Our Solar System is a vast and captivating cosmic framework, the home of our planet Earth and numerous other celestial bodies that dance in orbit around our star, the Sun. With a diameter of approximately 120-130 astronomical units (AU) or 0.0019 light years, it is located in the Orion Arm of the Milky Way. Orbiting around the center of the galaxy at a distance of about 26,700 light years with a speed of 230 km/s, the Solar System is estimated to take around 230 million years to complete one full orbit.

Solar System’s structure

The most probable theories about the formation of the Solar System describe its birth 4.6 billion years ago from the fragmentation and gravitational collapse of a giant molecular cloud with a diameter of 65 light years. At the center of our Solar System shines the primary celestial body, the Sun. The Sun is a yellow dwarf of spectral class G2 and contains 99.86% of all known mass in the Solar System. This incandescent gas sphere radiates light and heat into the surrounding space, while its gravity exerts gravitational control over all celestial bodies in the Solar System, keeping them in a constant orbit.

Most large objects in orbit are in a plane similar to that of Earth’s orbit, called the ecliptic. Typically, the orbital plane of planets is very close to that of the ecliptic, while comets and objects in the Kuiper Belt have a significantly greater angle.

Distance of a body from the Sun varies during its orbital motion. The point closest to the Sun in the orbit of a body is called perihelion, while the farthest is called aphelion. Solar System is conventionally divided into two zones. The inner Solar System includes the four rocky planets and the main asteroid belt. The rest of the system is considered the outer Solar System, including the Kuiper Belt and the Oort Cloud.

Planets and Satellites

All planets and most other objects orbit in the same direction as the Sun’s rotation, counterclockwise from an external observer’s perspective. Trajectories of objects orbiting the Sun follow Kepler’s laws. Planets’ orbits are nearly circular, while those of smaller bodies have greater eccentricity and can be highly elliptical.

Among the inner planets, we find Mercury, the closest planet to the Sun, a dry and rocky world. Venus is shrouded in a dense and extremely corrosive sulfuric acid atmosphere. Earth, our blue planet, hosts an extraordinary variety of life forms. Mars, with its red and dusty surface, has always sparked interest as a possible future human destination.

Solar System's illustration
Solar System’s illustration

Gas giants, Jupiter and Saturn, dominate the outer regions of the Solar System. Jupiter, with its imposing mass, is a gas giant with a powerful atmosphere and a series of interesting moons. Saturn, known for its spectacular rings, while Uranus is the least massive outer planet and the only one that orbits the Sun with an axial tilt greater than 90° relative to the ecliptic. Finally, Neptune, slightly smaller than Uranus, is denser and, like Jupiter and Saturn, features a large storm, the Great Dark Spot.

Many planets are accompanied by natural satellites. Ours, the Moon, influences Earth’s tides and has played a crucial role in the history of human space exploration. Moons of Jupiter and Saturn, such as Europa and Titan, are objects of scientific interest due to their peculiar atmospheres and potential conditions favorable to life.

Asteroids and Comets

In addition to planets and moons, the Solar System hosts a multitude of asteroids and comets. Asteroids are rocky masses wandering in space, while comets are icy celestial bodies that soar among the stars. The former can cross Earth’s orbit, offering unique celestial displays and sometimes causing meteoritic phenomena on our surface.

The main asteroid belt occupies the region between the orbits of Mars and Jupiter. It is presumed to be remnants of the Solar System’s formation, whose merging failed due to gravitational interference from Jupiter. The size of rocks in this belt can range from hundreds of kilometers to a few centimeters. All asteroids, except the largest, Ceres, are classified as minor bodies of the Solar System. The asteroid belt contains tens of thousands, perhaps millions, of objects above one kilometer in diameter. Nevertheless, the total mass of all asteroids in the main belt would hardly amount to more than a thousandth of Earth’s mass.

The Kuiper Belt is a large ring of debris similar to the asteroid belt but composed mainly of ice. It consists mainly of small Solar System bodies, although some of the largest objects in this belt may be reclassified as dwarf planets: for example, Quaoar, Varuna, and Orcus. According to estimates, there are over 100,000 objects with a diameter greater than 50 km in the Kuiper Belt, but the total mass of all objects in the Kuiper Belt is thought to be one-tenth, or even one-hundredth, of Earth’s mass.

Farthest Region of the Solar System

The point where the Solar System ends and interstellar space begins is not precisely defined. Boundary can be traced in two distinct ways: the solar wind or the gravity of the Sun. The outer limit traced by the solar wind, heliopause, reaches about four times the distance from Pluto to the Sun. However, the Hill sphere of the Sun, the actual radius of its gravitational influence, could extend up to a thousand times farther.

The heliosphere is divided into two distinct regions. The solar wind travels at about 400 km/s until it passes through the so-called termination shock, located between 80 and 100 AU from the Sun. Here, the wind slows dramatically, increases in density and temperature, and becomes more turbulent, forming an oval structure called the heliosheath.

The Oort Cloud is a large mass composed of billions of icy objects believed to be the source of long-period comets, surrounding the Solar System at about 1 light year. It is believed to consist of comets that were expelled from the inner Solar System due to gravitational interactions with the gas giants. Objects in the Oort Cloud are very slow and can be disturbed by rare events, such as collisions, the gravitational force of a passing star, or tidal forces exerted by the Milky Way

Stefano Gallotta
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