Alien planets

Also see: Extrasolar planet

Geology
Though there are speculation about life on gas giants, as far as we know a solid crust is necessary to the complex chemical interaction needed by life, and as a long-term storage of chemicals. It's still unclear what factors create a solid (terrestrial) rather than a gaseous (jovian) planet - since terrestrial planets in the Solar System are the closest to the Sun, it was assumed that planets with higher mass were naturally pushed away by their star, but now a number of planets are known to be both gaseous and very close to the star.

As a general rule, bigger terrestrial planets will have a stronger tectonic activity: for the square-cube law, a bigger planet has a smaller surface relatively to its volume, and therefore leaks away less internal heat. It should have a large molten core rich in metals and radioactive elements (which release more heat), a thick mantle and a thin, brittle crust that is easily cracked by mantle convection. In fact, Earth as a strong tectonic activity, as probably does Venus, while far smaller Mars and Mercury have none (though there are old volcanoes that witness a past activity). A large presence of water in the crust also contributes to tectonic activity, lubricating plates and making rocks more pliable.

Planetary development
As far as we know, most of planetary formation occurrs by accretion of a disk of gas and dust orbiting the main star. To retain its matter, instead of dispersing it, a planet needs a certain gravitational pull: this being directly linked to density, it needs to be composed by solid elements, common but heavy, such as silicon, aluminium, iron, calcium, carbon and magnesium. Terrestrial planets (Mercury, Venus, Earth, Mars) have a density comprised between 3900 and 5500 kg/m3.

As written above, it's now uncertain why protoplanets become terrestrial or jovian. While many gas giants close to their star are known, some studies suggest that they tend to be pushed away from it until they're farther than rocky planets, no matter where they formed.

Terrestrial planets are categorized, on the basis of their composition, in a number of categories:
 * Silicate planets, primarily made of solid and molten silicates, with a metallic (mostly iron) core, such as Earth, Mars, Mercury and Venus.
 * Iron planets, primarily made of heavy metals. They might form very close to their star (Mercury is a borderline iron planet, since is mass is 60-70% iron), and since they'd cool rapidly after their formation they wouldn't have plate tectonic or a magnetic field.
 * Carbon planets (purely theoretical), composed by carbon compounds surrounding a metal core. Formed by stars very rich in carbon, they could have rivers and seas of oil and a diamond-rich mantle.
 * Coreless planets, essentially silicate planets without a metal core. They might form far from their star, and they'd have no magnetic field.
 * Super-Earths, silicate planets with a mass significantly higher than Earth's (up to ten times higher). There are several known examples.

Rotation
Being produced by a nebula rotating around the star, all planets keep rotating around their own axis when they take shape. The axial tilt (see below) can vary wildly, and so can the direction and speed of rotation (barring tidal lock); however, rotation will probably be faster in gas giants than in rocky planets (less than 10 hours on Jupiter, over 5000 hours on Venus).

Besides the length of the day, wind velocity, the magnetic field etc., rotation also produces oblateness: that is, it makes the planet flatter at the poles and bulging out at the Equator. Earth's polar radius, for example, is 299/300 its equatorial radius. The precise value for oblateness can't be directly computed, but it varies between obmin and obmax (measured as the difference in metres between the two radii), where obmax = (5·pi2·r3)/(G·M·T2) and obmin = 0.315·obmax, where pi is 3.14159, r is the equatorial radius in metres, G is the gravitational constant, M is the planet mass in kg and T is the length of its day in seconds.

This difference also has consequences on gravity: the polar gravity is gp = ge·(2.5-K)·(1-ob), where ge is the equatorial gravity and K is a constant depending on the nature of the planet, being 0.5 for bodies with a core extremely denser than the surface (such as giant stars), 0.75 for gas giants such as Jupiter, 0.9 for rocky planets with a large metallic core such as Earth and 1.1 for rocky planets with a reduced core such as Mars.

Tectonic activity and orogeny
Also see: Continental drift

As hot molten rock rises from the core, loses heat and descends again, it creates lateral movements just under the solid crust (this process is called convection), breaking it apart in a number of plates. Currently, the surface of the Earth is broken up in seven major plates and eight minor ones, but with time they collide, merge or break apart again.

Two plates can meet, and move relatively to each other, in three ways: The highest possible mountain height is inversely proportional to the gravity (Mars' gravity is a third of Earth's, and its tallest mountain, Mount Olympus, is three times as tall as Mount Everest). Since orogen (mountain-building) materials have a finite resistence to compression, bigger planets will have shorter and broader mountain ranges. Stronger materials, also, will be able to form higher peaks: As an example, quartzite and limestone have roughly the same density, but quartzite has a compressive strength four times higher, so a limestone mountain will need to have a base surface four times wider than a quartzite mountain to reach the same height.
 * Transform (conservative) boundaries: the plates slide or grind past each other, creating a faultline (such as the San Andreas Fault in California).
 * Divergent (constructive) boundaries: the plates slide apart from each other, usually in oceans. There is much volcanic activity in the middle, whether it's under water (e.g., the Mid-Atlantic Ridge) and on land (e.g., the East African Rift Valley). New crust is generated from molten rocks, and a continent may be pulled apart.
 * Convergent (destructive) boundaries: the plates slide towards each other. If both plate edges are part of continents, a continental collision occurs, and the edges are lifted upwards, creating a new mountain range. If at least one of the plates is oceanic crust, subduction occurs: one of the edges slides beneath the other and melts into the mantle, releasing the water in its rocks and stimulating volcanic activity (as happened in the Andes range or the Japanese islands).

For planets with a lesser mass or a small metallic core, a different source of tectonic phenomena can be found in tidal friction. Orbital mechanical energy is dissipated as heat in the crust, and on a small planet with an extremely large and close moon (or a moon of a large and close planet) the presence of the other body can create tides of rock many metres high, melting and cracking parts of the crust. Tidal friction caused the strong volcanic activity on Io (a moon of Jupiter) and probably melted the inferior layers of ice on Europa (Jupiter's moon) and Enceladus (Saturn's moon).

Average temperature
Essential for any consideration on atmosphere and liquid bodies, the average temperature of the planet can be easily calculated with the formula T = 374·G·(1-A)·4√I, where A is the planet's albedo, I the relative insulation and G a factor to account for greenhouse effect. (Note: the temperature is in kelvin. To convert it to °C, subtract 273).

Albedo is the reflecting power of a surface, that is, the fraction of incoming light that gets reflected rather than absorbed - Earth's average albedo is 0.30-0.35; the Moon, devoid of clouds, has 0.12; Enceladus, ice-covered, has 0.96.

Insolation (light incoming from the star) is directly proportional to the 4th power of the star's temperature in kelvin: if it's n times the Sun temperature, then the insulation is n4. For example, with a temperature of 7200 K n is 1.2 and the relative insolation is 2.1. It's also inversely proportional to the square of the distance, so if a planet is twice as distant from the star as the Earth is from Sun it will receive an amount of energy four times smaller.

Finally, the factor G depends from the amount of greenhouse gases in the atmosphere: in most habitable planets it will be 1.1 - 1.2.

Thalassogens
Life will likely need a liquid medium to develop in, and to incorporate as a solvent to store and carry chemicals around the body. On Earth, water is the thalassogen, that is, the chemical substance whose liquid form creates oceans on its surface and takes part in organic processes; see the appropriate article about possible solvents other than water.

The first requisite for a thalassogen is, of course, being abundant in the universe. Barring noble gases such as helium and neon, which are liquid only at extremely low temperature and/or high pressure and cannot form compounds, the most common elements are hydrogen (H), oxygen (O), nitrogen (N), carbon (C), silicon (Si), magnesium (Mg), iron (Fe) and sulfur (S). Also, the molecule this elements make up has to be simple, or it will be degraded by heat, radiations and other reactants faster than it's synthetized.

Moreover, of course, it has to exist in a liquid state in the planet's temperature range: silicon, magnesium and iron form oxides, sulfides, hydrides and nitrides, but metal oxides and sulfides are solid until several thousands of degrees, while hydrides and nitrides are decomposed by heat before they liquefy or in presence of water.

It's likely that in an environment rich in hydrogen elements will tend to become as hydrogenated as possible: this means that oxygen, nitrogen, carbon and sulfur will tend to become, respectively, water (H2O), ammonia (NH3), methane (CH4) and hydrogen sulfide (H2S). Other likely simple thalassogens, already present in interstellar space, are carbon monoxide (CO), sulfur dioxide (SO2), cyanogen (CN) and hydrogen cyanide (HCN), while chemical reaction on planetary surfaces are likely to produce carbon dioxide (CO2), nitrogen dioxide (NO2) and carbon disulfide (CS2).

Liquidity ranges
This table (adapted from Xenology) sums up the temperature in which these chemicals remain liquids: (Notes: these temperatures increase at higher pressure; critical temperature is the temperature at which a chemical is liquid at any pressure; critical pressure is the pressure at which a chemical is liquid at any temperature; carbon dioxide cannot exist as a liquid at less than 5.1 atm)

If a compound has a low liquidity range (as with cyanogen) it's less likely to be found in liquid form, and the planet on which it's found has to mantain even temperatures, through fast spinning and/or a thick atmosphere. Under this aspect, carbon disulfide, sulfur, carbon dioxide (under a greater pressure), perhaps sulfur dioxide and of course water have much better chances as thalassogens.

Appearance of the sea
Liquid carbon dioxide, sulfur dioxide and ammonia all are clear and colourless in small quantities, but when in large quantities they'd form blue oceans, very similar to Earth's water seas; other thalassogens, though, could have a very different aspect: carbon disulfide, thanks to a large number of suspended sulfur particles, would be light yellow near to coasts, and light green in deeper seas.

Molten sulfur would go through a number of colours along with temperature changes: between 113°C and 157°C it's a thin, pale yellow liquid; from 157°C to 197°C it's extremely thick and dark red; between 197°C and 227°C it's less viscous and black; from 227°C to 445°C it returns thin again.

Atmosphere
It's very likely that chemicals in a gaseous state are also needed for organic processes; these will form an atmosphere around the planetary surface. Of course, atmospheric chemicals need to be reasonably abundant in the universe, and exist as gases in the planet's temperature range. Noble gases (helium, neon, argon, krypton and xenon) are always so, except at extremely low temperatures, and they'll probably be a part of any atmosphere, but they won't be able to take part in complex, if any, chemical reactions.

Gas conservation
Thermal evaporation is another important factor. The higher is the temperature of the atmosphere, the more energy the gas molecules have, and the easier is for them to escape the planet's gravitational pull. ''Note: a chemical cannot be a gas at a pressure above the critical pressure. However, it can be a gas at a temperature under the boiling point, such as water on Earth at less than 100°C, thanks to the evaporation from liquid reservoirs such as oceans.''

This table orders various bodies of the Solar System by mass and temperature, and inserts for those values the approximate minimum molecular weight that a body with those characteristics may retain as gas. For example, a body with the same approximate mass as Venus but a temperature above 730°C can hold only gas molecules with a molecular weight of 70 or more, such as krypton and xenon, while Venus itself, being colder, can hold all gases with a molecular weight above 30, including ozone, nitrous oxide, oxygen, etc. The bodies that end up with a 0 can hold all gases: they become gas giants, such as Jupiter; those that end up with an ∞ can't hold any gases, and they become airless bodies, such as the Moon.

See here an interactive plot of the atmospheric retention.

Evolution of the atmosphere
On the other hand, very heavy gas are actually rare on most planets, though they're easier to retain. Our early atmosphere is believed to have been stripped away by solar wind, the stream of charged particles emitted by G-type stars like the Sun, blue giants and oldest main-sequence stars (also see here). The elements that survived (C, H, N, O) likely did so as liquids or solids, and then rebuilt a new atmosphere rich in nitrogen, carbon oxides and hydrogen compounds.

The presence of liquid thalassogens also plays a role: planets too close to their star, as Venus is, cannot have liquid water, and therefore volcanic outgassing (carbon dioxide and sulfur compounds) is gathered in the atmosphere, forming a thick layer of clouds that, in turn, raises temperature even more through greenhouse effect. Water vapor is split by radiations in hydrogen (which is lost to space) and oxygen, which (being so reactive) combines with minerals in the ground, disappearing from atmosphere.

The opposite case is Mars, a planet much farther from its star. All water is stored as ice around the poles, but the case is very similar: carbon dioxide builds up in the atmosphere. Mars, though, does not have a strong volcanic activity, and therefore it remains cold (with some active volcano, perhaps, it could have managed to melt its icecaps and form oceans).

In the middle, there is Earth. Carbon dioxide is dissolved in liquid water as carbonic acid (HCO3), which reacts with minerals on the seabed; it forms carbonate rocks (chalk, limestone, etc.), storing an enormous quantity of carbon deep in the crust, blocking a runaway greenhouse effect and balancing temperature. When it becomes too cold, less water evaporates, wind and rain reduce, less carbon is locked as carbonate and it's therefore free to build up in the atmosphere, warming the planet again - and vice versa when it becomes too warm.

As for other gases, hydrogen is always lost to space (except in gas giants) and nitrogen, being inert, accumulates in the air without reacting much with anything. Ammonia and methane behave in a very similar way to carbon dioxide, while oxygen is quickly depleted by its reaction with many other elements (that is, unless it's constantly released anew by photosynthesis).

Biological schemes
Once that life is established on a world, it will start to modify the chemical balance of the atmosphere. There will likely be a complex cycle between organism of whatever metabolic oxidant they'll be using to produce energy, oxygen in our case, while relatively inert gases (such as nitrogen) will mostly remain as they are.

We can build a simple scheme of the chemicals in air and in the seas that partake to living processes in autotrophs (such as our plants) and eterotrophs (such as our animals). Here, autotrophs absorb, store and transpire; eterotrophs eat, inhale and exhale.

Wind and air circulation
Among the most significant factors that influence weather is air circulation. We can expect faster winds on a planet that has a thinner atmosphere (in which case they'll still have less strength), spins faster, has strong temperature differences on its surface and receives more energy from its star. Conversely, we'll find slower winds on a planet with thicker atmosphere (which will even out the temperature and will make wind similar to slow but strong sea currents), slower spin and less irradiation.

Of course, fast wind and rotation will produce more hurricanes, tornadoes and typhoons, but day and night will have more even temperatures. This effect will be also produced by extensive oceans and by atmospheric chemical with high heat capacity, meaning they need more heat than others to warm up by the same temperature (helium, ammonia, methane and water vapour have higher heat capacity than terran air; hydrogen has it 14 times higher).

Baroclinic weather systems


Planets with slower rotation, (relatively) negliglible internal heat, lower atmospheric pressure and/or heat capacity and a solid surface tend to have a baroclinic flow system. Climate is powered by a large temperature difference between Equator and Poles (> 10°C). Warm air rises from the Equator, creating the Hadley cell; cold air falls on the Poles, creating the polar cell; in the middle, the Ferrel cell is pushed as a gear between the two.

On Earth, winds in the Hadley and polar cell flow mainly from east to west (and would flow in the opposite direction in a planet that spins in the opposite way), while in the Ferrel cell they flow mainly from west to east. At the boundary between the Ferrel and Hadley cell, cold, dry, falling air removes moisture and creates a belt of deserts where tornadoes and hurricanes are common; at the Equator, warm, moist, rising air generates nearly constant rains, and creates instead a belt of rainforests and similar biomes. Winds also drive sea currents, which tend to flow in their same direction, until they reach a shore and bend north or south, thereby warming up or cooling down the near lands.

Being great heat retainers, oceans keep the coastal regions close to an intermediate temperature, while inland is subjected to wider swings, both daily and seasonal. This also creates a secondary wind that flows from sea to land during the day and from land to sea during the night. On a particular coast of very large land masses (eastern on Earth, western on planets with opposite rotation), strong winds can draw moist air from the ocean during the summer, creating a period of intense rain, the monsoon.

Barotropic weather systems
Conversely, planets with faster rotation, significant internal heat, higher atmospheric pressure and/or heat capacity, especially if they don't have a definite solid surface (therefore, especially gas giants) tend to have a barotropic flow system. There is a small difference in temperature between Equator and Poles (< 5°C), and climate is powered by vertical pressure differences. The atmosphere is divided in zones of moist, warm, rising air (low pressure) and belts of dry, cold, falling air (high pressure); strong winds flow at the boundaries between zones and belts, creating huge and long-lasting hurricanes such as Jupiter's Great Red Spot.

An intermediate situation between a baroclinic and a barotropic system can be found in terrestrial planets with a very thick atmosphere (and thus a small temperature difference between Poles and Equator) and a slow rotation on its axis. In this case, there would be a large number of climatic belts, all of the same width (say, five belts per hemisphere of 18° each). The Poles would still be dry and the Equator wet, but in the middle there would be several alternating zones of forests and deserts/steppes (see here).

Other weather systems
Finally, on a planet with a very thin but non negligible atmosphere, such as Mars, winds can build up much faster thanks to the low inertia (it takes very little energy to warm up or cool down martian air), and expand to enormous storms that cover the entire planet. It's been speculated that world whose thalassogen has a narrow liquidity range (e. g., methane on Titan) might be scourged by abrupt storms and floodings where the thalassogen rapidly melts and/or explodes in steam even with a moderate warming.

A tidally locked planet wouldn't have a significant air circulation in the interior of each side, while the edge would constantly experience torrential rain, hurricanes and extremely strong wind that would always flow from the cold side to the hot side in the lower atmosphere, and in the opposite direction at higher altitude. If it has a thick atmosphere, the very slow rotation would mean that it'd be dominated by local convection cells, rather than horizontal belts.

Precipitations and weather
Rain (or snow, if the temperature is low enough to freeze the thalassogen) occurs in three circumstances:
 * Convective rain occurs when a moist volume of air is warmer than its surroundings, and therefore it rises, cooling and dropping as rain. Convective clouds (such as anvil-shaped cumulonimbi) are tall and narrow, and produce an intense but short-lived rain. They're common at low pressure latitudes, where rising cell edges meet: at the Equator or between the Ferrel and polar cells.
 * Stratiform rain occurs when arm air, generally tropical, meets colder air, rises and cools.
 * Orogenic rain occurs when warm and moist air meets a mountain range. Ascending, the air cools down and falls as rain on the seaward side of the mountains; reached the top, it starts descending again, but having depleted its moisture it becomes a hot and dry wind. In this way, mountain ranges can create deserts: e. g., the desert of Atacama west of the Andes.

As for other meteorological phenomena, any planet with a not-too-thin atmosphere should be rich in clouds and fog. Rainbows just require clouds, rain and sunlight, but they'd be more vivid on a small planet with weak gravity and larger raindrops, and multiple rainbows could occur if the sky contains more than one lightsource (other stars, bright moons, reflective rings). Lightning would be stronger and more common on worlds rich in dust and with frequent sandstorms, which generate intense electric fields, and where atmosphere has a higher breakdown voltage (that is, more in hydrogen, helium and neon than in oxygen, nitrogen or fluorine).

Auroras require a magnetic field, which would be stronger in massive worlds with a rapid rotation and great metallic cores, and near to hot stars with a stronger solar wind; mirages would occur in a dense atmosphere subjected to stratified heating - in an extremely thick and hot atmosphere as on Venus, the horizon would appear above the observer, and bending upwards at the sides, and objects beyond the horizon might be visible.

This site summarizes in a table the main factors that influence precipitations: those on the left increase rain, those on the right decrease it (note: east and west are inverted in planets that spin in the opposite direction).

Seasons
Seasons are periodical changes of climate and day length, given by a planet's axial tilt (23.4° for Earth). Since sunlight rays are functionally parallel to each other, they strike the surface of a (approximately spherical) planet with different angles: they fall perdependicular to the ground on the tropical belt, and they spread out nearly parallel to it in the polar zone, which is therefore warmed much less.

Since a planet moves around its star, the sunlight tilt varies in the same location throughout the year.

Since Earth has a low orbit eccentricity (the orbit is very close to a circle), the distance from the Sun doesn't affect seasons much, but it would for a planet with a more elliptical orbit. In this case, season would come at the same time on all the world, and since planets move slower when more distant from the star, winter would be much longer than summer.
 * On the tropical belt (from the Equator to x° degrees of latitude on each hemisphere, where x° is also the axial tilt) sunlight always falls nearly vertical in every month of the year, so the climate is always warm and it never changes much.
 * On the temperate belts (from x° to (90-x)° degrees of latitude on each hemisphere) sunlight is more direct on summer and more spread out in winter. Also, the time a point on the surface spends on the enlightened side of the planet is longer on summer (making days longer than nights) and shorter in winter (making nights longer than days). When one of the hemispheres experiences summer, on the other it's winter.
 * Inside the Polar Circles (from 90°-x° degrees of latitude to the Pole on each hemisphere), not only sunlight is always very low on the horizon, but for half a year that area is all on the side that faces the sun and for half a year it's all on the opposite side, so summer and winter become six months of day and six months of night.

A planet with a pefectly vertical axis wouldn't have seasons as Earth does: the tropical belt would be much hotter and narrower, and the rest of the world significantly colder. With an high axial tilt (such as Uranus, whose poles point towards the Sun once a year each), the effect in the Polar Circles would be severe: they'd be extremely hot for half a year and extremely cold for the other half, while the Equator would always receive little sunlight and would be moderately cool.

Ice and polar caps
For most of its history, Earth didn't have permanent ice. The few periods when it does, extensively, are called glacial ages. At least five are known: Huronian (2400-2100 Ma), Cryogenian (850-635 Ma), Andean-Saharan (460-430 Ma), Karoo (360-260 Ma) and Quaternary (2.6 Ma-current). The recurrence of glacial ages is linked to a number of factors such as long-term solar activity cycles and changes in the atmospheric carbon dioxide concentration.

As a thalassogen, water has the unique property of floating, when solid, above its liquid form: that allows to ice to cover seas and protect them from complete glaciation, preserving liquid water even at the Poles. On the other hand, ice also reflects much more light than water does, meaning that a world with growing ice tends to warm less and less, triggering a global cooling (or warming, if the opposite occurs) that can last for millions of years.

Colour of the sky
The blue colour of our sky (when saw through human eyes, of course) is due to Rayleigh scattering, a phenomenon due to particles suspended in the atmosphere. Sunlight with a shorter wavelength is scattered more easily, so blue is scattered much more than green, yellow and red; violet and UV light are scattered even more, but the human eye is less sensitive to violet and not at all to UV light. An atmosphere of a certain consistency is still needed: the thinner air is, the darker the sky appears (until it becomes black in the absence of air, as on the Moon).

Bodies significantly larger that light's wavelength, though, distinguish the colours in different ways: groups of small and large water drops, like those that make up fog and clouds, appear of a milky white. The colour of particles themselves, of course, is also very important: martian sky is dyed pinkish-gray by the dust (rich in red iron oxide) blown around by sandstorms. A very dense aeroplankton would also give the sky its colour, for example green in the event of chlorophyll-endowed organisms, as it happens on Extraterrestrial's Blue Moon.

In a similar way, different atmospheric gases could change the colour of the sky. Most likely choices (oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapour) are transparent, but chlorine is light green, fluorine is pale yellow, and nitrogen dioxide would make an orange-brown sky. Unless they're extremely thin, though, these gases would make air completely opaque after a few metres. Methane and ammonia are weaker absorbers, and they'd provide a vivid blue-green sky even in large quantities (as they do on Uranus and Neptune), while ozone would probably appear reddish.

Ultimately, sky light comes from the main star(s), and different classes of stars emit light in different wavelengths. M-class stars have their emission peak in red, K-class in orange, G-class in green-yellow, F-class in blue and A-class, B-class and O-class in UV light. M-class and K-class therefore emit little light suitable for scattering, and the sky on their planets (unless they have an extremely thick atmosphere, in which case it'd tend to white) would be darker than ours; biggest F-class and hotter stars would probably make skies appear violet even to human observers.

Under a M-class or K-class sun, reds would appear brighter and blues darker; shadows would be blurry, and the sun would seem bigger and redder. An F-class sun would light up blues more, and would cast sharper, bluish shadows (also see here).