Astronomers discovered a new ring system around the dwarf
planet Quaoar on the edge of the solar system.
The rings were discovered by accident
as the planet passed in front of a star during an occultation event.
But this ring system is really weird because it orbits much
further out from its planet than was thought possible.
So this discovery may just overturn over 200 years
of our understanding of how rings can form around planets.
The discovery was announced by a team of astronomers
led by Bruno Morgado from the federal University
of Rio de Janeiro in the February four issue of Nature It's
not a discovery of something that anybody seriously thinks is impossible.
Rather, it's an opportunity to learn something new.
So let's learn about what we know so far.
The dwarf planet Quaoar was discovered way back in 2002 by Chad
Trujillo and Mike Brown, and they found the object orbiting
at a distance of 43 astronomical units from the Sun.
That's 43 times further from the Sun than Earth.
And that puts it firmly in the Kuiper Belt, which is a region of small,
icy bodies that orbit out past Neptune.
The dwarf planet is well, it's dwarf.
At about 1100 kilometers in diameter, it's about half the size of Pluto.
It was given the name Quaoar after the deity
of the indigenous Tongva people of Southern California.
In 2007, Mike Brown discovered a satellite of Quaoar
that was dubbed Weywot, named after the deity’s son.
In 2021, Morgado and collaborators used the 10.4 meter Gran
Telescopio CANARIAS to probe Quaoar’s atmosphere during a stellar occultation.
The idea is that as the planet passes in front of the star,
the star's light dims a little bit at first
as it passes through the planet's atmosphere,
and then it finally disappears as the main body of the planet blocks the star.
By analyzing the rate at which starlight dims
and by comparing the dimming in different filters, you can work out things
like the atmosphere's density, pressure, and composition.
Quaoar is small, so the whole occultation lasted for less than a minute.
But when they went back and looked at the light curve,
they noticed that there were these two tiny dips in the starlight
just before and after the occultation.
The best way to explain those dips are that Quaoar has a ring!
Even better, they went back and inspected the data from previous occultations
going back to 2018, and sure enough they found that there were those
same pre- and post-occultation dips in those light curves as well!
That makes Quaoar only the third small solar system body
to have a ring discovered around it.
The first occurred back in 2013 when the center asteroid
Chariklo occulted a star.
The light curve showed that there were actually two dips,
both occurring before and after the main occultation.
That means Chariklo actually has two rings,
one that's relatively dense and the other much thinner.
In 2022, the James Webb Space Telescope detected the rings again
in another occultation, and this time the asteroid passed
just underneath Webb's line of sight to the star.
So it didn't see a main occultation, but
it could still detect the tiny dips caused by the rings.
Another ring system was discovered around the dwarf planet Haumea
in 2017, during another stellar occultation.
So rings around small solar system bodies aren't necessarily new,
although it's still pretty amazeballs that small bodies
like these can actually have rings.
But even then, Quaoar’s ring system turns out to be really strange because it
orbits way further out from its planet than was previously thought possible.
You see, rings are thought to form when a small body
- like a moon - gets too close to its planet.
At some critical distance from the planet, tidal forces exerted on the moon
become so strong that the moon gets ripped apart and forms a ring.
And this critical distance from the planet was first calculated
by French astronomer Edouard Roche in 1848.
Now, exactly what distance the Roche limit is depends on how dense
and how rigid the moon is relative to its planet.
But for a typical moon around a planet, the Roche limit typically
works out to a distance of between two and four planetary radii.
Quaoar’s ring, on the other hand, lies more than seven radii from the planet!
And rings outside the Roche limit aren't impossible per se,
but they should be unstable.
At that distance, Ttdal forces from the planet
just aren't strong enough to keep the ring shredded, so clumpy regions in the ring
will eventually form and start to attract other ring particles.
Those particles build into a larger clumps, which keep attracting
more ring particles until it eventually forms a new moon.
And it doesn't take very long to form a moon this way either.
A low mass ring like this should form a moon and maybe just a few weeks.
The odds of catching a ring that's just a few days or weeks old
are vanishingly small.
And yet there it is, seemingly present since the first occultation in 2018.
So what's going on?
Well there are, in fact, very tenuous rings around Saturn
that exist beyond its Roche limit.
Saturn's E ring consists of tiny particles
that are expelled from the geysers of the moon Enceladus.
Most of the E ring material is either melted away by sunlight
or accumulates on the moons that orbit within it.
The only reason the ring is still there is because it's constantly
being replenished by Enceladus’ geysers.
That doesn't seem to be the case with Quaoar’s ring, though.
The ring blocks out just enough starlight to suggest it's densely populated with
icy boulders up to kilometers in size rather than diffuse microscopic particles.
So this seems to be a proper dense ring that's in the wrong place.
And right now, it's not clear how you can get such
a dense ring to remain stable outside the Roche limit.
Any external perturbations should cause the ring to clump up in places
which would lead to accretion, which would form a moon.
But there are some possible clues here.
One is that the ring may be located near two possible regions
where the particles can enter into one of two resonances
with either Quaoar or its moon Weywot.Tresonances are periodic alignments
between celestial bodies such that they exert regular, periodic
gravitational tugs on each other at repeating intervals.
Because the alignment always repeats when the bodies return to the same point
in the orbit, the gravitational tug is always in the same direction.
Resonances do play a role in maintaining ring systems.
For example, let's say there's a particle near
the inner edge of the Cassini division in Saturn's rings.
When the particle nears the inner edge, it enters
into a 2 to 1 resonance with its moon Mimas.
And that means the particle completes two orbits for every single orbit of Mimas.
With each pass, Mimas pulls the particle closer
until it eventually crosses the Cassini division.
At that point, the particle falls out of resonance with Mimas and
takes up a new orbit in the outer ring. Quaoar’s ring
seems to
lie near the inner 6 to 1 resonance with Weywot.
So in that case, the ring would complete six orbits
around Quaoar for every single orbit of Weywot.
The ring is also located near
the distance where the orbital period of the ring body
is three times longer than the rotational period of the planet.
This is called a “Spin-Orbit Resonance”.
The idea goes like this: Quaoar is a small body
and therefore probably has some kind of an irregular shape.
If so, then one side of the planet could exert a little extra gravitational
tug on the ring particles as it swings around every three orbits.
And this same 3 to 1 coincidence
has also been found in the rings of Chariklo and Haumea;
they're both in a 3 to 1 spin orbit resonance with their host planets.
So the ring may be in just the right location where either
or both of these resonances perturb the ring system
just enough to prevent them from accreting and forming a moon.
But those resonances should be taken with a grain of salt because we don't yet
know enough about Quaoar’s true shape and we don't even know
enough about Weywot’s orbit to say for sure.
So we can't say exactly where these resonances really are
in the Quaoar system, let alone whether or not they could be
enough to maintain the ring this far from the planet.
Another thing is that the two dips in the light curve caused by
the rings are roughly the same depth, but they have different durations.
And this suggests that the rings are clumpy;
they must be more narrow on one side and more spread out on the other.
It's reminiscent of the arcs in Neptune's Adams ring
or the clumps in Saturn's F ring.
The light curves indicate that Quaoar’s ring is less than ten
kilometers wide at its most narrow point,
and that's about one ten-thousandth of the ring’s diameter.
Collisions in this narrow region should cause the ring to spread
rapidly in the radial direction and possibly dissipate.
It's very unlikely, though, that we would just happen
to discover a ring that's about to disappear.
So more likely,
there has got to be some other mechanism that's keeping the ring so narrow.
In the 1980s, astronomers at the Lick Observatory
conducted some laboratory experiments to understand
how ice particles in Saturn's rings might behave when they collide.
They found that the colder the particles were, the harder
the ice, and therefore the more elastic the collisions were.
Elastic collisions are those that bounce apart,
much like the way steel balls do when they collide.
On the other hand, inelastic collisions tend to stick together,
much like the way to led balls will after they collide.
The ring particles should be made of ice, and at more
than four times Saturn's distance from the Sun,
should be frozen hard as the hardest granite here on Earth.
Given the ring's uneven shape and likely hardness of its particles, the team ran
some n-body simulations to work out how they might behave outside the Roche limit.
And they found that if the particles collided too slowly,
their mutual gravity would hold them together, and that would
allow them to eventually attract other particles and form a moon.
But if the particles collided faster, their collisions would be elastic enough
to bounce them apart before gravity had time to hold anything together.
But the simulations also suggest that if the collisions were too elastic,
then the particles would spread
so far apart from each other that the ring dissipates completely.
But again, the chances we just happened to discover a ring
that's about to disappear like this seems very unlikely.
Chances are that the real mechanism that keeps Quaoar’s ring
stable has got to be a lot more complex.
It may involve one or both of these resonances between Quaoar and Weywot,
or perhaps there are some additional satellites at play there
that we just haven't yet found.
Or maybe it's just something else entirely.
Obviously, there's a lot more observations that are going to be needed
to test out these ideas.
The good news is that there happen to be some more occultations coming up;
two in May and one in September of 2023.
Those occultations will help to further narrow down Quaoar’s
shape and further characterize the rings.
As a matter of fact, the James Webb Space Telescope just made a really nice
measurement of asteroid Chariklo‘s rings in its first year of operations.
Hopefully it'll be used to monitor Quaoar’s
rings in a future occultation during Cycle 2.
Webb's Cycle 2 observations should be getting started this summer, so
right in time for that September occultation.
That would be great if it could happen because
there's so much more to be learned.
But if nothing else, Quaoar demonstrates that the classical Roche limit
may not be the only way to maintain a ring system.
My thanks as always to the generous supporters of Launch Pad Astronomy
on my Patreon.
And I want to just take a moment and say,
sorry it's been a while since I've made a video.
I got a little sick this past January and I spent most of the month on the couch.
What can I say?
It happens. But I'm back and I'm glad to be doing this stuff.
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Until next time, stay curious, my friend.