Scale 1 on Earth means 1 Astronomical Unit (AU), which is the distance from the Earth to the Sun. Example of Saturn, 10 AU = 10 times the distance between Earth and Sun
The Oort Cloud, also known as the Öpik-Oort cloud, is a hypothetical spherical cloud of trans-Neptunian objects . It has not been directly observed. It lies at the outer reaches of our solar system. With a diameter of one light-year, it is located about a quarter of the distance from Proxima Centauri, our nearest star. To give you an idea of its size relative to the Sun, let's look at some details.
We have Mercury, Venus, Earth, and Mars, in that order, relative to the Sun. A ray of sunlight takes 8 minutes and 19 seconds to reach Earth's surface. Beyond that, between Mars and Jupiter, lies the asteroid belt. Beyond this belt come the four gas giants: Jupiter, Saturn, Uranus, and Neptune. Neptune is approximately 30 times farther from the Sun than Earth is from Earth. Sunlight takes about 4 hours and 15 minutes to reach it. If we consider our planet farthest from the Sun, the outer reaches of the Oort Cloud would be 2.060 times the distance from the Sun to Neptune . This highlights the importance of the Oort Cloud and its effects on the Solar System.
Where is its existence deduced from?
In 1932, astronomer Ernst Öpik postulated that long-period comets originated within a large cloud beyond the confines of the solar system. In 1950, astronomer Jan Oort independently proposed the theory, resulting in a paradox. Oort asserted that meteoroids could not have formed in their current orbits due to the very astronomical phenomena that govern them, thus maintaining that their orbits, and all of them, must be stored within a large cloud. This colossal cloud is named after these two great astronomers.
Oort investigated two types of comets: those with orbits smaller than 10 AU and those with long-period (nearly isotropic) orbits, which are larger than 1.000 AU and can even reach 20.000 AU. He also observed that they all came from all directions. This led him to deduce that, if they came from all directions, the hypothetical cloud should be spherical. To better understand how they form, you can consult more information about comets in the solar system.
What exists and does the Oort Cloud encompass?
According to hypotheses about the origin of the Oort Cloud, it is linked to the formation of our solar system , resulting from large collisions and the ejected material. The objects that form it initially formed very close to the Sun. However, the gravitational pull of the giant planets distorted their orbits, sending them to their current locations in the distant solar system.
Comet orbits, simulations by NASA
Within the Oort cloud, we can differentiate two parts:
- Internal / Indoor Oort Cloud: It is more gravitationally related to the Sun. Also called the Hills Cloud, it is shaped like a disk. It measures between 2.000 and 20.000 AU.
- Oort Cloud Outer: Spherical in shape, more related to the other stars and the galactic tide, which modifies the orbits of the planets making them more circular. Measures between 20.000 and 50.000 AU. It should be added that it is really the gravitational limit of the Sun.
The Oort Cloud encompasses all the planets in our solar system, dwarf planets, meteoroids, comets, and trillions of celestial bodies larger than 1,3 km in diameter. Despite such a significant number of celestial bodies, the distance between them is estimated to be tens of millions of kilometers. Its total mass is unknown , but using Halley's Comet as a reference, it has been estimated at about 3 × 10²⁵ kg, roughly five times the mass of Earth. For more information about this famous comet, see Halley's Comet and its significance.
The Tidal effect in the Oort Cloud and on Earth
Just as the Moon exerts a force on the seas, raising the tides, it has been deduced that this phenomenon occurs on a galactic scale . The distance between two bodies reduces the gravitational influence one has on the other. To understand this phenomenon, we can look at the gravitational force that the Moon and the Sun exert on Earth. Depending on the Moon's position relative to the Sun and our planet, the tides can vary in magnitude. An alignment with the Sun exerts such a strong gravitational influence on our planet that it explains why the tides rise so much.
In the case of the Oort Cloud, let's say it represents the oceans of our planet. And the Milky Way would represent the Moon. That's the tidal effect. What it produces, as the graphic description shows, is a deformation towards the center of our galaxy. Considering that the Sun's gravitational force becomes weaker the farther we move away from it, this small force is also enough to perturb the motion of some celestial bodies, causing them to be sent back towards the Sun. For more information on the effect of tides on celestial bodies, you can consult the article on the tidal effect.
Cycles of extinction of species on our planet
Scientists have observed that approximately every 26 million years , a pattern repeats itself: the extinction of a significant number of species. While the exact cause of this phenomenon remains uncertain, the tidal effect of the Milky Way on the Oort cloud could be a hypothesis worth considering.
Given that the Sun revolves around the galaxy, and in its orbit tends to pass through the galactic plane with some regularity, these extinction cycles could be described. It has been calculated that every 20 to 25 million years, the Sun passes through the galactic plane. When that happens, the gravitational force exerted by the galactic plane would be sufficient to perturb the entire Oort Cloud. This would shake and disturb the bodies within the cloud. Many of them would be pulled back toward the Sun, which could have devastating effects on our planet, similar to what is discussed in the panspermia theory.
Alternative Theory
Other astronomers believe the Sun is already close enough to this galactic plane. Their theory is that the disturbance could originate from the galaxy's spiral arms. While there are numerous molecular clouds, these are also teeming with blue giants. These are very large stars with very short lifespans, as they rapidly consume their nuclear fuel. Every few million years, some of these blue giants explode, creating supernovae . This would explain the strong disturbances affecting the Oort Cloud.
Be that as it may, we may not be able to perceive it with the naked eye. But our planet is still a grain of sand in infinity. From the Moon to our galaxy, they have affected from their origin, the life and existence that our planet has endured. A huge amount of things are happening right now, beyond what we can see.
