Earth Trojan Asteroids: The Companions Sharing our Orbit

Trojan asteroids sharing Earth's orbit — The **Trojan asteroid 2010 TK7** (in the small green circle) orbits around the Earth's **Lagrange point \(L_4\)**. 2010 TK7 was first photographed in the infrared in October 2010 by the Wide-field Infrared Survey Explorer.
Image source : NASA/JPL-Caltech/UCLA (new window)

Scientific summary

Astronoo's article presents the Earth Trojan asteroids, celestial objects sharing our planet's orbit around the Lagrange points \(L_4\) and \(L_5\). These bodies, trapped by a subtle gravitational balance, are extremely rare due to strong perturbations from Venus and Mars. Observations from the WISE space telescope and the Pan-STARRS survey have confirmed two stable Trojans, 2010 TK7 and 2020 XL5, while other candidate objects are undergoing dynamic simulations to validate their status. Studying these populations offers a natural laboratory to test models of the inner solar system formation and long-term gravitational capture mechanisms.

Why are Earth Trojan asteroids so difficult to detect and what do they teach us about our solar system?

The article explores the nature of Earth Trojan asteroids, these discreet companions that share our orbit around the Sun. But why are they so rare and what do they reveal about our cosmic neighborhood? The answer lies in the fragile stability of these objects. While Jupiter has thousands of them, Earth has only two confirmed to date due to repeated gravitational perturbations from Venus and Mars, which quickly eject these bodies from the Lagrange points \(L_4\) and \(L_5\). Observing them is a challenge: located 60° ahead or behind Earth, they appear near the Sun in the sky, requiring twilight observations or space-based surveys. Their study is nonetheless fundamental because these asteroids are fossils from the formation of the solar system, trapped for billions of years. By analyzing their composition and orbit, astronomers can test models of planetary migration and chaotic dynamics, while also preparing future space missions to these nearby targets.

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The Secret Points of the Earth occupied by the Trojans: L₄ and L₅

Did you know that Earth is not alone in its orbit? Trojan asteroids, true cosmic companions, discreetly share our trajectory around the Sun.

Lagrange points: gravitational wells

The Lagrange points \(L_4\) and \(L_5\) of the Earth-Sun system are gravitational equilibrium positions located 60° ahead and behind Earth (~150 million km) on its orbit. Theoretically stable for small perturbations, these points form potential wells where small celestial bodies can be lodged: the Trojan asteroids.

A dynamic equilibrium in motion

From the perspective of Earth and the Sun, these points keep the asteroids in place thanks to a subtle balance between gravity and centrifugal force, making these points stationary for a small body. These equilibrium points are not fixed locations in absolute space, but move in orbital synchrony with Earth around the Sun. They thus offer a privileged site to host asteroids in a semi-stable manner for several thousand years.

Late and difficult observations

It was only in the 21st century that the first Earth Trojans were detected there, due to major observational constraints: the points \(L_4\) and \(L_5\) are close to the Sun in the terrestrial sky, making them difficult to observe with ground-based instruments. One must scan the sky at dusk or dawn, under unfavorable lighting conditions. These areas must have hosted many temporarily captured objects over time, subject to planetary perturbations.

Why are they called "Trojans"?

A historical convention born with Jupiter

The term "Trojan" comes from a naming convention introduced at the beginning of the 20th century, following the discovery of asteroids sharing the orbit of Jupiter. In 1906, the asteroid Achilles was identified near Jupiter's Lagrange point \(L_4\). By tradition, objects located at \(L_4\) were named after Greek heroes (Achilles, Ulysses…), while those at \(L_5\) received names from the Trojan camp (Hector, Priam…).

Since then, the term "Trojan" has been used by extension to designate any small body orbiting around the Lagrange points \(L_4\) or \(L_5\) of a planet. We thus speak of Jupiter Trojans, Mars Trojans, Neptune Trojans… and now Earth Trojans.

Earth Trojans: Earth's Invisible Cohabitants

A rare and difficult-to-observe population

Trojan asteroids are therefore celestial bodies gravitationally trapped around the Lagrange points \(L_4\) and \(L_5\) of a planet. In the solar system, Jupiter has thousands of them, but Earth has only a few confirmed ones to date.

Their dynamics are explained within the framework of the restricted three-body problem, where the third body (the asteroid) has a negligible mass. It is trapped around points \(L_4\) and \(L_5\) when the centrifugal force exactly counterbalances the combined attraction of the Sun and Earth.

2010 TK7: First Identified Earth Trojan

An asteroid discovered in WISE archives

The first Earth Trojan (2010 TK7) was almost missed: it was spotted thanks to retrospective analyses of data from the WISE space telescope! Discovered in 2010, 2010 TK7 is the first identified Earth Trojan. It also follows an orbit around point \(L_4\), but with a much higher inclination (nearly 21°), making its dynamics more chaotic and its stability shorter.

Its trajectory forms a complex "bean-shaped" pattern in a rotating reference frame, oscillating around \(L_4\) on timescales of several millennia. Although its size is modest (~300 m), its discovery paved the way for a new class of co-orbital objects.

2020 XL5: The Stable Companion of Earth

The largest known Earth Trojan

The asteroid 2020 XL5, discovered by the Pan-STARRS telescope in 2020, is the largest and most stable of the known Earth Trojans. It orbits around the Lagrange point \(L_4\), 60° ahead of Earth, on an inclined and moderately eccentric orbit.

Dynamic simulations show that its trajectory is stable for about 4,000 years, after which it could leave the co-orbital region. With an estimated diameter of 1.2 km, it could represent a potential target for a space mission.

All Earth Trojan Asteroids

Since the discovery of 2010 TK7, astronomers have identified several other Earth Trojan candidates. The table below summarizes all confirmed or potential objects, with their main orbital characteristics and degree of stability. Only two of them are officially recognized as stable Trojans, the others being classified as quasi-Trojans, horseshoe co-orbitals, or quasi-satellites.

Known and candidate Earth Trojan asteroids
Name	Semi-major axis (AU)	Eccentricity	Inclination (°)	Lagrange point	Estimated diameter (m)	Orbital stability	Status
2020 XL5	1.00021	0.514	13.8	L₄	1180	~4,000 years	Confirmed Trojan
2010 TK7	1.00037	0.190	20.9	L₄	300	~10,000 years	Confirmed Trojan
2015 XX₁₆₉	0.9999	0.180	8.5	L₅ (quasi)	—	~1,000 years	Horseshoe co-orbital
2010 SO₁₆	1.0001	0.075	14.5	L₄ (quasi)	—	~350,000 years	Trojan candidate (uncertain)
2023 FW₁₃	1.002	0.21	13.0	L₄ (provisional)	—	To be confirmed	Quasi-satellite

N.B.:
A quasi-satellite is an object that orbits the Sun on a trajectory similar to Earth's, but appears as a retrograde satellite when observed from our planet. A horseshoe co-orbital alternates between the L₄ and L₅ points over time, without being stably trapped around a single Lagrange point. Statuses "candidate" and "uncertain" indicate that additional dynamic studies are needed to confirm their classification.

References

FAQ: Everything you need to know about Earth Trojan asteroids

What is an Earth Trojan asteroid?

An Earth Trojan asteroid is a small celestial body that shares Earth's orbit around the Sun, stably positioned around the Lagrange points \(L_4\) or \(L_5\). These points are located 60° ahead and behind Earth, forming an equilateral triangle with the Sun and Earth. These objects are gravitationally trapped and can remain in these zones for thousands, even millions, of years.

Why are Earth Trojans so difficult to observe?

The Lagrange points \(L_4\) and \(L_5\) are close to the Sun in the sky as seen from Earth. They are only visible at twilight or dawn, in low light conditions, making them very hard to detect. Moreover, these asteroids are small (a few hundred meters to a kilometer) and their brightness is very faint, requiring powerful telescopes and systematic surveys like those conducted by WISE or Pan-STARRS.

What is the difference between 2010 TK7 and 2020 XL5?

2010 TK7 is the first Earth Trojan discovered (in 2010). It is about 300 meters in diameter, has a highly inclined orbit (21°), and is less stable (a few millennia). 2020 XL5, discovered in 2020, is larger (1.2 km), has a less inclined orbit (13.8°), and is more stable (about 4,000 years). 2020 XL5 is considered the best candidate for a potential space mission.

Do these asteroids pose a danger to Earth?

No, Earth Trojan asteroids pose no danger to our planet. They are trapped in stable gravitational zones and do not have trajectories that bring them close to Earth. They remain at a distance of about 150 million kilometers (the same as Earth from the Sun) and are not at risk of colliding with our planet.

How many Earth Trojans do we currently know of?

To date, two Earth Trojans are confirmed: 2010 TK7 and 2020 XL5. A few other objects, such as 2015 XX₁₆₉, 2010 SO₁₆, and 2023 FW₁₃, are considered candidates or quasi-Trojans, but their status is not yet definitively established. Ongoing dynamic studies will confirm or rule out their belonging to this family.