Planet

What Exactly Is a Planet?

A planet is a large, rounded astronomical body in orbit around a star, stellar remnant, or brown dwarf. The Solar System has eight planets by restrictive definition: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

How Do Planets Form?

The nebular hypothesis proposes that planets form from interstellar clouds collapsing into protoplanetary disks, where material accumulates through accretion. This process is like a cosmic nursery, where smaller particles come together to form larger bodies.

Planets in Our Solar System

The term planet comes from the Greek ‘wanderers’ and initially referred to the Sun, Moon, and five visible points of light in antiquity. Planets have historically had religious associations and were recognized as a separate entity when heliocentrism replaced geocentrism in the 16th century.

Categories of Planets

The planets can be divided into categories based on their composition: terrestrials (rock and metal), gas giants (primarily hydrogen and helium), ice giants (water, methane, and ammonia), and dwarf planets. Dwarf planets include Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.

Exoplanets: Beyond Our Solar System

The level of metallicity in a star determines its likelihood of having a substantial planetary system. Metal-rich stars are more likely to have planets than metal-poor stars. An exoplanet is a planet outside the Solar System. As of 6 October 1995, the first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred when Michel Mayor and Didier Queloz announced the detection of 51 Pegasi b.

Characteristics of Exoplanets

The first confirmed discovery of an exoplanet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz announced the detection of 51 Pegasi b. Since then, more than 100 planets have been identified that are approximately the same size as Earth, with one in five Sun-like stars expected to have an Earth-sized planet in its habitable zone.

Planetary Atmospheres and Rotation

A planet’s defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal.

Rotation and Axial Tilt

The rotation period of a planet is known as a stellar day, with most planets rotating counter-clockwise around the Sun. Venus and Uranus are exceptions, with Uranus having an extreme axial tilt that makes its rotation convention ambiguous.

Planetary Interior and Magnetic Fields

The interior of a planet differentiates by density, causing higher density materials to sink towards the core. Smaller terrestrial planets lose most of their atmospheres due to accretion, but can be replaced by outgassing from the mantle and comets.

Magnetic Fields

Gravity causes planets to be pulled into a roughly spherical shape, so a planet’s size can be expressed roughly by an average radius. However, planets are not perfectly spherical; for example, the Earth’s rotation causes it to be slightly flattened at the poles with a bulge around the equator.

Planetary Science and History

The concept of a planet has evolved over time, with changes in what counts as a planet. Ancient civilizations believed Earth was the center of the Universe, and planets were thought to orbit around it. The Babylonian Venus tablet of Ammisaduqa is the oldest surviving planetary astronomical text, dating back as far as the second millennium BC.

Modern Discoveries

The ancient Greeks developed their own planetary theory, which included the Earth, Sun, Moon, and planets revolving around a ‘Central Fire.’ Pythagoras and Aristarchus of Samos proposed heliocentric systems. Greek astronomers developed mathematical schemes for predicting planet positions based on geometry.

Planetary Classification

The IAU definition has not been universally used or accepted. In planetary geology, celestial objects are defined as planets by geophysical characteristics. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight.

Planetary Names

The naming of planets has a rich history, with names often derived from ancient mythologies and cultures. Earth’s name comes from the Old English word ‘eorþe’ meaning ground or dirt. Non-European cultures use various planetary-naming systems.

Conclusion

In summary, our understanding of planets has evolved significantly over time, from early beliefs to modern scientific discoveries. The definition and classification of planets continue to be a subject of debate among astronomers and planetary scientists, reflecting the dynamic nature of our exploration of the cosmos.