CELESTIAL BODIES: CHARACTERISTICS AND TYPES - ARTE - 2025

The celestial bodies are all those natural objects that belong to the universe, have measurable size and have mass, therefore they are capable of interacting gravitationally with each other. Designating astronomical objects as celestial bodies derives from the fact that they can be seen from Earth, occupying different positions in the sky.

Some authors consider that a celestial body is an entity or individual body, different from an astronomical object. Others even claim that the heavenly bodies are only those that belong to the solar system. In this case, only one star would be considered a celestial body: the Sun, the others would not.

Figure 1. Venus and Mars in the night sky. Source: Wikimedia Commons. Al-Demon / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

In this sense, the beautiful cluster of the Pleiades (the Seven Goats or the Seven Sisters) would not be a celestial body, although we can see it from Earth. But an individual star in the cluster would be a celestial body, unless we limit ourselves to the solar system.

Throughout this article we will consider as celestial bodies those that can be distinguished with the naked eye, with instruments or are detected by their effects on other known bodies, that have individual character and that also possess the characteristics described at the beginning: having mass and measurable size. With mass comes the ability to gravitationally interact with other celestial bodies.

Types of celestial bodies

We will classify the celestial bodies into two large groups:

-Those that emit their own light

-Those that reflect the light emitted by other stars.

In the first group we have the stars, while in the second there is a great variety of bodies that includes: planets, natural satellites, dwarf planets, asteroids, meteoroids, comets and trans-Neptunian objects.

Except for planets, satellites, and dwarf planets, all other objects are classified as minor bodies in the solar system.

Stars

Illustration of the solar system; it shows the Sun - a star -, inner planets, asteroid belt, outer planets, Pluto, and a comet. This image is not to scale.

They are gigantic balls of gas, mainly hydrogen and helium, the lightest elements that exist, inside which there is a nuclear reactor that continuously transforms them into heavier elements.

Thanks to these reactions, the star gives off enormous amounts of energy, in the form of light and heat.

During their life, the stars remain in equilibrium thanks to the action of the force of gravity that compresses them, and the pressure of the constituent gases, which tends to expand them.

There are all kinds of stars, from supergiants to dwarfs, as well as colors, which are determined by the temperature of the stellar atmosphere.

The life of stars depends on their initial mass: smaller stars, whose mass is similar to or less than that of our Sun, are stable and have longer lives. Very massive stars, on the other hand, have short lives and tend to end in catastrophic supernova events.

Planets

Jupiter and the Galilean moons.

Planets are bodies that lack their own light, since they do not have a fusion reactor at their center. They orbit the central star, which in the case of the solar system is the Sun.

The planets of the solar system are divided into two categories: rocky terrestrial planets and giant planets, also called Jovian planets, because Jupiter is the prototype. The former are dense and small, the latter are light and much larger.

For a long time the only known planets were the 8 planets that orbit the Sun, the Earth included.

But an extrasolar planet was detected for the first time in 1992, and more are discovered with each passing year. Almost all of them are of the Jovian type, that is, gas giants in which life as we know it is hardly found.

However, terrestrial-type planets are known, being the planet Kepler-438b, 473 light years away, the one that most closely resembles Earth to date.

Natural satellites

Almost all the planets in the solar system have natural satellites, rocky bodies that orbit the planet, instead of around the Sun. Only Mercury and Venus, the closest to the Sun, lack satellites.

Figure 2. Jupiter and its moon Europa. Source: Giphy.

Rocky planets like Earth and Mars have few satellites, but giant planets number them by the dozen.

The largest of all is Ganymede, one of Jupiter's four Galilean moons, named for Galileo, the first to see them through his telescope.

Tiny planets

It is a more recent category of celestial bodies, the best known member of which is the ancient planet Pluto.

Although they have almost all the characteristics of the larger planets, it is considered that the dwarf planets lack a "clean" orbit, that is, exclusively dominated by the gravity of the sun king.

So far the dwarf planets are, in addition to Pluto, Ceres, ancient asteroid, Eris, Makemake and Haumea. Of these, only Ceres is close to Earth, since it belongs to the asteroid belt. The rest are beyond the orbit of Neptune, the planet furthest from the Sun.

Asteroids

An asteroid is a rocky body in the solar system, larger than a meteoroid, but smaller than a planet. Most of them are in the asteroid belt, the boundary that separates the terrestrial planets from the Jovian planets.

The rest are scattered around the orbit of Jupiter and to a lesser extent of the other planets. Neptune's orbit constitutes the natural boundaries.

They are very numerous, to date about 600,000 are known, of which about 10,000 have orbits close to that of the Earth.

Their shapes and sizes are diverse. Palas and Vesta are the largest, with a diameter of approximately 500 km. That is why they can rarely be seen with the naked eye, unless they pass close to Earth.

Meteoroids

They are rocky celestial bodies, smaller than asteroids, but larger than cosmic dust. They reach a maximum of 50 m in diameter.

These include fragmented asteroid and comet remnants, debris from the formation of the solar system, or rocks ejected from planets by colossal impacts.

When they reach the vicinity of Earth or any other planet and enter the atmosphere they are called meteors. The friction with atmospheric molecules heats them up and vaporizes them, due to the great speed with which they enter. The fragments that manage to hit the surface are known as meteorites.

Kites

Comet Lovejoy. Source: Wikimedia Commons. John Vermette / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

They are stars made of rock, ice, gas and dust that orbit around the Sun following some conical-type trajectory, almost always elliptical with great eccentricity, which means that they move considerably away from the Sun.

When their orbits bring them closer to the star king, the heat of the Sun and the solar wind give rise to the characteristic hair and tail, but at the same time fragment the comet.

Much of the debris they leave during their visits remains in Earth's orbit. This is how many periodic showers of stars originate in the night skies.

Trans-Neptunian objects

Sedna's orbit in red, compared to Pluto (green) and the inner planets (yellow). Source: Wikimedia Commons. NASA / JPL-Caltech / R. Hurt (SSC-Caltech) / Public domain.

As their name implies, they are rocky celestial bodies that lie after the orbit of Neptune.

Pluto and its satellite Charon, the plutinos, the plutoids, which are the dwarf planets like Eris, Makemake, and Haumea, are also trans-Neptunian objects.

Then there are Sedna, one of the most distant objects in the solar system known so far, and the bodies that populate the Kuiper belt, the Oort cloud and the scattered disk.

References

1. Díaz-Giménez, E. 2014. Basic Notes on Astronomy. Published by the University of Córdoba, Argentina.
2. Pasachoff, J. 2007.The Cosmos. Thomson Brooks-Cole.
3. Powell, M. The Naked Eye Planets in the Night Sky (and how to identify them). Recovered from: nakedeyeplanets.com
4. Seeds, M. 2011.The Solar System. Seventh Edition. Cengage Learning.
5. Wikipedia. Trans-Neptunian objects. Recovered from: es.wikipedia.org.