Our solar system is quite large with a diameter of about 200,000 AU or roughly three light-years across. We have a star, the Sun, at our center and a collection of rocky and gas planets orbiting it. Beyond our furthest planet Neptune is the Oort Cloud, but the distance between is tremendous. So what objects do we find in-between?
Subscribe and join us weekly for astronomical facts and interesting science.
If you have questions you would like answered on Astro Brief, email them to Dr. Mike Reed at mikereed@missouristate.edu.
Astro Brief is a podcast collaboration between KSMU, the Missouri Space Grant, and MSU's Department of Physics, Astronomy and Materials Science. Hosted by Dr. Mike Reed, Astro Brief focuses on astronomical events, the field of astronomy, and astronomy-related guests. It airs Thursdays at 9:45 am on KSMU.
Transcript
In this episode we're going to talk about trans-Neptunian objects and some recent observations of Quaoar's moon, Wei Wot.
But first, what are trans-Neptunian objects, or TNOs? To make it even more confusing, how do TNOs compare to Kuiper Belt objects or KBOs, or compare to scattered disked objects or SDOs, and then what are Centaurs and Ort Cloud Objects?
Phew, let's define all these things.
A lot of this is based on distance and orbital shape. Our solar system is a disk on the inside and roughly a sphere at the extreme outside. The disk gets fatter the farther out from the sun you get and is somewhat defined by two main objects — Jupiter and Neptune. Jupiter has more than twice the mass of all other planets put together and so its gravity is important for small objects in our outer solar system. Neptune is the farthest major planet we know of. Jupiter is at 5.2 AU — where 1 AU is the Earth-Sun distance — and Neptune is at 30 AU.
Any object that orbits between Jupiter and Neptune is considered a Centaur, the largest of which is Chariklo at just under 190 miles in diameter. So Centaurs are pretty small and are roughly rocky icy asteroids. It's expected that most Centaurs will have their orbits affected by one of our gas planets within some millions of years. In the long run, that means they are temporary, though new ones make their way into that area so it's not going to run out.
Next is the Kuiper Belt, and basically this is just the asteroid belt, but beyond Neptune, out to about 50 AU. As its name implies, it's a belt — so flattened like the plane of our solar system — but a bit fatter because it's farther away. Pluto is an example. Kuiper Belt objects that get scattered into an orbit out of the plane of the belt become a scattered disk object. The dwarf planet Eris is a great example with an orbit that's inclined 44 degrees out of the ecliptic plane.
The Oort Cloud is a roughly spherical halo of asteroids and comets ranging from about 2,000 to 200,000 AU, basically a quart of the way to our nearest neighbor star. We really only detect these via long period comets.
A trans-Neptunian object is anything beyond the orbit of Neptune, which also includes Kuiper Belt, scattered disk, and Oort Cloud objects, so it's quite a broad term.
In my mind, when I think of TNOs, I tend to think of small asteroid-like objects with a mixture of rock and ice. Just single, small, non-round crater things like you might see in the movies, very distant from any other objects because space is huge out there.
But in fact, they're much more interesting than that!
While they're quite small and very distant, at least 149 of them have moons, sometimes so large that really they're more like binary systems. The dwarf planet Haumea and TNO Lempo both have two moons, and the dwarf planets, and also TNOs Haumea and Quaoar, both have rings, as does the Centaur Chariklo.
A 2023 paper by Dr. C.L. Pereira of the National Observatory of Brazil and collaborators, published in Astronomy and Astrophysics, describes the two rings around Quaoar. Quaoar itself is just 680 miles across, and it has a ring that is only 6 miles across — about 1,600 miles from Quaoar — and a second ring that's about 37 miles wide and 2,500 miles from Quaoar. Way more interesting than just single rocks in space, but those are really thin orbiting a small object at a huge distance from us.
How do we get such good information?
That leads us to a more recent article by Dr. Estela Fernandez-Valenzuela of the Florida Space Institute and collaborators, published in the Astrophysical Journal Letters. They use stellar occultations to measure the size of Quaoar's moon, Weywot. Stellar occultations is when the object goes between us and a star, temporarily blocking the star's light. When many observatories along the predicted path look for the occultation, it can be a very powerful method. That's really how most properties of TNOs are measured.
The group predicted three occultations for Weywot, but only successfully measured one, yet by using the occultation they did measure, they could update Weywot's orbit to understand how they missed the other two.
Weywot is about 93 miles in diameter, and what's really interesting is that it's made of really, really dark material, only reflecting about 2-8% of the light reaching it. Quaoar is also dark, reflecting only about 11% of the incoming light, but their difference is enough to suggest they did not form together. One idea is that Weywot impacted Quaoar, but more of a skimming impact that did not destroy Weywot, but did throw up enough material to form the rings.
Who knew the outer solar system could be so interesting?