Are There Trojan Asteroids Outside Jupiter’s Orbit?
Are There Trojan Asteroids Outside Jupiter’s Orbit? https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2025/03/19115538/Saulos-post-2.jpg 512 341 Asteroid Day https://asteroidday-uploads.s3.eu-central-1.amazonaws.com/wp-content/uploads/2025/03/19115538/Saulos-post-2.jpg
Learning Objective:
Discover that other planets also have Trojan asteroids, enhancing our understanding of the gravitational interactions between planets and smaller celestial bodies.
Overview:
While the well-known Lagrange points are most often associated with Jupiter, they also allow other planets to capture and hold asteroids in shared orbits.
What Are Trojan Asteroids?
Trojan asteroids are small bodies that share a planet’s orbit around the Sun, residing at specific points called Lagrange points. These points are located 60 degrees ahead of or behind the planet, offering insight into the solar system’s formation and the gravitational dynamics of celestial bodies.
The location of the five Lagrange points (NASA/Lucy mission – images not shown to scale)
Though we typically associate Trojan asteroids with Jupiter, they can also be found around other planets. Here is a look of some of the others:
Venus
The asteroid 2013 ND15, discovered in 2013 by the Pan-STARRS project, follows the same orbit as Venus at the L4 Lagrange point. With an estimated diameter of 40 to 100 metres, it is relatively small compared to other asteroids. Although its presence as a Trojan may be temporary, 2013 ND15 remains a rare example of a Trojan asteroid associated with a planet closer to the Sun than Earth.
Earth
Two Trojan asteroids have been confirmed around Earth: (706765) 2010 TK7, discovered in 2010 by a team of astronomers using NASA’s NEOWISE, and (614689) 2020 XL5, identified in 2020 by the Pan-STARRS project. The first has a diameter of about 300 metres, while the latter is about four times larger. Both asteroids remain at stable positions, offering valuable insight into Earth’s gravitational interactions.
Heliocentric orbit diagram of 2020 XL5 alongside the inner planets (Cosmos Centre for Astrophysics and Supercomputing / Britannica / Mutual Assurance)
Mars
Home for almost 20 identified Trojan asteroids. The first, 5261 Eureka, was discovered in 1990 by David H. Levy and Henry Holt at the Monte Palomar Observatory. Notably, Eureka has a natural satellite, discovered in 2011. Another significant discovery is 2023 FW14, a 300-metre asteroid confirmed as a Trojan in 2023 by teams from the Instituto de Astrofísica de Canarias and the Universidad Complutense de Madrid.
The most recently confirmed Mars Trojan is 2023 FW14, an object with a diameter of just over 300 metres. In the same year it was discovered, researchers from the Instituto de Astrofísica de Canarias and the Universidad Complutense de Madrid confirmed its Trojan characteristics.
Saturn
For years, Saturn had no known Trojan asteroids, but that changed in 2024 when an international team of astronomers and astrophysicists confirmed the characteristics of 2019 UO14.
Discovered in 2019 by Pan-STARRS, this asteroid measures just over 12 km in diameter. The late discovery of Trojans around Saturn is likely due to the gravitational influence of Jupiter, which can destabilise asteroids in nearby orbits. However, Saturn’s own gravitational pull plays a crucial role, and more Trojans may yet be found.
Uranus
Only two Trojan asteroids have been confirmed to share Uranus’ orbit. In 2013, astronomers from Canada and France announced the discovery of the Trojan nature of (687170) 2011 QF99, an asteroid approximately 60 kilometres in diameter. It was first detected in 2011 during a deep survey of trans-Neptunian objects, marking a significant addition to the study of planetary Trojans.
In 2017, astronomers from the Universidad Complutense de Madrid identified a second Trojan. The asteroid (636872) 2014 YX49 had been observed a decade earlier by the Mount Lemmon Survey, but its classification remained uncertain until its Trojan status was confirmed. With an estimated diameter of between 70 and 80 kilometres, it was found to be positioned 60 degrees ahead of Uranus in its orbit around the Sun.
The small dot highlighted in the image represents the asteroid 2011 QF99 (Canada-France-Hawaii Telescope / CSMonitor)
Neptune
It was the next planet, after Jupiter, where Trojan asteroids were discovered, with the first identified object dating back to the early 21st century. In addition to being the first known case outside Jupiter’s orbit, Neptune has since become the planet with the second-largest number of confirmed Trojans, with around 30 identified so far. Among them, 385571 Otrera stands out as a key discovery. Almost 100 kilometres in diameter and first observed in 2004, it was the first Neptune Trojan to be officially named.
Following the precedent set for Jupiter’s Trojans, the International Astronomical Union (IAU) established a specific naming convention for Neptune’s Trojans. While the Trojans of Jupiter are named after Greek warriors at the L4 Lagrange point and their Trojan War counterparts at L5, Neptune’s Trojans are named after the Amazons, the legendary female warriors of Greek mythology. Unlike Jupiter’s classification, however, Neptune’s Trojans follow this naming tradition regardless of their position ahead of or behind the planet in its orbit.
The orbits of Neptune’s Trojans, represented by green dots, along with the four giant planets, are shown.
What about Mercury?
So far, no Trojan asteroids have been confirmed to be associated with Mercury. Its proximity to the Sun creates extreme conditions that make the formation and long-term stability of such objects highly challenging. However, some asteroids do cross Mercury’s orbit. These Mercury-crossing asteroids interact with the planet’s gravitational field but do not occupy the stable Lagrange points that define Trojan asteroids.
Co-orbital Asteroids
There are asteroids that share the same orbit as a planet but are not necessarily located at Lagrange points. These co-orbital asteroids may be slightly ahead of or behind the planet, but unlike Trojans, they are not restricted to these specific points. While Trojan asteroids tend to remain relatively stable due to the balance of gravitational forces at Lagrange points, co-orbital asteroids are far more unstable and may eventually drift away under the influence of gravitational interactions.
The Changing Number of Trojan Asteroids
Not all Trojan asteroids are permanent. Many remain gravitationally bound to a planet for only decades or centuries before being ejected or displaced into regions beyond the planet’s influence, ultimately losing their Trojan status.
The number of known celestial bodies across various categories continues to grow, largely due to advances in astronomical technology that enable new discoveries and more precise identification. However, the ever-changing nature of planetary systems means that surprises are always possible. Collisions with other bodies, gravitational interactions, and perturbations caused by neighbouring planets can occasionally alter the population of Trojan asteroids in unexpected ways.
Learn more about this topic by visiting these websites:
LEARN – What are Trojan asteroids?
LEARN – Where Are Asteroids Located?
LEARN – What are Near-Earth Asteroids (NEAs)?
LEARN – What are Potentially Hazardous Asteroids?
LEARN – What is Asteroid Orbital Resonance?
