Formation of a Gas Giant with an Initial Primary Atmosphere

A primary atmosphere, often called a primordial or proto-atmosphere, is an

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

of a

protoplanet A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitatio ...

that forms by accretion of gaseous matter from the

protoplanetary disk A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may not be considered an accretion disk; while the two are sim ...

Gas giant A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranu ...

planets including

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

Saturn Saturn is the sixth planet from the Sun and the second largest in the Solar System, after Jupiter. It is a gas giant, with an average radius of about 9 times that of Earth. It has an eighth the average density of Earth, but is over 95 tim ...

, have primary atmospheres. Primary atmospheres are very thick compared to secondary atmospheres like the one found on

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

. The primary atmosphere was lost on the

terrestrial planets A terrestrial planet, tellurian planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate, Rock (geology), rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner plane ...

of the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

due to a combination of surface temperature, mass of the

atoms Atoms are the basic particles of the chemical elements. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other ...

and the escape velocity of the planet.

Formation and Composition

Protoplanetary disk formation

Primary atmospheres begin to form during the early stages of a solar system's development. As a star forms from a collapsing cloud of gas and dust, the remaining material flattens into a rotating disk around the star, known as the

. This disk is rich in gases like hydrogen and helium, which are the most abundant elements in the universe. Formation of a Primary Atmosphere

Accretion of gases

Planets start to form within this disk through the process of accretion. As dust and solid materials coalesce to form

planetesimals Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...

and eventually protoplanets, these bodies begin to exert gravitational forces. The gravity of these growing protoplanets attracts surrounding gases from the protoplanetary disk. Larger planets, particularly those forming in the colder outer regions of the disk, are capable of attracting more substantial envelopes of gas, leading to the formation of thick primary atmospheres. Gas accretion processes can be influenced by mass and temperature of the protoplanet as well as chemical and physical conditions of the planetary disk. Accretion can be dependent on the host star's

solar radiation Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared (typically p ...

and wind environment and the temperature and density of the surrounding gases in the planetary disk. A protoplanet will continue to accrete gases to its surface while resources are available. Its primary atmosphere can be lost however, due to the onset of atmospheric escape or

outgassing Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (whic ...

of gases with low boiling points called

volatiles Volatility or volatile may refer to: Chemistry * Volatility (chemistry), a measuring tendency of a substance or liquid to vaporize easily ** Volatile organic compounds, organic or carbon compounds that can evaporate at normal temperature and pre ...

. Atmospheric escape will occur after intense exposure to

X-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

and

UV radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

and

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

, driving gases to reach a terminal

escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for an object to escape from contact with or orbit of a primary body, assuming: * Ballistic trajectory – no other forces are acting on the object, such as ...

. The secondary atmosphere can also be initiated from outgassed volatiles from the planetary

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (laboratory), a highly specialized shared research resource * Core (manufacturing), used in casting and molding * Core (optical fiber ...

and mantle like

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

and

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

or from volatiles contributed during collisions. The development of a proto-atmosphere is dependent on where it is formed within the protoplanetary disk or after it dissipates, resulting in the formation of a secondary atmosphere.

Gas Accretion and Planetary Characteristics

Accretion

The composition of a primary atmosphere is primarily hydrogen and helium, with minor amounts of other volatiles like water vapor, methane, and ammonia, depending on the temperature and region of the protoplanetary disk. These atmospheres are generally thick and extended, enveloping the young planet in a dense layer of gas.

Characteristics

A protoplanet's ability to keep its dense hydrogen/helium rich envelope is dependent on mass, rate of accretion, temperature, and its position within the planetary disk. The structure and mass for a hydrogen-rich atmosphere will also depend on ambient disk conditions, energy supply rates, and opacity of dust grains. Primary atmospheres are characteristic for the giant planets of the

as compared to the

that are hydrogen-poor and thin. Gas giants Jupiter and Saturn have maintained a thick primary atmosphere due to their unstable inner core. Their core is unstable because these giants accrete gaseous bodies faster than solid bodies. This results in hydrogen and helium compositing 85% and 60% of their atmospheres respectively. Ice giants

Uranus Uranus is the seventh planet from the Sun. It is a gaseous cyan-coloured ice giant. Most of the planet is made of water, ammonia, and methane in a Supercritical fluid, supercritical phase of matter, which astronomy calls "ice" or Volatile ( ...

and

Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...

, which have heavier cores, are each 10% rich in hydrogen and helium.

Planetary Evolution and Exoplanetary Insights

Using current detection and characterization techniques to understand planetary atmospheres of the Solar System are useful to understand

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first confirmed detection of an exoplanet was in 1992 around a pulsar, and the first detection around a main-sequence star was in 1995. A different planet, first det ...

atmospheres are potential habitability. Atmospheric accretion and loss processes can vary based on the planetary body and can shape their evolution. For example, large planets with masses heavier than Earth like Jupiter and Saturn sufficiently capture hydrogen and helium. However, planets lighter than Earth can lose their primary atmosphere by burning excess gas or by

hydrodynamic escape In atmospheric science, hydrodynamic escape refers to a thermal atmospheric escape mechanism that can lead to the escape of heavier atoms of a planetary atmosphere through numerous collisions with lighter atoms, typically hydrogen. This mechani ...

if they are orbiting a rapidly spinning star. Planetary cores with masses larger than Earth can accrete large amounts of gas thus reducing hydrodynamic escape and keeping its primordial atmosphere intact. Using

Kepler Johannes Kepler (27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws of p ...

and other

satellites A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scientif ...

, planets of sub-Neptune and Neptune-size have been found that have hydrogen/helium envelopes.

References

{{Reflist Atmosphere Planetary atmospheres Atmospheric sciences