Earth is currently orbited by a single, familiar natural satellite: the Moon. The idea of a “second moon” often sparks curiosity, leading to questions about Earth’s past, potential temporary companions, or even hypothetical celestial bodies. While Earth does not possess another permanent moon, scientific understanding encompasses both ancient, catastrophic events that shaped our planet and the transient visitors that occasionally grace our skies. Exploring these different phenomena helps to clarify what a “second moon” might entail in an astronomical context.
Defining “Second Moon”
The concept of a “second moon” carries different meanings within scientific discourse. One interpretation refers to a large, ancient planetary body that played a role in Earth’s early formation, such as a protoplanet that existed billions of years ago. Another understanding relates to much smaller, temporary celestial objects that are gravitationally influenced by Earth for a limited period, often just a few months or years. These temporary companions, unlike our Moon, do not maintain stable, long-term orbits around Earth.
The Ancient Progenitor: Theia
The most widely accepted scientific explanation for a large, ancient “second moon” involves a hypothetical protoplanet named Theia. This Mars-sized body is central to the Giant Impact Hypothesis, which describes the formation of Earth’s Moon. Theia is believed to have collided with the early Earth, Proto-Earth, approximately 4.5 billion years ago. This colossal impact occurred about 20 to 100 million years after the Solar System began to form.
Theia is thought to have originated from a Lagrange point within the Earth-Sun system, where it would have maintained a relatively stable orbit. Over time, gravitational perturbations, possibly from Jupiter or Venus, likely disturbed Theia’s orbit, sending it on a collision course with Proto-Earth. Modern simulations suggest it was an oblique or even a head-on collision, occurring at a moderate velocity.
The Aftermath: What Happened to Theia
The collision between Theia and Proto-Earth caused both planets to re-melt and eject massive amounts of vaporized rock and debris into orbit around Earth. Much of Theia’s iron core is believed to have merged with Earth’s core, which helps explain Earth’s unusually large core. The ejected material, a mixture of Theia’s mantle and crust and a significant portion of Proto-Earth’s mantle and crust, formed a vast orbiting ring of debris. Over a relatively short astronomical timescale, this orbiting material coalesced under its own gravity to form the Moon. Theia’s components were integrated into both the newly formed Earth and the Moon.
Earth’s Transient Companions
Beyond the ancient, lost protoplanet Theia, the term “second moon” can also refer to small, temporary celestial bodies. These are not true moons, but rather asteroids or space rocks that temporarily enter Earth’s gravitational sphere of influence. These transient companions are typically categorized as “mini-moons” or “quasi-satellites.” Their orbits are not stable, and they are eventually ejected back into heliocentric orbits around the Sun due to gravitational tugs from the Sun and the Moon.
Mini-moons, such as 2020 CD3, are small near-Earth objects that are briefly captured into orbit around Earth. 2020 CD3, for instance, was a small asteroid, estimated to be between 1 and 6 meters across, that orbited Earth for about three years before escaping in 2020. These objects typically make a few looping orbits before departing, with most remaining captured for only a few months to a few years. Another recent example is 2024 PT5, a 10-meter asteroid expected to be temporarily captured by Earth’s gravity in late 2024 for about two months.
Quasi-satellites, like 3753 Cruithne and 469219 Kamoʻoalewa, are objects that share Earth’s orbital period around the Sun, making them appear to orbit Earth from our planet’s perspective, even though they are primarily orbiting the Sun. Cruithne, for example, is a 5-kilometer asteroid that completes a bean-shaped, horseshoe-like orbit relative to Earth, taking about 770 years to complete its pattern. Kamoʻoalewa, discovered in 2016, is a more stable quasi-satellite, approximately 40 to 100 meters in diameter, which appears to circle Earth as it orbits the Sun. Its spectral characteristics suggest it might be a fragment ejected from the Moon’s surface due to an impact. These objects are not gravitationally bound to Earth in the long term, but their unique orbital dynamics make them temporary celestial neighbors.