When a young solar system gets going it’s little more than a young star and a rotating disk of debris. Accepted thinking says that the swirling debris is swept up in planet formation. But a new study says that much of the matter in the disk could face a different fate.
It may not have the honour of becoming part of a nice stable planet, orbiting placidly and reliably around its host star. Instead, it’s simply discarded. It’s ejected out of the young, still-forming solar system to spend its existence as interstellar objects or as rogue planets.
The study comes from Avi Loeb and Amir Siraj, names that may be familiar to Universe Today readers. Loeb and Siraj are both from the Center for Astrophysics (CfA) at Harvard and have collaborated on research before. Their new study is titled “Preliminary Evidence That Protoplanetary Disks Eject More Mass Than They Retain.” It’s available on the pre-print site arxiv.org and hasn’t been peer-reviewed yet.
Loeb and Siraj point to the existence of interstellar objects like Oumuamua and 2I/Borisov to make their case. So far, there’s no conclusive proof for the origin of these objects and their brethren. Researchers have come up with different origins, and have brought evidence to bear, but so far there’s no consensus. Oumuamua could be an interstellar dark hydrogen iceberg, an object similar to Pluto, or even a type of interstellar ‘dust bunny.’ And comet 2l/Borisov is likely a rogue interstellar comet, the first one we’ve observed.
On the left is an artist’s illustration of the interstellar object ‘Oumuamua. On the right is an image of interstellar comet 2I/Borisov. Image Credit Left: European Southern Observatory / M. Kornmesser. Image Credit Right: By NASA, ESA, and D. Jewitt (UCLA) – Public Domain
Stellar mass budgets show that neither exo-Oort Clouds nor protoplanetary disks can provide enough mass to explain interstellar objects and the rogue planet population. So maybe our stellar mass budgets are wrong? Maybe the bulk of the material in protoplanetary disks is ejected and becomes interstellar objects like ‘Oumuamua, 2I/Borisov, and rogue planets, with some of those planets being many times larger than Earth.
Much of the paper is based on scientific estimations, and much of it is preliminary. The authors make that clear in the paper’s title. Scientists don’t yet have a clear understanding of how many interstellar objects and rogue planets there are. But you have to start somewhere, and this paper is a kind of starting point.
Their paper opens with, “If interstellar objects originate in protoplanetary disks, they can be used to calibrate the fraction of mass that such disks eject.” From there they dig deeper.
“The origin of interstellar objects is an unsolved mystery,” they write. “Neither exo-Oort clouds nor protoplanetary disks are capable of filling the mass budget necessary to produce the inferred interstellar object population.” That only leaves two broad possibilities for their origins. One is different stellar mass budgets, which may be implausible. The other is different survival probabilities for interstellar objects across vast distances and time scales.
This preamble sets up the researchers’ main question: “How much mass per star is required to produce interstellar objects?”
The first hurdle in the way of answering that question is the fact that we only really know about two interstellar objects: ‘Oumuamua, which was discovered in 2017, and comet 2I/Borisov, which was discovered in 2019. And scientists only have observed estimates for their sizes. ‘Oumuamua is estimated to be between 20 to 200 meters, and Borisov’s nucleus is estimated to be between 0.4 to 1 km. There’s also a third potential interstellar object named CNEOS 2014-01-08, but its status as an interstellar object is not confirmed.
We only have estimates of how many of these interstellar objects there are, including rogue planets. For objects like ‘Oumuamua and Borisov, the
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