One of the most powerful weapons of the empire in Star Wars is known as the Death Star; a space station with enough destructive power to take down entire planets. The plot of several episodes of the saga revolves around the Rebel Alliance’s struggle to put an end to this weapon of mass destruction that, in fiction, materializes in the form of a high-powered hyperlaser. In astrophysical reality and with gravity assistance, it is relatively easy to tear a planet apart: it could collide with a large object; be swallowed up by its star; fall into the jaws of a black hole; or just enter what is called the Roche limit of a bigger star. As in many aspects of our lives, it is also much easier to imagine the process of destroying than that of building. Perhaps it has to do with the fact that the latter is a simmering transformation. Let’s make the effort today, how is the process of formation of a planet and what does it require?
Before our telescopes allowed us to detect planets outside the Solar System, theoretical models informed us that their existence must be part of the star formation process: they are made at the same time. We expected the presence of planets out there, the question was how many. Today we have the numbers that, although they are being refined at every moment, allow us to infer that they are built at the same time as most of the stars because wherever we look and we have the ability to detect them, we see them. We find them of all sizes and orbits and some, like the super-earths that we do not have in our system, are among the most abundant. But, despite all that we have advanced, there is still something that we do not know deeply and it has to do with the fact that practically all the detected planets are relatively old, they are planets that are the age of their star, billions of years in most of the cases.
Watching a planet cook is a very difficult process to unravel as it is happening. We need to look around young stars that are still surrounded by the material from which they grow and which hides and obscures their surroundings. In addition, the common techniques to find planets are less effective, among other things, because if the planet is young, the star also tends to have increases in magnetic activity that can hide the planetary signals. For these reasons, among others, we only have solid direct imaging detection of a planet still submerged in its natal disk (we call it a protoplanetary disk). Its about PDS 70 system, which is five million years old (most of the known exoplanets are billions of years old) and where, moreover, recently, it has been detected, around one of the two planets of the system, the disk where a moon could be forming.
Much of the effort and progress in studying planets in formation is based on astronomical observations of protoplanetary disks. We know well that in astrophysics life does not give us to wait for things to change in the sky, that is why we reconstruct the timelines by looking at different objects at different stages of their lives. This is how we join the points that allow us to reconstruct its evolution. In this way the structures like rings, holes, spiral arms, asymmetries in the discs of young stars They give us information not only on the existence of protoplanets that are sculpting these structures, but also on the processes necessary for their existence.
It all starts when the clouds of material in the interstellar medium are cool enough to collapse under the influence of gravity. The physics that causes the formation of a disk in the process of the formation of a star can be summarized in one line: conservation of angular momentum. Planets are formed on that disk, but how?
Let’s think of Earth as an example. The starting point to build it are small particles of solid material, which we call dust, which occur in stars like the Sun at the end of their days and in supernova explosions. Raw powder is made from grains that can be a billion times smaller than one meter. When dust is present in the region the birth disk of a planet has to grow to sizes of six million meters which is the radius of the Earth (6,371 km). To put it into the form of something that to our regret is all too familiar to us, we have to grow an object the size of the Earth from particles of solid material the size of a coronavirus.
Growth, according to our models and observations, does not happen all at once. First the dust grains need to coagulate into larger particles in dense regions, where they also acquire molecular ice sheets. Small particles collide gently due to the Brownian movementnamed after Robert Brown, a botanist who demonstrated that the movement of the tiny pollen particles he was looking at under the microscope had nothing to do with life. In this case, the microscopic solid particles move randomly immersed in the gas. The collisions between them make them larger and, with greater size, they collide with each other at a higher speed until they acquire sizes of millimeters or centimeters. The critical step is to make them grow to the size of a planetesimal that is a solid body between 100 and 1000 m and that is held together by its own gravitational attraction and not by the tension of the material. The problem is that planetesimals have to grow fast. Clocks in the chemistry of the cosmos tell us that on scales of a few million years they formed in the solar nebula.
Planetesimals are literally building blocks and once they reach that size, building planets is simply assisted by gravitational processes. It is the properties of the disk itself that determine where planets are formed and what they are made of: those that dictate whether we have planets like Jupiter or Earth and at what distance from the star and made of what material. Protoplanetary disks are very thin and contain only 1 to 10% of the mass of the star at their center. The disc also contains details that we still do not understand well. It is precisely that step from centimeter to kilometer, from the large dust grain to the planetesimal, that reveals the structure of the disk with its spirals and walls, with its rings and holes. That is where we believe these medium-sized particles accumulate in order to grow. Large particles that would travel directly to be destroyed on the surface of the star to fulfill what would be their physical destiny if no one stops them.
Eva Villaver She is a researcher at the Astrobiology Center, dependent on the Higher Council for Scientific Research and the National Institute for Aerospace Technology (CAB / CSIC-INTA).
Cosmic Void It is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It is intended to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillion atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up of Pablo G. Pérez González, researcher at the Center for Astrobiology; Patricia Sánchez Blázquez, Professor at the Complutense University of Madrid (UCM); and Eva Villaver, researcher at the Center for Astrobiology
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