Earth Science 22.8 and 22.17

Man-made satellites orbit all of the inner planets (Mercury, Venus, Earth, Mars) and some outer planets (Jupiter, Saturn).

Satellites capture photos and collect important data.

This allows us to study these planets from the comfort of Earth—no field trip needed!

The four inner planets:

• Mercury, Venus, Earth, and Mars.

• Also called terrestrial planets because they are similar to Earth.

Characteristics of inner planets:

• Solid, dense, rocky composition.

• No rings, smaller in size, and shorter orbits.

• Slow spin; Venus spins backward.

The inner planets are shown here to scale, from Mercury to Mars.

All are made of cooled igneous rock with inner iron cores.

Earth has one moon, while Mars has two small moons.

Mercury and Venus have no moons.

The four outer planets:

• Jupiter, Saturn, Uranus, and Neptune.

• Also called gas giants because they are mostly made of gases (hydrogen and helium).

Characteristics of outer planets:

• Larger, longer orbits, and faster spins than inner planets.

• All have numerous moons and planetary rings.

The outer planets are mostly made of hydrogen and helium.

The Sun and outer planets were able to keep these light gases due to their stronger gravity.

The inner planets, with weaker gravity, lost their hydrogen and helium.

All of the outer planets have many moons.

They also have rings made of dust and particles.

Only Saturn's rings are easily visible from Earth.

Inner planets:

• Shorter orbits, slower spins, made of rock and metal, no rings.

Outer planets:

• Longer orbits, faster spins, made of gases, have rings and numerous moons.

Outer planets are called gas giants because they are mostly made of hydrogen and helium.

Exoplanets are planets outside our solar system.

First exoplanet discovered: Tadmor (Gamma Cephei Ab)

• First detected in 1988, confirmed in 2002.

As of 2022, over 5,000 exoplanets have been confirmed!

Gliese 436: A dwarf star, reddish due to its cool temperature.

The recently discovered planet orbiting Gliese 436 is the size of Neptune and orbits every 2.6 days.

Exoplanets are difficult to see directly because they are:

• Far away and tiny.

How we detect them:

• Slight motion of a star due to the planet’s gravitational pull.

• Dimming of a star’s light when a planet passes in front of it.

If life exists elsewhere, it’s most likely on an Earth-like exoplanet.

Kepler-22b:

• Found in the Earth-zone of a Sun-like star.

• Potential for liquid water, a key ingredient for life.

Fun fact: The Milky Way galaxy likely contains around 100 million planets!

Exoplanets are planets that orbit stars outside our solar system.

As detection methods improve, the number of confirmed exoplanets is increasing rapidly.

If life exists elsewhere, it may be found on Earth-like exoplanets like Kepler-22b.