Question

Compare and contrast the inner planets and the outer planets.

Short answer

Inner planets are rocky and dense with fewer moons, while outer planets are large, gaseous, and have many moons.

Step-by-step solution

Define Inner and Outer Planets

The inner planets, also known as terrestrial planets, include Mercury, Venus, Earth, and Mars. The outer planets, known as gas giants and ice giants, include Jupiter, Saturn, Uranus, and Neptune. These are categorized based on their proximity to the Sun and their distinct characteristics.

Examine Characteristics of Inner Planets

Inner planets have rocky surfaces, are smaller in size compared to the outer planets, and have relatively higher densities. They lack ring systems and have fewer moons. These planets are closer to the Sun and have shorter orbital periods.

Examine Characteristics of Outer Planets

Outer planets are significantly larger, composed mainly of gases and ices, which contribute to their lower density. They possess ring systems and numerous moons. Being farther from the Sun, their orbital periods are much longer compared to the inner planets.

Compare the Composition

Inner planets are composed mostly of rock and metal, leading to a solid surface, while outer planets consist mainly of hydrogen, helium, and other gases, with possible icy cores. This fundamentally distinguishes their structure and appearance.

Compare Atmospheric and Orbital Properties

The atmospheres of inner planets are much thinner or even non-existent (like Mercury), whereas outer planets possess thick gaseous atmospheres. Inner planets orbit closer and faster around the Sun, while outer planets have wide, circular orbits.

Key concepts

Terrestrial Planets

The terrestrial planets, also known as the inner planets, consist of Mercury, Venus, Earth, and Mars. These planets share several common characteristics that set them apart from their outer planetary neighbors. Firstly, terrestrial planets have rocky surfaces, meaning you could theoretically stand on them, given the right conditions. They are composed mainly of silicate rocks and metals, contributing to a relatively high density compared to the gas giants.
These planets are closer to the Sun, which results in several interesting aspects of their environmental conditions and physical makeup. Due to their proximity, they have shorter orbital periods. For instance, Mercury, the closest planet to the Sun, orbits it in just 88 Earth days.
In addition to size and composition, terrestrial planets lack significant ring systems and generally have fewer moons. This can be attributed to their smaller gravities that are less effective at capturing moons or keeping ring particles in stable orbits, compared to the massive outer planets.

Gas Giants

Gas giants are part of the outer planets and consist of Jupiter and Saturn. These planets are much larger by volume and mass compared to terrestrial planets. Unlike rocky planets, gas giants do not have a well-defined solid surface. Instead, they are composed primarily of hydrogen and helium, with possible liquid or metallic hydrogen layers beneath their thick atmospheres.
Jupiter, for example, is so large that it could hold over 1,300 Earths inside it. This massive size contributes to their strong gravitational pulls, which allows them to have numerous moons and impressive ring systems. Saturn, famous for its beautiful rings, has over 80 known moons.
The substantial distance of gas giants from the Sun results in much lower surface temperatures and longer orbital periods. Jupiter takes about 12 Earth years to complete a single orbit around the Sun. The greater distance also affects the composition of their atmospheres and the prevalence of certain elements and compounds.

Planetary Composition

When discussing planetary composition, the differences between inner and outer planets become strikingly apparent. Terrestrial planets are primarily composed of rock and metal. Elements such as iron, nickel, and silicates dominate their structure, providing their solid and robust surfaces.
In stark contrast, the outer planets' composition is overwhelmingly gaseous. Gas giants are predominantly made of hydrogen and helium, which are the lightest and most abundant elements in the universe. This gives them an overall lower density. Ice giants like Uranus and Neptune, while also containing hydrogen and helium, have significant amounts of water, ammonia, and methane ices.
This fundamental difference in composition leads to vastly divergent planetary characteristics, affecting everything from their density and atmosphere to surface conditions and the nature of potential rings and satellites.

Orbital Characteristics

Orbital characteristics further distinguish inner from outer planets. Inner planets have shorter, faster orbits due to their proximity to the Sun. Mercury, the fastest orbiting planet, completes a full revolution around the Sun in only 88 days. This rapid orbit contributes to their relatively warmer surface temperatures.
Outer planets, however, reside at much greater distances from the Sun. Their orbits are not only longer – taking years, sometimes even decades, to complete – but also wider and more circular. Jupiter, the largest planet, has an orbital period of about 12 Earth years, while distant Neptune takes an astonishing 165 Earth years to orbit the Sun.
The vast distances and elongated periods affect not only the time it takes for these planets to complete an orbit but also the energy they receive from the Sun, influencing climate and atmospheric dynamics.

Atmospheric Properties

The atmospheric properties of planets provide insight into their nature. Inner planets like Mercury and Mars have very thin or nearly non-existent atmospheres, primarily because their lower gravitational pulls are less capable of retaining atmospheric gases over time. Earth and Venus are the exceptions, possessing thicker atmospheres capable of supporting distinct surface conditions, such as water bodies and cloud systems.
In contrast, outer planets boast thick, expansive atmospheres rich in hydrogen and helium. These atmospheres can become extreme in certain conditions, leading to powerful winds and storms; for example, Jupiter is renowned for the Great Red Spot, a massive storm lasting for centuries.
The dense atmospheres of gas giants also enable the appearance of colorful bands and patterns, caused by varying chemical compositions and cloud heights. This makes them visually distinct in the night sky, even through a simple telescope.