The Oort Cloud
Most comets we see originate from one place: the Oort Cloud. It’s a shell of billions to trillions of comets that encompasses the Solar System. In fact, it’s so big that Voyager 1 won’t reach it for 300 years (and the outer edge for 30,000 years), meaning it won’t have enough energy to send back data about it, leading also to no direct observations having ever been taken (or ever to be taken, with current technology) [1][5].
So how do we know it’s there? In 1950, Dutch astronomer Jan Oort theorised a cloud which extends 15 trillion kilometres from the Sun (over 1.5 light years!) because of a type of comet known as a ‘long-period’ comet: those that take thousands of years to orbit the Sun and which don’t come from the Kuiper belt (known as ‘short-period’ comets) which only extends 0.05% the distance of the Oort Cloud [2][3]. Oort noticed some striking features about these long-period comets: their orbits didn’t hint at an interstellar origin (so it’s in the Solar System), they came from all directions (so it’s a sphere), and the furthest point of their orbits (their aphelion) was around 50,000AU (7.5 trillion km) [4].
These long-period comets are arguably more exciting than short-period comets for one special reason: they’re usually pristine, formed at the beginning of the Solar System. “So was everything else in the Solar System” you might say, but unlike other objects orbiting the Sun, like short-period comets, objects in the Oort Cloud are made up of the fragments of planets ejected (many ejected into interstellar space) 4.6 billion years ago during their formation. Known as planetesimals, the ones that managed to keep locked on the Sun’s gravity gained eccentric elongated orbits[5]. Their orbits have then settled out into the Oort Cloud, only to be plucked from there by gravitational forces to become comets, possibly due to the pull of the Milky Way, or the hypothetical object called ‘Planet 9’, several times the size of Jupiter but never directly observed [1].
However, the important point to make is that, because these comets take so long to orbit and they spend so much time so far from the Sun, they barely get tarnished by the harsh solar wind and radiation that create those characteristic tails seen on comets, meaning they’re made up of material from the beginning of the Solar System as it was when it was ejected all those billions of years ago [6]. This can then shine a light onto the formation of the early Solar System, and the formation of life.
Diagram of the Oort Cloud showing how big it is in comparison with the orbits of Pluto and other Solar System Objects. Credit: NASA
An Ancient Messenger
One such object came to within naked-eye viewing distance of Earth back in early 1997: Comet Hale-Bopp. At 4.5 billion years old and with an origin in the Oort Cloud, it was comet Hale-Bopp’s very first close approach to the Sun (perihelion) in its lifetime, taking 2,533 years to do the full orbit (not yet complete) [7][6]. And the fact it’s its first journey is important, since this made Comet Hale-Bopp the most pristine comet ever found, having had very little in the way of solar wind or radiation.
How do we know? Using a technique called polarimetry, scientists could measure how polarised the light was (light oriented in one plane) which reflected off the coma (the ‘atmosphere’ of ice, gas and dust produced by the comet’s interaction with solar wind), thereby showing how smooth the coma is and so how much space weathering (solar wind, radiation etc.) the comet itself has experienced over its lifetime: the more polarised the light, the smoother the coma, the less chance the object has come close to a body emitting high levels of radiation before (like the Sun), the more pristine the object [6][9].
This meant that Hale-Bopp’s pristine coma could be observed using infrared spectroscopy (looking at what wavelengths of light has been removed by the coma) to study the make up of the comet, and so what was around in the early Solar System and where the comet originated from. Results showed it originated between the orbits of Jupiter and Neptune before being ejected into the Oort Cloud [7].
Comet C/1995 O1 (Hale-Bopp).
Credit: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria, CC BY-SA 3.0
Foreign Time Capsule
Comets don’t just come from the Oort Cloud and the Kuiper Belt however; they occasionally make the journey from a totally different planetary system. Discovered in mid-2019, Comet Borisov is the first comet of interstellar origins ever observed, known to be interstellar because of eccentricity: how elongated an object’s orbit is. An eccentricity of 0 is a perfect circle around the body it’s orbiting; an eccentricity of 1 is very elongated but not too much as to not orbit the object at its perihelion; and an eccentricity of greater than 1 is an orbit which is so elongated that the object will only be going around the body at the perihelion of its orbit once, meaning its origins are interstellar: Borisov had an eccentricity of more than 3 [8]!
However, that’s not the only secret Borisov had hidden up its sleeve: the light reflecting off its coma was recently found to be the most polarised ever observed. This means that it is the most pristine comet ever observed [9].
Put the two facts together and you get an untouched artefact from another planetary system emitting gas and dust which was collected when that planetary system was forming, thereby showing astronomers here on Earth what another planetary system’s early formation was like. Scientists have so far found Borisov’s coma is made up of compact pebbles around 2mm wide, as apposed to the comae from long period comets originating from this Solar System, which are made of irregular bits of material from 2mm wide to 1m wide.
This suggests that Borisov’s home planetary system witnessed large impacts which crushed matter into the small dense pebbles observed [10].
Comet C/2019 Q4 (Borisov). Credit: ESO/O. Hainaut, CC BY 4.0
An Interstellar Boomerang
Comet Borisov isn’t the only interstellar messenger we’ve come across though. There’s one more in the from of the asteroid ‘Oumuamua (Hawaiian for “a messenger from afar arriving first” [14]).
‘Oumuamua is an asteroid with a few differences: first off, its shape. ‘Oumuamua was observed with large fluctuations in its brightness over time, suggesting the asteroid’s length (800 metres) is up to 10 times its width. As well as this shape, the changes in brightness suggest it doesn’t rotate on one axis like most objects in the Solar System: it tumbles, completing 1 rotation every 7.3 hours by moving on 2 axes.
It’s also outgassing material: something usually seen on comets, leading it to be categorised as one at first. Although the asteroid was a long way from Earth, even from its closest approach (leading to observations of a small coma being unlikely), ‘Oumuamua was seen to be accelerating in a different way to the one expected (meaning earlier predictions of its trajectory were wrong), suggesting an outgassing of material. Despite this, it was reclassified as an asteroid since a coma was not visible, leading to Comet Borisov being the first interstellar comet to be observed.
Moreover, since ‘Oumuamua has no visible coma, it’s a lot dimmer than Borisov, meaning that after January 2018 it became invisible to all telescopes on our planet (although current predictions say its currently just past the orbit of Uranus [13]) [11][12].
Combined image of 1I/2017 U1 (`Oumuamua) (in blue) observed by the Very Large Telescope and Gemini South Telescope, with star trails smearing the background.
Credit: ESO/K. Meech et al., CC BY 4.0
Mission to… somewhere?
In 2029, ESA are going to launch a mission to investigate a “dynamically new comet or interstellar object” [15]. The only thing is the mission is yet to have a target.
This is because it’s part of ESA’s new fast-class in its Cosmic Vision Programme: a way of intercepting a pristine comet and studying it whilst it’s nearby. Normally lots of planning and preparation is needed to have a space mission, but time is simply not on scientists' side when it comes to fast interstellar and long period comets, taking a short time to whip around the Sun, and taking a very long time to come back again (sometimes never coming back), and by the time they’re back the surface is tarnished and pristine remnants of the early Solar System destroyed [16].
What comets turn into when they are damaged by solar wind and radiation: meteor showers (shooting stars) like this 4 hour exposure of the Leonid meteor shower in 1998 (the remnants of Comet Tempel–Tuttle.
Credit: Juraj Tóth/Astronomical and geophysical observatory Comenius University,
Such a high risk mission does come with very high reward, and with technology continually improving, we should expect to see and investigate more of these amazing pristine wonders of our Universe, and delve into the history of not just our own Solar System, but that of other planetary systems.
by George Abraham, ADAS member.
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Click here for the next news article
Click here to see where Comet Borisov is in its journey out of the Solar System, here to see where it is in our skies, and here for a video by ESO about the comet.
Click here to see where ‘Oumuamua is in its journey out of the Solar System, here to see where it is in our skies and here to watch ESO's video on the discovery of 'Oumuamua.
Click here to see where ‘Oumuamua is in its journey out of the Solar System and here to see where it is in our skies (not that it's visible!).
Click here to see where Hale-Bopp is in its journey to the Oort Cloud and here to see where it is in our skies.
References
"What is the Oort Cloud?" BBC Sky at Night Magazine. Archived from the original on 2nd April 2021.
"Oort Cloud: The Outer Solar System's Icy Shell". Space.com. Archived from the original on 2nd April 2021.
"Kuiper Belt". NASA Solar System Exploration. Archived from the original on 2nd April 2021.
"Oort Cloud" Swinburne Cosmos. Archived from the original on 2nd April 2021.
"Oort Cloud". NASA Solar System Exploration. Archived from the original on 2nd April 2021.
"First interstellar comet may be the most pristine ever found". ESO. Archived from the original on 2nd April 2021.
"Comet Hale-Bopp: the story of a visitor from the edge of the Solar System". BBC Sky at Night Magazine. Archived from the original on 2nd April 2021.
"Bizarre comet from another star system just spotted". National Geographic. Archived from the original on 2nd April 2021.
"Interstellar comet Borisov is the most pristine space object ever seen". New Scientist. Archived from the original on 2nd April 2021.
"Interstellar interloper 2I/Borisov may be the most pristine comet ever observed". Space.com. Archived from the original on 2nd April 2021.
"What We Know--And Don't Know--About 'Oumuamua". Solar System Exploration NASA. Archived from the original on 2nd April 2021.
"10 Things: Mysterious 'Oumuamua". NASA Solar System Exploration. Archived from the original on 2nd April 2021.
"'Oumuamua (A/2017 U1)". JPL Solar System Dynamics. Archived from the original on 2nd April 2021.
"ESA's New Mission to Intercept a Comet". ESA Cosmic Vision. Archived from the original on 2nd April 2021.