Understanding the Oort Cloud
So, let’s delve into some of the specifics regarding the Oort Cloud:
a spherical shell of cometary bodies believed to surround the sun far beyond the orbits of the outermost planets and from which some are dislodged when perturbed to fall toward the sun – as per Merriam Webster
Distance and scale
The Oort Cloud represents the most distant region in our solar system. It can be hard to comprehend just how far away it is. It extends around one-quarter to halfway from our Sun to the next star.
To truly understand the distance to the Oort Cloud, it can help to set kilometers and miles aside, and to use an astronomical unit instead, which is AU – this is the unit that is defined as the distance between the sun and the Earth. Therefore, one AU is approximately 150 million kilometers or 93 million miles.
To get a comparison and some perspective regarding this, the elliptical orbit of Pluto carries around 30 to 50 astronomical units from the Sun. Nevertheless, the inner edge of the Oort Cloud is expected to be situated between 2,000 and 5,000 AU from the sun. The outer edge is situated somewhere between 10,000 and 100,000 AU from the sun.
We are sure that these distances can be quite challenging to visualize, so you may prefer to use time as a ruler in this case. At the current speed of approximately a million miles per day, the Voyager 1 spacecraft from NASA is not going to enter the Oort Cloud for around 300 years. Furthermore, it would not exit the outer edge for around 30,000 years.
Even if it was possible for you to travel at the speed of light, which is around one billion kilometers per hour or 671 million miles per hour, you would need to pack for a very length expedition if you were going to take a trip to the Oort cloud.
Once the light leaves the Sun, it takes just a little bit more than eight minutes to reach Earth, and approximately 4.5 hours to get to the orbit of Neptune. Just under three hours once it has passed Neptune’s orbit, the light from the sun will pass beyond the outer edge of the Kuiper Belt.
Once another 12 hours have passed, the sunlight will reach the heliopause, where the solar wind, which is a torrent of charged particles that flow away from the Sun at approximately a million miles per hour, will smoosh up against the interstellar medium. Interstellar space is beyond this boundary, i.e. where the magnetic field of the Sun will not hold any sway. The sunlight has now been traveling away from the Sun for approximately 17 hours.
Less than one Earth day since leaving the Sun, the sunlight will have already traveled further from the Sun when compared with any human-made spacecraft. However, somewhere it is going to be between 10 and 28 days before the same sunlight gets to the Oort Cloud’s inner edge, and it could even be as long as one year and a half before the sunlight passes beyond the outer edge of the Oort Cloud.
Formation of the Oort Cloud
The leading idea behind the formation of the Oort Cloud states that these icy objects are not always so far from the Sun. After the planets were formed 4.6 billion years ago, the region in which they were created still contained plenty of leftover chunks. These leftover chunks are known as planetesimals. Planetesimals were created from the same material as the planets did. The gravity of the planets, primarily Jupiter then dispersed the planetesimals in every which manner.
“Astronomers suspect that the Oort Cloud could extend as far as three light-years from our solar system. That is more than halfway to the nearest stars, the Centauri triple star system, which is slightly more than four light-years from Earth. If we assume that the Centauri star system is also surrounded by a sphere of comets, then there might be a continuous trail of comets connecting it to Earth. It may be possible to establish a series of refueling stations, outposts, and relay locations on a grand interstellar highway. Instead of leaping to the next star in one jump, we might cultivate the more modest goal of “comet hopping” to the Centauri system. This thoroughfare could become a cosmic Route 66.” – Michio Kaku, The Future of Humanity: Terraforming Mars, Interstellar Travel, Immortality and Our Destiny Beyond Earth
Some planetesimals were ejected fully from the solar system, and at the same time, some of the others were sent out into eccentric orbits where the Sun’s gravity held them, yet they were sufficient enough away that galactic influences also tugged onto them. The tidal force from our galaxy itself was the biggest influence.
In short terms, gravity from the planets would shove a lot of icy planetesimals away from the Sun, and the gravity from the galaxy is probable to have resulted in them settling in the solar system’s borderlands, where planets were not able to perturb them anymore. They became what we know of today as Oort Cloud. Again, this is the leading idea, yet the Oort Cloud may also capture objects that did not get formed in the solar system.
Where does the Oort cloud come from?
The Oort Cloud is believed to have been created after planets were formed from the primordial protoplanetary disc roughly around 4.6 billion years ago. The most broadly accepted hypothesis is that the Oort Cloud’s objects first coalesced a lot closer to the Sun as part of the same process that created the planets and minor planets. Once they had been formed, powerful gravitational interactions with Jupiter and other young gas giants would scatter the objects into a very wide parabolic or elliptical orbits that were consequently modified by perturbations from giant molecular clouds and passing stars into long-lived orbits that were separate from the gas giant region.
Recent research has been cited by NASA and this hypothesizes that a big number of Oort cloud objects have occurred as a consequence of materials being exchanged between the Sun and its sibling stars, as they drifted apart and formed, and it is believed that a lot (potentially most) of Oort cloud objects didn’t get formed in near distance to the Sun. It has been suggested by simulations of the Oort cloud evolving from the start of the Solar System to the present peaked approximately 800 years once they had been formed, as the pace of collision and accretion slowed down and depletion started to overtake the supply.
Julio Angel Fernandez put together models that indicated that the scattered disc, which is the chief source for periodic comets in the Solar System, could also be the chief source for Oort cloud objects. As per the models, approximately half of the objects scattered travel outward in the direction of the Oort Cloud, whereas approximately a quarter shifted in an inward direction to the orbit of Jupiter. A quarter was then forced out on hyperbolic orbits. The scattered disc could still be providing material to the Oort Cloud today. In fact, once 2.5 billion years have passed, it is estimated that around one-third of the population of the scattered disc is probably going to end up in the Oort cloud.
Models that have been established on computers have indicated that cometary debris collisions throughout the formation period have a much bigger role to play than we thought to begin with. According to these computer models, the number of collisions early during the history of the Solar System was so substantial that the majority of comets were destroyed prior to ever reaching the Oort cloud. As a consequence, the current cumulative mass of the Oort cloud is nowhere near as big as it was suspected to be previously. The Oort cloud’s estimated mass is only a little part of the 50 to 100 Earth masses of material that has been ejected.
It is believed that comets have two separate origin points within the Solar System. Short-period comets, i.e. those that have orbits of up to 200 years, tend to be accepted to have emerged via either the scattered disc or the Kuiper belt, which are a couple of flat discs of iced debris that are connected beyond the orbit of Neptune at 30 au and extending jointly out further than 100 au from the Sun.
Very long-period comets, for example, the Cataline C/1999 F1, whose orbits last for millions of years, are believed to have originated straight from the Oort cloud including the PANSTARRS C/2017 T2, K2 C/2017, Siding Spring C/2013 A1, Comet ISON, Elenin C/2010 X1, and McNaught C/2006 P1.
“From a score of well-observed original orbits it is shown that the “new” long-period comets generally come from regions between about 50000 and 150000 A. U. distance. The sun must be surrounded by a general cloud of comets with a radius of this order, containing about 1011 comets of observable size; the total mass of the cloud is estimated to be of the order of 1/10 to 1/100 of that of the earth. Through the action of the stars, fresh comets are continually being carried from this cloud into the vicinity of the sun.“ – Jan Oort
The orbits in the Kuiper belt are pretty stable, and so very few comets are believed to originate there. Nevertheless, the scattered disc is dynamically active and is a lot more likely to be the location of origin for comets. Comets go through from the scattered disc into the area of the outer planets, resulting in what we know as centaurs. These centaurs are typically sent further inward so that they can become short-period comets.
There are two varieties of short-period comets: Halley-family comets and Jupiter-family comets, i.e. those that have semi-major axes that are less than 5 AU. Halley-family comets, which are named for Halley’s Comet, their prototype, are not usual in that despite the fact that they are short-period comets, it is hypothesized that their ultimate origin does not lie in the scattered disc. Instead, they lie within the Oort Cloud. Based on their orbits, it is suggested that they were long-period comets that were captured by the giant planet’s gravity and then sent into the inner part of the solar system.
This process could have also generated the present orbits of a considerable fraction of the comets within the Jupiter family, despite the fact that most of these comets are believed to have originated within the scattered disc.
Oort has noted that the number of comets that return is far fewer than was predicted by his model. This problem, which is noted as a cometary fading, has not been resolved yet. No dynamical procedure is known to explain the smaller number of observed comets that we are seeing in comparison to what was estimated by Oort.
Understanding tidal effects
The majority of the comets that were seen close to the Sun tend to have reached their existing positions by the gravitational perturbation of the Oort Cloud by the Milk Way exerting tidal force.
Just as the tidal force of the Moon will deform the oceans in the Earth, resulting in the tides rising and falling, the galactic tide will distort the orbits of bodies in the outer part of the Solar System.
In the Solar System’s charted regions, these impacts are negligible in comparison to the Sun’s gravity, yet when it comes to the system’s outer reaches, the gravity of the Sun is weaker and the Milky Way’s gravitational field’s gradient will have a significant impact.
Galactic tidal forces will stretch the cloud along an axis, with a direction toward the galactic center, compressing it along the other two axes. These small perturbations can often shift orbit within the Oort cloud to bring objects closer to the Sun.
The section whereby the gravity of the Sun will concede its influence on the galactic tide is known as the tidal truncation radius. It is placed at a radius between 100,000 and 200,000 AU and marks the Oort Cloud’s outer boundary.
Some scholars have brought forward the theory that the galactic tide may have been a contribution to the Oort cloud forming by increasing the perihelia of the planetesimals with large aphelia. To explain this in simpler terms, the large aphelia is the largest distance to the Sun, whereas the perihelia are the smallest distance to the Sun.
The impact of the galactic tide is quite complicated and depends largely on the behavior of individual objects within the planetary system. However, cumulatively, the impact can be rather significant. In fact, as much as 90 percent of all comets originate from the Oort cloud could be a consequence of the galactic tide.
Statistical demonstrations of the orbits that have been observed of long-period comets have argued that the galactic tide is the chief means by which the orbits are perturbed towards the Solar System’s inner section.
“There exists also another type of galaxy agglomeration. These have diameters between roughly one and ten times those of large clusters, but have much smaller densitities; they are usually irregular, with patchy density variations and no central concentration. The larger and most conspicuous of these agglomerations may contain several clusters, which explains why they have been given the name “superclusters.” In their longer dimensions, crossing times exceed the age of the Universe. They are thus unrelaxed. Unrelaxed apperance, together with large size, might be taken as a definition of superclusters.“ – Jan Oort
What should you know about the Oort cloud?
It can be a bit overwhelming to take in all of this information about the Oort Cloud. So, let’s break things down a little bit and reveal what you should know about the Oort Cloud below:
- Far, far away – The Oort Cloud is a circular layer of icy objects that surround our sun, a star, and is likely to occupy space at a distance that is between approximately 2,000 and 100,000 astronomical units (otherwise known as AU) from the Sun.
- Predicted realm – The Oort Cloud is a predicted collection of icy objects that are situated further away from everything else in the solar system. It fits with observations of comets in the solar system’s planetary region, yet scientists have not observed any object within the Oort Cloud itself.
- Long-way round – Long-period commits are likely to come from the Oort Cloud, which is often known as a cometary reservoir. If you have never heard of a long-period commit before, this is a comet that takes over 200 years to orbit the Sun.
- Bigger and closer – When comets from the Oort Cloud get close to the Sun, the ices on their surface start to vaporize, which produces a coma, which is a cometary atmosphere, and often two tails, one gas and one dust, which can reach hundreds or sometimes millions of kilometers or miles in length. The activity will subside, and the coma will collapse when the orbit of the comet carries it far enough away from the Sun.
- Large numbers – There have been predictions from experts that show that the Oort Cloud could contain over a trillion icy objects.
- Exo-comets – Astronomers have viewed evidence that comets are disintegrating around the stars, in the same manner that Comet ISON did when it grazed the Sun back in 2013. By utilizing spectrometry to study the chemical composition of these comets, scientists have the ability to make comparisons of the birth of our solar system with those in other planetary systems. A planetary system is a collection of asteroids and planets, etc, which orbit stars or a star. The planetary system that orbits the Sun, which is known as “Sol” in Latin. This is why we call our planetary system the solar system.
- Primitive – A number of the molecules that are located on comets formed prior to the Sun being born. They would not be able to survive at the pressures and temperatures located on or around the Earth. By assessing the conditions under which primitive cometary molecules are able to form, scientists are able to better understand what the environment of our solar system was like when it was born, which offers clues about how it has formed and since evolved.
- Deep thinkers – The Oort Cloud has been named for Jan Oort, who is a Dutch astronomer, who predicted the existence of the Oort Cloud back in the ‘50s.
- Dark and cold – The comet-like, frozen bodies of the Oort Cloud are not able to support life as we know it.
- A long trip – no missions have been sent to explore the Oort Cloud as it stands. However, five spacecraft will get there eventually. They are the Pioneer 11, Pioneer 10, New Horizons, Voyager 2, and Voyager 1. The Oort Cloud is so distant, nevertheless, that the power sources for all five spacecraft are going to be dead centuries before they are able to reach the inner edge of the Oort Cloud.
Further exploration to the Oort Cloud
We have touched upon the Oort Cloud and further exploration in the previous section, yet it is certainly worth expanding on in further detail. As it stands, space probes have not reached the area of the Oort Cloud.
The fastest and furthest of the interplanetary space probes that are leaving the Solar System at the moment is Voyager 1. It is estimated that it could reach the Oort cloud in approximately 300 years. However, it would take 30,000 years in total to pass through the Oort cloud.
Nevertheless, around 2025, the Voyager 1’s radioisotope thermoelectric generators are not going to be able to supply sufficient power to operate any of the scientific instruments, which prevent deeper exploration by the Voyager 1.
The other four probes that we mentioned that is escaping the solar system at the moment – Pioneer 11, Pioneer 10, New Horizons, and Voyager 2 – are either already or predicted to be non-functional once they get to the Oort cloud.
During the 1980s, there was a concept that a probe would be able to reach 1,000 AU within 50 years. This was known as the TAU. One of the missions that TAU would have would be to look for the Oort Cloud.
During the Announcement of Opportunity in 2014 for the Discovery program, an observatory called Whipple Mission was proposed for the purpose of detecting the objects within the Oort cloud and the Kuiper belt. It would be used for monitoring distant stars within a photometer, searching for transits that were as much as 10,000 AU away.
The observatory that was suggested for orbiting of the halo around L2 with a proposed five-year mission. It was put forward that the Kepler observatory may have been able to detect objects within the Oort Cloud.
Complex but not unique?
Professor of numerical star dynamics, Portegies Zwart, has an interesting take on the Oort Cloud:
“With our new calculations, we show that the Oort cloud arose from a kind of cosmic conspiracy 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.
“Despair often got the better of us. Only when the calculations were completed, did all the pieces of the puzzle suddenly fall into place and it all looked quite natural and self-evident. That is, I think, one of the most beautiful aspects of being a scientist. You suddenly realize how distorted our thinking concerning this problem was, until it actually turned out to be rather natural.”
The discovery of mysterious Oort cloud objects in 2014
There is no denying that we are learning more and more about our solar system and planet all of the time, and this is something that was illustrated perfectly back in 2014 when we saw mysterious Oort cloud objects, which astronomers around the world found incredibly excited.
The objects that were like commits that originated in our solar system’s farthest reaches were not what we expected them to be. Unlike other types of objects from this expansive field of icy bodies on the outer edge of the solar system – which we have established is the Oort cloud – the bodies of these objects were much more like those formed nearer to the sun, rather than being as we expected them to be.
It was this finding that supported the idea that Jupiter and other big planets like this moved around huge amounts throughout the early days of the solar system, which were incredibly chaotic, with the planets flinging asteroids in an outward direction as they went. This theory has also suggested that the icy bodies were thrown in an inward direction, which could also explain how we ended up getting water on our planet.
An astronomer from the University of Maryland, College Park, Micheal Kelley, spoke about how exciting this is and what he thought about the events that had unfolded.
“If this one asteroid in particular really is made of inner solar system material that got out to the Oort cloud and has worked its way back in, it could refine theories about how the solar system formed.”
Another take is from an astronomer from the University of Central Florida, Orlando, Yanga Fernández, who said the following:
“It’s a disconnect between two things we thought were pretty solid. It’s a very unusual result, and it will be extremely interesting if people get data that corroborate it.”
“The origin of the Oort cloud is going to tell you something about how things were moving around in that era right after the planets started to form.”
“That’s a very long-term goal.”
Final words on the Oort cloud
So there you have it: everything you need to know about the Oort cloud and what this exciting field of study holds for us. There is no denying that we still have a lot to learn about the outer edges of our solar system, yet we understanding more and more about it every day, and so it is certainly a truly fascinating area of study. Let’s just hope that things do not end up the way they did in the Don’t Look Up film! We won’t give you any spoilers from it but it is definitely something you should go and watch.