The Oort Cloud may not be as icy as we imagine.
The zone of little worlds outside of Neptune’s orbit appears to have a least some rocky composition, which is a growing hypothesis strengthened with a new meteoroid observation led by Canadian researchers.
Simply put, the find is astrounding. The maxim that the Oort Cloud is made of icy bodies has been drilled into even beginning astronomy students’ heads for generations. But like with many other astronomy findings of late, we’re learning that we need to introduce more subtlety into these proclamations because the observations are not matching up with predictions.
The team observed a fireball over Alberta on Feb. 22, 2021 that – despite its rocky composition – has a predicted orbit coming well within the Oort Cloud, although there is an error bar associated with that observation.
The University of Alberta caught the fireball using its Global Fireball Observatory and partnered with Ontario’s Western University, which also has decades of expertise in direct meteor and meteoroid observations. The approximately 2 kg (or grapefruit-sized) meteoroid dropped far down in Earth’s atmosphere, indicating it was rocky, and the orbit that generated its path through the atmosphere matched those for comets that originate in the Oort Cloud.
This isn’t the first time a strange object like this was observed, said Denis Vida, Western University’s lead researcher in a study published in Nature Astronomy on Monday (Dec. 12). The comet C/2014 S3 (PANSTARRS), despite an orbit suggesting it came from the Oort Cloud, had a spectrum that suggested rocky elements, he told SpaceQ.
But it was unclear in 2014 what the cometary observations meant, Vida said, as the spectrum colors are often “all over the place,” showing carbonaceous or rocky elements due to possible contamination from flying through the solar system.
The new fireball work shows more promise: the meteoroid came directly from the Oort Cloud, and by entering the atmosphere of Earth the researchers could “probe its physical strength” to show it was rocky – even though no meteorites were ever recovered, Vida said.
“It is just another piece of evidence that there is really, truly rocky material in the Oort Cloud – and not just that. We were also able to constrain a ratio. That was not done before,” Vida said.
The error margin is between 1% and 20%, which is big for now, Vida acknowledged – but it’s only because a couple of objects of this kind have been observed. (The samples included a meteorite that was collected in Alberta in the 1970s or 1980s and only recently reconsidered, Vida said.)
The error bar will narrow the next time a similar object is visible, and the next time, and so on.
“If we get more objects and more calibrated observations, that’s just going to make that estimate more precise,” Vida said. “But even this large ratio is enough for us to constrain the possible models of the formation of the solar system.”
Very simply put, the traditional model of the solar system suggests that rocky bodies form closer to the sun and icy bodies further away – as ice tends to melt close to the sun’s radiation and pressure. But newer models are showing subtleties, too.
One big question in theory is why Mars and the asteroid belt have such small masses relative to Jupiter (on one side of that zone of the solar system) and the accumulated masses of Mercury, Venus and Earth (on the other side). Researchers can’t easily make that work with the predicted composition of the primordial disk that formed our solar system, Vida said.
Newer research suggests there must be “missing” mass, based on the composition of the sun, which in turn shows the predicted gases that were present in that region as the disc was coalescing into planetesimals, planets, asteroids and moons. Jupiter’s massive gravity probably took that mass away, Vida, but even at that the thinking generally breaks into two camps.
One theory group suggests that the asteroid belt region was always small because Jupiter’s gravity prevented anything from forming there in the first place. The other theory group suggests Jupiter moved around rocky materials that formed there; some of the ejected material fell into the sun while the rest of the rocky materials were thrown into the Oort Cloud.
If Jupiter flung rocky material into the Oort Cloud, this could account for Vida’s team observations and suggest that the Oort Cloud is not wholly made up of icy bodies. That said, Vida emphasized more observations are needed to constrain the Oort Cloud’s composition – and that when it comes to predicting solar systems or Oort Clouds around exoplanets, there is too little evidence right now to draw broader implications.
There are few detailed observations available now of exoplanet systems, and exotic planet forms like “hot Jupiters” – gas giants close to their parent suns – are just one example of how solar system dynamics can change completely with a few variations in planet types and sizes.
“There’s a lot of work, a lot of discussion, a lot of models,” Vida said of the exoplanet question. “I would say we’re producing a part [of it], but we’re still waiting for a comprehensive model that basically puts everything together and explains everything in a good way.”