Forming Planetary Systems, Exoplanets and Life

How planets are born

Around a young star, gas and dust slowly gather to build a planetary system. Gravity, tidal forces and countless collisions shape the planets, moons, rings and asteroid belts we see today. This module covers how planetary systems form, how astronomers find planets around other stars (exoplanets), and the biggest question of all: could there be life out there?

The forces at work

A few key ideas explain much of what happens in a planetary system:

What tidal forces can do

A moon strays too close to a giant planet and is pulled apart into a stream of debris. What is the name for the distance inside which this happens?

• The Roche Limit
• A Lagrangian point
• The Habitable Zone
• The escape velocity

The Roche Limit and rings

The Roche Limit is the distance from a planet inside which its tidal forces are stronger than the gravity holding a smaller body together. A moon or comet that crosses it is torn apart, and the debris can spread into a ring. This is one reason the giant planets, such as Saturn, have ring systems.

Match each feature to its cause

Round or lumpy?

Why are big worlds round but small ones potato-shaped?

How a planetary system forms

Put the main stages of planetary-system formation in order, from first to last.

• A giant cloud of gas and dust (a nebula) collapses under gravity
• The material flattens into a spinning disc around the new star
• Dust grains collide and stick into larger clumps
• Clumps grow into planetesimals, then protoplanets
• Planets sweep up and clear the leftover debris from their orbits

Finding other worlds

Planets orbiting other stars are called exoplanets. Three main methods find them:

What does life need?

Select the TWO conditions most important for life as we know it.

• Liquid water
• A source of energy
• A system of planetary rings
• At least ten moons

Complete the search

A planet orbiting another star is called an _____. The method that watches for a dip in starlight as a planet passes in front is the _____ method. The zone around a star where liquid _____ could exist is called the _____ Zone. The _____ equation estimates how many communicating civilisations there might be in our Galaxy.

Which worlds might host life?

Tap the TWO moons that scientists think could have conditions for life, because they may hold liquid water.

• Europa, a moon of Jupiter with a suspected ocean beneath its icy crust.
• The Moon, which has no atmosphere and no liquid water.
• Enceladus, a moon of Saturn that sprays out plumes of water.
• Mercury, the small, scorched planet closest to the Sun.
• A hot gas giant with crushing pressure and no solid surface.

A Drake calculation

A student estimates the number of contactable civilisations with a simplified Drake equation. They multiply: 40 suitable stars, times 0.5 that have planets, times 2 planets each that could support life, times 0.5 where life begins, times 0.1 that become intelligent and send signals. Work out 40 x 0.5 x 2 x 0.5 x 0.1.

Is anyone out there?

Apply what you have learned to each situation.

• Astronomers see a star dim very slightly, at regular intervals. Which method has found a planet?
• A planet orbits its star at just the right distance for liquid water to exist. Where is it?
• Scientists listen for signals from intelligent life elsewhere in the Galaxy. What is this search called?

Explain the search for life

Explain how astronomers search for planets around other stars and judge whether those planets could support life.

• Name one method used to detect exoplanets, and describe how it works
• Explain what the Habitable (Goldilocks) Zone is and why it matters
• Give two conditions a planet needs to support life
• Explain what the Drake equation estimates, and one benefit or danger of finding alien life