Spectrophotometry of Jupiter Trojan with the Gaia DR3 catalog

¹ LIRA, Université Paris Cité, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS, 92190 Meudon, France
² Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France

^★ Corresponding author: sonia.fornasier@obspm.fr

Received: 17 June 2025
Accepted: 26 August 2025

Abstract

Context. Jupiter Trojans are considered to be among the most pristine bodies in the Solar System. They are believed to have originated from the Kuiper belt and subsequently captured by Jupiter during planetary migrations.

Aims. We present the spectral characterization of Jupiter Trojans using data from the third data release by the Gaia mission (DR3) spectral catalog and from the literature, along with an analysis of the spectral slope correlations with physical parameters and a comparison with the main properties of outer Solar System bodies.

Methods. The Gaia DR3 minor bodies spectral catalog comprises 478 Jupiter Trojans. Gaia spectrophotometry is available in 16 spectrophotometric points covering the 0.33–1.08 μm range. In this sample, we retained Trojans with a signal-to-noise ratio of S/N > 20 and we visually inspected the quality of those with lower S/N values. This process reduces the Trojans Gaia database to 320 objects. To enhance the statistical analysis, we also included the visible spectra (in the ~0.45–0.95 μm range) of Trojans available in the literature. The final dataset includes 519 Trojans, comprising 291 in the leading swarm (L4) and 228 in the trailing swarm (L5), which we classified using the Bus-Demeo and Mahlke classification schemes.

Results. The Trojan population is dominated by featureless asteroids with red spectral slopes belonging to the D-type, with an important fraction (~40%) belonging to the Z-class (in Mahlke taxonomy), characterized by very red slopes. The L4 swarm shows a higher spectral variability and a higher amount, by a factor of 2, of less spectrally red asteroids belonging to the C, P, and X classes that are mainly associated with family members. Once excluding peculiar objects such as the members of Eurybates family, known to be dominated by C-type asteroids, the two swarms have indistinguishable average slopes and very similar albedo values (7.86±0.15% and 7.35±0.15% for the L4 and L5 swarms, respectively). We did not observe any spectral color bi-modality distribution, conversely to the results reported in the literature, which had previously led to the suggestion that Trojans originated from two different regions of the protoplanetary disk: one before and one behind the stability lines of some volatile ices, such as H₂S. The spectral slope distribution is peaked at 9–11%/1000 Å and it is very narrow compared to that of Transneptunian objects (TNOs) or cometary nuclei.

Conclusions. In the visible range, L4 and L5 Jupiter Trojans show very similar average slope and albedo values, even if the L4 swarm have a higher spectral variability mostly associated with some peculiar families. This points toward a common origin of the bodies of the two swarms, likely from the Transneptunian region. The visible spectral slopes of Trojans match those of the less red TNOs, notably those classified as bowl-type by Pinilla-Alonso et al. (2025, Nature Astronomy, 9, 230), and lack of extremely red and organic rich surfaces observed in the TNOs. We suggest that the Jupiter Trojans might have been captured by Jupiter from the Centaurs and scattered disk population. We propose that the lack of extremely red objects among Trojans could be due to the removal of the organic-rich crust through the sublimation of volatiles and collisions as TNOs migrated inward in the Solar System.