The Oort cloud

After 70 years, a timeline emerges for the mysterious Oort cloud that surrounds our solar system

Jessica Richburg

A sphere dotted with gas, ice and huge bits of debris surrounds our solar system. This thick sphere of stellar detritus, the Oort cloud, is the farthest entity that is still deemed part of the solar system – and begins at about 40 times the distance of the sun to the former planet Pluto. Despite all the interest, though, no one knew which came first – the Oort cloud or the planets – and how things progressed.

There may be a little less uncertainty now, according to a preprint in arxiv by Simon Portegies Zwart and other researchers at Leiden University. They have concluded that the Oort cloud is a remnant of the time that the solar system sprung out of a cluster of newborn stars – cutely termed a stellar nursery. The planets? Well, they slowly coalesced some time later – give or take a few billion years.

Now, looking back in time is hard enough when digging through ruins of past cities – at least the matter is still there. It is harder when trying to make out where a careening comet or patch of gas started out.

Zwart, a professor of computational astrophysics at Leiden University, and his team started with a full image of their jigsaw puzzle and then worked backward. The effort was to create a complete chronology of the Oort cloud.

The Oort cloud??!

Jan Oort, who discovered the Oort cloud
Jan Oort, who discovered the Oort cloud. Pic courtesy, Wikimedia

Its existence was first suggested by Ernst Öpik, an Estonian astronomer. in 1932. Dutch astronomer Jan Hendrick Oort rejuvenated the idea in 1950. Despite some evidence, there have been no direct observations of the Oort cloud.

Some of the gas, ice and debris occasionally jostled out of it swings into the inner solar system. Comets that come calling many years apart, such as Halley’s, are believed to be products of the Oort cloud. This has earned it the moniker “the birthplace of comets.”

The solar system is more or less in a flat plane, all the major planets being at the same level. But the Oort cloud is a sphere enveloping it.

While scientists knew about the Oort cloud for decades, its history was an enigma an intractable riddle wrapped in a mystery that long defied explanation.

Simon Portegies Zwart
Simon Portegies Zwart, the lead member of the team that put together a coherent story of the history of the Oort cloud. Pic courtesy Simon Portegies Zwart

Stellar detective

The fact that so much of the Oort cloud is unknown is what first drew Zwart and his team to the subject.

He says that astronomers have bumped up against an assortment of roadblocks, including the Oort cloud’s own virtual invisibility and its huge distance from Earth. Its amorphous consistency also meant that astronomers learned about it from the few comets from it they could track.

“It is like listening to one tone in the entire spectrum of music to deduce all the music that has been composed by any human in the history of mankind,” Zwart says, “That is how absurd the whole situation is. There are 100 billion comets, probably, in the Oort cloud, and yet we only know about 100 of them.”

Now, astronomers did come up with the histories of a few objects, making for a patchwork of information. Zwart and his team brought that data together into what might be a coherent story.

Mapping the past

The researchers used a software called AMUSE: the Astrophysics Multipurpose Software Environment. AMUSE uses publicly available codes that address gravity, star evolution and other factors. It has incorporated at least two codes that solve the same physics.

With AMUSE, the team generated two sets of simulations.

The first set, consisting of two simulations, studied the evolution of a disk of an isolated solar system. It started where the sun might have been a billion years ago and ends a billion years later.

The second set, with 224 simulations, studied how the solar system developed in its birth cluster.

Fireworks in a nursery

A stellar nursery in nebula NGC 346
Gas collapsing in the preparation to form infant stars in a stellar nursery in nebula NGC 346. Pic courtesy NASA, ESA and A. Nota (STScIESA)

The team concluded that the sun was born in the aforementioned stellar nursery with a whole lot of other baby stars at the center of the galaxy. That was where a huge ball of gases, roiled by gravity, formed into clump of matter that became stars.

While still developing a solar system, the sun drifted away from the area. The researchers believe the surrounding stars in the nursery could have disrupted any forming cloud around it. So the comparatively fragile Oort cloud must have formed when the sun struck out on its own. That may have taken hundreds of million of years. Meanwhile, the sun was doing its own star trek to our current address: the Orion Arm of the galaxy.


The Oort cloud in relation to the solar system
The Oort cloud in relation to the solar system. Pic courtesy ESA

Zwart and his team calculated where about 70% of what makes up the Oort cloud today came from. That would be the region that now lies near Uranus and Neptune, and a cluster of asteroids called the Centaur family that lie between Jupiter and Neptune.

The researchers believe that the Oort cloud is still rather fragile. They think that as more comets and debris are pulled into the sun, it will lose mass and finally disappear.

While their main work was on the origins of the Oort cloud, they also considered the solar system’s history. They found support for the assertion that a planetoid named Sedna was pulled in from another star. Since it is in a stable area in terms of gravity, that could have been billions of years ago.

Beyond fashion

“I like to study that which others might not consider interesting, and then see, actually, that they are very interesting,” Zwart says. He gives the analogy of a biologist who makes an active choice to study particularly ugly birds. While it is natural for other biologists to be drawn to prettier subjects, true curiosity calls for seeing the value in these “ugly” birds.

“That shows courage – to study something that no one seems interested in,” he says. “But, you know, it’s about the science; it’s not about me.”

Still, there are downsides to venturing into uncharted territory.

The Oort cloud compared to where Voyager 1 is today
The red arrow indicates the location of space probe Voyager 1, which will reach the Oort cloud in about 300 years. Pic courtesy NASA / JPL-Caltech

Vast pieces of the timeline were missing, and the astronomers had to study those “soft spots” closely. They had to fill that in based on what preceded and what followed.

“You analyze your data and all the pieces improve at the same pace,” Zwart says. “You have a model that sort of works, but not quite. You start looking. Where does it go wrong. Can I improve that part? There’s no one starting point.”

The end – for the moment

While there was no clear, single starting point to this simulation, there certainly was a tentative end.

Most of the time, a scientist deems an experiment complete when there is a reasonably stable answer to whether the hypothesis holds. While a study based on minimal data like this, it may seem hard to reach any conclusion. But Zwart says that was not happened in the case of his team.

“There’s a moment when you know this must be right,” he says, “That’s the moment you realize, this must be the way it works.”

Jessica Richburg is a certified high school technical writing and grammar teacher with a degree in English.

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