In recently published research using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers announced that they had found the first clear detection of a circumplanetary, moon-forming disk surrounding the exoplanet PDS 70c — a first in astrophysics.
The circumplanetary disk, or CPD, has been seen in past research from other groups. However, due to the inability to tell the disk apart from its surrounding environment, the presence of the disk around PDS 70c could not be confirmed.
The CPD in question surrounds PDS 70c, a young exoplanet orbiting PDS 70, a 10 million-year-old K5 spectral type, low-mass T-Tauri star located 360 light-years away in the constellation Centaurus.
PDS 70c is one of two young, Jupiter-like planets currently orbiting PDS 70. Discovered in 2019 using the Very Large Telescope’s (VLT) Multi Unit Spectroscopic Explorer (MUSE), PDS 70c is thought to have a mass of two Jupiters and orbits at a distance of 34 AU from its star. The other planet, PDS 70b, is similar in size to PDS 70c, although it orbits closer to the parent star.
Circumplanetary disks, like circumstellar disks, are the birthplaces of natural satellites such as moons and other small rocky bodies. CPDs consist of various rock, gas, and other material that can “clump” together to form planetary bodies. Additionally, CPDs regulate the amount of material that falls onto a newly forming planet like PDS 70c.
Using data collected from their observations of PDS 70c and its CPD, a team of astronomers, led by Dr. Myriam Benisty of the University of Grenoble and the University of Chile, were able to determine that the CPD has enough mass to form three satellites the size of Earth’s moon.
But how exactly could these natural satellites form, when could we see them, and how?
To answer these questions, NASASpaceflight sat down with Dr. Stefano Facchini, ESO Fellow and co-author of the study.
“So far, the only system where two planets have been directly detected as they are growing and forming is PDS 70. Which is a young star, very similar to what the sun used to be when it was young. So these two planets are very massive,” said Dr. Facchini.
“What happened is, a couple of years ago, our team realized through an image of this system, taken by the ALMA facility, that there was some cold dust emission around one of these planets, planet c, PDS 70c. And so what we did recently was ask for a lot more telescope time at even higher angular resolution, so with better detail, to really see if there was a point source emission or some emission of dust where planet c is.”
At first, Benisty et al. were skeptical of the image they received from the telescope, but then they realized the image truly showed a point source emission. And that’s where the fun started.
1/ Astronomers made the first clear detection of a moon-forming disc around an exoplanet, using @ALMAObs , in which ESO is a partner.
— ESO (@ESO) July 22, 2021
“What happened is that an astonishing image came out of this new data, where we realized that this point source emission was real because there was some skepticism in the community. But instead, we realized it was real. And why is it so important? Because this would be, and is, the first direct image or detection of material of dust that is orbiting a growing planet,” Dr. Facchini said. “So we targeted this system because it’s the best system known to date in the sky to really see planet and moon formation in the act.”
“And in this dust, we think moons are forming.”
However, to have the data to analyze, the team first had to get it from somewhere. For Benisty et al., that location was the Atacama Large Millimeter/submillimeter Array (ALMA) telescope located in the Atacama Desert in northern Chile. ALMA is an astronomical interferometer consisting of 66 total radio telescopes/antennas. The telescope operates at wavelengths of 3.6 to 0.32 millimeters and is extremely sensitive. It has been instrumental in supporting some of the greatest astrophysics science throughout the past decade.
“[The antennas] operate in millimeter wavelength range, so not in the visible light but in longer wavelengths which cannot be picked up by the human eye but can be picked up by instruments,” Dr. Facchini said.
“So we have this array of antennas. We have 66; we [used] about 45 for these observations. And the way we use it, and the way this facility operates is that we can combine the light from the different telescopes, these different antennas, and we can play a game or use some mathematics to combine the light from these antennas in a smart way, in such a way that the angular resolution we obtain is the same as if we had a huge telescope with a diameter that is equal to the distance between the two farthest antennas we have.”
“So, the bigger the mirror, the better resolution you have.”
Hence why 45 antennas were needed to collect enough information for analysis. With the farthest distance between two antennas being approximately 8 km, Benisty et al. were basically working with an 8 km wide mirror.
“We use the telescope in this way, and in particular, in this study, we used it to probe what we call the continuum emission. It’s the emission that the cold dust emits thermally. Just because it’s cold, it’s like at -250° Celsius, it emits a thermal emission at millimeter or submillimeter wavelengths. And so we used this telescope to observe the emission from these small dust grains [in the CPD].”
With the right telescope, the team was able to collect astonishing data on the PDS 70 system and PDS 70c itself, mainly its CPD. As data were collected and analysis ensued, Benisty et al. were able to clearly detect a CPD around PDS 70c — the first time a CPD was confirmed around a body outside of our solar system.
So if CPDs are thought to be the birthplaces of natural satellites around planets, could moons form around PDS 70c from its CPD?
“The honest answer is, from a theoretical point of view, we think moons around PDS 70c can form from what theory tells us,” Dr. Facchini said.
When looking at the planet and the disk further, PDS 70c’s CPD is a late-stage disk.
“So what we think is that when there is a massive planet such as Jupiter, Jupiter carves a gap in the disk where it’s harbored. It carves a huge gap in the density. So it removes gas and dust from its vicinity, its orbit. But still, some gas and dust can go towards the planet. As the gas approaches, it starts orbiting around the planet and forms a small disk, which we call a circumplanetary disk around the planet.”