What are the differences between a star and a planet?
Last Updated: February 6, 2023
Our universe is host to billions of celestial bodies. From various types of galaxies and meteors to planets and stars, observing them can help us develop our understanding of the universe. However, they can often be difficult to distinguish from Earth.
How can we tell the difference between stars and planets, and how can this help us to identify them in the night sky?
What are stars?
Stars are huge celestial bodies made primarily of hydrogen and helium gas. They produce light and heat from their nuclear cores and are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen across the universe.
It is not known how many stars exist, but the number is bound to be astronomical – there are an estimated 300 billion stars in the Milky Way galaxy alone.
What are planets?
Planets are even more common than stars across the universe; based on the assumption that all stars have at least one or more orbiting planets, the number of planets will outweigh the number of stars. What defines a planet, however, is widely debated amongst astronomers.
The criteria for a planet have varied over time as new discoveries and technological advances add to our constantly evolving understanding of the universe – as shown by the demotion of Pluto in 2006. The most recent definition of a planet, as adopted by the International Astronomical Union in 2006, states a planet must orbit a star, be spherically shaped due to gravity, and be big enough that its gravity clears away smaller objects from its orbit.
As our understanding of planetary systems deepens and expands, the definition of a planet will also continue to shift and adapt.
Each star in the universe could host several planets.
How do stars form?
Stars are formed over billions of years. Inside hydrogen-based dust clouds called nebulae, gravity creates pockets of dense matter over thousands of years. This causes the nebulae to become unstable and eventually collapse under their own weight. Although difficult for astronomers to see due to the obscuring of the dust, these have been identified as protostars, the first form stars take.
As protostars get smaller, they spin faster, causing increased pressure and rising temperatures. Over millions of years, the temperature continues to rise and the heart of the protostar begins gathering more dust and gas. Once the core reaches 15 million degrees Celsius (27 million degrees Fahrenheit) nuclear fusion – and the longest stage of a star’s life – begins.
Stars spend 90 per cent of their life in this stage, known as the main sequence. During this period, a star will convert hydrogen to helium and radiating x-rays, emitting enormous amounts of energy, heat and light. The outflow of energy from the core of the star provides the necessary pressures to stop the star from collapsing under its own weight.
Related reading: From Birth to Death: The Complete Lifecycle of Our Sun
The formation of a star can also emit waste materials, impacting the space around it. For example, not all the dust and gas gathered by the hot core of a protostar will become a star. Some may remain as dust, or group to form comets, asteroids, or even entire planets.
How do planets form?
During the transition from protostar to main sequence stage, extremely hot winds are ejected which causes some particles to crash into one another. Once enough particles have clumped together larger objects are formed called planetesimals. These are the building blocks of planets.
In the colder areas of the newly forming star tiny fragments of ice can attach to planetsimals, creating colder regions which allow gas molecules to slow down and attach themselves to the planet. These planets form quickly and attract most of the gas in the area.
This is how gas giants, such as Jupiter, Saturn, Uranus and Neptune (the farthest planet from the Sun) are believed to have been formed. Rocky planets, such as Mercury, Venus, Earth and Mars, are formed much slower in the warmer areas closer to the core of the star.
19 key differences between stars and planets
Stars and planets are two of the most important components of our universe. While they both have a crucial role in the cosmos, there are numerous differences between them. In the below list, we will explore 19 of the most notable differences between these fascinating celestial objects.
- Origin: Stars form from the collapse of clouds of gas and dust, while planets form from the leftover material around a star.
- Size: Stars are much larger than planets, with the average diameter of a star being 1.4 million kilometers compared to the average size of a planet which is 15,000 kilometers.
- Mass: Stars have much greater mass than planets, with the largest stars having hundreds of times the mass of all the planets in a solar system combined.
- Composition: Stars are made mostly of hydrogen and helium, while planets have a wide range of compositions, including rocky, metallic, and gaseous.
- Energy source: Stars generate their own light and heat through nuclear fusion, while planets reflect light from an external source, like the Sun.
- Location: Stars are often at the center of a solar system, while planets orbit around them.
- Life cycle: Stars have a well-defined life cycle, from birth to death, while planets have a stable, long-term existence, as long as nothing disrupts their orbits or nothing too large hits their surface.
- Number: There are billions of stars in the universe, while the number of planets is estimated to be even higher.
- Gravity: The gravity of stars is much much stronger than the gravity of planets.
- Surface: Stars have no solid surface, while planets have a solid or semi-solid surface.
- Magnetic field: Stars have very strong magnetic fields, while planets have much weaker ones.
- Temperature: The temperatures on the surface of stars are much higher than the temperatures on planets, reaching millions of degrees Celsius.
- Atmosphere: Stars have a thin, highly ionized atmosphere, while planets have thicker atmospheres that can support life.
- Rotation: Stars rotate slowly, while planets can have rapid rotation.
- Moons: Many planets have moons, while stars do not have any natural satellites, although you could argue that planets could be considered a star’s moon?
- Rings: Some planets have rings, while stars do not have rings.
- Evolution: The evolution of stars and planets is completely different, with stars evolving from birth to death over billions of years, while planets remain largely unchanged.
- Types: There are many different types of stars, including red giants, white dwarves, and neutron stars, while there are only a few different types of planets, including rocky, gas, and ice giants.
- Life: Planets can sustain life if the conditions are right. Stars can not due to a lack of surface and the intense heat and radiation emitted by their cores.
Stars are indeed much much larger than rocky and gas planets.
Can a star turn into a planet?
Stars and planets appear to be very separate, distinct celestial bodies. However, astronomers have discovered one category of stars that has the ability to transform into a planet – the brown dwarf.
There are a wide variety of categories that stars can be classified as, ranging from white dwarves to red giants. These help astronomers to understand how the star was formed and its current position in its life cycle. The brown dwarf, however, is still hotly debated within the astronomical community.
Brown dwarves do not meet the criteria for either stars or planets. Unlike planets, they are often located at the centre of a solar system and have a much greater mass than average planets. They are smaller than regular stars and do not have enough mass for the nuclear fusion of regular hydrogen to begin.
Brown dwarves do, however, glow.
Although they do not have enough mass for the nuclear fusion of regular hydrogen, brown dwarves can ignite the nuclear fusion of heavy hydrogen – hydrogen with a neutron in its nucleus as well as a proton, making it much heavier. As heavy hydrogen is much rarer in the universe the brown dwarf will burn up its store quickly, resulting in a loss of heat and light emission much earlier in its life than another category of star. When this happens the brown dwarf behaves and looks more like a planet, resulting in a collection of planets orbiting a larger central planet.
As no star is emitting heat to the system, it becomes very cold and dark. This darkness makes it much more difficult for astronomers to identify them. Although only 3000 brown dwarves have been identified by telescope as of 2018, NASA believes there are far more in our universe – we just cannot see them.
How to tell the difference between stars and planets in the night sky?
A large number of stars and planets can, however, be seen from Earth. The majority of celestial bodies visible are stars; NASA estimates that approximately 10,000 stars are visible from Earth through a telescope, largely due to their greater size and the energy they emit. The colour of a star can also vary depending on its temperature – cooler stars can appear orange or red while warmer stars may appear to have a white or blue hue. Stars are also more likely to appear in small clusters. Computer models of protostars by NASA predict the collapsing gas and dust may break up into two or three separate bodies which stay grouped together.
Although the vast majority of celestial objects that can be seen from Earth are stars, some planets are also visible. While stars seem to twinkle due to the atmospheric scintillation, planets emit a more stable light. The position of the celestial body in the night sky can also help to identify it; planets move considerably in a short space of time due to their orbits, whereas the position of stars appear to move over a longer period of time due to the Earth’s orbit and axis rotation. As planets – including the Earth – move around the Sun at different rates, their visibility varies throughout the year, and from year to year.
Due to the constantly evolving research conducted by astronomers, our understanding of stars and planets, including how to identify them from Earth, has deepened and expanded. As technology advances and more discoveries are made, this will continue to develop, allowing us to learn about – and see – more of the Universe than ever before.
StarLust has a number of stargazing guides that can help you identify on which dates planets will be visible from where you are and also offer advice on how to locate specific planets and constellations.
Both planets and stars appear as points of light in the sky. Stars, however, twinkle a lot while planets do not.
Written by Amy Donnelly
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