Earth and Space Sciences: Stars vs Planets

Stars are giant, luminous spheres of plasma. There are billions of them — including our own sun — in the  Milky Way Galaxy. And there are billions of galaxies in the universe. So far, we have learned that hundreds of stars also have planets orbiting them.

A star develops from a giant, slowly rotating cloud that is made up entirely or almost entirely of hydrogen and helium. Due to its own gravitational pull, the cloud begins to collapse inward, and as it shrinks, it spins more and more quickly, with the outer parts becoming a disk while the innermost parts become a roughly spherical clump. 

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The word goes back to the ancient Greek word planēt, and it means "wanderer."

A more modern definition can be found in the Merriam-Webster dictionary which defines a planet as "any of the large bodies that revolve around the Sun in the solar system."

In 2006, the International Astronomical Union (IAU) - a group of astronomers that names objects in our solar system - agreed on their own definition of the word "planet." This new definition changed caused Pluto's famous "demotion" to a dwarf planet.

The definition of a planet adopted by the IAU says a planet must do three things:

1. It must orbit a star (in our cosmic neighbourhood, the Sun).
2. It must be big enough to have enough gravity to force it into a spherical shape.
3. It must be big enough that its gravity has cleared away any other objects of a similar size near its orbit around the Sun.

Source NASA

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1. Origin

Stars are formed from huge clouds of gas and dust that collapse under the force of gravity and heat up, igniting nuclear fusion in their cores. Planets form from leftover materials that didn't contribute to a star's formation.

2. Composition

Most of the found stars consist primarily of hydrogen and helium. Speaking of planets, there are two types based on their composition — gas planets (that also consist of hydrogen and helium) and terrestrial (rocky) ones.

3. Orbit

Stars don't orbit planets, but planets usually orbit stars. However, there are exceptions, such as rogue (or free floating) planets. They're not gravitationally bound to any star or brown dwarf and casually wander through space on their own. Yes, that's possible! Even our Sun used to have more planets. Rogue planets appear when several large planets fight for a place around a single star and eventually kick their rivals out of that planetary system.

4. Lifespan

While planets have a stable, long-term existence as long as nothing destroys or captures them, stars have a well-defined life cycle from birth to death. This cycle depends on the star’s size — the bigger the star, the shorter its lifetime. For instance, the most massive stars can die after only a few million years, while a Sun-like star can live for about 10 billion years.

5. Diameter

Usually, stars have a bigger diameter than planets. However, there are exceptions, such as white dwarf stars. They are remnants of stars that were once like the Sun but died, shedding their outer layers and leaving only the core behind. That core is only about the size of the Earth. If the star had planets bigger than the Earth orbiting it before it died, it's possible that some of them will survive, and you’ll get a planet bigger than its star.

6. Mass

Stars always have more mass than planets. As mentioned above, if a gaseous planet gains as much mass as a star, it most likely becomes a star. As for rocky planets, there is no known rocky planet with a mass anywhere near that of a star.

7. Atmosphere

Stars’ atmosphere is mostly composed of hot gasses and plasma. Planets, on the other hand, have atmospheres that vary in composition and density. For example, the Earth’s atmosphere is 99% nitrogen and oxygen, while Venus’ and Mars’ atmospheres have more than 98% of carbon dioxide and nitrogen.

8. Habitability

Stars can't be habitable because of the lack of surface area and the intense heat and radiation emitted from their cores. And there are also unhabitable planets with extreme temperatures, lack of breathable air, or toxic environments. But we're a living example of how some planets, like the Earth, can support life.

9. Temperature

Stars are incredibly hot and have high temperatures; planets have relatively low temperatures. But there are curious cases. For example, in 2017, scientists found KELT-9b — a planet with a "surface" temperature of over 4,000°C, almost as hot as our Sun. The reason KELT-9b is so hot is that its star is hot itself, and the planet is very close to it.

10. Number in the Universe

Planets are more common than stars in our universe. Most likely, the total number of planets exceeds the number of stars by a factor of 100 to 100,000. What's even more surprising is that there may be even more rogue planets (not orbiting any star) than stars in the Milky Way

Source Starwalk