SHOCKING: Astronomers Detect Dusty Disk Around Exoplanet Forming a Moon
Seth Pincock
For the first time, astronomers have detected a ring of dust circling around another planet—in
an entirely different solar system, 2 quadrillion miles away.
The planet Saturn in our own solar system is well-known for its brilliant system of
rings.
These rings are made up of billions of chunks of ice, rock, and dust.
Although they are nowhere near as big or bright, the other gas giant planets in the solar system—Jupiter,
Uranus, and Neptune—are also encircled by their own systems of dusty rings.
But Saturn’s rings are very different from the recently discovered ring around planet
PDS 70c.
This new discovery could offer some clues as to what happened during the very early
history of our solar system.
The solar system we live in is very old.
The Sun, the planets, moons, and asteroids have been here for billions of years—four
and a half billion years, to be precise—and will be here for billions of years more, and
aren’t expected to change all that much.
The story is very different for PDS 70, a star 370 light-years away in the constellation
Centaurus.
This star is very young—so young, in fact, that it isn’t even a proper star yet.
PDS 70 is a part of a class of stars known as T Tauri stars.
Stars like our Sun are called main-sequence stars.
Our Sun’s core is so incredibly hot and dense that it is able to compress hydrogen
atoms together to create helium atoms, in a process called nuclear fusion.
Every time hydrogen is fused together into helium, some energy is released.
This highly-efficient fusion furnace in the Sun’s core is what has kept it burning bright
for billions of years.
But a T Tauri star is still in the infant stages of its evolution.
It is not able to produce nuclear fusion in its core—not yet.
Every star begins the same way—as a cloud of gas and dust in space, known as a nebula.
Eventually, gravity begins to pull that cloud together, compressing the gas into a tight
sphere.
As compression continues, it grows hotter and hotter until it grows bright, almost like
a star.
This is a fledgling T Tauri star.
They are hot and bright, but the core is not yet hot enough nor dense enough to crush hydrogen
atoms together.
Before fusion begins, T Tauri stars are also very unstable.
T Tauri stars are known to vary in brightness quickly and chaotically, periodically giving
off sudden bursts of powerful solar wind.
PDS 70 is still in this violent, formational phase.
It is only an estimated 5.4 million years old.
That may sound like a very long time by human standards, but it is practically the blink
of an eye in astronomical terms.
PDS 70 has only been around for 0.1% of the time that the Earth has.
By the time PDS 70 formed, the dinosaurs had already been dead for 60 million years, and
the first early ancestors of humans had begun to evolve.
PDS 70 is so young that it is still in the process of forming its planetary companions.
After a T Tauri star collapses down from its stellar nursery, some of the leftover gas
and dust will collect into a wide, flat ring, known as a circumstellar disk.
The disk is the source of material for new planet formation.
Gravity will gather the gas and dust together into small clumps, which will grow and grow,
gathering more material like a snowball rolling down a hill.
Over the course of several million years, those clumps, or planetesimals, will eventually
grow to the size of entire planets.
A circumstellar disk has been observed in orbit around PDS 70 since 2006.
The disk is around 140 times wider than the Earth’s orbit around the Sun.
Astronomers have also observed some strange gaps in PDS 70’s circumstellar disk—in
2018, they found the culprit.
The dust in that gap had been swept up to form a Jupiter-sized planet: PDS 70b.
The next year, an even larger planet, PDS 70c, was also detected.
PDS 70c is around 4 and a half times more massive than Jupiter, and circles its small,
orange sun in a wide, 190-year orbit.
But more exciting than the discovery of two new planets is what Dr. Myriam Benisty’s
team at the Atacama Large Millimeter Array, or ALMA, has discovered in the space around
planet PDS 70c: its own miniature ring of dust.
This is exciting for two reasons.
One: it means that the planet is still growing.
It is adding more mass to itself little by little every day as its gravity sucks in more
of the surrounding gas and dust.
And two: this dusty ring is evidence of the ongoing formation of moons.
The gas giants in our solar system, like Jupiter and Saturn, are the parents to a few dozen
moons each.
Jupiter alone has 79 known moons, the largest of which is even larger than the planet Mercury.
How does a planet collect so many companions?
Gravity is certainly a factor.
Jupiter is the largest planet in the solar system and, with a mass 300 times that of
Earth, has a very strong gravitational pull—strong enough to hold on to plenty of moons.
But there must be another factor at play as well.
Looking at the orbits of Jupiter’s largest moons, they all lie in the same plane, precisely
aligned with Jupiter’s equator.
The most logical explanation is that all these moons formed there.
The prevailing theory has been that, during Jupiter’s formation from the circumstellar
disk, a similar circumplanetary disk formed around the planet.
It was out of this circumplanetary disk that Jupiter’s moons were created.
For the very first time, we have concrete evidence to support this theory.
The PDS 70 system has been described by Dr. Sabastiaan Haffert of the University of Arizona
as “the textbook example of planet formation.”
Dr. Jason Wang of CalTech said “We know so little about satellite formation.
[PDS 70c] is certainly a stepping stone in understanding when and how efficiently satellites
form around planets.”
So, since the planet Saturn is surrounded by a ring of dust, does that mean it is going
to form a new moon?
The short answer is: probably not.
In fact, it’s probably the opposite.
Saturn has not had its famous rings for very long—probably only the past 100 million
years.
It likely gained these rings when one of its moons strayed too close to the planet and
was ripped apart by tidal forces.
The debris that forms Saturn’s rings still orbits too close to the planet for it to gather
itself back together to form another moon.
Instead, all of that debris is slowly falling down onto the planet.
In the next few hundred million years, Saturn’s rings may become a thing of the past.
Unfortunately, this means that there is no suitable analog for a circumplanetary disk
to study in our own solar system.
Because of that, and because the PDS 70 planetary system is so distant, much of it remains clouded
in mystery.
There are still a lot of unanswered questions about PDS 70c’s circumplanetary disk.
Astronomers aren’t quite sure what the disk is made of, how big it is, or even how massive.
Dr. Benisty’s team has made some rough estimates to try to get a better idea of what is happening
inside the system.
They guess that the disk might be as wide as Earth’s orbit, and contain about three
times the mass of Earth’s moon.
Their estimated mass, if it’s correct, is actually surprisingly small, especially compared
to the mass of the planet itself.
Such a small circumplanetary disk-to-planet mass ratio would indicate that the planet
has already used up most of the material—it is nearing the end of the formation process.
While PDS 70c is probably still growing, it is very slow, and it probably won’t grow
much larger than it already is.
The same goes for whatever moons might be growing within the dusty circumplanetary ring.
The hidden moons are most likely close to being fully-formed.
In a few million more years, the dust will all have been collected by these moons or
have fallen down onto the planet itself.
Given planet PDS 70c’s massive size, the moons could be the size of some planets in
our solar system—potentially even larger than than the planet Mars.
But planet c is not the only planet in the PDS 70 system.
PDS 70 b and c orbit together in a nearly two-to-one orbital resonance, meaning that
for every orbit that planet c makes, planet b makes 2.
But, unlike its companion, PDS 70b does not show any evidence of a circumplanetary disk.
This could simply indicate that it is finished building all its moons.
Another possibility is that its dust ring was stolen, either by the stronger gravitational
influence of the star or by planet c.
Since planet c is farther from the star than planet b, it might capture most of the infalling
dust from the outer regions of the solar system, only leaving a small stream of dust to pass
through for planet b to use.
Whatever the case may be, if there are any moons in orbit around the planets of system
PDS 70, it will be impossible to detect them with our current technology.
It will likely be several more years before the first announcement of the discovery of
an exomoon.
Still, the discovery of this circumplanetary disk is a landmark achievement in the history
of exoplanetary science.
Studying the PDS 70 system is akin to looking back at a time when our own solar system was
still in its infancy.
By learning more about this distant, newborn planetary system, we might be able to fill
in some unanswered questions and create a more complete picture of how planetary systems
across the cosmos come to be.