A natural satellite, commonly called a moon, is an astronomical object that orbits a larger celestial body, such as a planet. Earth’s Moon is a massive, permanent natural satellite that formed early in the Solar System’s history. The question of a “second moon” has multiple answers, depending entirely on the object’s scale and permanence. These range from a body nearly the size of Mars to small asteroids temporarily captured by Earth’s gravity.
The Giant Impact Hypothesis
The most significant historical body considered a “second moon” was the protoplanet Theia. This ancient body, estimated to be about the size of Mars, existed approximately 4.5 billion years ago. The prevailing scientific explanation, the Giant Impact Hypothesis, suggests Theia collided with the proto-Earth.
The impact occurred at an oblique angle, generating immense energy that vaporized a significant portion of both Theia and Earth’s crust. This event ejected a massive cloud of molten rock and gas into orbit, forming a debris disk. This ejected material eventually coalesced to form the single, large Moon we see today.
Evidence comes from Moon rocks collected during the Apollo missions. Samples show that stable isotope ratios of elements like oxygen and titanium are nearly identical between Earth and the Moon. This chemical similarity suggests the Moon formed from a thorough mixing of material from Earth and Theia. The Moon’s lack of volatile elements, vaporized by the high-energy impact, also supports this hypothesis. Theia was incorporated into the formation of both the Earth and its current Moon.
Quasi-Satellites and Earth’s Gravitational Influence
Other objects, known as quasi-satellites or pseudo-moons, are often mistaken for a second moon because they appear to follow Earth’s orbital path. These near-Earth asteroids are not gravitationally bound to Earth; their primary orbit is around the Sun. They possess an orbital period almost exactly the same as Earth’s, keeping them in Earth’s general vicinity.
These objects are influenced by specialized regions in space known as Lagrangian points, which are points of gravitational equilibrium. Quasi-satellites often orbit near the L1 and L2 points, or follow a horseshoe-shaped orbit between the L4 and L5 points. Objects located at L4 and L5, which form an equilateral triangle with the Sun and Earth, are also known as Trojan asteroids.
A well-studied example is the quasi-satellite 469219 Kamoʻoalewa, estimated to be 40 to 100 meters across. Its oscillating path makes it appear to circle Earth from our perspective, but its orbit is too distant and unstable to be a true satellite. Kamoʻoalewa and similar objects fundamentally orbit the Sun, with Earth’s gravity only slightly nudging their path.
Transiently Captured Mini-Moons
The most modern and frequently occurring answer to the question of a second moon involves small asteroids that are temporarily captured by Earth’s gravitational pull. These objects are known as transiently captured objects, or “mini-moons”. These mini-moons are typically small asteroids, often only a few meters in diameter.
The Earth-Moon system’s combined gravity occasionally captures a passing near-Earth object into a temporary, complex orbit. Scientists estimate that at any given time, there should be at least one natural object about a meter across orbiting Earth. These temporary satellites usually remain in orbit for only a few months, or perhaps up to a year, before the gravitational forces of the Sun and Moon pull them away.
The first confirmed example of this phenomenon was the asteroid 2006 RH120, which was captured in 2006 and escaped in 2007. Another example, 2020 CD3, was discovered in 2020 and was only a couple of meters wide. These mini-moons are extremely difficult to detect due to their small size and faintness, but their discovery confirms that Earth frequently, though briefly, hosts additional natural satellites.