An (exo)moon is born – TecMundo

What if it was possible to witness the birth of a star? From a planet? And even a moon?

Things around here in the Solar System today are pretty stable, they don’t change much. That means we can’t see many celestial bodies forming around the neighborhood, but it also means it’s stable enough for life to thrive, so it’s not all bad.

But thanks to advances in technology and human knowledge, we’ve managed to sneak a peek at neighboring systems. Not just a peek, but more and more we can learn about other planetary systems, other stars, other galaxies. Astronomy grows more each year.

With these first observations, we began to ask ourselves: “how common is a planetary system like our Solar System?” The structure of the Solar System is well structured and known: star in the center, 4 rocky planets and 4 gaseous planets, in addition to the smaller bodies. Does this structure repeat itself in other parts of the Milky Way?

Solar System: Sun, planets and dwarf planets. It is possible to identify the 4 rocky planets (Mercury, Venus, Earth and Mars) and the 4 gas planets (Jupiter, Saturn, Uranus and Neptune). Distance is out of scaleSource: Astro – UFRGS

With the first exoplanets discovered, the first answer might have been no. We started to see planets the size of Jupiter or larger, closer to their stars than Venus! These planets are called hot Jupiters. This didn’t make sense with our planetary formation theory. The formation of gaseous planets like Jupiter needs to happen far from the star so that the atmosphere is created without evaporating.

It turns out that the theory of how planetary systems are formed cannot be valid for our Solar System alone. Physics and chemistry are the same everywhere in the Universe. So, the time has come to update our knowledge to frame other types of planetary systems. Today we understand that, within the planetary system in formation, you can have “planetary migration”. This happens when planets travel within your system, changing their distance to the star. Glad it wasn’t our case!

Artistic conception of a hot Jupiter close to its star.Artistic conception of a hot Jupiter close to its star.Source:  NASA

More than 4000 exoplanets have been detected in recent years. The prefix “exo” indicates that it is outside the Solar System. Now most, if not all, of these exoplanets are formed systems, like the Solar System, and won’t go through much change. But what if we could see these systems in the making? The “birth” of exoplanets? This too has been increasingly possible!

The earlier stage of planetary formation is a gas- and dust-filled disk called a “proto-planetary disk.” From that disk the planets will form. With observations from the ALMA telescope, we were able to find more and more of these disks. Finding several objects of the same class at different times in their evolution, it is possible to assemble the puzzle of the story. This is quite common in astronomy and you can read more in this column!

Artistic impression of a proto-planetary disk.Artistic impression of a proto-planetary disk.Source:  ESO

Planetary formation is a study that has been advancing a lot! And even if the details aren’t complete, we know more and more. Today we understand that on the protoplanetary disk, small bodies begin to come together. Remembering that everything on the protoplanetary disk is orbiting the central star: gas, particles, dust. Things are not static. With the collision and growth of these little bodies, at some point gravity starts to be interesting enough to attract other materials. So, these planetesimals orbiting the star start a job of cleaning up their orbit, and this creates holes in the protoplanetary disk, which have already been observed! Over time, the disk dissolves and only the planets are left.

And the moons? Both those of the Solar System and other systems? How are they formed? ALMA’s last observation will surely bring many answers to this question! For the first time clearly, the formation of an exomoon was detected! In one of these protoplanetary disks, we can see a planet and, beside it, another structure. Astronomy today understands that as the exoplanet forms, it also forms a disk. From that disk moons can be born! Of course, this also encompasses the formation of moons in the Solar System. But not all.

Although Earth is not one of the largest planets in the Solar System, it does have the fifth largest natural satellite! That’s impressive. The mass ratio of the Earth to the Moon is not common, so how the Moon formed is a big question for astronomy. It is now understood that the formation of the Moon took place after a large late impact of a celestial body with the Earth. This impact loosened the surface part of the Earth which, over time, attracted and formed our dear Moon. In fact, without the Moon stabilizing the Earth’s rotation, life probably would not have prospered here!

Earth rising from the Moon. Photo taken during the Apollo 8 mission.Earth rising from the Moon. Photo taken during the Apollo 8 mission.Source:  NASA

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