets are extremely diverse as a class of celestial object, existing across a wide range of masses, sizes, and compositions (Fig. 1). See also: Brown dwarf; Exoplanets; Solar system
Planets attain their myriad characteristics all from a common origin as material around a star, either leftover from a star's initial star formation or hypothetically from the shedding of mass by an aging star. This article will focus on the planets (and other related bodies) in our solar system as quintessential examples of the origin, development, and nature of these objects.
Origins and taxonomy of the Sun's planets
The standard model of solar-system formation assumes that the early solar nebula that gave rise to the Sun and the planets condensed from a diffuse interstellar cloud of gas and traces of other materials. The collapse of the cloud could have been initiated by cooling (if thermal pressure in the cloud could no longer balance the cloud's self-gravity) or by perturbations that created a local high-density region. In any case, once self-gravity initiated condensation, the cloud's collapse would have been a runaway process. The cloud would have increased its rotational speed as it condensed (like a skater pulling in her arms), eventually forming a spinning disk, known as a protoplanetary disk, with most of its mass in the center. This central mass became the basis for a proto-Sun, and the molecules and atoms in the rest of the disk settled into the central plane of the disk. Some molecules would have condensed into solid particles, and the particles would have accreted into planetesimals, the building blocks of planets. See also: Molecular cloud; Protostar
Hydrogen and helium are thought to have made up 98% of the solar nebula, followed by water (H2O), methane (CH4), and ammonia (NH3), which are the hydrogenated forms of carbon, nitrogen, and oxygen, respectively; silicates (rocks); and metals (mostly as iron and nickel). These various constituents condensed into solid particles in the central plane of the disk, but because metals, rocks, and ices solidify at different temperatures, there was a compositional gradient as a function of distance from the proto-Sun. Beyond the “frost line” (perhaps 2 to 5 fives times the average Sun-Earth distance of 150 million km (93 million mi), also known as a an astronomical unit, or AU), volatile compounds such as water, methane, and ammonia could condense into ices. This meant that the availability of solid material was several times higher outside of the frost line than inside it. The planetary cores outside the frost line were therefore more massive, enough to attract and retain atmospheres of hydrogen and helium. See also: Astronomical unit; Carbon; Helium; Hydrogen; Iron; Nickel; Oxygen
The solar system's eight (major) planets fall into two basic groups: the small, dense, terrestrial planets—Mercury, Venus, Earth, and Mars—and the giant or Jovian planets Jupiter, Saturn, Uranus, and Neptune. The terrestrial planets are all located relatively close to the Sun, whereas the lower-density giant planets extend outward from Jupiter to great distances (Fig. 2). Terrestrial planets have relatively shallow atmospheres compared to the bulk of their constituent material, whereas gas giants have deep, voluminous atmospheres compared to their non-gaseous interiors. See also: Jupiter; Mars; Mercury (planet); Neptune; Saturn; Uranus; Venus
Terrestrial planets (Mercury, Venus, Earth, and Mars) are thought to have formed in a region dominated by metallic and rocky planetesimals, and their surface gravities were not sufficient to hold onto hydrogen and helium gases. In contrast, the gas giants (Jupiter and Saturn) accreted massive cores with extremely high escape velocities. They retained hydrogen and helium atmospheres. Their current compositional makeup is similar to that of early solar nebula: 98% hydrogen and helium with traces of other constituents. Uranus and Neptune are sometimes called “ice giants” because their composition is similar to that of comets. They have less hydrogen and helium than Jupiter and Saturn do, but they accreted from numerous volatile-rich icy planetesimals to become an order of magnitude more massive than each of the terrestrial planets. Finally, less-massive objects such as Pluto or Eris are also the accretion products of icy planetesimals but did not grow to the extent of a Neptune or Uranus. See also: Solar system
Definition of “planet”
The word “planet” derives from the Greek word planasthai, meaning “to wander,” and it was used by ancient stargazers to identify bright starlike objects that appeared to wander against the background of fixed stars.
Until recently, astronomers also considered Pluto to be a planet. However, the discovery of Kuiper Belt objects with orbits similar to Pluto's and in particular the discovery in 2003 of Eris, a very distant solar-system object comparable than Pluto, triggered a protracted debate about whether Pluto truly qualified as a planet. In 2006, members of the International Astronomical Union (IAU) defined a planet in our solar system as: "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 neighborhood around its orbit” through gravitational scattering. In this context, "hydrostatic equilibrium" yields roughly spherical shapes for slowly rotating objects and ellipsoids for fast-rotating ones.
The astronomers also defined a new class of objects, dwarf planets, to designate smaller, less massive bodies that are "round" (in hydrostatic equilibrium) but unable to clear their orbital neighborhood as they circle the Sun.
Thus, the term “planet” now formally applies only to the solar system's eight largest worlds. Pluto, Eris, and the large asteroid Ceres became the first objects to be called dwarf planets, a designation later given as well to the large, recently discovered objects Haumea and Makemake in the Kuiper Belt. See also: Ceres; Pluto
The IAU has not defined an upper size or mass limit for a planet. However, many astronomers informally favor a limit of 13 times the mass of Jupiter (presuming the object to have similar elemental ratios). Bodies with greater masses can generate energy through the fusion of deuterium in their cores and thus are considered to belong to a class of substellar objects called brown dwarfs. See also: Brown dwarf
Each of the planets from Earth to Neptune is accompanied by one or more secondary bodies called satellites, and more conventionally called and thought of as "moons." Many of the smallest satellites, for instance around the solar system's gas giants, are not observable from Earth but were instead discovered during spacecraft visits. See also: Satellite (astronomy)
The gas-and-dust disk that surrounded the infant Sun also gave rise to a multitude of smaller bodies, which fall into two main groups. The asteroids, of which nearly 250,000 have well-determined orbits, are rocky and largely confined between the orbits of Mars and Jupiter; objects in the Kuiper Belt, about 1400 of which are now known but which may number in the billions, are primarily icy and lie beyond the orbit of Neptune. See also: Asteroid; Kuiper belt
Possible unknown planets
During the nineteenth century, an unexplained irregularity in the motion of Mercury was thought by some investigators to be caused by an unknown planet circulating between the Sun and Mercury, called Vulcan, which was looked for in vain. This irregularity was satisfactorily explained in 1915 by Albert Einstein's general theory of relativity. It is now certain that no intra-Mercurial planet larger than 30 mi (50 km) can exist. See also: Relativity
The discovery of objects in the Kuiper Belt beginning in 1992 has spurred efforts to find large bodies beyond the orbit of Neptune. Eris, an object just slightly smaller than Pluto but more massive, was discovered in 2003 at a distance of 97 AU, nearly 21/2 times Pluto's mean heliocentric distance. Theoretical modeling of the formation of the Kuiper Belt and of outer-planet evolution suggests that even larger Kuiper Belt objects await discovery. See also: Kuiper Belt; Large Kuiper Belt objects; Pluto
For instance, in 2016, based on abnormal and unexplained orbits of objects in the Kuiper Belt, researchers proposed the existence of a body dubbed Planet Nine. This world could possess as many as 10 Earth-masses, making it a "super-Earth," a type of body commonly found in exoplanet searches but unprecedented in our solar system. Its orbit is estimated to take it between 200 AU and 1200 AU from the Sun, and thus at a minimum of five times Pluto’s average distance. Searches for Planet Nine remain ongoing.
Planetary orbits and motions
The motions of planets in their orbits around the Sun and other stars are governed by three laws of motion discovered by Johannes Kepler at the beginning of the seventeenth century. See also: Celestial mechanics; Kepler's laws
First law: The orbit of a planet is an ellipse, with the Sun or host star at one of its foci. See also: Ellipse
Second law (the law of areas): As a planet revolves in its orbit, the radius vector (the line from the Sun or star to the planet) sweeps out equal areas in equal intervals of time.
Third law (the harmonic law): The square of the period of revolution P is proportional to the cube of the orbit's semimajor axis a; that is, for all planets the ratio P2/a3 is a constant.
The position of a planet in its orbit and the orientation of the orbit in space are completely defined by seven orbital elements (Fig. 3). These are (1) the semimajor axis a, (2) the eccentricity e, (3) the inclination i of the plane of the orbit to the plane of the ecliptic, (4) the longitude Ω of the ascending node N, (5) the angle ω from the ascending node N to the perihelion q, (6) the sidereal period of revolution P, or the mean (daily) motion n = 2π/P, and (7) the date of perihelion passage T, or epoch E.
The intersection NN′ of the plane of a planet's orbit and the plane of the ecliptic (defined by Earth's orbit) is the line of nodes. The planet crosses the plane of the ecliptic from south to north at the ascending node N and from north to south at the descending node N′. The longitude of the ascending node is the angle Ω measured in the plane of the ecliptic from the vernal equinox ∊. The location of the plane of the orbit in space is defined by i and Ω, the orientation of the ellipse in this plane by ω, its form by e, its size by a, and the position of the planet on the ellipse by P and T (and by the time t). See also: Orbital motion
The orbital elements of the planets are deduced from accurate telescopic observations of the planets’ positions in the sky with respect to background stars, together with tracking data for spacecraft in the planets’ vicinity and radar-derived range and velocity measurements. The orbital elements allow the motion of each body to be calculated into the past and future; a set of these calculated positions, published in tabular form, is called an ephemeris. See also: Ephemeris
The main orbital characteristics of the planets and dwarf planets are given in Table 1.
|Planets||Mean distance from Sun (semimajor axis of orbit)||Sidereal period of revolution||Synodic period||Mean orbital velocity||Orbital eccentricity||Orbital inclination|
|AU†||millions of mi||millions of km||years||days||days||mi/s||km/s||0 = perfect circle||degrees|
†Astronomical Unit, denoting the mean Earth-Sun distance as 1.
*No other bodies have been designated dwarf planets, but several likely candidates exist.
As previously noted, planets come in our solar system come in at least two basic varieties: small and rocky or big and gaseous. It is additionally theorized that exoplanets could exist with entirely liquid “surfaces,” consisting of voluminous oceans, or magma. These basic planet types arise as a function of mass, radius, constituent materials, orbital distance from a host star, and other variables, Some key physical characteristics of the Sun’s planets are provided in Table 2.
|Planet||Equatorial radius, re||Ellipticity||Volume||Mass||Density||Rotation||Obliquity†|
|(Earth = 1)||mi||km||(Earth = 1)||(Earth = 1)||g/cm3||mi/s||km/s||period ("day")||degrees|
|Mercury||0.38||1515||2440||0.000||0.056||0.055||5.43||2.8||4.4||58 d 15.5 h||0.1|
|Venus||0.95||3760||6052||0.000||0.857||0.815||5.20||6.4||10.4||243 d 0.5 h||177.4‡|
|Earth||1||3963||6378||0.0034||1.000||1||5.52||7||11.2||23 h 56 m 23 s||23.45|
|Mars||0.53||2110||3396||0.0065||0.151||0.107||3.93||3.1||5||24 h 37 m 23 s||25.19|
|Jupiter||11.21||44,423||71,492||0.0651||1321||317.71||1.33||37||59.5||9 h 55 m 30 s¶||3.12|
|Saturn||9.45||37,449||60,268||0.098||764.||95.152||0.69||22.1||35.5||10 h 39 m 22 s¶||26.73|
|Uranus||4.01||15,882||25,559||0.0229||63.||14.536||1.27||13.2||21.3||17 h 14.4 m¶||97.86‡|
|Neptune||3.88||15,389||24,766||0.0171||58.||17.147||1.64||14.6||23.5||16 h 6.6 m¶||29.56|
|Ceres||0.08||303||487||0.066||0.0004||0.0002||2.08||0.3||0.5||9 h 4 m 27 s||3|
|Pluto||0.18||715||1150||?||0.006||0.002||2||0.8||1.3||6 d 9 h 17.6 m||119.6‡|
|Haumea||0.09||360||575||?||0.001||0.0001||∼3||?||?||3 h 54 m 56 s||?|
|Makemake||0.11||440||710||?||0.001||0.0005||?||?||?||7 h 46 m 16 s||?|
|Eris||0.19||745||1200||?||0.007||0.0028||?||?||?||25 h 55 m||?|
†Obliquity is the tilt of the equator with respect to the orbital plane.
‡Venus, Uranus, and Pluto are considered to have retrograde rotation.
¶Rotation period of a non-solid planetary body's core deduced from magnetic field; differs from equatorial rotation period.
*No other bodies have been designated dwarf planets, but several likely candidates exist.
During their formation, all of the planets underwent differentiation, meaning that their partially molten interiors segregated into discrete layers of differing composition and density. For the four inner planets, the result was a molten, iron-rich core overlain by a dense, viscous mantle and a solid, relatively buoyant crust. The hydrogen-helium mixture that dominates Jupiter and Saturn exists in both planets as an exterior layer of gas and an interior layer in which hydrogen is believed to assume a solid state akin to the arrangement of atoms in a metal. Ice and rock are concentrated in their cores. Uranus and Neptune, along with distant outer dwarf planets like Pluto, contain roughly equal mixtures of rock and ice that have differentiated into discrete layers.
The solar system's largest satellites—Earth's Moon; Jupiter's Io, Europa, Ganymede, and Callisto; Saturn's Titan; and Neptune's Triton—also have interiors that are partially or completely differentiated. Therefore, they are (or once must have been) at least partially molten. Consequently, their surfaces can exhibit geologic features akin to those found on the terrestrial planets. In this sense, geologists consider them planetary bodies; indeed, several are large and massive enough to be classified as planets were they to orbit the Sun. See also: Moon
The interiors of planets are difficult to study directly. Even on Earth, samples obtained by drilling probe less than 1% of Earth's radius. Seismic sensors can detect compression and transverse waves that propagate through an object. The speed and direction of these waves depend on density and density gradients. Sensors can detect waves that are reflected off interfaces (changes in state or composition within the planet). The Apollo and Viking missions deployed seismic sensors on the Moon and Mars, respectively, but in the latter case measurements were contaminated by buffeting from Martian winds. For bodies besides Earth and the Moon, astronomers must observe spin rates, oblateness, and gravitational fields (found by precisely measuring velocity changes of spacecraft in their vicinity) to determine the distribution of mass in the planets’ interiors. See also: Earth's interior; Seismology
Moments of inertia and distribution of mass
A spherically symmetric planet has a spherically symmetric gravitational field. Most planets, however, are oblate—they are slightly wider at the equator than they are from pole to pole. Saturn is the most oblate, with an equatorial radius that is 10% larger than its polar radius. As a result, Saturn's gravitational field is a little stronger in the plane of Saturn's equator than it is above either of Saturn's poles (at a given distance from the planet).
The gravitational field of an oblate planet depends on the distribution of mass in its interior (Fig. 4). If the planet has a dense core, then less mass is distributed around its equator. The gravitational field around a hypothetical differentiated “dense-core” planet will be more spherical than it would be around a “uniform-density” planet of the same oblateness. Measurements of the gravitational field in the vicinity of a planet can be made by studying the motion of a nearby object—a natural moon or an artificial spacecraft—that in turn reveals details about the state of the planet's interior.
If we assume that a planet's interior is in hydrostatic equilibrium (that is, inward-pointing forces due to gravity are balanced by outward-pointing pressure-gradient forces), then one can estimate the radial distribution of mass from two observables: an object's spin rate and its oblateness. To illustrate, consider a nonrotating, Jupiter-size planet. It would be a giant onion of spherical layers, with each layer growing denser toward the center of the planet. The density profile can be modeled—including changes of state (such as the onset of metallic hydrogen) where the pressure and temperature are high enough—but cannot be confirmed if the planet is spherical. Once we start spinning this gas giant, we can calculate the balance between self-gravity and centripetal acceleration at each point and compare the predicted shape of the planet to the oblateness we observe. Jupiter and Saturn, for example, would be even more oblate if they were composed of hydrogen and helium to their cores. We deduce that Jupiter and Saturn have dense, rocky cores (about 10–20 Earth masses in the case of Jupiter) from the fact that each of the objects is less oblate than they would be if they had no rocky cores.
Asteroids and small moons are generally not in hydrostatic equilibrium because their intrinsic material strength prevails over self-gravity. The radius at which self-gravity overcomes intrinsic strength is roughly 200–400 km (125–250 mi).
Jupiter and Saturn have similar compositions and are nearly the same size, yet Jupiter is nearly three times more massive. Why is there such a difference in densities? Consider a small planet made of hydrogen to which we slowly add more and more hydrogen. For very small planets, the radius will scale with the cube root of the mass, since the density will be roughly constant in this regime. As the mass of the planet increases, however, the density of the core increases, and the radius will be proportional to the mass raised to a power less than 1/3. Eventually, the compression is so great that the addition of more hydrogen simply increases the average density, and the radius is unchanged. This is the case for hydrogen-based planets having between 100 and 1000 Earth masses, including Saturn and Jupiter. Objects in this mass range have nearly constant radii, independent of their mass. The addition of even more hydrogen (beyond 1000 Earth masses) actually shrinks the radius; this is the regime of white dwarves and eventually black holes or neutron stars.
A polytrope is a hypothetical object in which the pressure is proportional to the density raised to a power. This power is usually written as 1 + 1/n, where n is the polytropic index. For a hydrogen-dominated object such as Jupiter, n = 1 is a reasonable approximation to its equation of state. Jupiter's radius is very close to the mass-independent radius of an n = 1 polytrope, but Saturn's radius is smaller. This is evidence that Saturn contains more elements (relative to Jupiter) that are heavier than hydrogen. See also: Polytropic process
Composition and phase changes
A polytrope is a useful approximation for examining the interiors of Jupiter and Saturn. In particular, it helps us determine where phase changes occur within the gas giants. At a pressure of about 2 Mbar (2 × 1011 N/m2), hydrogen gas undergoes a phase change to state called metallic hydrogen. We can estimate the pressure from the density, and the density of an n = 1 polytrope has a radial dependence proportional to sin (Ar)/Ar, where A is a constant and r is the radius. For Jupiter, we find that the pressure exceeds 2 Mbar inside 80% of Jupiter's radius, which means that just over half of Jupiter (by volume) is its metallic hydrogen core. For Saturn, the metallic transition takes place at the 50% radius, so Saturn's metallic hydrogen core occupies about 1/8 of its total volume.
The solid surfaces of Mercury, Venus, Earth, and Mars bear features that have been shaped primarily by three major processes: impact cratering, volcanism, and tectonism (crustal movement). Whereas impact cratering (discussed below) is a consequence of external forces, each terrestrial planet bears a signature of volcanism and tectonism that is a manifestation of the amount of heat flowing outward from its interior. Minor surface-forming processes include erosion by wind and (on Earth and Mars) by water. See also: Meteorite; Volcano
Once differentiation was complete, the decay of radioactive isotopes within each body generated heat that partially melted portions of the mantle. The resulting magmas, hotter and less dense than their surroundings, rise through the crust and can erupt on the surface as volcanic flows. These flows serve to cool the planets’ interiors. Mercury and Mars, which have relatively high surface-to-volume ratios, have cooled to the point that volcanism essentially ended long ago. By contrast, internal heat continues to drive active volcanism on both Earth and Venus. Much of Venus is covered by lava flows that erupted within the past 700 million years.
All four inner planets have undergone tectonism to some degree. Mercury exhibits the least, primarily in the form of thrust faults caused by a slight decrease in the planet's radius as it cooled. Mars exhibits large crustal bulges that reflect upwelling within its mantle. Tectonic deformation of surface features on Venus is widespread. Earth is the most tectonically active planet. Its surface is covered by interlocking crustal plates that shift in position due to motions within the mantle below. New crust forms along midocean ridges, and old crust is recycled into the mantle along subduction zones. This plate motion is the primary means by which Earth dissipates its interior heat. See also: Plate tectonics
The most distinctive features on nearly all solid surfaces in the solar system are impact craters. Except for objects that have undergone recent resurfacing, such as Earth or the Jovian satellite Europa, all solid surfaces reveal their history though impact craters. See also: Lithosphere
During an impact event, the kinetic energy of the impactor is rapidly partitioned into heating the impactor, heating the target, compacting the target, comminuting the target (breaking it up into a powder or small pieces), and ejecting target material. Small impacts are in the “energy scaling” regime. Most of the energy goes into heating, compacting, or comminuting material. The craters formed in this regime are simple, bowl-shaped craters.
Above a certain threshold, about half of the impactor's energy goes into excavating and launching material from the impact site; these craters are in the “gravity regime.” The transition from energy scaling to gravity scaling depends mainly on the acceleration due to gravity at the target's surface. For the Moon (with a surface gravity of 1.6 m/s2), the transition occurs at 15 km (9 mi). Objects with higher surface gravities will have the energy scaling versus gravity scaling transition occur at smaller crater diameters. For the Earth and Mercury, the surface accelerations due to gravity are 9.8 and 3.7 m/s2, respectively, and the transition diameters are about 2 and 5 km (1.2 and 3 mi).
Large craters, those formed in the gravity regime, are complex craters (Fig. 5). They often have central peaks made from rock that has frozen in mid-rebound. They have terraces and flat bottoms. The crater sides slumped and filled in the crater floor. The shallow slopes of complex crater walls are often substantially less than the angle of repose, evidence that the postimpact rock is—for a moment—more fluid than sand.
Catastrophic disruption and reaccretion
A catastrophic disruption is defined as a collision in which the largest fragment is less than half the mass of the parent object. Evidence for catastrophic disruption abounds in the solar system. For example:
1. The Earth-Moon system is thought to be the result of a Mars-sized impactor hitting the early Earth. Much debris was ejected into space, and some of that debris (outside the Roche limit) accreted to form the Moon. See also: Moon
2. The planet Uranus spins on its side (its north pole lies almost in the plane of the solar system, whereas most planets’ north poles point out of the solar system). This orientation must be the result of a massive impact to the early Uranus.
3. Pluto, like Uranus, spins on its side. In addition, Pluto's three satellites were almost certainly formed from postimpact debris that was orbiting Pluto. All of Pluto's satellites are outside Pluto's Roche limit (as expected, since they would not have reaccreted inside the Roche limit).
4. Asteroid families are groups of asteroids whose orbits indicate that they were once part of the same progenitor. In some cases, the orbits can be traced backward to determine the precise time of the breakup.
5. Meteorites are rocks from interplanetary space that reach Earth's surface. One class of meteorites consists almost entirely of iron. These are thought to have formed in the interiors of objects that were large enough to undergo gravitational differentiation. Dense material (iron) settles to the core, and lighter material (rock) forms the crust. The existence of iron meteorites implies that large, differentiated objects roamed the asteroid belt at one time but underwent catastrophic disruption and their cores became the source for iron meteorites. See also: Meteorite
Some planets have interiors that are both fluid and conducting. For Earth and Venus, the cores consist of iron alloys that are denser than silicates (rocks) and have collected to form iron-rich cores. For Jupiter and Saturn, their metallic hydrogen cores are conductors. (Metallic hydrogen is so dense that some electrons are not bound to a proton but are free to move throughout the medium.) A body with a conducting fluid interior is a candidate for having a dynamo.
Characteristics of a dynamo
A dynamo is a mechanism that converts mechanical energy into electrical energy, including the specific example of a convecting planetary interior generating electrical currents and magnetic fields. The quantitative details of how dynamos work are an ongoing subject of study, but here are some general principles:
1. A planetary dynamo can exist when a conducting fluid moves across magnetic field lines. The magnetic field induces current loops in the fluid, which in turn generates a new magnetic field that is added to the ambient magnetic field. The process is self-exciting; all that is required is a sufficiently conductive fluid in motion. See also: Faraday's law of induction
2. Planetary magnetic fields require energy sources; otherwise they would decay on time scales much shorter than the age of the solar system. For example, in the absence of a dynamo, the decay time for the magnetic field generated in Earth's core is on the order of 10,000 years. The implication is that a planet's magnetic field must be generated continually.
3. Dynamos can occur when the magnetic Reynolds number (equal to the product of the typical velocity and length scale of the motion in the conducting fluid, divided by the magnetic diffusivity) is greater than 10 or 100 and when fluid motions have certain characteristics. These fluid motions could likely be convection in the presence of a sufficiently large Coriolis force. See also: Convection (heat); Coriolis acceleration
4. Venus's slow rotation does not explain its lack of a dynamo. This planet's 243-day sidereal period is sufficient to generate the necessary Coriolis forces. In fact, for a given temperature gradient, slower rotation rates lead to faster convective velocities. A plausible explanation for the lack of a Venus dynamo is that its liquid core is not undergoing convection.
5. The electrical conductivity of the fluid might not be high (though good conductors make the most efficient dynamos). Because high electrical conductivity correlates with high thermal conductivity, a liquid core with high electrical conductivity might cool by conduction as opposed to convection. It appears that terrestrial planets are close to the conduction-versus-convection threshold (that is, the Earth is barely in the convective regime). Gas giants are securely in the convective regime.
Charged particles (electrons, protons, ions) are constrained to move along magnetic field lines. To first order, the planets’ magnetic fields are all predominantly dipoles (that is, they mimic the fields produced by simple bar magnets), though the dipoles of Uranus and Neptune are offset significantly from these planets’ centers. Charged particles inside a planet's magnetosphere are guided along magnetic field lines until they intersect the planet's atmosphere, often near the north and south magnetic poles. Atoms in the atmosphere are then ionized and/or excited by collisions with the incoming charged particles. The excited atoms emit the auroral light when they return to their normal energy levels.
Since the source of charged particles is typically larger than the planet (on Earth, the solar wind is the main source of charged particles), the auroras are produced in symmetric pairs, aurora borealis and aurora australis, at both magnetic poles. On Jupiter and Saturn, auroras are created by particles created within their magnetospheres as opposed to trapped solar-wind particles. Some of these particles are electrons that are sputtered or ejected off satellites; the ejected particles follow field lines from the satellites’ surfaces to specific locations on each planet. In the case of Jupiter, the satellites Io, Ganymede, and Europa each produce an “auroral footprint” near Jupiter's broader auroral ring. The footprints show up as bright dots at the points where Jupiter's magnetic field lines from the satellites intersect Jupiter's atmosphere (Fig. 6). See also: Aurora; Solar wind
An atmosphere is a layer of gases enveloping a celestial body, such as a planet or a moon. The atmospheres of the terrestrial planets consist primarily of carbon dioxide, nitrogen, water, and, in the case of Earth, oxygen; Mercury has a very tenuous envelope dominated by atoms of sodium and potassium. The atmospheres of the giant planets are composed primarily of hydrogen and helium, with lesser amounts of methane, ammonia, and water.
Atmospheric motions are driven by temperature gradients—in general, those existing between the warm equatorial regions and the cooler polar areas. An atmosphere thus tends to redistribute heat over the planetary surface, lessening the temperature extremes found on airless bodies. See also: Temperature
Directly through wind erosion or indirectly by precipitation, the atmospheres of the terrestrial planets are a major factor in modifying surface features and rearranging the distribution of surface materials. Mercury, being virtually airless, exhibits a relatively unmodified surface, very similar in appearance to that of the Moon. See also: Erosion; Wind
Continuing study of the thousands of known exoplanets is revealing that the diversity within our solar system is but a mere sampling of the possibilities in terms of orbital dimensions, mass, radius, composition, climate, and numerous other variables concerning planetary bodies. Awareness of planetary diversity will only grow in the decades ahead as new missions and instruments discover and more deeply characterize the nearby solar systems most accessible for detailed studies. See also: Exoplanet