Something us scientists love to do is to take questions from the public about the work that we do and the topics that we study. I know that when I give a public talk, the Q&A is almost always my favorite part. This week, Christina holds another such session after the success of her previous post on this topic.
by Dr. Christina Smith
A few months ago (or maybe longer than a few…) I did a post called “Answer me these questions… five?” where I answered questions that people had asked me on social media. Its my turn again to do a post and I thought why not reprise this topic as it was quite fun the first time!
So, without further ado…
1) What is the Oort Cloud, does it exist, and if so, how do we know? --Martin
Excellent question! So, in a completely illogical fashion, I’ll answer the last part of your question first. The Oort Cloud is currently a theoretical idea – though one with a lot of backing in the community and is considered generally as a thing that’s very likely to exist. So, what is it? Well, it’s a roughly spherical cloud of small icy bodies (comets and comet-like things) and is thought to extend from ~20,000 astronomical units (1 astronomical unit is the average distance between Earth and the Sun) to 100,000 astronomical units. If astronomical units aren’t your bag, then that’s 0.3 to 1.6 light years (for reference Neptune sits at a lovely 0.00047322 light years from the Sun – or about 30 astronomical units - right now according to Google). For additional reference and to blow your mind just a little bit, Voyager 1 is not even close to reaching the Oort Cloud. It’s about 300 years away from the inner edge. And about 30,000 years from the outer edge. Yes, you read that right: 300 years before it gets to it and 30,000 years before it gets out the other side. So, the Oort Cloud is not only pretty far away, its also pretty darn thick.
I should also side-note here with a fun fact that the rest of the Solar System is pretty flat, like a disc. The planets mostly sit in the plane of the Solar System, as does the asteroid belt and the Kuiper belt. [Refresher as a Solar System Road Map: starting at the Sun and moving outwards, we have the Sun, Mercury, Venus, Earth, Mars, the asteroid belt, Jupiter, Saturn Uranus, Neptune, Kuiper Belt (sorry Pluto, I’m lumping you in with the Kuiper Belt Objects, don’t mind me while I duck and cover)].
Anyway, scientists think that the Oort Cloud is the origin of what we call “long period comets” also known as “comets that take more than 200 years to go around the Sun”. [Fun fact: the “short period comets”, or “comets that take less than 200 years to go around the Sun” are thought to come from the Kuiper Belt – that cluster of comet-like things and the odd minor planet out beyond Neptune] Back in the 1950s a scientist named Jan Hendrik Oort hypothesized the existence of what we now call the Oort Cloud of comet-like objects way out there because he noticed that comets had a few things in common. He noticed that they came from all directions rather than in from the plane of the Solar System (remember what I told you about the rest of the Solar System sitting – roughly – in a single plane like a disc?), and that the shapes of their orbits didn’t indicate that they were coming from interstellar space (the space between stars, or in this case our Sun and another star) – in fact their orbits seemed to cluster with the furthest point at 50,000 astronomical units (seem like a familiar kind of distance?). Oort then figured that, given the number and frequency of comets that there must be this big cloud of comet objects out there with the odd few getting knocked into the inner part of the Solar System (not physically knocked, I’m talking a gravitational knock – something else’s gravity from outside the Solar System disturbs it and sets it off on a path towards the inner part of the Solar System) and boom, you have comets that come in from all directions with the kinds of features we see.
As a nice visual to kind of round off this answer, the image below shows you the Oort Cloud on the right and the inner part of the Solar System (Sun to Kuiper Belt) expanded out. Hope that answers your question!
As always, here are some fun references I used to help me:
https://solarsystem.nasa.gov/solar-system/oort-cloud/in-depth/
https://www.esa.int/ESA_Multimedia/Images/2007/09/The_Oort_cloud
http://astronomy.swin.edu.au/cosmos/O/Oort+Cloud
Oort Cloud region of the Solar System (with the 8 planets and Kuiper Belt shown to orient you).
https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2014/12/kuiper_belt_and_oort_cloud_in_context/15106869-1-eng-GB/Kuiper_Belt_and_Oort_Cloud_in_context_pillars.jpg
2) What is the latest thinking about Saturn’s rings? How old and where did they come from and why Saturn? -- Alec
So, first thing… Fun fact: Saturn isn’t the only planet in our Solar system that has rings! In fact, all four of the giant planets (Jupiter, Saturn, Uranus, Neptune) have ring systems. Saturn’s is by far the largest and most spectacular – and also the only one that was known about until the 1970s (William Herschel claimed to have seen rings around Uranus back in the late 1700s though the community is split on whether that would have been possible at the time with the equipment available to him). Jupiter, Uranus, and Neptune have much smaller ring systems that are also darker so generally harder to detect so that’s why it took so long for them to be discovered. Uranus was the second planet discovered to have rings, though its rings are quite thin-stringy looking things so they’re pretty different to Saturn’s rings (which are, as you see in pictures, quite wide sheet-like things). Also, fun fact about Uranus, it orbits the Sun on its side so it’s rings are at 90 degrees to the rest of the plane of the Solar System (see the above answer for a side-jaunt into what the plane of the Solar System is). Jupiter’s rings were discovered in 1979 in Voyager 1 images and it wasn’t until 1989 that Voyager 2 flew past Neptune and saw its ring system there, completing the set of ringed giant planets.
Rings aren’t thought to be a permanent fixture of the planets though – they’re thought to be transient, so if you were to time travel forwards or backwards you might not necessarily see Saturn with the brightest or largest ring system. So how old are Saturn’s rings thought to be? 10-100 million years old (think: the time of the dinosaurs) according to some new work on data from the Cassini spacecraft, though there is some disagreement on that as before those results came out it was thought that the rings formed around the same time, or a little later than, the rest of the Solar System (about 4.6 billion years ago). So I’d probably say stay tuned on that one – I wouldn’t go all in on young or old just yet.
How are ring systems formed? Well standing theories include the pulverisation of asteroids and comets as they get too close to a giant system due to the giant planet’s gravity, and the remnants form these rings – the ring systems we see are made up of chunks (grain of sand to mountain -sized) of ice and rock. A different theory, particularly for Saturn, is that the rings (around a given planet) formed from a single violent event - like the destruction of one comet or moon. This could explain Saturn’s rings’ younger age if the dinosaur-age holds true. Essentially, we aren’t (to the best of my knowledge) completely sure yet, so stay tuned!
And here are some handy helpful references:
https://solarsystem.nasa.gov/planets/saturn/in-depth/
https://solarsystem.nasa.gov/news/813/nasas-cassini-data-show-saturns-rings-relatively-new/
https://www.newscientist.com/article/2106108-saturns-rings-may-be-from-the-whirl-of-a-passing-icy-rock/
http://coolcosmos.ipac.caltech.edu/ask/195-How-many-planets-in-the-Solar-System-have-rings-
So I hope that satisfactorily answered those questions! If you have any more, I’m happy to answer!
The rings of Uranus and Neptune: https://room.eu.com/images/contents/UranusNeptuneRings2.jpg
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