Lesson 3 of 6 · Temperature, solid cores & a limited gas supply
Why some worlds become giants
8 minute lesson · 3 checks along the way
Your goal for this lessonExplain how available materials, growth rates, and disk lifetime influence planet types without turning a formation model into a fixed recipe.
Follow a linked name to explore its archive card. “Back to your lesson” brings you to the same part of the reading.
Words with a dotted underline open a short definition right here in the lesson.
Temperature changes the available ingredients
The inner part of a young disk is generally hotter than the outer part. Some rock-forming materials can remain solid at temperatures where water ice cannot. Farther out, ice can add to the supply of solid material. This helps explain why formation conditions vary across a disk, but the boundary is not a permanent wall dividing all rock from all ice.
A marks conditions where a particular substance changes between ice and vapor. Its location depends on temperature, pressure, and time; different substances have different snowlines. Solids can drift, gas can circulate, and planets can move. Therefore a present-day orbit does not uniquely specify the composition or location of every ingredient from which a planet was assembled.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Grow a core while the gas is available
In core-accretion models, a growing concentration of solids builds a large planetary seed. Gravity holds an atmosphere around it, and cooling can let that atmosphere contract and acquire more gas. Under favorable conditions, gas accumulation becomes rapid. The supply must still exist in the surrounding disk. Core mass alone is not an on/off switch independent of temperature, opacity, gas flow, or growth history.
Jupiter and Saturn contain large hydrogen-and-helium envelopes. Uranus and Neptune contain a smaller proportion of those gases, while the terrestrial planets are dominated by rock and metal. Their differences constrain formation models, but do not specify one universal sequence. In sufficiently massive, cooling disks, direct gravitational of gas is another investigated route to some giant planets, especially in environments difficult for slow core growth.
Compare the evidence
Does the proposed growth leave time for gas capture?
Both cards are original fictional scenarios. Their common disk has no gas left after five million years; the source explains why gas availability matters, not these invented dates.
Seed A: early arrival
- When and where
- Fictional time: 2 million years · Toy disk with gas gone at 5 million years
- Purpose
- Check an available growth window.
- What it supports
- A gas-accreting seed appears three million years before the stated cutoff.
- What remains uncertain
- An opportunity is not a guaranteed giant; cooling, rates, and reservoir size still matter.
Seed B: late arrival
- When and where
- Fictional time: 8 million years · The same toy disk
- Purpose
- Check a missing material supply.
- What it supports
- The seed appears three million years after surrounding gas has vanished.
- What remains uncertain
- It cannot acquire a disk-gas envelope from that absent source under the supplied assumptions.
What can the simple clock comparison reject, and what can it not establish?
Compare your reasoning
It rejects late capture from a gas reservoir defined as absent. It does not establish successful early capture or claim a sharp five-million-year deadline for real systems.
Work through a decision
Test a proposed planet against the clock
Imagine a deliberately simplified disk whose surrounding gas is completely gone five million years after its chosen starting time. Model A produces a suitable gas-accreting seed at two million years. Model B produces the same seed at eight million years. A leaves three million years in which disk-gas capture might occur; B has missed that supply entirely. These numbers are invented, not dates measured for Jupiter.
This comparison rejects B’s proposed late capture from that vanished disk. It does not prove that A creates a giant: the capture rate, cooling, and available gas still matter. A real disk disperses over a range of times and places instead of obeying a single sharp deadline. A good model must join growth to the evolving environment rather than calculate each in isolation.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Do not confuse assembly with a finished interior
As bodies grow, impacts and radioactive decay can heat them. If sufficient melting and separation occur, dense metal can move inward while less dense rocky material forms outer regions. This sorting is . Size, composition, and timing affect heating and cooling, so not every surviving small body is an unchanged grain pile or a fully layered miniature Earth.
The Dawn photograph shows Vesta’s surface, not a cutaway of its interior. NASA reported a 2025 reanalysis of spacecraft motion and imaging that challenged an earlier simple large-core interpretation. Competing histories can fit different parts of its record. Use this case to separate a well-defined physical process, differentiation, from the harder inference about how completely that process occurred inside one particular object.
Dawn’s clear-filter image from July 24, 2011 records Vesta’s irregular, cratered surface. The scale bar describes the source view. Surface relief is visible; an iron core is not. Interior models require additional tracking, rotational, and compositional evidence.
Look closely
Which part of a layered-interior drawing could you directly verify from this photograph?
See what to notice
The image constrains the visible surface and shape. It does not directly show internal layer boundaries, their composition, or the size of a core. Those are inferences to test with other measurements.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.