Concept

Solar System

Overview

The Solar System is the gravitationally bound system of the Sun and the masses that orbit it, most prominently its eight planets, of which Earth is one. The Solar System is an isolated single-star planetary system (not part of a larger star system) within the Milky Way Galaxy. The system formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, creating the Sun and a protoplanetary disc from which the orbiting bodies assembled.

The Sun accounts for 99.86% of the Solar System's total mass. Inside the Sun's core, hydrogen is fused into helium, releasing energy that is emitted through the Sun's photosphere. This creates the heliosphere and a decreasing temperature gradient across the Solar System.

The next most massive objects of the system are the eight planets, which by definition dominate the orbits they occupy. Closest to the Sun in order of increasing distance are the terrestrial planets – Mercury, Venus, Earth and Mars. These four planets are part of the inner Solar System. Earth and Mars are the only planets that orbit within the Sun's habitable zone, in which sunlight can keep surface water liquid under atmospheric pressure. Beyond the frost line at about five astronomical units (AU), are the planets of the outer Solar System: two gas giants (Jupiter and Saturn) and two ice giants (Uranus and Neptune). Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.

Objects of planetary mass that do not dominate their orbit but directly orbit the Sun are called dwarf planets. The International Astronomical Union's Minor Planet Center lists Ceres, Pluto, Eris, Makemake, and Haumea as dwarf planets. Four other Solar System objects are generally identified as such: Orcus, Quaoar, Gonggong, and Sedna. Less massive than the dwarf planets are the vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. The dwarf planet Ceres and many of these smaller bodies are located in the asteroid belt (between Mars's and Jupiter's orbit), while all other dwarf planets are members of populations of trans-Neptunian objects, which may be found in the Kuiper belt just outside Neptune or in the further scattered disc.

Many objects in the Solar System do not orbit the Sun directly and are instead natural satellites, commonly called 'moons', of larger bodies. These can be found throughout the Solar System in sizes from planetary-mass moons at their largest to much less massive moonlets at their smallest. The largest two moons (Ganymede of Jupiter and Titan of Saturn) are larger (though less massive) than the smallest planet (Mercury), while the seven most massive, which includes Earth's Moon, are more massive and larger than any of the dwarf planets.

Within the heliosphere, the Solar System is constantly flooded by the charged plasma particles of the solar wind, which, along with interplanetary dust, gas and cosmic rays, form an interplanetary medium between the bodies of the Solar System. At around 70–90 AU from the Sun, the solar wind is halted by the interstellar medium, resulting in the heliopause and the border of the interplanetary medium to interstellar space. Further out somewhere beyond 2,000 AU from the Sun extends the outermost region of the Solar System, the theorized Oort cloud, the source for long-period comets, stretching to the edge of the Solar System, the edge of its Hill sphere, at 178,000–227,000 AU (2.81–3.59 ly), where its gravitational potential becomes equal to the galactic potential. The Solar System currently moves through a cloud of interstellar medium called the Local Cloud. The closest star to the Solar System, Proxima Centauri, is 269,000 AU (4.25 ly) away. Both are within the Local Bubble, a relatively small 1,000 light-years (ly) wide region of the Milky Way.

Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).

In the original texts

Passages from public-domain books where Solar System is discussed.

Still more clearly was it thus with Jupiter and his satellites or Saturn with his rings and satellites. Remembering that the ultimate units of matter of which the Solar System is composed are of the same kinds, and that they act on one another in conformity with the same laws, we see that, remote as the case is from the one we are considering in all other respects, it is similar in the respect that during…
Herbert Spencer, The Principles of Biology, Volume 1
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Considered genealogically, the received theory respecting the creation of the Solar System is unmistakably of low origin. You may clearly trace it back to primitive mythologies.
Herbert Spencer, Essays
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Why should we speak of it? The universe is an assemblage of solar systems which we have every reason to believe analogous to our own. No doubt they are not absolutely independent of one another. Our sun radiates heat and light beyond the farthest planet, and, on the other hand, our entire solar system is moving in a definite direction as if it were drawn. There is, then, a bond between the worlds.
Henri Bergson, Creative Evolution
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If the velocity of the solar system, that is to say if the constant part existed alone, the observed direction would be invariable. This position that one would thus observe is called the _mean_ apparent position of the star. Taking account now at the same time of the two parts of the velocity of the earth, we shall have the actual apparent position, which describes a little ellipse around the mean apparent…
Henri Poincaré, The Foundations of Science
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In virtue of this law, given the state of the solar system throughout any finite time, however short, its state at all earlier and later times is determinate except in so far as other forces than gravitation or other bodies than those in the solar system have to be taken into consideration. But other forces, so far as science can discover, appear to be equally regular, and equally capable of being summed up in…
Bertrand Russell, Our Knowledge of the External World as a Field for Scientific Method in Philosophy
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Philosophers who discussed Solar System

Related ideas

Opposing perspectives

Planet

Place in the tradition

Part of Agnosticism.

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