Planet: Definition

DEFINITION

The planet is a celestial body orbiting around a star. There are different definitions of the planet, depending on the size of the star and its mode of formation. By the broadest definition, any celestial object massive enough to be spherical and clean enough to orbit a star is a planet.

What is considered a planet according to NASA's definition

The planet we inhabit is called Earth. It is the third planet in the solar system, located approximately 150 million kilometers from the Sun. Earth is a rocky planet, with an atmosphere composed mainly of nitrogen and oxygen. Life was possible there thanks to the appearance of water, around 4.6 billion years ago.

But what we consider a planet according to NASA's definition is very different from what we see in the solar system. Indeed, NASA considers a planet to be a celestial body orbiting the Sun, having sufficient mass for its gravity to maintain it in a quasi-spherical shape and having eliminated most of the surrounding matter by its own gravitational field.

According to this definition, there are eight planets in the solar system, namely: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. However, in 2006, the planet Pluto was declassified by NASA, as it no longer met the criteria for the planet definition. Indeed, Pluto is much smaller than the other planets in the solar system and its orbit is very elliptical, which means that it sometimes passes closer to the Sun than Neptune.

But the definition of the planet does not stop there. Indeed, there are several scales that make it possible to classify the planets. The best known is the Jovic scale, created in 1992, which classifies planets according to their size. According to this scale, there are five categories of planets: brown dwarfs, super-Earths, gas giant planets, rocky planets and dwarf planets.

The first category, brown dwarfs, includes celestial bodies whose mass is less than the planet definition limit, which is about 13 Earth masses. Brown dwarfs are therefore celestial bodies orbiting the Sun, but which do not have enough mass to be considered planets.

The second category, super-Earths, includes planets whose mass is between the planet definition limit and approximately 10 times the mass of Earth. Super-Earths are therefore planets more massive than Earth, but less massive than giant planets.

The third category, gas giant planets, includes planets with masses greater than 10 times the mass of Earth. Gas giant planets are therefore very massive planets, composed mainly of gas.

The fourth category, rocky planets, includes planets whose mass is less than the planet definition limit, but greater than the brown dwarf definition limit. Rocky planets are therefore planets more massive than brown dwarfs, but less massive than super-Earths.

The final category, dwarf planets, includes planets whose mass is less than the brown dwarf definition limit. Dwarf planets are therefore very low-mass planets, composed mainly of rock.

In summary, the planet we inhabit, Earth, is a rocky planet in the super-Earth category, according to NASA's definition. It is located about 150 million kilometers from the Sun and is part of the solar system, which includes eight other planets.

What is considered a planet according to the definition of the International Astronomical Union

The International Astronomical Union (IAU) is the organization that officially defines what a planet is. According to the IAU, a planet is “a celestial body orbiting the Sun, sufficiently massive for its gravitational field to maintain it in a spherical shape, and sufficiently distant from other objects in the Solar System for its integrity to be preserved by its own gravity “. This definition therefore excludes the Moon, because it orbits the Earth, not the Sun. Likewise, it excludes asteroids, because they are not massive enough to be spherical in shape.

However, this definition has caused a lot of controversy, because it also excludes Pluto, which was considered a planet for many years. In 2006, the IAU therefore decided to modify its definition and introduce the new category of “dwarf planets” to include Pluto. According to the new IAU definition, a dwarf planet is “a celestial body orbiting the Sun, massive enough for its gravitational field to hold it in a spherical shape, but not massive enough for it to burn hydrogen as a star “. This definition therefore includes Pluto, but also excludes asteroids.

Alongside the IAU, there is another scale, called the Klass scale, which classifies objects in the Solar System into different categories. According to this scale, a planet is “a massive object orbiting the Sun, massive enough that its gravitational field gives it a spherical shape, but not massive enough that it burns hydrogen like a star.” This definition therefore includes Pluto, but also excludes asteroids. It is therefore very similar to the new IAU definition.

In summary, what counts as a planet according to the IAU definition depends a lot on the mass of the object in question. If the object is massive enough, it is considered a planet, but if the object is too small, it is considered an asteroid.

What we consider to be a planet according to the definition of different researchers

According to most researchers, a planet is a massive celestial body large enough for its gravity to hold it together, whether in the form of a sphere or an ellipsoid. This definition therefore excludes asteroids and comets, even if some researchers consider comets to be dwarf planets.

Other researchers, on the other hand, consider that asteroids and comets can also be considered planets, because they are also massive and large enough that their gravity holds them together. This definition therefore also includes asteroids and comets.

Some researchers go even further and consider that any celestial object that is massive and large enough for its gravity to hold it together is a planet, whatever its shape. This definition therefore also includes stars and galaxies.

In summary, what we consider to be a planet according to the definition of different researchers is quite variable. For some, a planet is just a massive celestial body large enough for its gravity to hold it together, in the form of a sphere or ellipsoid. For others, a planet can be any celestial object that is massive and large enough that its gravity holds it together, regardless of its shape.

What we consider to be a planet therefore depends on the definition we adopt. According to NASA, any celestial object orbiting the Sun that is massive enough that its gravity gives it a nearly spherical shape is considered a planet. According to the International Astronomical Union, a planet is a celestial body whose mass, composition, and history are sufficiently different from those of other celestial bodies that it can be considered a distinct object. According to different researchers, a planet is a massive object orbiting a star that has enough gravity to make its shape almost spherical, but does not have enough gravity to eliminate other celestial bodies from its orbit. influence.