In the Universe, nothing is immutable. This is because the physical processes that affect the different masses of the cosmos are dynamic and change over time. This is the case with regard to the Solar System, as well as all other solar systems in the Universe. As the host star evolves, the planets orbiting it gradually move away from it. But what are the reasons?
On January 3, 2019, the Earth reached the point in its orbit closest to the Sun:perihelion. Any object orbiting a single mass (like the Sun) performs an ellipse on which there is a particular point bringing it closest to this mass:the periapsis (or periapsis). For the past 4.5 billion years, Earth has orbited the Sun in an ellipse, like all the other planets orbiting their host star.
However, the Earth's orbital path does not remain constant over time but spirals outward. In 2019, its perihelion was 1.5 cm further away than the previous year, which was itself further away than the year before, etc. But it's not just about the Earth; all planets move away from their host star.
The phenomenon responsible for the orbits of all planets in all planetary systems is the same:gravity. Whether described in Newtonian terms, where masses attract each other, or in terms of general relativity, where masses follow the geodesics of spacetime, the larger mass dominates and influences the orbits of the others. .
If the central mass did not change and was the only factor in play, the force of gravity would remain constant over time. Each orbit would continue in a perfect closed ellipse, forever.
Each planetary system contains other masses:planets, moons, asteroids, centaurs, Kuiper belt objects, satellites, etc. These masses contribute to disturb the orbits and lead to their precession. This means that the closest point of attachment—the periapsis in general or the perihelion for an orbit relative to the Sun—rotates in time.
Orbital mechanics, in various ways, affect the precession of the equinoxes. Earth, for example, had its perihelion and December solstice aligned just 800 years ago, but they are slowly drifting apart. With a period of 21,000 years, the precession of Earth's perihelion not only alters the closest point in Earth's orbit, but also the location of the pole stars.
Other factors also impact the Earth's orbit:
Related topic:The Earth is slowing down, and the Moon is causing it
These last two effects, however, are important only under extreme conditions, such as very close to a large and compact mass, or in the early stages of planetary system formation, when protoplanetary disks are present and still massive.
Today, the Earth (and all the planets) is so far from the Sun and surrounded by such a rare quantity of matter, that the time scale of a spiralization is several hundreds of billions of billions of years more longer than the current age of the Universe. Since the protoplanetary disk completely evaporated some 4.5 billion years ago, there is almost nothing left to dissipate the angular momentum of the planets.
The most important effect contributing to the spiralization is the emission of the solar wind, which strikes the Earth and attaches to it, leading to a slight loss of angular momentum. In general, the Earth does not spiral towards the Sun, but in the opposite direction, like all the other planets. Every year, the Earth moves 1.5 cm away from the Sun, and the reason for this is the Sun itself.
At the core of the Sun, the process of nuclear fusion occurs. Every second, the Sun emits about 3.8×10 26 joules of energy, which are released by the conversion of mass to energy in the heart. The mass-energy equivalence E=mc² is the main cause; nuclear fusion is the process, and the continuous emission of solar energy is the result.
The conversion of matter into energy results in a considerable loss of mass from the Sun. During the 4.5 billion years of the history of the Solar System, the Sun, due to the process of nuclear fusion, lost about 0.03% of its initial mass, which is comparable to the mass of Saturn.
Every second, the Sun loses about 5.9 million tons of matter, which decreases its gravitational pull on every object in the Solar System. If this attraction remained unchanged, there would be a very, very slow interior spiralization, due to the effects of friction, collisions and gravitational radiation. But with the gravitational changes of the Sun, the Earth, like all planets, slowly moves away from its star, spiraling outward.
Although the effect is small, this variation of 1.5 cm per year is easily calculable. 4.5 billion years ago, the Earth was therefore about 50,000 km closer to the Sun than today. And that distance will continue to grow as the Sun burns its fuel. These calculations are unambiguous, although scientists have yet to directly measure this progressive distancing experimentally.
