Neptune is famous for its vivid blue color, but the asteroids orbiting close to it are definitely not.
An international team of astronomers recently took a look at Neptunetrojan asteroids and found that they all appear to be some shade of red, much redder than most asteroids in the Solar system. They published their results on February 14 in the journal Monthly Notices of the Royal Astronomical Society: Letters (opens in a new tab).
The Neptunian Trojans are a cloud of asteroids whose orbit around the sun is parallel to that of Neptune. They hang out at the gravitationally stable points between Neptune and Sunor between Neptune and the dwarf planet Pluto. First discovered in 2001, fewer than 50 of these rock bodies have been described to date.
The reason for this is not that Neptunian Trojans are rare; This is probably because it is difficult to detect space rocks that are so small and so far away. These asteroids tend to be between 50 and 100 kilometers (31 to 62 miles) across and orbit at a distance of 4.5 billion kilometers (2.8 billion miles) from the sun. Before this research, astronomers had studied only a dozen of these asteroids and had to use some of The largest and most powerful telescopes on Earth to do it.
“In our new work, we have more than doubled the sample of Neptunian Trojans studied with large telescopes.” Bryce Bolin (opens in a new tab)astronomer at NASA’s Goddard Space Flight Center and lead author of the study, said in a statement (opens in a new tab).
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Bolin’s team synthesized data collected by four telescopes: the Palomar Observatory telescope in California, the Gemini North and South telescopes in Hawaii and Chile, and the keck telescope in Hawaii, more than two years. The researchers tracked 18 Neptunian Trojans and analyzed their color. They found that most were significantly redder than most asteroids, including four that were extremely red.
That crimson color indicates that Neptunian Trojans are rich in volatile compounds like ammonia and methanol. Ices made of these chemicals are very sensitive to heat and will quickly turn to gas when exposed to sufficient solar radiation. Because of this, astronomers expect asteroids closest to the sun to have much less red tint; its ammonia and methanol have already evaporated.
Sure enough, researchers have observed something of an ombré progression of red asteroids, starting with slate-grey rocks in the inner solar system and moving toward a dark red beyond Pluto’s orbit.
Some of Neptune’s reddest asteroids likely formed even farther from the sun in the early days of the solar system, before migrating inward and becoming trapped in Neptune’s orbit, the researchers added. Studying them could open a window into how asteroids formed in the early solar system and how their composition has changed over the past 4.6 billion years.