Moons abound in our solar system; almost every planet has at least one. Moons can form in many ways, but one of them is that they are born with the planet (wait a second and I will come back to it later). Given the number of moons we have locally, we think they also abound in other solar systems.
We have not seen any exomoon yet. They are ridiculously hard to detect. But new observations indicate that at least one newborn planet surrounding a nearby star is still surrounded by a disk of dust, the kind of things we think they are forming. In other words, we may be seeing the birthplace of an exomoon or exomoons.
To be clear, we do not know if moons are or have been formed from this material. But this is the first time a disc around a planet is detected with confidence. So it's a good bet.
The observations were made using the large set of Atmama / submillimeter Array (ALMA) dishes in the high desert plains of Chile. ALMA looks at the light far beyond what our eyes can see, with wavelengths of about one millimeter or less. This exceeds the infrared and microwave frequencies of the electromagnetic spectrum, but its length is not as long as that of radio waves.
This is the ideal wavelength for searching for dusty environments around young exoplanets. Stars do not emit much light to the millimeter, but the dust surroundings after their birth, they shine brightly. It's the same dust that planets form.
In a word, clouds of gas and dust can collapse (perhaps they are struck by the wind of a powerful nearby star, a supernova or two colliding clouds). When they do, the material flattens to form a giant disk in rotation, usually tens or hundreds of billions of kilometers in diameter. The star is formed in the center. Further down the disc, clusters of substance begin to form, aggregating over millions of years to form planets. If a planet is massive enough (about ten times the mass of the Earth), then it can create a vacuum in the dust ring, collecting materials by gravitation.
This material forms a disc around the planetcalled circumplanetary disk (or CPD). Much of the material on this disc eventually falls on the planet, but some may also clump together during their orbit, forming moons.
It was thought that many of the observed young exoplanets had CPP, but the observations were not safe enough to be able to make this assertion … until now.
The PDS 70 is a very young star, probably aged about 5 million years, located about 370 light years away. A disc of material around the star was seen for the first time in 2002. It is actually two rings of dust and gas, a closer to the star and one more big, much farther apart, separated by a great void. In 2018, a planet in active formation was observed at about 3.5 billion kilometers from the star (a little farther than Uranus compared to the sun), directly detectable in the infrared images taken at the Using the very large telescope. Called PDS 70b, it was found that his growth was fairly fast, which indicates that he is really very young.
Then, in 2019, a second planet was also found in orbit around the star. Called PDS 70c, it is farther away than PDS 70b, gravitating around the star located inside the inner edge of the outer ring, about 5.3 billion km from the star (a little further than Neptune of the Sun).
The new ALMA images go further than that. They detect the outer ring and the two planets. But there is a twist. At these wavelengths, the planet should be relatively weak. However, the PDS 70c is much brighter than expected. The simplest explanation is that it is surrounded by its own disk of hardware, of its own CPD. This disc is much too small to be solved by ALMA, but the observations correspond to it, it is a disc made of hot dust. This is the first time that such a disk is detected with confidence.
The amount of material in the disc is low, indicating that the planet may be in the advanced formation phase and has already siphoned off most of the CPAP material. But this disc, in addition to feeding the planet, is also the place where astronomers think moons are formed.
So, again, we can not say for sure that there are moons out there, but I would venture to say that's the way to bet. The disk around the PDS 70c probably has a total mass of about one tenth of the Earth (dust and gas); for comparison, the moon has a mass of 1/80th Earth. So there is a lot of material for the making of the moon (to be fair, it is the dust that constitutes the bulk of the moon, and there is much less dust than gas, still enough to form moons) .
Oddly enough, no such disc has been seen around the PDS 70b. But even stranger is that ALMA has seen a bright spot of what might be dust near but apparently not in the same place as the planet. It is only possible that it coincides with the planet (the exact location is difficult to determine in the data), but it is more likely that it has moved away from it. What could it be? Astronomers are not sure, but they think that it could be a material that trails behind the planet as it revolves around the Sun, trapped in a special gravitational region called Lagrange point (specifically L5), where gravity and centrifugal forces balance, allowing to accumulate there. But it's not at all clear.
So, wow. Many things happen in this system, made even more surprising insofar as we only know 70b for a year and 70c for less than that! As I wrote yesterday, we have barely begun to find exoplanets en masse, with more than 4,000 people now known. We're so good at observing them, creating telescopes, detectors, and software to watch and analyze them, that discoveries are pouring in now.
I imagine that we will soon see even more discs around the planets, with enough mass to create a system of moons. And we are at the forefront of the search for moons already formed in the observations.
We think that the planets are more numerous than the stars in the sky. The moons will certainly be more numerous than the planets. And we found that a dozen moons of our own solar system had oceans of liquid water beneath their frozen surface. When I reflect on the implications of this for finding life in the universe, my heart flies away.
It may be a long time from now, but I really think this time is coming. And what time it will be.