Astronomers have discovered the two reddest objects in the asteroid belt, and their origin story might tell us more about planet formation.

None of the asteroids in the main belt are quite like the two that astronomers have just discovered. These extremely red oddballs could mean that planet formation theories are on the right track.

Between Mars and Jupiter lies the main asteroid belt, which is mostly comprised of dark, carbon-rich or stony, silicate-rich rocks. But while these types are the most common, they’re not the only class of asteroids living in the belt. D-type asteroids – carbon-rich rocks that contain volatiles such as water and carbon dioxide — are sprinkled throughout the main belt but are more common toward the giant planets and can even be found caught along Jupiter's orbit as Trojans.

an asteroid like object floats through space with a bright star in the background
An artists impression of an icy trans-Neptunian object that orbits past Neptune, with the faraway Sun in the background.
NASA / ESA / G. Bacon (STSci)

These D-type asteroids, unlike their cousins, are very red, and until now, they were the reddest objects known to exist in the asteroid belt.

Now, a team of astronomers led by Sunao Hasegawa (Institute of Space and Astronautical Science, Japan) has found two even redder objects in the main belt, named 203 Pompeja and 269 Justitia. Objects this red must have journeyed inward from beyond Neptune’s orbit, and their presence in the main belt strengthens the case for giant planet migration early in the solar system’s history. Hasegawa and colleagues have published this study in the July 20th Astrophysical Journal Letters.

Outer Solar System Origins

Hasegawa’s team found Pompeja serendipitously while using NASA’s Infrared Telescope Facility and the Seoul National University Astronomical Observatory (Republic of Korea) to survey asteroids larger than 100 km (60 miles). At 110 km wide, the red Pompeja stood out from the crowd and prompted the study of the known asteroid Justitia, which is smaller in size. The analysis of these objects led to the unexpected discovery of their red color.

a chart of the solar system shows where the snow line are in relation to types of asteroids.
This diagram shows how different asteroids and centaurs are organized in the solar system and where they form in relation to the snow lines.

Until Pompeja and Justitia came along, astronomers thought D-type asteroids were the reddest objects in the asteroid belt — but not the reddest in the solar system. Outer solar system objects can beat out the D-types, since asteroids’ colors tend to redden the farther they orbit from the Sun. The presence of complex organic compounds – molecules with lots of carbon atoms in them, like benzene – causes this increased surface redness.

For complex organics to exist on an asteroid, the body must form past certain snow lines, where the molecules exist as ices rather than vapors. The water snow line, where water freezes, is right past the main belt. The carbon dioxide snow line is farther out, toward Saturn. Simple carbon-containing compounds such as methane form even farther than that. This increasing distance to the ice boundaries means that the most complex organic materials freeze only in the outermost reaches of the solar system.

The two newly discovered asteroids resemble two redder classes of objects in the outer solar system: Centaurs and trans-Neptunian objects (TNOs), which orbit near the giant planets and out past Neptune, respectively. For Pompeja and Justitia to be so red, they must hail from the solar system’s edge. So how did the newly discovered oddballs end up where astronomers found them?

Migration Patterns of Planets

Current planet formation theories state that, after the early stages of planet growth created a compact giant-planet lineup, these outer planets began to migrate and spread out, interacting with one another and mixing up asteroids along the way. This stirring implanted objects near Jupiter and beyond into the asteroid belt from time to time.

the spectra of Pompeja and Justitia is shown as clearly more red than other asteroids and as red as TNOs and Centaurs
When an asteroid is very red, this means it gets more reflective to light as its wavelength increases. In these plots, it is clear that Pompeja and Justitia are much redder than other types of main belt asteroids past the visible light range (Left), and are just as red as TNOs and Centaurs (Right) despite their placement in the asteroid belt.
Hasegawa et al. 2021

The presence of Pompeja and Justitia in the main asteroid belt implies that distant objects migrated inward, far away from home. The migration of Jupiter and other giant planets must have coaxed these outer solar system bodies into making such a long journey, as predicted.

“I interpret this discovery as further, very strong circumstantial evidence in support of these current models,” says Sean Raymond (Astrophysics Laboratory of Bordeaux, France), who was not involved in the study.

If Pompeja and Justitia are truly red wanderers from outside Neptune’s orbit, they confirm scientists’ current understanding of planet formation. By visiting the main belt, these asteroids offer an opportunity for astronomers to study the conditions of the outer solar system from afar.


Image of Peter Rowen

Peter Rowen

August 5, 2021 at 1:31 pm

Fascinating objects! And they seem well-positioned for more in-depth study. Amazing how much still remains to be discovered relatively close to home.

By the way, contrary to what the article states, Trojan asteroids do not orbit Jupiter. Rather, they orbit the Sun at the same distance as Jupiter, but 60 degrees offset, either ahead or behind Jupiter in its orbit.

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Lauren Sgro

August 6, 2021 at 10:57 am

Hi Peter! Thank you so much for catching this - we have updated the wording to define Trojans more accurately.

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Bob Jones

August 6, 2021 at 7:06 pm

"The two newly discovered asteroids..."

I think the new discovery was how red they are. Both asteroids were previously known.

(203) Pompeja was discovered by Peters in 1879 at Clinton, NY, and (203) Justitia was discovered by Palisa in 1887 at Vienna.

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Yaron Sheffer

August 7, 2021 at 6:59 am

Excellent catch!

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