Neptunian Trojans are trans-Neptunian objects in 1:1 mean-motion resonance with the ice giant Neptune. These asteroids are generally thought to have been captured from the ancient protoplanetary disk into co-orbital resonance with Neptune during its outward migration. It is possible, therefore, that the color distribution of Neptunian Trojans is a constraint on the location of any color transition zones that may have been present in the protoplanetary disk. In support of this possible test, planetary scientists from NASA and elsewhere obtained new observations of 18 Neptunian Trojans, more than doubling the sample of Neptunian Trojans with known visible colors to 31 objects.
An artist’s concept of a Neptunian Trojan asteroid. Image credit: NASA / ESA / G. Bacon, STScI / C. Fuentes, Harvard-Smithsonian Center for Astrophysics.
The newly-observed Neptunian Trojans are between 50 and 100 km in size and are located at a distance of around 4.5 billion km from the Sun.
Asteroids orbiting this far away are faint and so are challenging for astronomers to study.
Before the new work, only 13 Neptunian Trojans had been studied, requiring the use of some of the largest telescopes on Earth.
The new data were gathered over the course of two years using the WASP wide field camera on the Palomar Observatory telescope, the GMOS cameras on the Gemini North and South telescopes, and the LRIS camera on the Keck telescope.
Of the 18 observed Neptunian Trojans, several were much redder than most asteroids, and compared with other asteroids in this group looked at in previous studies.
Redder asteroids are expected to have formed much further from the Sun; one population of these is known as the Cold Classical trans-Neptunian objects found beyond the orbit of Pluto, at around 6 billion km from the Sun.
Multi-band images of Neptunian Trojans 2015 VW165, 2014 QO441, 2014 YB92 and 2014 SC374 taken with the Palomar 200-inch, Gemini and Keck telescopes. Image credit: Bryce Bolin.
The newly-observed Neptunian Trojans are also unlike asteroids located in the orbit of Jupiter, which are typically more neutral in color.
The redness of the asteroids implies that they contain a higher proportion of more volatile ices such as ammonia and methanol.
These are extremely sensitive to heat, and can rapidly transform into gas if the temperature rises, so are more stable at large distances from the Sun.
The location of the asteroids at the same orbital distance as Neptune also implies that they are stable on timescales comparable to the age of the Solar System.
They effectively act as a time-capsule, recording the initial conditions of the Solar System.
The presence of redder asteroids among the Neptunian Trojans suggests the existence of a transition zone between more neutral colored and redder objects.
The redder Neptunian asteroids may have formed beyond this transition boundary before being captured into the orbit of Neptune.
Neptunian Trojans would have been captured into the same orbit as the planet Neptune as the ice giant planet migrated from the inner Solar System to where it is now, some 4.5 billion km from the Sun.
“In our new work, we have more than doubled the sample of Neptunian Trojans studied with large telescopes,” said Dr. Bryce Bolin, a researcher at NASA’s Goddard Space Flight Centre.
“It’s exciting to find the first evidence of redder asteroids in this group.”
“Because we have a larger sample of Neptunian Trojans with measured colors, we can now start to see major differences between asteroid groups.”
“Our observations also show that the Neptunian Trojans are also different in color compared to asteroid groups even further from the Sun.”
“A possible explanation may be that the processing of the surfaces of asteroids by the Sun’s heat may have different effects for asteroids at varying solar distances.”
The team’s work was published in the Monthly Notices of the Royal Astronomical Society: Letters.
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B.T. Bolin et al. 2023. Keck, Gemini, and Palomar 200-inch visible photometry of red and very-red Neptunian Trojans. MNRASL 521 (1): L29-L33; doi: 10.1093/mnrasl/slad018
