Theia (planet)
Theia is a hypothetical planetary body, comparable in size to Mars, believed to have existed in the early solar system and followed an orbit similar to Earth. Central to the giant-impact hypothesis, Theia is thought to have collided with the Earth around 4.5 billion years ago, resulting in debris that contributed to the formation of the Moon. Initially proposed in the mid-1970s, the hypothesis suggested a glancing blow; however, recent research indicates a head-on collision, which would have significantly altered Earth's composition by merging parts of Theia with the planet's mantle.
The collision occurred during a time when Earth was still forming and largely molten, allowing it to absorb Theia's iron core while ejecting some of its own material. This ejected debris eventually coalesced to form the Moon. Notably, studies comparing oxygen isotopes from Earth's mantle and lunar rocks have shown that both share similar signatures, implying that both bodies contain mixed materials from Theia and Earth. In addition to contributing to the Moon's formation, Theia may also have played a role in the origins of Earth's water and various chemical components.
Theia is named after a figure from Greek mythology, reflecting the tendency to draw connections between celestial bodies and mythological stories. Recent studies have even suggested that remnants of Theia could still be present in Earth's lower mantle. Overall, Theia is a crucial element in understanding the early dynamics of our solar system and the formation of Earth and its Moon.
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Theia (planet)
Theia is a hypothetical space object about the size of Mars that many astronomers believe once existed in the solar system, following an orbit similar to that of Earth. According to a theory known as the giant-impact hypothesis, Theia collided with Earth almost 4.5 billion years ago, sending debris hurtling into space. Fragments of Theia would have mixed with pieces of Earth’s mantle that were also thrust into the atmosphere from the collision, eventually coalescing to form Earth’s moon. The impact was initially thought to have been glancing rather than head-on, but evidence published by Edward D. Young and colleagues in 2016 supports the idea of a head-on collision that resulted in parts of Theia being forcibly merged with Earth.
Overview
The solar system was created about 4.6 billion years ago, when a massive cloud of interstellar matter succumbed to gravity and collapsed, forming a newborn star—the Sun—surrounded by a rotating disk of the matter left over from the cloud. Over time, through a succession of collisions, this remaining matter clumped together in discrete units that grew larger and larger, eventually becoming asteroids, planets, and moons.
Astronomers who subscribe to the giant-impact hypothesis believe that the collision between Earth and Theia occurred during this time of planetary formation, when Earth was almost but not quite fully formed, and Theia was likely on its way to becoming a planet in its own right. Earth would still have been largely molten rock, allowing it to absorb Theia’s iron core and much of its mantle while simultaneously ejecting a portion of its own mantle. While some of the combined material from Earth and Theia would have been ejected beyond Earth’s gravitational pull, a significant amount remained and began orbiting Earth, eventually clumping together to form the moon—the same process that formed the rest of the solar system, but much faster and on a much smaller scale.
The giant-impact hypothesis was first proposed in the mid-1970s by American astronomers William K. Hartmann and Donald R. Davis. Originally it was believed that Theia merely glanced off the Earth, leaving little impact on the planet itself but ultimately forging the moon from its molten fragments in space. However, in January 2016, University of California, Los Angeles (UCLA) professor Edward D. Young and colleagues published research in the journal Science that supports the idea that Theia crashed into Earth head-on and actually altered the composition of the planet.
Young’s research team compared six volcanic rocks from Earth’s mantle to seven rocks retrieved from the moon during several Apollo missions. Their analysis found that all the samples contain the same relative amounts of different oxygen isotopes. Each solar-system body has its own unique ratio of oxygen isotopes, which serves as a kind of fingerprint that allows scientists to identify the body of origin of any oxygen-containing material. Therefore, if the moon contained matter that originally came from Theia and Earth did not, the oxygen composition of the Earth rocks would be different from that of the moon rocks. However, Young’s team discovered the opposite: both bodies of matter share the same ratio of oxygen isotopes, suggesting that both Earth and the moon contain the same amounts of matter from Earth and from Theia. To achieve such a thorough mixing of planetary matter, a collision must have been head-on.
Some researchers have suggested that the majority of the water found on Earth today originated from ice or minerals that came from Theia. Other than this, and the chemical contributions that it made to both Earth and the moon, Theia no longer exists. Astronomers believe that Theia would probably have evolved into a full-sized planet if it had not collided with Earth in the early days of the solar system.
Theia, like many other solar-system bodies, was named after a mythological figure. The Theia of Greek mythology was a Titan, one of a race of giants said to have given birth to the gods; among her children was the moon god Selene. In 2023, a study suggested that a series of unusual blobs located in the Earth's lower mantle may be remnants of Theia.
Bibliography
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