Tuesday

Raven lays eight cards on the Sky Deck step, one per planet, each with its diameter written on it.

"Sort them," she says. Rocket shuffles them into two piles in seconds. "Four small ones. Four huge ones."

"What do you notice about where each pile lives?" Raven asks.

Rocket checks last week's model. "The small ones are the inner planets. The huge ones are the outer planets."

Nova hovers over the big pile. "Size is not the only difference," she says. "Could you stand on any of these?"

Rocket picks up the Jupiter card and frowns. "No surface? Then where would my feet go?" Raven grins. "Read the page."

NASA says the first four planets from the Sun are Mercury, Venus, Earth and Mars. They are called terrestrial planets because they have solid surfaces.

OpenStax Astronomy describes them as relatively small worlds made mainly of rock and metal.

Their solid surfaces carry the record of their history in the form of craters, mountains and volcanoes.

NASA says the giant planets of the outer solar system do not have hard surfaces. They have swirling gases above a core.

Jupiter and Saturn are gas giants. Uranus and Neptune are ice giants.

OpenStax says these four are much larger and made mainly of lighter ices, liquids and gases.

They are more like vast spherical oceans with much smaller, dense cores. There is no solid surface to land on.

NASA notes that Saturn is the only planet with an average density less than water. It would float, if a big enough tub existed.

Clear sorting. Tomorrow is Sky Deck Lab: you cut the planets out of paper at one scale and see them side by side.