A study of the colors of asteroids orbiting between Mars and Jupiter (known as the asteroid belt) has caught two interlopers red-surfaced, indicating they formed beyond Neptune and somehow sneaked into their current orbits. The discovery could lead to important insights into Solar System evolution. It also provides an opportunity to study objects whose origins lie far from the Sun without having to make a particularly daunting journey.
Asteroids come in classes based on composition and origin, and to some extent this is reflected in their colors. D-type asteroids have the reddest coloring in the inner Solar System, but we know many Trans-Neptunian Objects (TNOs) are redder still. A multinational team conducted a survey of the colors of D-type asteroids with diameters larger than 110 kilometers (66 miles), to see if any stood out.
In the Astrophysical Journal Letters, they report the asteroid 203 Pompeja, which barely scrapes into the size class, has a “much redder spectral slope” (measuring relative brightness at different wavelengths of light) than any other object they sampled. The authors then looked for smaller asteroids with the same spectral slope and found 54-kilometers-wide 269 Justitia fits the bill.
There are so many objects in the main asteroid belt the authors could not possibly observe them all. Consequently, it's very likely Pompeja and Justitia have many counterparts too small for suitable measurements.
The two big questions from this work are why the pair are so red, and how they got into the asteroid belt, assuming they didn't form there.
The authors conclude the red color indicates “The presence of a mixture of complex organics and other unknown ... materials.” Both orbit too close to the Sun to have maintained water ice, unlike the similarly colored TNOs or even Centaurs, which orbit between Jupiter and Neptune.
“So they must have formed in a very cold environment. Then the solar irradiation of the ice creates those complex organics.”
Comets are sometimes redirected from their orbits through close gravitational encounters with planets to end up in the inner Solar System, but usually with much more eccentric orbits than this pair. The authors believe their inward shift happened a long time ago, quite possibly as a consequence of Jupiter and Saturn's suspected orbital migration. Over billions of years the gravitational influence of the planets, even at a considerable distance, would have pulled them into near-circular orbits indistinguishable from other main belt asteroids with less exotic origins.
For the foreseeable future missions to objects beyond Neptune are likely to be limited to quick flybys like New Horizons' encounters with Pluto and the very red MU69. However, Pompeja and Justitia are as accessible as Ceres (the largest object in the asteroid belt) and Vesta (the second-largest), both of which got an extensive visit from Dawn, and are easier to get to than the Trojan asteroids, which share Jupiter's orbit around the Sun, a mission to which will be launched this year.