Moons are natural satellites that orbit planets and asteroids in our solar system. There are over 200 moons in our solar system, each with unique characteristics and properties based on what they orbit and how they formed.
What are the main characteristics of moons?
The key characteristics of moons include:
- Orbit – Moons orbit around planets, dwarf planets, or asteroids.
- Smaller than planets – Moons are much smaller than the planets they orbit.
- Made of rocks/ice – Moons are composed of silicate rocks, metals, and ice.
- Irregular shapes – They often have irregular, non-spherical shapes.
- Fewer moons farther out – Planets farther from the Sun tend to have fewer moons.
What are the 8 main types of moons?
Astronomers categorize moons into 8 main types based on their physical characteristics and how they formed. The 8 main types of moons are:
- Regular moons
- Irregular moons
- Prograde moons
- Retrograde moons
- Inner moons
- Outer moons
- Captured moons
- Collisional moons
Regular Moons
Regular moons have prograde, circular orbits around their planet. They formed from the accretion disk of dust and gas that surrounded the planet during its formation. Regular moons are generally located closer to the planet. Some examples include:
- Our Moon orbiting Earth
- Io, Europa, Ganymede & Callisto orbiting Jupiter
- Titan & Rhea orbiting Saturn
Regular moons tend to have spherical shapes due to their gravity being strong enough to force them into a spherical shape. They are also tidally locked with their planet, with one side permanently facing the planet.
Key Facts About Regular Moons:
- Formed from accretion disks around planets
- Prograde, circular, close-in orbits
- Spherical shape and tidally locked
- Includes many of the largest moons
Irregular Moons
Irregular moons have retrograde, distant, eccentric orbits around their planets. Their chaotic orbits imply that they were captured by their planet’s gravity rather than forming in place. Irregular moons typically have more elliptical-shaped, elongated orbits compared to regular moons. Some examples include:
- Neptune’s moon Triton
- Saturn’s moons Phoebe & Himalia
- Jupiter’s moon Ananke
Due to their small size and distant orbits, irregular moons have irregular, non-spherical shapes. Their distant orbits and odd inclinations also imply they were captured from the Kuiper belt or orbiting the Sun on their own before being captured.
Key Facts About Irregular Moons:
- Capture origin instead of forming locally
- Retrograde, eccentric, and highly inclined orbits
- Irregular, non-spherical shapes
- Typically smaller, more distant moons
Prograde Moons
Prograde moons orbit in the same direction as their planet rotates. Their prograde orbits indicate they likely formed from the accretion disk surrounding the planet rather than being captured. Some examples of prograde moons include:
- Earth’s Moon
- Most of Jupiter and Saturn’s regular moons
- Uranus’ moons Miranda, Ariel, Umbriel, Titania, and Oberon
These moons orbit their planets in the same clockwise direction that the planet itself rotates. Prograde orbits are considered “normal” for a moon if it formed alongside its planet from an accretion disk.
Key Facts About Prograde Moons:
- Orbit the same direction as their planet’s rotation
- Formed from the planet’s accretion disk
- Most regular, close-in moons have prograde orbits
Retrograde Moons
Retrograde moons orbit in the opposite direction from their planet’s rotation. Their “backwards” orbit indicates they were likely captured by the planet’s gravity rather than forming in place. Some examples include:
- Neptune’s moon Triton
- Jupiter’s moons Carme, Pasiphae, Sinope, Lysithhea, and Ananke
- Saturn’s moons Phoebe and Himalia
- Uranus’ moon Sycorax
These moons orbit against the natural rotation of their planet. Their retrograde motion hints at a capture origin, possibly from the Kuiper belt. Retrograde orbits tend to be found among irregular outer moons.
Key Facts About Retrograde Moons:
- Orbit opposite of their planet’s rotation
- Indicates a likely capture origin
- Typically outer, irregular moons
Inner Moons
Inner moons orbit very close to their host planet. They are characterized by prograde, circular orbits that lie inside the planet’s synchronous orbit radius. Some examples include:
- Mars’ moons Phobos and Deimos
- Most of Jupiter and Saturn’s major, regular moons
- Uranus’ moons Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, and Perdita
These inner moons likely formed from the debris disk around the planet during its formation. Their proximity gives them a strong gravitational relationship with their planet.
Key Facts About Inner Moons:
- Orbit very close to the planet
- Usually regular, prograde moons
- Formed from the planetary accretion disk
- Strong gravitational ties to planet
Outer Moons
Outer moons orbit much farther away from their host planet. They typically have orbits with higher inclinations, eccentricities, and semi-major axes compared to inner moons. Some examples include:
- Saturn’s moons Iapetus, Rhea, Dione, Tethys, Enceladus, Mimas, Janus, and Epimetheus
- Uranus’ moons Puck, Miranda, Ariel, Umbriel, Titania, and Oberon
- Neptune’s moons Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus
Outer moons like these exist at the edge of their planet’s gravitational sphere of influence. They tend to be irregularly shaped and have orbital eccentricities induced by solar perturbations.
Key Facts About Outer Moons:
- Orbit farther away from their planet
- More susceptible to solar perturbations
- Can be irregularly shaped with eccentric orbits
- At edge of planet’s sphere of influence
Captured Moons
Captured moons are asteroids or Kuiper Belt Objects (KBOs) that have been captured by a planet’s gravity. Their retrograde or highly inclined orbits provide evidence of their captured origins. Some examples include:
- Neptune’s moon Triton
- Saturn’s moons Phoebe and Himalia
- Uranus’ moon Sycorax
- Jupiter’s moons Carme, Ananke, and Sinope
These moons were wandering through space before being captured and orbiting their current planet. Their orbits are typically farther out, more eccentric, and more highly inclined than regular moons.
Key Facts About Captured Moons:
- Asteroids or KBOs captured by a planet’s gravity
- Have highly eccentric, inclined, retrograde orbits
- Evidence they previously orbited the Sun independently
Collisional Moons
Collisional moons are thought to have formed from the debris of collisions between asteroids/comets and preexisting moons. Their orbits and compositions provide clues about their origins. Some examples may include:
- Saturn’s moon Hyperion
- Jupiter’s moons Himalia and Elara
These moons have chaotic rotations and irregular shapes, likely caused by one or more major collisions in the past. The collisions sprayed debris into orbit around the planet which later coalesced into a moon.
Key Facts About Collisional Moons:
- Formed from the debris of collisions
- Have irregular shapes and chaotically rotating
- Believed to originate as reaccumulated collision fragments
Comparison of Moon Characteristics
Here is a table summarizing some key characteristics of the different types of moons:
| Moon Type | Orbit | Origin | Examples |
|---|---|---|---|
| Regular | Circular, prograde, close-in | Formed from accretion disk | Moon, Io, Europa, Titan |
| Irregular | Elliptical, retrograde, distant | Captured | Triton, Phoebe, Himalia |
| Prograde | Prograde orbit | Formed from accretion disk | Moon, Miranda, Titania |
| Retrograde | Retrograde orbit | Captured | Triton, Phoebe, Sycorax |
| Inner | Very close-in orbit | Formed from accretion disk | Phobos, Io, Europa |
| Outer | Distant orbit | Formed from accretion disk | Rhea, Oberon, Larissa |
| Captured | Highly inclined, eccentric orbit | Captured as asteroid/KBO | Triton, Himalia, Sycorax |
| Collisional | Chaotic orbit | Formed from collision debris | Hyperion, Himalia |
Conclusion
In summary, the 8 main types of moons found in our solar system are:
- Regular – Formed from accretion disk, prograde & circular orbits
- Irregular – Captured asteroids/KBOs, retrograde & eccentric orbits
- Prograde – Orbit direction matches planet’s rotation
- Retrograde – Orbit direction opposite to planet’s rotation
- Inner – Very close-in, prograde orbits
- Outer – More distant, prone to solar perturbations
- Captured – Asteroids/KBOs captured by planet’s gravity
- Collisional – Formed from debris of past collisions
Each type has unique properties and formation histories. Regular inner moons arise with their planet from accretion disks. Irregular outer moons are usually captured asteroids or Kuiper Belt objects. And collisional moons formed from the shrapnel of major collisions. Understanding moon types gives insight into their origins and the history of our solar system.
