a planet and a star

What are the differences between a star and a planet?

Last Updated: September 28, 2021

Our universe is host to billions of celestial bodies. Ranging from galaxies and meteors to planets and stars, observing them can help us to 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.

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 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.

The key differences between stars and planets

Although both celestial bodies are of significant size, stars and planets are considerably different. A key difference is a light source; stars emit light and heat, while planets reflect light from an external source such as the Sun. Stars also have a greater mass than planets. For example, the average diameter of a star is 1.4 million kilometres whereas an average-sized planet is an estimated 15,000 kilometres in diameter.

Related article: Discover the largest stars in the universe

The planets in order by size

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 majority of celestial bodies visible from earth are stars, planets can also be seen. Unlike stars, which appear to twinkle due to their helium emissions, the light of a planet appears more stable. 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. The StarLust website 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.

Venus in the night sky

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

I am a lover of folklore, archaeology & anthropology. When I am not studying history, I like to learn about space exploration and imagine what kind of future awaits us amongst the stars. Follow me on Instagram!

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