OC401: Where and What is the Oort Cloud?

David Noel
Ben Franklin Centre for Theoretical Research
PO Box 27, Subiaco, WA 6008, Australia.

This is the first in a suite of web articles about the Oort Cloud, the volume of space immediately outside our Solar System. The boundary between the Solar System and the surrounding Oort Cloud is called the Heliosphere.

The Heliosphere in Universal Space
Our Solar System is made up of the Sun, the Earth and other planets, and various moons, asteroids, comets, and such. It can be regarded as a bounded sphere in interstellar space, the "Heliosphere". The Oort Cloud is the region of space which lies outside the Solar System, beyond the Heliosphere.

Fig. OCA-F1. The Heliosphere. From [A6].

The Heliosphere is a real physical entity. It has a radius of about 100 AU, where 1 AU (Astronomical Unit) is the distance from the Earth to the Sun (about 150 million kilometres). Within the Heliosphere lie the 8 planets, all within a plane which more or less coincides with the equatorial plane of the Sun.

There are a number of ways in which conditions inside and outside the Heliosphere are different. The Sun emits a sweep of particles and radiation called the Solar Wind, which weakens as it approaches the edge of the Heliosphere, and stops altogether at the Heliopause, short of the boundary. A change in plasma density occurs on crossing the Heliosphere [reference A7], together with alterations in interstellar magnetic fields.

Two of the space probes sent out from Earth, Voyager 1 and Voyager 2, have now passed through the Heliosphere boundary. Voyager 1 exited in 2012, and Voyager 2 in 2018 [A7]. One Voyager scientists commented "There's a distinct boundary out there. It's just astonishing how fluids, including plasmas, form boundaries."

There is also a marked difference in the distribution of masses inside and outside the Heliosphere. Within the boundary, the Sun is the overwhelming gravitational master, and in the past has reformed the previously somewhat random orbits of the planets and other bodies, so that most masses now orbit on the Sun's equatorial plane, the Ecliptic. Beyond the Heliosphere, the Sun may still be a major gravitational player, but its influence weakens as you move out into the Oort Cloud proper.

Because of the Sun's past influence, the Heliosphere is now divided in two, with most of the mass it contains in the equatorial plane of the Sun. Above and below this plane, the two hemispheres consist of comparatively good vacuums, with very little matter.

The Oort Cloud -- Beyond the Heliosphere
If the Heliosphere, with a radius of about 100 AU, defines the Solar System, what do we call the space beyond it, stretching out towards the nearest stars? That vast and hitherto mysterious volume is what's called the Oort Cloud.

Fig. OCA-F2. The Heliosphere and the Oort Cloud. From [A1].

While the inner boundary of the Oort Cloud is well defined, it has only an arbitrary outer limit, assigned purely for convenience at 100,000 AU. That is, taking the outer boundary at just under half-way to the nearest star, a little under 2 light-years, allows us to do calculations on its properties. But there are no marked changes in properties or contents at the outer boundary, it's just a surface drawn in interstellar space.

It is the nature and contents of interstellar space which is the subject of this suite of articles. Formally, the name "Oort Cloud" refers to a selected volume of space around our own Solar System. To generalize and refer to the matter in all spaces around other solar systems, we may refer to this as the "Oort Soup". In later articles in this suite, we will show there are larger bodies within the Oort Soup, and may refer to these as the "Oort Worlds".

What's recorded about the Oort Cloud
If you look at standard sources such as Wikipedia, you will find that that recorded information on the nature and contents of the Oort Cloud is relatively sparse. Such sources will identify the Cloud as the source of comets, and will describe how it is named after the noted Dutch astronomer Jan Oort, and may speculate about its total mass. That is about it.

As an example, in 2015 Matt Williams gave the following summary on the Oort Cloud [reference A1]. Note the term "theoretical", implying nothing known for certain, only a matter of speculation.

"The Oort Cloud is a theoretical spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance of up to around 100,000 AU (2 light-years). This places it in interstellar space, beyond the Sun’s Heliosphere where it defines the cosmological boundary between the Solar System and the region of the Sun’s gravitational dominance" [A1].

In what follows, we will find that quite a lot is known about the Oort Cloud, but much of this information has been wrongly filed, wrongly attributed to other areas of cosmology. Or, the data is perfectly good, but it has been wrongly interpreted. But first, a bit about how the Cloud was named.

Jan Oort, noted astronomer
Jan Hendrik Oort (1900 -- 1992) was a noted Dutch astronomer who made significant contributions to the understanding of the Milky Way, and was a pioneer in the field of radio astronomy and comet science -- one obituary calling him "one of the century's foremost explorers of the universe" [A2]. Oort determined that the Milky Way rotates, and overturned the idea that the Sun was at its center. He also postulated the existence of a mysterious invisible dark matter, and was responsible for a number of important insights about comets.

Fig. OCA-F3. Jan Oort. From [A3].

After his early work, Oort went on to study comets, for which he formulated a number of revolutionary hypotheses. He hypothesized that the Solar System is surrounded by a massive cloud consisting of billions of comets, many of them “long-period” comets that originate in a space far beyond the orbits of Neptune and Pluto. This cloud is now known as the Oort Cloud [A2].

Oort was one of the few astronomers to see two visits of Halley's comet, in 1910 and 1986. The Oort Cloud, the Oort Constants, and the asteroid "1691 Oort", were all named after him. By the time of his death, at the age of 92, Professor Oort was recognised as one of the greatest astronomers of the 20th century. Both an observer and a theorist, Oort revolutionised astronomy through his ground-breaking discoveries [A4]. Colleagues remembered him as a tall, lean and courtly man with a genial manner.

Fig. OCA-F4. The vast Oort Cloud surrounding the Solar System. From [A1].

Matter in the Oort Cloud
So the established view of the Oort Cloud is that is huge, and contains a lot of comets. Its total mass has been guessed at since it was first mooted, usually by counting the number of comets which venture into the Heliosphere each year, estimating their average mass, and assuming that the rest of the Oort cloud contained only a similar population of bodies over its whole volume.

According to reference [A1], "The outer Oort cloud may have trillions of objects larger than 1 km, and billions that measure 20 kilometers in diameter. Its total mass is not known, but, assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects, it has the combined mass of roughly 3 ×1025 kilograms, or five Earths". Since our Sun has the mass of about 333,000 Earths, this would mean that the total vast Oort Cloud would contain only a tiny fraction of the mass of one star.

We will see later that such an estimate is wrong by many, many orders of magnitude -- in fact our Oort cloud contains something like 9 times the mass of our Sun. In the next segment, we'll go on to look at the whole question of mass in the universe. But first, a general observation of how we have learned about the Universe at large.

Until less than a hundred years ago, our total knowledge of the Universe beyond our Solar System came from the light given out by stars, from visible light. Visible light is only a tiny fraction of the electromagnetic spectrum, and we were totally reliant on this narrow window to look at the heavens, right up to the development of radio astronomy in the late 1940s.

Using only simple maths, if the radius of the Solar System is taken as 100 AU, and the radius of the Oort Cloud as 100,000 AU, then the volume of the Oort Cloud is 1000 cubed, or one billion times, that of the Solar System. So the Oort Cloud, as yet not directly observed, is almost all the Universe. We have been relying for our knowledge of the Universe on the very tiniest sample of the whole.

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References and Links

[A1]. Matt Williams. What is the Oort Cloud?. https://www.universetoday.com/32522/oort-cloud/ .
[A2]. Jan Oort. https://en.wikipedia.org/wiki/Jan_Oort .
[A3]. Jan Hendrik Oort, 1900-1992. https://www.esa.int/About_Us/ESA_history/Jan_Hendrik_Oort_Comet_pioneer .
[A4]. Jan Hendrik Oort: Comet Pioneer. https://www.esa.int/Our_Activities/Space_Science/Jan_Hendrik_Oort_Comet_pioneer .
[A5]. How many meteorites hit Earth each year?. http://curious.astro.cornell.edu/about-us/75-our-solar-system/comets-meteors-and-asteroids/meteorites/313-how-many-meteorites-hit-earth-each-year-intermediate .
[A6]. Heliospheric Physics. http://natural-sciences.nwu.ac.za/centre-space-research/heliospheric-physics .
[A7]. Voyager 2 reaches interstellar space. https://www.sciencedaily.com/releases/2019/11/191104112823.htm .

Go to the Oort Cloud Home Page

Go to the next article in the OC suite

Version 1.0 published November 2019 as Segment A of the book "The Oort Cloud: Almost all the Universe". AOI Press, ISBN 9798614884314.
Version 2.0 placed on web at "AOI.com.au", 2022 Jun 14.