Gas giants and ice giants
The giant planets in our solar system come in two flavors: mostly hydrogen gas (Jupiter, Saturn), and mostly ice (Uranus, Neptune). Consequently, they are called respectively gas giants, and ice giants.
Both types of giants formed in the region of the solar nebula where ices are solid — beyond the snow line, currently located inside the main asteroid belt near the dwarf planet Ceres.
gas giants must form quickly before the gas is gone
When protostars become T Tauri stars, collapsing toward internal temperatures at which they
begin nuclear fusion of hydrogen to helium, they produce strong stellar winds that blow gas out of the surrounding protoplanetary nebula. To become a gas giant, a protoplanet must become large enough (~10 Earth masses) to accrete large amounts of gas from the circumstellar disk before the gas is gone.
At core masses below this critical mass, gas pressure around the core creates a hydrostatic equilibrium in the gas, which keeps the rates of core and gas envelope growth about the same. Above the critical mass, hydrostatic equilibrium fails. Runaway growth of the gas envelope occurs, independently of core growth and limited only by the available gas; the protoplanet then consumes all the gas within its feeding zone.
The critical mass for runaway growth must be reached during the short amount of time (~3 million years) before the star’s T Tauri phase stellar winds blow away the circumstellar gas. Protoplanets that form at greater distances from a star have less dense material to feed on and form more slowly, so they may reach critical mass only after the gas has dissipated, in which case they wind up made mostly of solid materials -- predominately ice. This is what happened to Uranus and Neptune — they took longer to form than Jupiter and Saturn because they formed further out where the material was less dense. Protoplanets that form closer to the star (but still beyond the snow line) have the most material available for core formation. They can become large quickly, and can accrete large amounts of gas before it is dissipated by T Tauri stellar winds. Hence: gas giants form closer in, ice giants form farther out.
Core accretion and disk instability models
The commonly used model of how gas giants form is core accretion. Disk material aggregates electrostatically (bound by van der Waals forces) to form 1 cm to 1 m objects, which further aggregate by collisions into planetesimals (1 km and larger). Planetesimals coalesce gravitationally into cores (of multiple Earth masses), and gas accretes onto cores when the critical mass for runaway growth is reached. Ice giants form in the same way, but the gas is gone before they get beyond the hydrostatic equilibrium growth phase. Terrestrial planets form mostly from material inside the snow line, so they get little ice, and their masses are too small to retain any nebular hydrogen gas against thermal escape.
An alternative model for gas giant formation is disk instability or gravitational instability. In this model, something that looks a bit like the formation of spiral arms in a galaxy occurs in the circumstellar disk material, the arms fragment, and a section of gas suddenly collapses to a gas giant planet purely by self-gravitation — very much like forming a star. A gas giant formed by disk instability might still be able to acquire a core from rocky stuff falling into it later on, but it’s not clear whether this could ever create a sharply bounded core or something “mushy”.
JUNO AT JUPITER
NASA’s Juno Mission, due to arrive at Jupiter on this July 4, 2016, will orbit Jupiter in polar orbits
that make close approaches to the planet. Juno will measure the gravitational and magnetic fields of Jupiter near the planet, to improve our knowledge of the mass distribution and geomagnetic dynamo inside the planet. We should be able to establish better models of Jupiter’s (presumed) core and of how its powerful magnetic field is generated in the thick metallic hydrogen mantle. Juno will also capture visual images and study the cloud layers, charged particles and aurorae at Jupiter’s poles.
The ICE GIANTS
We have never put a spacecraft into orbit around either of our system’s ice giants and there are currently no missions planned to either of them. Voyager 2, launched in 1977, flew past Neptune on October 2, 1989, and is the only spacecraft ever to fly by the two ice giants. It had visited Uranus in 1986.
Voyager 2 measured the magnetosphere and radiation belts of Neptune, and found that the planet’s magnetic axis is tilted 47 degrees from its rotation axis. It is also not centered at the planet’s physical center, but displaced from the center by more than half (.55) of Neptune’s radius. The magnetic field
is generated in a zone above the core of the planet -- probably not a layer of metallic hydrogen, but a conductive liquid layer derived from icy materials. The field is irregular, asymmetric and unstable, changing chaotically as it interacts with the solar wind. Observations Voyager 2 made at Uranus also point to a messy, unstable, offset magnetic field generated from shallower layers rather than from deep inside the planet.
Neptune is at the upper end of an exoplanet mass range called “super-Earths”: objects ranging from Earth mass to Neptune mass. (Objects in this range that are closer in mass to Neptune are also called “mini-Neptunes”.) These are the most common exoplanets found in surveys so far, though observational biases inherent in current surveys are very likely under-detecting Earth-sized exoplanets in Earth-like orbits: we are better able to survey larger planets close to their stars, than smaller planets farther out. Our system does not have a super-Earth — unless one is lurking out at 200-2000 AU as recently proposed by Batygin and Brown. Most of the super-Earths we have found in other systems have probably migrated inwards after forming beyond their system’s snow line. Migration seems to be a very common theme for giant planets.