The coldest, darkest reaches of our Solar System – a region still to be visited by human spacecraft – is a strange, frigid cloud that contains material from other stars and an unconfirmed object known as Planet Nine that may actually be a planet-mass black hole. This forbidding uncharted region, an enormous bubble of material encasing the planets and our Sun, is known as the Oort Cloud. This far-away shell surrounding our Solar System underscores Edwin Hubble’s observation in 1936 that “with increasing distance, our knowledge fades, and fades rapidly. Eventually, we reach the dim boundary—the utmost limits of our telescopes. There, we measure shadows.”
3000 Times the Distance between the Earth and the Sun
The Oort cloud was hypothesized in 1950 by the Dutch astronomer Jan Hendrik Oort to explain why there continue to be new comets with elongated orbits in our solar system. The cloud, which starts at more than 3000 times the distance between the Earth and the Sun, should not be confused with the Kuiper belt –the rim of rock, grains and ice in which the dwarf planet Pluto is located and which orbits relatively close to the Sun at about 30 to 50 times the Earth-Sun distance. Even today, astronomers have yet to directly observe an object in the distant Oort cloud – they’ve only seen those Oort cloud comets that have been kicked into the inner Solar System due to gravitational interactions with nearby stars.
Origins a Mystery
The origins of the cloud are still a mystery, but if recent predictions are accurate, the Oort Cloud could contain material that is alien to our Solar System. A recent study has offered some insights into what it may take for the Oort Cloud to have formed. Simon Portegies Zwart , a Professor of Computational Astrophysics, and his colleagues at Leiden University in the Netherlands have managed to calculate the first 100 million years of the history of the Oort cloud in its entirety. Until now, only parts of the history had been studied separately. The cloud, which is expected to contain roughly 100 billion comet-like objects, forms an enormous shell at the edge of our solar system.
The Planet 9 Hypothesis –“An Intriguing Idea”
“The discovery of the new large Oort comet Bernardinelli-Bernstein does not have any bearing (that I know of) on the existence of a Planet 9,” wrote Gary M. Bernstein, Reese W. Flower Professor of Astronomy and Astrophysics at the University of Pennsylvania in an email to The Daily Galaxy. “In fact I’m not sure if there are any papers that connect the P9 question to the dynamics of the outer Oort cloud.
“But if you’re asking about P9,” Bernstein added. “I’ll give opinions that are based on the results of analyzing the “extreme TNO” discoveries of the Dark Energy Survey. I keep the proposal of an unseen Neptune-mass planet in the “intriguing idea” category. I wouldn’t bet my reputation – or anything of real value – on its existence, because none of the alignments of distant minor planets that inspired this hypothesis have been verified by a well-characterized search of the sky. When the Vera Rubin Observatory starts up around 2023 we will fairly quickly get a definitive thumbs up or down on the need for a Planet 9.
Have Neptune- or Earth-mass Planets Eluded Discovery?
“But even before P9 was hypothesized,” Bernstein continued, “we and others knew that we have only very sketchy knowledge of what lurks in our solar system more than 100 AU from the Sun. There could easily be Neptune- or Earth-mass planets that have eluded discovery so far, and that alone is a good reason to keep looking, in my view. These outer realms of the solar system must hold an abundance of debris from the early days of the solar system, and we have only some limited and indirect knowledge right now of what’s out there.
P9 –A Black Hole? “Not a Shred of Evidence”
“As for the idea that there is a P9 and it’s a black hole, there’s not really a shred of evidence to suggest that there’s anything more exotic than ice and rock in our solar system, so I’d consider this an “amusing thought” that doesn’t yet rise even to “intriguing.” The best part of the black-hole idea so far is that it led to a very fun “actual size” figure in a paper.”
About P9 and the black hole conjecture, Portegies Zwart told The Daily Galaxy: “In my opinion, the assumption of a 9th planet in the outer parts of the Solar system is just one solution to a very intriguing problem with the arguments of periceter of the trans-Neptunian Sedna-like objects, but by no means the only solution. Replacing a hypothetical planet by a black hole sound to me unnecessarily complicated.”
“I think there’s zero chance of a planet-mass black hole. There’s plenty of worlds in the Oort cloud, left from the birth of the Solar System, but that’s not one of them,” planetary scientist Michele Bannister, Rutherford Discovery Fellow at the University of Canterbury told The Daily Galaxy.
Comet-Like Objects Come from Two Places in the Universe
The Oort cloud, the Leiden simulations confirm, is a remnant of the proto-planetary disk of gas and debris from which the Solar system emerged some 4.6 billion years ago. The comet-like objects in the Oort cloud come from roughly two places in the Universe. The first part of the objects comes from close by, from the Solar system. These debris and asteroids have been thrown out by the giant planets. However, some of the debris did not succeed in doing so and is still in the asteroid belt between Mars and Jupiter. A second population of objects, the Leiden astronomers concluded, comes from other stars. When the Sun was just born, there were about a thousand other nearby stars in our parent birth cluster. The Oort cloud may have captured comets that originally belonged to those other stars.
“A Conspiracy of Nature”
“With our new calculations, we show that the Oort cloud arose from a kind of cosmic conspiracy,” says Portegies Zwart, “in which nearby stars, planets and the Milky Way all play their part. Each of the individual processes alone would not be able to explain the Oort cloud. You really need the interplay and the right choreography of all the processes together. And that, by the way, can be explained quite naturally from Sun’s birth environment. So although the Oort cloud is complicatedly formed, it is probably not unique.”
Planets, stars, and the Milky Way all had a role to play in its formation, he says. “The complication of the process surprised me. If you want to calculate the whole sequence in a computer, you will irrevocably run aground. That’s why, until now, only separate events were simulated.”
Mapping the Entire Genesis of the Oort Cloud
Portegies Zwart’s team started from separate events, as in previous studies, but for the first time they were able to connect the events with each other, using the end result of the first calculation as the starting point for the next calculation. In this way, they were able to map out the entire genesis of the Oort cloud.
Not Unique in the Cosmos
But the results mean it’s unlikely our solar system is the only one enveloped by a vast, icy cloud. “Once we had mapped out the various processes, they turned out to be a rather natural consequence of the evolution of the Solar System,” says Portegies Zwart.
“Stuff from Other Stars”
Their work has also made predictions about what the Oort Cloud may contain. If their predictions are accurate, the Oort Cloud could contain material that is alien to our Solar System: “Stuff from other stars,” says Portegies Zwart.
The idea that our Sun might have stolen material from elsewhere was first put forward about a decade ago, reports The BBC. “In the Sun’s birth cluster of stars, the sibling stars would have been snuggled up tight enough for their comet clouds to overlap and tangle,” says Michele Bannister, suggesting that some of our own comets may now be orbiting other stars in return. “Then they parted ways as the cluster dispersed.”
Exactly how much of the Oort Cloud comes from other stars remains a mystery, but a study from November 2020, suggests interstellar objects could outnumber those from our own Solar System. Another, which released preliminary results earlier this year, identified three stars that might have passed through the Oort Cloud.
“It would be very difficult to know which comets were not formed here, but perhaps future studies of real-time interstellar comet visitors will give us some insights into this,” says Kat Volk, a planetary scientist at the University of Arizona.
Oort Cloud Object at Outer Reaches of Our Solar System
Astronomers sifting through archival data from the Dark Energy Survey (DES) have spotted a large Oort Cloud object, C/2014 UN271, approaching the outer regions of the solar system. The discovery, reports Physics World, “has caused ripples of excitement within the planetary science community because of the object’s unusually large size – initial estimates suggest it may be as big as 130–160 km across, substantially bigger than some of the largest comets. Studying the object could also give researchers insights into an enigmatic process in the solar system. The DES project, which investigates the cosmological mystery of dark energy by photographing distant galaxies.
Understanding the pristine, primordial objects of Oort Cloud could give us some important clues about the origins of our Solar System and how it formed. “It would be really great to be able to drill a few holes in a few Oort Cloud objects and analyse the material,” says Portegies Zwart.
Source: Oort cloud Ecology II: The chronology of the formation of the Oort cloud. Accepted for publication in Astronomy & Astrophysics.
Image credit top of page: Impression of the Oort cloud. The density has been exaggerated. By artist Pablo Carlos Budassi [CC BY-SA 4.0] via Wikimedia