The space rocks may have come from beyond Neptune, and potentially offer hints at the chaos of the early solar system.
Two red things are hiding in a part of the solar system where they shouldn’t be.
Scientists led by Sunao Hasegawa from JAXA, the Japanese space agency, reported in The Astrophysical Journal Letters on Monday that two objects spotted in the asteroid belt between Mars and Jupiter appear to have originated beyond Neptune. The discoveries could one day provide direct evidence of the chaos that existed in the early solar system.
“If true it would be a huge deal,” says Hal Levison, a planetary scientist at the Southwest Research Institute in Colorado, who was not involved in the research.
Earth’s stellar neighborhood is fairly stable today. But 4 billion years ago, chaos reigned as the orbits of Jupiter and other giant planets beyond it may have shifted. The gravitational havoc caused by this planetary dance likely threw pieces of rock and ice all over the place.
“It was very dynamic,” said Karin Öberg, an expert in solar system evolution from Harvard University who was not involved in the new study.
Some of those rocks settled into the gap between Mars and Jupiter and became the asteroid belt. Most of the material is believed to be fairly similar hunks of inactive rock that failed to form planets.
But then there are two objects called 203 Pompeja and 269 Justitia.
They orbit at about 2.7 and 2.6 times the Earth-sun distance, well within the asteroid belt. 203 Pompeja, at about 70 miles across, appears to be structurally intact, whereas 269 Justitia, only 35 miles or so, is likely a fragment of a previous collision. Both have stable circular orbits, meaning they must have settled into this space long ago.
Both also have an unusual color. Objects in the inner solar system tend to reflect more blue light because they are devoid of organic material — things like carbon and methane — whereas objects in the outer solar system are redder because they have a lot of organics, perhaps the building blocks of life on Earth.
“In order to have these organics, you need to initially have a lot of ice at the surface,” said Michaël Marsset from the Massachusetts Institute of Technology, a co-author on the paper. “So they must have formed in a very cold environment. Then the solar irradiation of the ice creates those complex organics.”
These two rocks, as it turns out, are extremely red — more red than anything else seen in the asteroid belt. While tentative hints of other red asteroids have been found, these two appear to be special.
The finding, if correct, would offer evidence for planetary migration in the early solar system, particularly in support of an idea called the Nice Model, with Saturn, Uranus and Neptune all moving outward, and Jupiter inward slightly, over a few hundred million years. This would have perturbed organic-laden asteroids leftover from the formation of the planets, sending them pinging around the solar system.
“It’s an exciting discovery with implications for the origins of life,” Dr. Öberg said.
Most of these leftover objects in the present day are known as trans-Neptunian objects and orbit in the Kuiper belt beyond Neptune. Many are red in color, like Arrokoth, the rock that NASA’s New Horizons mission got a close-up of in 2019. 203 Pompeja and 269 Justitia both appear to match them.
“People have been talking about some fraction of asteroids coming from the Kuiper belt for quite a while now,” said Josh Emery, a planetary scientist from Northern Arizona University who was not involved in the paper. He said the research “definitely takes a step” toward finding evidence to support that hypothesis.
Not everyone is convinced just yet. Dr. Levison, who was also not involved in the paper, says objects should become less red as they approach the sun. Even captured asteroids in Jupiter’s orbit known as Trojans, thought to possibly be trans-Neptunian objects, aren’t this red. “It seems to be inconsistent with our models,” said Dr. Levison, who is the head of NASA’s Lucy mission, which is scheduled to launch in October to study Jupiter’s Trojans.
Dr. Marsset agrees that it’s not clear why they would be so red, but it is possibly related to how long it took them to become implanted into the asteroid belt. Some Trojans may also be as red, but haven’t been found yet.
To truly confirm the origin of 203 Pompeja and 269 Justitia, a spacecraft would likely need to visit them. Such a mission could potentially offer a glimpse at the outer solar system, but without spending a decade or more to fly there.
“You could flyby one of these strange asteroids, and a more typical asteroid for comparison,” Dr. Emery said. “That would be a really compelling spacecraft mission.”
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