Astronomers Have Discovered Earth’s Latest Quasi-Lunar Moon
The Earth has just added its seventh confirmed quasi-lunar moon. It is 2025 PN7, a small Apollo-type asteroid detected in August solely by its brightness, thanks to the Hawaiian Pan-STARRS 1 telescope.
After analyzing its trajectory, scientists concluded that the object maintains a 1:1 resonance with the Earth. In other words, it orbits the sun at the same time as our planet. From a distant perspective, this synchronicity makes it look as if the Earth is accompanied by a tiny asteroid—as if it had an additional moon.
Unlike the moon, quasi-lunar moons are not gravitationally bound to the Earth. They are ephemeral companions, in cosmological terms, following their own path around the sun. Only at certain times do they come close enough to appear bound. In the case of 2025 PN7, its minimum distance is 299,000 kilometers, while at its farthest point it can reach 17 million km. For comparison, the moon remains at an average distance of 384,000 km from Earth.
According to the article published in Research Notes of the AAS, the asteroid has been in a quasi-satellite phase since 1965, and is expected to remain so for 128 years. Some researchers estimate that 2025 PN7 will finally move away in 2083.
Why Does the Earth Have Quasi-Lunar Moons?So far, seven bodies have been confirmed that appear to accompany the planet in its orbit. Astronomers believe that more may be discovered in the future. Earth is a natural reservoir of quasi-lunars because the Earth's orbit is similar to that of certain nearby objects that inhabit the so-called Arjuna group of asteroids, a population that has only recently begun to be studied in greater detail.
The Arjuna group does not form a ring like the asteroid belt between Mars and Jupiter, but comprises a legion of near-Earth rocks that orbit the sun on a similar path as our planet. Occasionally, some of these asteroids coincide with our trajectory and, depending on their orbital dynamics, are classified as quasi-lunar or mini moons.
The main difference between the two categories is that quasi-lunar moons orbit the sun together with the planet. Mini moons, on the other hand, do “go around” the Earth, but usually in a horseshoe-type orbital motion. In addition, mini moons appear for brief periods of weeks or months before they move on forever.
The seven known quasi-lunar moons belong to the Arjuna group and share a 1:1 resonance with the Earth's orbit. They are: 164207 Cardea, 277810, 2013 LX28, 2014 OL339, 469219 Kamoʻoalewa, 2023 FW13 and 2025 PN7.
The Pan-STARRS observatory has established itself as one of the leading detectors of near-Earth objects. Its list of findings includes quasi-lunar moons, comets, and even supernovae, thanks to a 1.4 billion-pixel digital camera, the largest of its kind in the world.
In astronomy, there are different ways to acquire a moon. Some planets, such as Jupiter or Saturn, capture bodies orbiting close to them. Others, like Earth, formed their satellites after collisions in the infancy of the solar system. In any case, the essential rule is clear: To qualify as a real-deal moon, a satellite must be permanently bound to the planet's gravity.
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