“In this circumplanetary disk somehow, there’s a lot of dust collected,” Dr. Facchini said. “And this dust coagulates fairly quickly and forms moons. And that’s the dust we saw in this measurement.”
“There are still challenges from a theoretical point of view, but what we know is that these moons should be forming at what we call ‘late stages,’ when the planet is not accreting too much gas anymore. And that’s exactly what we see with PDS 70c. It’s still accreting some gas, but not too much. We see the dust around it, so from what we know, this planet could be forming moons. And likely, it is.”
When looking at conditions for moon formation in a CPD, temperature is a big factor. In Benisty et al.’s paper, the team goes into depth on the estimated temperature inside PDS 70c’s CPD. So what could certain temperatures or temperature fluctuations in the CPD mean for moon formation?
“We think the temperature can have a big effect. The disk has to be fairly cold to be able to form moons. That’s why I said we’re in the late stage of the CPD evolution, meaning that it’s cold. If it’s too hot, the disk… the CPD wouldn’t be a disk; it would be basically a crazy hot sphere-like thing orbiting around the planet. And the pressure there would be so crazy high that it would really struggle to form any moon,” said Dr. Facchini.
“In that case, what would happen is that the accretion gas going onto the planet is very high. You have a lot of material falling in. The planet would release a lot of energy. So the potential energy of the material falling onto the planet would be basically converted into radiation and temperature. And this would heat up the CPD so much that it would be a hot, warm sphere.”
“So we need this temperature to be low enough that an actual disk can form around the planet. So these temperatures are important.”
Understanding this theory and where PDS 70c is in its development, the team was able to develop a general idea as to where current moon development is based on CPD conditions. Additionally, the team was able to gain an idea of what the moons’ chemical composition could be.
“So we think, now, we have passed the phase… the big sphere phase. We’re over it. That’s what we think. So again, the temperature is cold enough that we think we’re good,” Dr. Facchini said.
“Also the chemical composition of the moons would be affected by the temperature of the CPD. So that’s also an interesting aspect that we’re working on as a team, to study the chemical environment of the whole PDS 70 system.”
During accretion, cosmic dust, gas, and rock are pulled into a forming planet, galaxy, or star — allowing it to grow and increase its mass. When accreting, planets typically pull this cosmic dust material from their CPDs. Earlier this year, PDS 70b’s accretion growth rate was measure using UV light for the first time.
If PDS 70c is no longer undergoing rapid accretion, the material in its CPD could possibly begin to form moons. However, like most other observational situations such as these, it can’t be confirmed without evidence.
“That would be the final proof that there are moons being formed in there. So can we see it? The honest answer is that it is extremely challenging, and I’m not sure we’ll ever see a moon around PDS 70c,” said Dr. Facchini.
Zoomed-out image of the PDS 70 system and a close-up image of PDS 70c. The bright coloring surrounding PDS 70c is the dust in the planet’s CPD. (Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)
“Also because… if moons are forming, it may take, you know, hundreds of thousands of years to form, so we would need to wait quite a long time to see that happening.”
However, if PDS 70c were to fully stop accreting, would the dust, gas, and other cosmic material in PDS 70c’s CPD begin to form moons, or would the material freely float into space as gases subside?
“So we think that, theoretically, the formation of moons actually needs the gas.”
“What happens is… there is some weird hydrodynamic effect that leads these pebbles to form — what we call planetesimals. So there are big rocks of like 100 km in size. In the case of a CPD, these would be satellitesimals. And in order for this to happen, we need the gas because we need these hydrodynamic effects to collect the dust into the planetesimals.”
“So the gas is needed for what we think is happening.”
However, these pebbles could do something quite unexpected based on Benisty et al.’s models and theories.
“These dust pebbles, once they reach the CPD, the theory predicts they could spiral inwards very quickly onto the planet. So fast that it wouldn’t have any time to form anything,” Dr. Facchini said. “But still we see moons everywhere. Jupiter and Saturn have, like, 80 moons roughly. More if you sum them up.”
Artist’s illustration of PDS 70c’s sibling planet, PDS 70b, pulling material from its own CPD and accreting. (Credit: NASA, ESA, STScI, Joseph Olmsted (STScI))
“So we know that this process has to be very efficient. And we see a CPD around PDS 70c. That’s why it’s such an important discovery. We have the first observational proof that it exists.”
When planets like PDS 70b and c form, they can pull in matter from the circumstellar disk they formed from into their own circumplanetary disk. This process creates cavities, or gaps, in the star’s circumstellar disk where the planet is orbiting.
This same idea can be applied to moon or satellitesimal formation inside of a planet’s CPD.
“We think that if a moon is massive enough, it will create a cavity,” said Dr. Facchini. “And that’s something we see now with recent observations. We see it very often around protoplanetary disks. We think that when we see a lot of them, we call them annuli. So there are concentric rings and gaps.”
Knowing what planetary bodies can do to circumstellar disks, astronomers believe that a large majority of gaps in disks are likely caused by planets. “What we think is that at least some of them, but probably the large majority of them, are caused by embedded planets that create these rings and gaps in the density structure. We think that exactly the same would happen with a moon around a planet.”
Additionally, if a moon or satellitesimal was to form, it would likely push the CPD ring out farther than the moon’s orbit.
“What people suggest is that, for example, once you form the first moon in the system, and the moon is orbiting, you create rings outside its orbit. And that helps with retaining a lot of dust. So maybe the dust we’re seeing with this new image, if you could zoom in even better, maybe 100 years from now we’ll have some facility able to do it, maybe we would be able to see rings and gaps and count how many exomoons are around the planet.”
This image of star HL Tauri shows the star’s expansive circumstellar disk. The small gaps and bands in the disk are likely due to forming planets. (Credit: ALMA (ESO/NAOJ/NRAO))
However, knowing that planets can create significant gaps in a star’s disk, would the added mass, or presence, of a moon around a planet increase the gap in the star’s circumstellar disk? “The answer is yes, in the sense that these planets had to start from somewhere,” Dr. Facchini said. “They didn’t have a lot of mass in the beginning.”
“You start with sand-like dust and very low-density gas. So, at some point, you have a seed of a few Earth masses of rock, probably. And then if you accrete a lot of gas, you create a giant planet, like [PDS 70c]. If you keep adding mass to the system, to the planet itself, or with a moon or with different moons or with the CPD even… as the planet plus moons plus CPD system grows in mass, this has an effect on the circumstellar disk.”
To this end, while the formation, sustainability, and long-term presence of a moon around PDS 70c is a subject that requires needs further study, the known presence of a CPD around the exoplanet helps push the idea of moon-formation forward.
Benisty et al.’s research was published in The Astrophysical Journal Letters in July 2021.
(Lead image: wide and close-up views of PDS 70c’s moon-forming disc as seen with ALMA. Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.)