volume (7) pt 3
Hello Earthlings.
We have made it to the final installment of a stroll through our solar system. On we persevere to the outskirts(!).
Are you ready?
let’s go
volume (7.3):
Our home stellar system: part 3(!)
We are coming to a close on our tour through our home stellar system. We started at the sun, our home star which is solely responsible for the planets themselves as well as all of the elements required for us lil ol humans to exist and live on to understand grains of sands in the vast cosmic tapestry. We flew through the terrestrials (Mercury, Venus, Earth, Mars) and noted similarities between the seemingly different rocky worlds of the sun. We stopped at the gas giants, not before waving to the celestial graveyard lovingly known as the [Main] Asteroid Belt, observing some similarity in the gas giants’ icy moons and our terrestrial planets. And now we come to our final group of the tour - the outer planets (or the ice giants), the TNOs - transneptunian objects, and the edges of our very solar system itself.
Welcome to the [inner] outer reaches of the Solar System earthlings.
I hope you’re dressed warm enough(!)
Uranus ⛢ [7th planet from the sun]:
ice giant planet (made mostly of gas, with a core)
orbit averages 1.784 billion miles from the sun (159.6 light-minutes)
1 day on Uranus lasts just 17 Earth hours, 1 year on Uranus lasts 84 Earth years.
27x (known) moons
Robot visitors:
Voyager 2 in 1989, and only Voyager 2(!)
Uranus has been studied by plenty of ground based and space based telescopes but has never been orbited, and has only once been flown by decades ago.
Read the full series [our solar system]: Uranus - coming soon right HERE
Neptune ♆ [8th planet from the sun]:
ice giant planet (made mostly of gas, with a core)
orbit averages 2.793 billion miles from the sun (4.1 light-hours)
1 day on Neptune lasts just 16 Earth hours, 1 year on Neptune lasts 165 Earth years.
14x (known) moons
Robot Visitors:
Voyager 2 in 1989, and only Voyager 2(!)
Like Uranus, Neptune has been studied by plenty of ground based and space based telescopes but has never been orbited, and has only once been flown by over three decades ago.
Read the full series [our solar system]: Neptune - coming soon right HERE
NOW APPROACHING THE [inner] SHORES OF OUR SOLAR SYSTEM… THERE’S PLENTY TO SEE OUT HERE, DON’T BE SCARED!
Kuiper Belt Objects [in the space past Neptune]:
The Kuiper Belt is kind of like the asteroid belt but much much colder, icier and lies beyond the orbit of Neptune. It’s where Pluto lives(!). It’s also one of the least well explored areas of our Solar System. The Kuiper Belt is believed to be home to millions of small, icy objects that are relics from our early forming solar system. In addition to rock and water ice, Kuiper Belt Objects (KBOs also known as Trans-Neptunian Objects or TNOs) contain a variety of other frozen compounds like ammonia and methane.
The Kuiper Belt is estimated to reach from right past the orbit of Neptune, 30 AU or 2.78 billion miles (4.48 billion km) from the Sun, extending all the way out to 1,000 AU or 92.95 billion miles (149.59 billion km) from the Sun. That’s pretty darn far out, but would you even believe me if I told you that light from the Sun traveling out to the Kuiper Belt only takes 5.77 light days to reach its very furthest assumed edges at about 1,000 AU? Really, we haven’t even travelled a light-week yet, I told you don’t be scared! That puts the total length of the Kuiper belt itself at about 970 AU or 90.17 billion miles (145.11 billion km) or 5.6 light days. Like I said… one of the largest structures in the whole darn Solar System.
The first robot to visit the Kuiper Belt was Pioneer 10 (in 1983, when it crossed into the space beyond Neptune’s orbit). But that robot didn’t stop to study any of the celestial objects there. Most of what we know about the Kuiper Belt comes from ground based telescopes and the good ol’ Hubble Space Telescope. Voyager 2 briefly visited Neptune’s moon Triton (in 1989), and Cassini visited Saturn’s moon Phoebe (in 2004) both (moons) could be worlds originally from the Kuiper belt that have since escaped(!) or have been kidnapped(!) however you might want to tell yourself the story… *spoiler alert* it wasn’t just Jupiter’s big @$$ gravity this time(!) - it was all four of the giants dancing! Only one spacecraft has studied the Kuiper Belt. NASA’s New Horizons flew past Pluto in July of 2015; sending back the first clear, close-up images of the tiny world (and the now famous one right below of Pluto’s big old heart). Then in 2019, New Horizons flew by the Kuiper Belt object (486958) 2014 MU69 (known lovingly to the team that set New Horizons upon it as Ultima Thule) later named Arrokoth.
Without further ado, let me introduce you to the dwarf planets of the Kuiper Belt:
Pluto (♇):
classified scholarly as a dwarf planet
made of ice and rock
orbit averages 3.67 billion miles from the sun (5.5 light-hours)
1 day on Pluto lasts 6.4 Earth days, 1 year on Pluto lasts 248 Earth years.
5x (known) moons (Charon, Hydra, Kerberos, Nix, and Styx)
Robot Visitors:
New Horizons is the only robot to visit Pluto.
Eris:
classified scholarly as a dwarf planet
The first dwarf planet in fact. If you’re one of the people still mad about Pluto not being a proper planet anymore, blame Eris. Discovered in 2003, initially thought to be slightly bigger than Pluto, Eris was once going to be considered as the 10th planet of the solar system. Upon further examination it was determined that Eris was indeed smaller than Pluto and it also had its own moon(!) - now called Dysnomia. When Eris was first discovered the team responsible for the find referred to the object as Xena (as in the warrior princess on TV at the time, yes, truly) and they subsequently named its moon Gabriella after Xena’s sidekick… After many months of debate as to how to classify Eris the International Astronomical Union (IAU) announced that the object that will be called Eris shall be classified along with Pluto and Ceres (from the Asteroid Belt) as dwarf planets. And that’s how dwarf planets became a thing.
made of ice and rock
orbit averages 4.01 billion miles from the sun (9+ light-hours)
1 day on Eris lasts 25.9 hours, 1 year on Eris lasts 557 earth years
1x (known) moon
Haumea:
classified scholarly as a dwarf planet; Haumea is the first (and only) TNO/KBO to be found to have rings(!)
made of ice and rock
orbit averages 4.01 billion miles from the sun ( light-hours)
1 day on Haumea lasts just 4 hours (making it one of the fastest spinning objects in the solar system!), 1 year on Haumea lasts 285 earth years
2x (known) moons; Namaka and Hi'iaka
Makemake:
classified scholarly as a dwarf planet
made of ice and rock
orbit averages 4.253 billion miles from the sun (6 hours 20 light minutes)
1 day on Makemake lasts 22.48 hours, 1 year on Makemake lasts 305.34 earth years
1x (known) moon
OTHER KUIPER BELT OBJECTS…
As previously mentioned, there are oodles of TNOs/KBOs in the space past Neptune. Every few years now, some new even farther out object is observed. Other than dwarf planets, astronomers have broken up the TNO/KBO objects into these groups (so far):
Classical KBOs: [red circles in the image above] referred to as such due to their “classic” orbits (read: like the planets - almost circular and all lying within the same plane of space). There are two types of Classical KBOs; hot and cold. Cold Classical KBOs are the more authentically “classic” - their orbits are almost circular (keeping them roughly the same distance from the Sun at all points in their go-arounds) and they do not tilt away from the planetary plane. Hot Classical KBOs have more elliptical orbits (they do vary in their distance from the Sun in their go-arounds) and they do tilt away from the planetary plane.
Resonant KBOs: [white circles in the image above] these objects are in orbits that are tightly controlled by the biggie Neptune. There are several groupings (or resonances or ratios) - like 1:1, 4:3, 3:2, 2:1. Pluto, for example is in a 3:2 resonance with Neptune, meaning that Pluto goes around the Sun twice every time Neptune goes around the sun three times. In fact, there are so many of these 3:2 Resonant KBOs that astronomers have given them yet another more specific scholarly categorical name: the Plutinos.
Scattered Disk KBOs: [magenta circles in the image above] this region of the Kuiper belt is home to objects that have been scattered by Neptune into orbits that are highly elliptical and highly tilted on the planetary plane. Many Scattered Disk KBOs venture hundreds of AU away from the Sun and high above the plane of the planets (at their farthest points in their orbits) only to come back around to a closest point near the orbit of Neptune. The orbits of a lot of the stuff being flung around in the Scattered Disk are still evolving. Compared to the Classical region where KBO orbits are more stable, the Scattered Disk is a region of slow evolution. Eris is an example of an object in the scattered disk and is the largest known member of this population.
Centaurs: [orange triangles in the image above] though technically not occupying the space beyond Neptune (these objects orbit in the space between Jupiter and Neptune), Centaurs are considered to be former Kuiper Belt dwellers whose movements are entirely the effect of the two giants’ (Jupiter and Neptune) gravitational relevance. Almost the opposite of Scattered Disk KBOs whose orbits are also effected by the giant planets, Centaurs’ interaction brought them closer to the sun instead of further away. The Centaurs are also going through an evolution of change - most are likely to someday be ejected from the solar system or flung closer to the Sun; becoming a comet or perhaps crashing into a planet!
Detached KBOs: [cyan triangles in the image above] these objects have orbits that never bring them closer than about 40 AU to the Sun (Neptune, remember is about 30 AU from the Sun). Because their orbits don’t come anywhere close to Neptune’s, these objects seem unlikely to have been flung from the Kuiper Belt by the giant planet itself. Scientists believe that its likely some other force at play. Have you heard of the yet-to-be-directly-observed planet 9? Please go contribute to finding it! Perhaps it is the force of an undiscovered giant planet in a very distant orbit, maybe it’s the gravity of passing stars, or perhaps the gravitational perturbations as the Kuiper Belt was forming long ago. Sedna is an example of a detached KBO. The closest it comes to the Sun is 76 AU, but at its farthest it ventures out to about 1,200 AU.
NOW APPROACHING THE [outer] SHORES OF OUR SOLAR SYSTEM…
The Oort Cloud [an icy bubble all around us]
*over loudspeaker* This is the last stop, The Oort Cloud, I repeat, last stop, the Oort Cloud, please get off my spacebus! *static*
The Oort cloud: a roughly spherical “cloud” of small icy objects that are inferred to revolve around the Sun at distances estimated to range from 1,000 AU to 100,000 AU from the Sun (0.03 light-years to 1.6 light-years from the Sun). The Oort Cloud has been calculated to be the source of most of the historically observed long-period comets (think Hale-Bopp, Lovejoy, or the recent Neowise), whose orbits take them more than 200 years to complete. The objects of the Oort Cloud may number in the trillions, with a total mass of 10-100 times that of Earth, but the structure has never been observed directly.
Here is where I would have told you that the Oort Cloud should be considered the edge of our Solar System, because it is [only partially as it turns out] representative of the Sun’s sphere of influence (SOI)- in other words the extent of its gravitational relevance; the bubble of space inside which the Sun is the main force of gravity acting upon any objects. That’s what I would call the edge of our Solar System. But here’s the thing…
There is an equation which describes where the gravitational edges of two celestial bodies in space (one major body and one minor body - like the Sun and the planets) meet, this shape is known as the (gravitational) SOI. The edge of the SOI is where the gravitational strength of the minor body (in this case the Sun) is equal to the gravitational strength of the major body (the Milky Way galaxy’s central supermassive blackhole). In order to calculate the Sun’s actual SOI you would have to use the supermassive black hole (SMBH) at the center of the Milky Way as your major body in your two body SOI equation. LOL Ok! Sure thing! Let’s just do it right?
Thing is, not only do astronomers not know the exact mass of the SMBH that resides in the center of our galaxy (the existence of an SMBH in the center of our galaxy was only truly confirmed around 2008, and its mass is estimated to be ~4.1 million solar masses), that would also mean that the Sun’s SOI lives somewhere between here and the center of the Galaxy (not necessarily representative of the Oort Cloud, but perhaps still possible)… AND although mathematically the equation could be solved if given the SMBH mass, ignoring the effects of the 100s of thousands of other stars between here and there would just be foolish to talk about. The interaction between stars in their local neighborhood, although certainly all being acted upon too by the gravity of the SMBH, are probably influencing stellar structures - like say “exo-Oort Cloud(s)” should other stars, too, have such structures.
HOLD UP. SO WHERE(!!!) IS THE END OF THE SOLAR SYSTEM?(!)
I THOUGHT I GOT OFF AT THE LAST BUS STOP?(!)
HOW DID I END UP IN INTERSTELLAR SPACE?(!)
Didn’t she say don’t be alarmed, we’ve barely travelled any light time?
I need new glasses, but it looks to me as though the shores are fuzzy in the end. Perhaps like the waves crashing on a beach, solar systems may well ebb and flow into each other in rhythmic or chaotic ways yet to be illuminated by cosmic bioluminescence or captured by our robot explorers.
Cheers to the brainf*#k of conceptualizing the edges of stellar systems. Sorry I totally dropped you off in interstellar space, but I promise I’ll come get you in a light-week. While you wait, radio signal back in the comments your thoughts on what/where/why/who/when should be considered the end of the Solar System(!).
“The Solar System is off center, and consequently man is too…“ - Harlow Shapley