Certains scientifiques suggèrent que certaines planètes, comme la Terre et Mars, étaient autrefois plates ou en forme de disque lors de leurs premières étapes de formation. Cette théorie propose que, par des processus tels que les collisions ou les forces de marée, ces disques plats se sont finalement transformés en les formes sphériques que nous reconnaissons aujourd'hui. Le concept remet en question les idées traditionnelles sur l'évolution planétaire et met en évidence la nature dynamique des corps célestes. Des recherches et des preuves supplémentaires sont nécessaires pour mieux comprendre cette hypothèse. Dans l'ensemble, c'est un concept fascinant qui ajoute une autre couche de complexité à notre compréhension de la formation des planètes. Animation créée par Sympa.
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00:00The Earth is not flat, but Jupiter could have been. Instead of large round spheres, the gas giants of our solar system may have started their lives as simple crepes.
00:10Jupiter is one of our oldest neighbors. It is 4.6 billion years old, just like our sun. And when it was still young, it probably formed in a protoplanetary disk.
00:23Everything starts with stars. When a star is still forming, it does not look like a round object. It is more like a large disk of matter.
00:31At this stage, hot winds blow, composed of charged particles. The dust of this disk contains elements such as carbon and iron.
00:40Some of them collide and remain together, forming larger objects.
00:45The dust turns into pebbles, the pebbles turn into rocks, and the rocks hit each other, becoming larger and larger.
00:53The gas present in the disk helps all these solid pieces to agglutinate. Some detach, but others remain together.
01:00And it is they who become the basis of the planets. They are called planetesimals.
01:05Even gas giants like Jupiter began as tiny particles of dust, smaller than a human hair.
01:13Finally, they formed their own disk of matter.
01:17Then they began to rotate around our sun, growing as they accumulated gas and rocks, like a big snowball.
01:29Gas giants are special. They are born from the coldest parts of the disk.
01:34In these cold regions, the molecules are slower, which makes them easier to capture.
01:39The water can freeze, and tiny pieces of ice stick together and mix with dust.
01:45These salt snowballs then form the cores of vast planets like Jupiter, Saturn, Uranus and Neptune.
01:54In the warmer regions, close to the star, telluric planets like Mercury, Venus, Earth and Mars begin to form.
02:03After the birth of the ice giants, there was not much gas left for these tiny celestial bodies,
02:08and it could take tens of millions of years for such planets to form after the birth of a star.
02:15Our sun was growing at the same time, sucking in the gas close and pushing the distant objects even further.
02:21After billions of years, the disk has completely changed,
02:25turning into a spherical star with a bunch of giant and tiny planets, asteroids, moons, meteoroids and comets around.
02:38Recently, simulations have shown that these proto-planets, as we call these first dust conglomerates,
02:44did not start by looking like the planets we know.
02:47In the case of gas giants like Jupiter, they would look more like crushed balls or M&Ms.
02:54When the sun was still young, the gas and dust disk that surrounded it cooled down and became unstable.
03:01It began to break into large pieces.
03:04These gathered under the effect of an implacable gravity to create Jupiter.
03:09Then it became a spherical gas giant over time.
03:13Numerous anomalies can occur during this process of planet formation.
03:17Have you ever wondered why Venus or Uranus turned in the opposite direction?
03:22Usually, when things are formed from a rotating gas disk, they tend to turn in the same direction.
03:29If you spin a bunch of balls tied to a string, for example, they will all turn in the same way.
03:35So, theoretically, all the planets should also turn in the same direction.
03:41But there are also many objects that move quickly in our solar system, like comets and asteroids.
03:48When they collide with the planets, especially during their first days, this impact can lead them to turn in the opposite direction.
03:55Venus and Uranus probably survived a large-scale collision.
03:59Fortunately, they were not ejected into space.
04:03The gravity of the sun and neighboring planets kept them in their place.
04:10There are also what are called gravitationally locked planets.
04:15These are celestial bodies that rotate so that one side is always facing their star, while the other remains in perpetual darkness.
04:24Thus, one side is always very hot, while the other is extremely cold.
04:28If we lived on such a planet, we could only exist on a thin band between these two extremes.
04:34These planets form when they are very close to their star.
04:38The gravitational forces at work are intense and, over time, slow down the rotation of the planet until it corresponds to the time it takes to orbit around its star.
04:48Imagine that you are turning on your chair.
04:51Someone approaches you and, holding your chair in his hands, begins to turn with you.
04:56In this way, you will face each other all the time.
04:59Synchronous rotation planets work a little like this.
05:03Our moon is also in synchronous rotation with the Earth.
05:06That's why we only see one side of it.
05:11We have discovered more than 5,000 exoplanets outside our solar system.
05:16Some of them have very strange orbits.
05:19There are some, for example, that have incredibly long orbits, thousands of years to make a single revolution around their star.
05:27Or very unequal orbits, similar to those of comets.
05:30Or those planets that are called Jupiter Hot, which are very close to their star, much closer than Mercury is to our sun.
05:38But these planets could not have formed where they are now.
05:42As their solar system evolved, they, for one reason or another, changed position.
05:48This rearrangement is called planetary migration.
05:53There are three ways in which this migration can occur.
05:57First of all, due to the gas and dust in rotation around the planet.
06:02When a planet collides with this matter, it can create spiral patterns in the gas.
06:08These patterns can either bring the planet closer to the center or move it away, depending on how they mix.
06:14This is called a type I migration.
06:18This is what Jupiter experienced when it got closer to the sun billions of years ago.
06:23And this also explains the existence of Jupiter Hot.
06:27Secondly, large planets can move smaller ones, changing their trajectory.
06:32And thirdly, the gravity of the star can attract the planet, making its orbit circular.
06:41Have you ever heard of wandering planets?
06:44Imagine a lonely planet, floating in the vastness of space without being attached to any star.
06:50These are the wandering nomads of our galaxy, condemned to wander eternally.
06:55And there are so many of them.
06:57There could be more planets in freedom than planets linked to a star.
07:01We are talking about thousands of billions of wandering bodies in our only milky way.
07:07It is not uncommon for them to be as massive as our largest planet, Jupiter.
07:12But most of them would have a size more comparable to that of the Earth.
07:16There are even some that could have a thick atmosphere capable of keeping them warm.
07:21Although they are far from all stars.
07:23Some of these planets may have aurorae to catch their breath,
07:26while others may be entwined with moons containing liquid Leo, a potential harvest for life.
07:32There is even a chance that they can inhabit an extraterrestrial life.
07:36These planets could be hit by other stars or even entire planetary systems during their crazy race through space.
07:43Sometimes, they can be captured by the gravity of a star for a certain time,
07:47before being ejected back into space.
07:50But how are they born?
07:52Sometimes, during this chaotic formation process,
07:55all planets fail to stay close to their related stars.
07:59Some of them are expelled from their solar system
08:02due to the powerful gravitational attraction of other planets or passing stars.
08:07These ejected planets then become wandering planets.
08:14In 2012, astronomers discovered a solar system dating from the beginning of the universe.
08:19This system includes a star and two planets.
08:22It was nicknamed the Fossil System.
08:25Its star is incredibly old,
08:28about 13 billion years old,
08:30almost as old as our universe itself.
08:34This system is mainly composed of hydrogen and helium.
08:37It is unusual because planets are generally formed from gas clouds containing heavier elements.
08:44It was then that we understood that the way planets were formed before
08:48was different from the way they are formed today.
08:52We know that the stars containing the most metals are the most likely to have planets.
08:57In astronomical terms,
08:59metal means any chemical element other than hydrogen and helium.
09:04But in the primitive universe, there were not many heavy elements.
09:08Most of them were created inside the stars
09:11and then dispersed in space when they exploded.
09:15So, when did the very first planets form?
09:19This newly discovered system helps to answer this question.
09:23These two giant planets are orbiting around a star
09:26that is incredibly poor in metals and extremely old.
09:30This should be very rare, or even impossible,
09:33but they exist, nevertheless.
09:35This may mean that there are more planets in poor metal systems than we thought.
09:40Studying them will help us learn more about the formation of planets.