Moons are in our solar system; Almost every planet has at least one. There are many ways that men can form, but one way is that they are born together with the planet (hang for a second and I come to it below). Given how many moons we have locally, we also see that they are superfluous in other solar systems.
We haven't seen any exomons yet. They are ridiculously hard to detect. But new observations indicate that at least one new-born planet that circles a nearby star is still surrounded by a pond, exactly the kind of thing that we think moons form out of. In other words, we can see the birthplace of an exomon or exomoner .
To be clear, we do not know whether men are actually or have been formed by this material. But it's the first time a disc around a planet has ever been detected with confidence, so it seems like a fair bet.
The observations were made with the Atacama Large Millimeter / submillimeter array (ALMA), a collection of dishes in high desert plains in Chile. ALMA looks at the light far beyond what our eyes can see, with wavelengths around one millimeter or less. It is out past the infrared and microwave part of the electromagnetic spectrum, but not quite as long as a wavelength as radio waves.
This is the perfect wavelength to look for the dusty surroundings of young exoplanets. Stars do not put much light into the millimeter range, but the dust surrounding them after they are born shines brightly there. This is the same dust form from.
In a nutshell, clouds of gas and dust can collapse (perhaps they are hit by the wind from a strong nearby star, or a supernova or two clouds collide). When they do, the material is flattened into a giant rotating disc, usually tens of hundreds of billions of kilometers across. The star forms in the middle. Further into the record, clumps of things begin to form and aggregate for millions of years to form planets. If a planet is massive enough (about ten times the earth's mass) then it may require a gap in the dust ring and collect material from it gravitationally.
This material forms a disk around the plane called a circumplanetary disk (or CPD). Many of the material from this record eventually ends up on the planet, but some can also clump when it orbits and forms moons.
Several young exoplanets were observed to have had CPD, but the observations were not safe enough to claim … until now.
PDS 70 is a very young star, probably about 5 million years old, located about 370 light years away. A disc of material around the star was first experienced in 2002. The disc is actually two rings of dust and gas, one near the star and one larger one much further out, with a large gap between them. In 201
Then a second planet was found in 2019 also the round of the star. Called the PDS 70c, it is longer than the PDS 70b, which orbits the star just inside the inner edge of the outer ring, about 5.3 billion kilometers from the star (slightly longer than the Neptune is from the sun).
The new ALMA images go beyond that. They discover the outer ring and both planets. But it's a twist. At these wavelengths, the planet is expected to be relatively weak. However, the PDS 70c is much lighter than expected. The simplest explanation is that it is surrounded by its own disc of material, its own CPD. This disc is far too small to be solved by ALMA, but the observations fit in with being a disc made of hot dust. This is the first time such a disc has been discovered with confidence.
The amount of material in the disc is low, indicating that the planet may be in the late stages of formation and has already seeped out most of the material from CPD it can. But this record, in addition to feeding the planet, is where astronomers believe men are forming.
So again we can't say there are moons there, but I dare say that it is a way to bet. The disk around the PDS 70c probably has a mass of one tenth or so of the total earth (dust plus gas); for comparison, the moon has a mass of 1/80 th earth. So there are lots of multipurpose materials there (to be fair, that's the dust that makes up the bulk of the moon, and there's much less dust than gas, still enough to form moons.)
Incredibly, no such disc was seen around PDS 70b. But even the weather is that ALMA saw a clear spot of what could be dust near but apparently not in the same place as the planet. It is hardly possible that it coincides with the planet (the exact location is difficult to determine in the data) but it is more likely to shift from it. What could it be? Astronomers are not safe, but they speculate that it may be material behind the planet as it orbits the sun, captured in a particular gravity called the Lagrange point (specifically L5), where gravity and centrifugal forces balance, allowing particles to accumulate there. But it is not at all clear.
So, wow. Much is going on in this system, made even more amazing by just knowing 70b for a year and 70c for less than that! As I wrote yesterday, we have just barely started to find exoplanets in bulk, with 4,000 + now known. We become so good when we observe them, create telescopes and detectors and software to observe and analyze them, that the discoveries are now pouring in.
I imagine we will see even more disks around planets soon, with more than enough mass to create a system of moons. And we are on the bleeding edge of finding already formed moons in observations as well.
We believe that planets exceed stars in the sky. Moons will surely surpass planets. And we have found that a dozen moons in our own solar system have sea of liquid water under their frozen surfaces. When I think of the consequences of finding life in the universe, my heart is hovering.
It may be a long time yet, but I really believe that time will come . And what time will it be.