Gas giant planets may start their lives as warm ocean worlds

As proto-planets grow, increased pressure warms them up, causing ices to sublimate.
By Laurel Kornfeld | Nov 08, 2017
According to a new theory of planet formation proposed by John Chambers of the Carnegie Institution of Washington in Washington, DC, gas giant planets like Jupiter in our solar system may start their lives as warm ocean worlds with water vapor atmospheres.

When these proto-planets, formed by the accretion of dust and ice pebbles, grow to a size somewhat larger than Earth, they warm up, and their growing pressure melts their ice, creating oceans.

Some of the water and other liquids sublimate and become gaseous, forming a water vapor atmosphere.

Because water vapor is a greenhouse gas, the more water vapor in the atmosphere, the warmer the proto-planet becomes.

Even small proto-planets with masses ranging from 0.08 to 0.16 Earth masses can reach temperatures ranging from 32 to 704 degrees Fahrenheit (o to 347 degrees Celsius), Chambers noted.

The majority of planet formation models used by scientists begin with planetesimals of about one kilometer that come together to create planets.

In contrast, Chambers' theory proposes the process begins with tiny pebbles.

His computer simulation used a Sun-like star with an orbiting proto-planet made up of 50 percent rock and 50 percent ice, approximately three times as distant from the star as the Earth is from the Sun.

The pebbles accreted, forming a small proto-planet with a thin atmosphere. Pressure levels became high enough to melt the proto-planet's ice once the world reached 0.084 Earth masses.

As pressure levels further increased, the proto-planet became more massive to the point that its ocean went from being a liquid to an intermediate state between liquid and gas that has properties of both. That state is known as "supercritical."

Hydrogen and helium mixed with this supercritical fluid, dissolving boundaries between the world's surface and atmosphere.

Gas giants grow rapidly in a runaway process once they reach about two to five Earth masses, quickly absorbing gas from the proto-planetary disk around the young star.

While it is unclear whether Jupiter formed this way, recent data returned by NASA's Juno orbiter show the gas giant to have a large, fuzzy, diffuse core that may even be partially dissolved rather than a compact, small core or no core.

If Jupiter formed via Chambers' model, its core would at most have several Earth masses.

Chambers' next step will be to study exoplanets and input observational data into his planet formation model.

His findings are scheduled for publication in The Astrophysical Journal.

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