PISF: Desert Planets

Monochromatic Planet

A planet or moon that has only one ecosystem.

Why is Earth NOT a monochromatic planet?

Earth has seasonal variation, which creates different types of ecosystems.

Cause of Earth's Seasons

The tilt of Earth’s rotation axis causes seasonal changes, not Earth's distance from the Sun.

Axis Tilt

Earth’s axis points in the same direction throughout the year, causing changes in its orientation relative to the Sun.

Northern Summer / Southern Winter (June)

Sunlight falls more directly on the Northern Hemisphere, making it summer there, while the Southern Hemisphere receives less direct sunlight, making it winter.

Spring/Fall Equinox

Happens twice a year when sunlight falls equally on both hemispheres—March (spring in the Northern Hemisphere, fall in the Southern) and September (fall in the Northern Hemisphere, spring in the Southern).

Northern Winter / Southern Summer (December)

Sunlight falls less directly on the Northern Hemisphere, making it winter, while the Southern Hemisphere receives more direct sunlight, making it summer.

Solstices

The two points in Earth’s orbit where sunlight is most extreme for each hemisphere (June and December).

Equinoxes

The two points in Earth’s orbit where both hemispheres receive equal sunlight (March and September).

Interpreting the Seasons Diagram

• Earth's size relative to its orbit is microscopic, meaning both hemispheres are nearly the same distance from the Sun.

• The side view of Earth's orbit helps visualize the tilt, while a top-down view shows Earth's nearly circular orbit.

Solar Constant

he energy (flux) received at the top of Earth’s atmosphere from the Sun at a distance of 1 astronomical unit (AU), equal to 1366 W/m².

Why does Earth have many different climates?

Climates result from local factors like topography and water availability, but most importantly, they depend on the amount of sunlight received, assuming a thick enough atmosphere to absorb and redistribute energy.

Sun’s Energy Reaching Earth’s Surface

Approximately 50% of the Sun’s energy makes it to Earth’s surface.

Mars: A Real Desert World

• Thin atmosphere

• No surface liquid water

Does Mars Have Seasons?

• It has a nearly identical axis tilt to Earth.

• Its very elliptical orbit causes longer seasons compared to Earth.

Why are desert planets so popular in science fiction?

• Extreme and harsh environments serve as a powerful narrative device for character development.

• Creates an illusion of an alien environment since most people have never seen a desert.

• Offers an interesting backstory for the planet’s evolution.

• Can make the planet itself feel like a character with its own “Hero’s Journey.”

How do desert planets shape storytelling?

• A desert’s harsh environment levels the playing field for characters in conflict.

• The planet’s history often involves tragic planetary degradation, whether by natural forces (stars, orbits) or human influence (wars).

• Examples: Dune’s Arrakis, Star Wars’ Tatooine, War of the Worlds’ Mars.

Arrakis (Dune) and Canopus

• Arrakis orbits Canopus, a bright star 310 light-years away.

• Canopus is 8× the Sun’s mass, 70× the Sun’s radius, and 10,000× more luminous.

• Problem: Short stellar lifetime (~10 million years), making it an unstable long-term home for a planet.

Vulcan (Star Trek) and the 40 Eridani System

• Vulcan is located in the 40 Eridani system, a triple star system 17 light-years from the Sun.

• In 2018, a planet (~8× Earth’s mass) was claimed to exist around 40 Eridani A in a 42-day orbit.

• In 2023, this was refuted—the planetary signature was actually caused by stellar activity.

Pathways to Losing Surface Liquid Water

• Vulcan: Flares from one of the Eridanus system stars.

• Arrakis: Biological invasive species—sandtrout—altered the ecosystem.

• Tatooine: Surface bombardment during a war with another planet or radiation modulations from its binary star caused water evaporation.

Why is there oxygen in the atmosphere of desert planets?

• Desert planets likely had oceans and thriving ecosystems in the past.

• As water evaporates, it gets broken apart in the upper atmosphere.

• Hydrogen escapes into space, while oxygen remains in the atmosphere.

Can desert planets remain habitable?

• Abe et al. (2011) research suggests that dry planets may have increased habitability.

• Less humidity = less greenhouse warming, which allows for longer water retention on the surface.