Abstract
Trojan asteroids are small bodies orbiting around the L4 or L5 Lagrangian points of a Sun-planet system. Due to their peculiar orbits, they provide key constraints to the Solar System evolution models. Despite numerous dedicated observational efforts in the last decade, asteroid 2010 TK7 has been the only known Earth Trojan thus far. Here we confirm that the recently discovered 2020 XL5 is the second transient Earth Trojan known. To study its orbit, we used archival data from 2012 to 2019 and observed the object in 2021 from three ground-based observatories. Our study of its orbital stability shows that 2020 XL5 will remain in L4 for at least 4 000 years. With a photometric analysis we estimate its absolute magnitude to be {H}_{r}=18.5{8}_{-0.15}^{+0.16}, and color indices suggestive of a C-complex taxonomy. Assuming an albedo of 0.06 ± 0.03, we obtain a diameter of 1.18 ± 0.08 km, larger than the first known Earth Trojan asteroid.
Highlights
Trojan asteroids are small bodies orbiting around the L4 or L5 Lagrangian points of a Sunplanet system
The nominal orbit of 2020 XL5 has been computed at the epoch MJD = 58444.1 using the European Space Agency’s (ESA) AstOD orbit determination software[19,20], based on the methods described in the literature[21], taking as input the full observations dataset described in the observation sections of Methods
2.25 × 10−5 deg Keplerian orbital elements of 2020 XL5 and their uncertainties, at the epoch MJD = 58444.1, computed with the European Space Agency’s (ESA) AstOD orbit determination software[19,20], based on the methods described in the literature[21], taking as input the full observations dataset described in the observation sections of Methods
Summary
Trojan asteroids are small bodies orbiting around the L4 or L5 Lagrangian points of a Sunplanet system. Despite failing in the detection of new ET asteroids, some of these surveys provided population constraints regarding their number and their size[12,13,14,15] The reason behind this low discovery success rate is related to the unfavorable viewing geometry of an object orbiting EarthSun’s L4 or L5 points as seen from our planet[17]. These objects are often observable very close to the Sun (i.e., at low Solar elongations) and under large phase angles (the Sun-objectobserver angle), meaning that a significant fraction of the object is shadowed as seen from the Earth, which in turn implies the object being faint. The orbit behavior suggested that 2020 XL5 could have been a candidate to become the second known ET, but the orbit uncertainty due to the short arc covered with observations was still too large to confirm a current Trojan engagement with Earth[18]
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