Theia is the name given to a hypothetical ancient protoplanet that existed in the early solar system. Its existence is the central premise of the prevailing scientific explanation for the formation of our Moon. This theoretical body is not directly observable today, but its characteristics and fate are inferred from detailed analysis of the Earth-Moon system, including orbital dynamics, chemical composition, and computer modeling.
Physical Characteristics and Scale
Theia is generally envisioned as a terrestrial protoplanet comparable in size to Mars. Its estimated mass was between 10% and 15% of the proto-Earth’s mass, though some models suggest it could have been up to 45%. Like other rocky planets, Theia was a differentiated body with a dense metallic core surrounded by a silicate mantle.
The intense energy and heat of the nascent solar system meant that Theia’s surface was geologically active and hot. Its outer layer was composed of rocky material, primarily silicates. This internal structure, with the heaviest elements like iron forming a distinct core, was necessary for the subsequent planetary collision to produce the Moon.
Proposed Origin and Formation
Current models suggest that Theia formed in a gravitationally stable region of space known as a Lagrangian point, specifically L4 or L5, relative to the Earth and the Sun. These points, sometimes called Trojan points, are located 60 degrees ahead of and behind Earth in its solar orbit. Theia gradually grew in this orbital pocket by accreting material over millions of years.
The planet’s growth eventually destabilized its position within the L4 or L5 point, causing it to drift out of its co-orbital path. Gravitational perturbations from Venus or Jupiter nudged Theia onto a chaotic trajectory that led toward a collision with the proto-Earth. Isotopic analysis suggests that Theia formed in the inner solar system, possibly even closer to the Sun than Earth, before its orbital shift.
The Aftermath: Where Did Theia Go?
The ultimate fate of Theia was a catastrophic collision with the proto-Earth, known as the Giant Impact, approximately 4.5 billion years ago. This impact is typically modeled as a high-velocity, glancing blow rather than a direct head-on strike. The immense energy of the collision vaporized a significant portion of both Theia and the Earth’s outer layers, ejecting a massive plume of molten and gaseous rock into orbit.
The majority of Theia’s material was redistributed throughout the newly formed Earth-Moon system. The dense iron core of Theia largely merged with the proto-Earth’s core, contributing to the size and mass of our planet’s metallic interior. The lighter, vaporized silicate mantle material from Theia and Earth formed a massive debris disk around the Earth, which quickly coalesced under gravity to form the Moon.
Intriguingly, recent seismic studies suggest that not all of Theia’s material was thoroughly mixed or vaporized. Two large, dense blobs of material deep within Earth’s lower mantle, known as Large Low-Shear-Velocity Provinces (LLSVPs), are hypothesized to be remnants of Theia’s mantle that sank but did not fully dissolve. These structures are evidence that portions of the lost world may still exist within our planet.
Scientific Clues Supporting Theia’s Existence
The hypothesis of Theia’s existence is strongly supported by chemical and physical evidence derived from lunar and terrestrial samples. The most compelling evidence is the near-identical isotopic composition of oxygen in rocks from the Earth and the Moon. Since isotopes act as a planetary fingerprint, this similarity suggests that the materials forming both bodies were thoroughly mixed in a single, violent event.
The Moon’s relatively low density and small iron core also align with the Giant Impact model. Because the Moon formed primarily from the ejected silicate mantles of the two colliding worlds, its composition is naturally iron-poor, as the iron cores largely stayed with the Earth. Furthermore, the Earth-Moon system possesses an anomalously high angular momentum, a predicted outcome of a massive, high-speed impact.
While the oxygen isotopes are nearly identical, subtle variations in other elements, such as molybdenum and iron isotopes, have been detected in lunar rocks. These slight differences provide chemical evidence that the impactor, Theia, was a distinct body that formed in a slightly different part of the inner solar system before its encounter with Earth.