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The Case of PDS 70 and a Moon-forming Disk

The things we look for around other stars do not necessarily surprise us. I think most astronomers were thinking we’d find planets around a lot of stars when the Kepler mission began its work. The question was how many — Kepler was to give us a statistical measurement on the planet population within its field of stars, and it succeeded brilliantly. These days it seems clear that we can find planets around most stars, in all kinds of sizes and orbits, as we continue to seek an Earth 2.0..

The continuing news about the star PDS 70, a young T Tauri star about 400 light years away in Centaurus, fits the same mold. Here we’re talking not just about planets but their moons. No exomoons have been confirmed, but there seems no reason to assume we won’t begin to find them — surely the process of forming moons is as universal as that of planet formation. The interest is in the observation, how it is made, and what it implies about our ability to move forward in characterizing planetary systems.

The process takes time, and results can be ambiguous. Back in 2019, PDS 70 was the subject of work performed at Monash University (Australia), led by Valentin Christiaens. The story received an exomoon splash in the press: The researchers believed they were looking at a circumplanetary disk around one of two gas giants forming in this system (see Exoplanet Moons in Formation?).

Everything pointed to a moon-forming disk around one of two young gas giants in the system, though the conclusion could only be considered tentative. I want to mention this because work from the European Southern Observatory that we’ll discuss today also finds a circumplanetary disk at PDS 70, though not around the same still-forming planet examined in the Christiaens et al. study. What an intriguing system this is!

While Christiaens and team looked at PDS 70b, the ESO work examines new high-resolution images of the second gas giant, PDS 70c, using data obtained through the Atacama Large Millimetre/submillimetre Array (ALMA). Led by Myriam Benisty (University of Grenoble and University of Chile), the international team now declares the detection of a circumplanetary disk — though not yet a moon — unambiguous. Says Benisty:

“Our work presents a clear detection of a disk in which satellites could be forming. Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with the planet and we are able to constrain its size for the first time.”

Image: This image shows wide (left) and close-up (right) views of the moon-forming disk surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away. The close-up view shows PDS 70c and its circumplanetary disk center-front, with the larger circumstellar ring-like disk taking up most of the right-hand side of the image. The star PDS 70 is at the center of the wide-view image on the left. Two planets have been found in the system, PDS 70c and PDS 70b, the latter not being visible in this image. They have carved a cavity in the circumstellar disk as they gobbled up material from the disk itself, growing in size. In this process, PDS 70c acquired its own circumplanetary disk, which contributes to the growth of the planet and where moons can form. This disk is as large as the Sun-Earth distance and has enough mass to form up to three satellites the size of the Moon. Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

The high-resolution data allow Benisty and team to state that the circumplanetary disk has a diameter of about 1 AU, with enough mass to form up to three moons the size of our own Moon. The planetary system forming around this star is reminiscent of the Jupiter and Saturn configuration in our own Solar System, though notice the size differential. The disk around PDS 70c is 500 times larger than Saturn’s rings. The two planets are also at much larger distances from the host star, and appear to be migrating inward. We are seeing the system in the process of formation, which should offer insights into how not just moons but planets themselves form around infant stars.

Interestingly, the second world here, PDS 70b, does not show evidence of a circumplanetary disk in the ALMA data. One supposition is that it is being starved of dusty material by PDS 70c, although other mechanisms are possible. Here’s a bit more on this from the paper, noting an apparent transport mechanism between disk and forming planet:

These ALMA observations shed new light on the origin of the mm emission close to planet b. The emission is diffuse with a low surface brightness and is suggestive of a streamer of material connecting the planets to the inner disk, providing insights into the transport of material through a cavity generated by two massive planets.

And as to PDS 70b:

The non-detection of a point source around PDS 70 b indicates a smaller and/or less massive CPD [circumplanetary disk] around planet b as compared to planet c, due to the filtering of dust grains by planet c preventing large amount of dust to leak through the cavity, or that the nature of the two CPDs differ. We also detect a faint inner disk emission that could be reproduced with small 1 µm dust grains, and resolve the outer disk into two substructures (a bright ring and an inner shoulder).

The Monash University team in Australia was able to image PDS 70b in the infrared and, like the ESO astronomers, was able to find a spiral arm seeming to feed a circumplanetary disk, while making the case for PDS 70b as the world with the disk. Remember that the two teams were working with different instrumentation and at different wavelengths — the Monash researchers operated at infrared wavelengths to analyze the spectrum of the planet produced by SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) at the Very Large Telescope in Chile. The ESO team used data from ALMA.

So do we have one or two circumplanetary disks in this system? We’ll see how this is resolved as the investigation of the planets around PDS 70 continues through a variety of instruments. For the importance of the system is clear, as the Benisty paper argues:

Detailed studies of the circumplanetary disks, and of the leakage of material through the cavity, will provide strong constraints on the formation of satellites around gas giants, and on the ability to provide the mass reservoir needed to form terrestrial planets in the inner regions of the disk. Upcoming studies of the gas kinematics and chemistry of PDS 70 will complement the view provided by this work, serving as a benchmark for models of satellite formation, planet-disk interactions and delivery of chemically enriched material to planetary atmospheres.

The paper is Benisty et al., “A Circumplanetary Disk around PDS70c,” Astrophysical Journal Letters Vol. 916, No. 1 (22 July 2021). Abstract.

{ 6 comments… add one }
  • James Essig July 28, 2021, 13:00

    The formation of Moons is a fun thing to contemplate.

    There are about 10 EXP 24 stars in the observable portion of our universe which theoretically seems to be a tiny portion of our universe. This is about as many stars as the number of grains of fine table sugar that would cover the entire United States 100 meters deep. The number of planets orbiting stars in our cosmic light-cone is about perhaps 10 times greater yet or equal to the number of fine grains of table sugar that would cover the entire United States 1,000 meters deep. Assuming our solar system is typical in moon count, the number of moons orbiting planets in our cosmic light-cone is roughly ten times greater yet or about equal to the number of fine grains of table sugar that would cover the entire United States 10,000 meters deep.

    Can you imagine that! Consider flying over the entire continental United States and looking down from a Boeing 747 flying at typical 37,000 feet and just a kilometer below you see a white covering which goes on hour after hour for 5 hours as you fly across the United States at 600 miles per hour.

    As much as I am a fan and theorist on light-speed inertial space travel, and also extreme sub-light-speed travel with extraordinarily great Lorentz factors, I become awed with I realize that simply achieving a velocity of about 0.866 c and a Lorentz factor of two could carry us to most of these worlds yet still remain within a common light-cone. Obviously, cryogenic sleep or a sustainable world ship would help at 0.866 c ; but, I see no reason why this cannot at some point be accomplished.

    Planetary moons to me have great allure ever since I saw the movie Avatar.

    Studying the formation of moons is an important part of our cosmic future.

    • Michael Fidler July 29, 2021, 9:18

      Do not forget all the rogue planets with moons and planets around brown dwarfs which would make the sugar 100 kilometers deep.

      • James Essig July 29, 2021, 11:19

        That is true, Definitely lots of worlds to set up shop. Regarding our own moon right here near Earth. When NASA’s Artemis Program hopefully returns in 2024, I am going to throw a nice cookout party and try to find some prime steaks to do on the grill.

  • Michael Fidler July 29, 2021, 9:57

    Moons and small rogue planets may be the best outpost for the interstellar traveler. Easy access to organic compounds and water ice may be the way interstellar species survive. Once in a new system the area in the outer asteroid belt would be the best place for organic carbon based compounds and water without the colder ices interfering. Would this be the best area for other civilizations to settle first before terraforming the inner planets?

    James Essig you might find this interesting:

    New quantum research gives insights into how quantum light can be mastered.

    “The researchers are also working on how to pull photons from a vacuum by modulating the quantum metasurface.”

    “The quantum vacuum is not empty but full of fleeting virtual photons. With the modulated quantum metasurface one is able to efficiently extract and convert virtual photons into real photon pairs,” says Wilton Kort-Kamp, who works in the Theoretical Division at the Lab’s Condensed Matter and Complex Systems group.

    Harnessing photons that exist in the vacuum and shooting them in one direction should create propulsion in the opposite direction. Similarly, stirring the vacuum should create rotational motion from the twisted photons. Structured quantum light could then one day be used to generate mechanical thrust, using only tiny amounts of energy to drive the metasurface.”

    Space-Time Quantum Metasurfaces.


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