How Terrestrial Planets Form - Josh Habka

Terrestrial planets formed through multiple stages of planetary accretion from planetesimal gas and dust throughout billions of years.
How Terrestrial Planets Form - Josh Habka

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  • Terrestrial planets, like those in our solar system, are believed to form closer to the Sun, while gas giants form farther out.
  • Rocky planets can form through the accretion of materials in the protoplanetary disk surrounding a newborn star.
  • Inner planets, being smaller with lower gravitational pull, are unable to retain significant atmospheres, unlike gas giants.
  • Gas giants have large cores and capture hydrogen and helium from the surrounding gas cloud during their formation.
  • Jovian planets may have formed from the breakup of protoplanets, with smaller particles forming moons and larger ones condensing into planetesimals.
  • Our solar system's terrestrial planets consist of two groups: the inner rocky planets (Mercury, Venus, Earth, Mars) and the outer Jovian planets.
  • Planetary embryos likely experienced massive impacts over millions of years to form rocky planets and moons.
  • Accretion is the gradual buildup of materials to form planets, layer by layer, over billions of years.
  • Terrestrial planets are composed of rocky mantles with metallic cores, with variations in composition depending on factors like proximity to the star and availability of materials.
  • The formation of planets around stars has implications for the abundance and distribution of elements like iron and silicon.
  • The process of planetary formation from a protoplanetary disk surrounding a star raises questions about whether it exhausts stars of certain elements.
  • Geologic timescales for planetary formation span billions of years, allowing for processes like differentiation to create diverse planetary structures.

Rocky planets can be formed through two radically different processes, but it is unclear which built our Solar System's landmasses. In our solar system, the rocky planets formed at the warmest interior part of the protoplanetary disk, whereas the gas giants formed farther out. Inner planets are far smaller than outer ones. As such, they have a relatively lower gravitational pull, unable to draw a lot of gas into an interior planet's atmosphere.

The inner protoplanets' rocky cores also include gases that are present in their environments. Still, unlike the inner planets, because of solar radiation effects and their greater distance from the Sun (Figure 1b), the outer planets did not lose the gases they acquired. This contrasts with the outer, giant planets, which have a primordial atmosphere; a primordial atmosphere is captured directly from the primordial solar nebula. While the rocky planets possess secondary atmospheres produced by outside factors, gas giants only possess the primary atmospheres created by the solar nebula.

As for gaseous planets, astronomers think gaseous planets are formed similarly to rocky planets, however, because they have very large cores (about 10 times that of the mass of the Earth), they take up hydrogen and helium from an initial cloud of gas. The gas giants were completely formed several million years after the start of the planet's formation, as the planet-forming discs from the sun were dissolved (preventing the later formation of the gaseous planets). We believe the planets underwent the final gas-disk dissipation-driven impact of a giant, resulting in the moon's formation.

It is possible that when Jovian proplanets broke up, smaller particles from the surrounding disk formed into some of the Moons now orbiting the individual outer planets. Even larger particles would have condensed under gravity to create planetesimals, which would have hit each other to form the solid inner planets.

The shapes made sense because individual outer planets had many moons and rings that orbited on the same plane, much as planets in our Solar System orbit the sun on the same plane. To make that work, a planet might simply form out of a loop of planetesimals starting somewhere in the middle between Venus's orbit and the orbit of the Earth. Solar systems can form out of three rings of planetesimals: the inner rings of which contain some Earth masses in the rocky planetesimals, which would grow rocky planets, a second with 50-100 Earth masses, which would grow the cores of the giant planets, and the outer rings with the icy planetesimals, which would form comets and Kuiper Belts.

It is important to note that our Solar System's terrestrial planets are composed of two distinct groups; the four planets closest to the Sun are rocky terrestrial planets, while the four farthest from the Sun are Jovians. The inner planets of Mercury, Venus, Earth, and Mars The outer planets are called the terrestrial planets because the inner planet of Mercury has a compact, rocky surface. The outer planets Venus and Mars, formed by the collision between smaller grains of dust and larger grains of the Solar Nebula, have formed larger grains over millions of years.

Researchers said that planetary embryos may have hit gigantic impacts in 30 to 100 million years to form the rocky planets and moons present in today's Solar System. New research suggests that clumps of tiny, glassy particles could have been responsible for forming giant asteroids and planetary embryos that hit each other to form rocky planets such as Earth. Scientists are investigating how small groups of materials may have contributed to planet formation.

Current theories about planet formation describe worlds growing out of tiny seeds of rock and ice plants within the swirling discs of gas and dust surrounding newborn stars. According to one part of the science classroom, planets and stars would form out of pieces of dust originating in the Sun about 4.6 billion years ago. Planets are formed by either collision between embryonic planets in the inner Solar System or from the accretion of millimeter-sized pebbles displayed by the Sun outwards.

The buildup of materials to form planets this way is called accretion. The accretion process means the material is collected over time, layer by layer, to form a solid planet. Over time, those masses build up to planets in an accretion process, which means growth through gradually adding layers of matter.

When you add iron, nickel, oxygen, carbon, and other elements that form rocks, you will see enough matter in the Sun to form thousands of rocky planets. On Earth, we see the core of our planet, which has a middle mantle layer and a rocky crust. A silicic planet A solid planet, such as Venus, Earth, or Mars, is composed mostly of silicon-based, rocky mantles with metallic (iron) cores.

Mercury, the planet in the Solar System, has a metal core that is 60%-70% of the mass of the planet, sometimes called an iron planet, although its surface is made of silicates, which are poor in iron. Iron planets are thought to form in high-temperature regions near the star, such as Mercury, and if the protoplanetary disc is iron-rich. In fact, earlier studies showed that gas giant planets preferentially form around iron-rich stars.

Work by a team including Johanna Teske has shown that smaller, rocky planets, like our sun, do not preferentially form around stars that are abundant in metal elements like iron and silicon. A team including Johanna Teske found that stars with Earth-sized rocky planets are generally similar in their chemistry to stars with planets the size of Neptune and stars without planets, but not to stars with gas giant planets. In new research, the team showed that Earth and Mars are overwhelmingly composed of planetary bodies that have been accreted from the inner Solar System, including materials from the innermost disk not sampled by meteorites, and only a small fraction of a planet's mass comes from bodies in the outer Solar System.

Planetary formation by a gas-and-dust disk surrounding a new star, as described above, has also raised the question of whether this process in and of itself exhausts stars for elements concentrated in the planets.

The heat given off by inner planets means only materials with higher melting temperatures, like metals and rocks, are likely to survive. If all the moons were formed simultaneously with the iron core (as is believed for the Earth), and the two Innermost Planets have no moons, they should also contain no iron core. Considered as such since a gaseous planet could be considered as one formed from solid, rock-like material, like Earth.

Note that the geologic timescale is in hundreds to millions of years, while rocky planets are formed in billions of years, allowing a process of differentiation to create structures that we see on rocky planets.

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