How Stars and Planets Differ

Planets and stars may look very much alike in the night sky, but they are very different in their nature.

As strange as it may seem, the biggest difference between the two is size. Planets can be gaseous or rocky, but in the former case, if they are below a certain size, they cannot become stars. A great example can be found in our own solar system. Jupiter, one of the planets in our solar system, is made primarily of gas. Had it been a little larger, it would have had enough mass for nuclear burning to begin, and our solar system would have 2 suns, and not just one. Yet, as large as Jupiter is, it is tiny in comparison to our sun. Our sun is a rather small star, but there are those that are far smaller.

A star is formed when dust and gas, mostly hydrogen, becomes concentrated in one place. Gravitational forces pull the dust and gas gradually toward the center, a process that can take tens of thousands to millions of years. As the gas collapses in toward the center, it creates heat, and as it becomes more dense, at a point of critical mass, the heat that is generated, coupled with the pressure of billions of tons, is sufficient to cause the gas to begin burning with a nuclear fire. This is the basis of the Hydrogen Bomb, where hydrogen atoms are fused to create Helium, only on an immensely larger scale. This is a birth of a star. The additional gases and dust continue to condense into globes, and so long as critical mass is not reached, these become planets, orbiting around the star.

This is what has happened in our own solar system. In some other star systems, one or more of the planets do reach critical mass and become stars. Most star systems, in fact, are made up of two or more stars, created in this way.

Planets, on the other hand, are the result of gas and debris gathering, but not in a great enough quantity to reach critical mass. Some of these can be tiny, as Mercury is, barely the size of our Moon, or they can be huge, like Jupiter which is very much larger than the Earth. In our solar system, and most scientists believe that this is true of most stellar systems, the inner most planets tend to be rocky, mostly made of the ancient dust, as are Mercury, Venus, Earth, and Mars. The outer planets, like our own Jupiter, Saturn, Neptune, and Uranus, tend to be made of large amounts of gas around a solid core.

Contrary to popular belief, you can’t tell a planet in the night sky due to how much it “twinkles”. The twinkling is caused by differing thermal layers in the Earth’s atmosphere, in much the same way that objects seem to be distorted when viewed over the top of a hot road. Since the planets as well as the stars are outside of our atmosphere, they all twinkle. The amount they twinkle also makes little difference, since this depends on the amount of light that is reflected from the sun. (All planets that can be see from earth, can only be seen due to the light that is reflected from our own sun.) This is true of the Sun and Moon as well, but because of their brightness and apparent size, it isn’t very noticeable. Venus, which is actually the most luminescent object in the night sky from Earth, also twinkles. But because of its brightness, this is also less noticeable. The other planets that can be seen from the earth by the naked eye; Mercury, Mars, Jupiter, and Saturn, twinkle far more and are often not discernible against the background of stars, unless a person knows exactly where to look for them.

Being able to tell the difference between planets and stars in the night sky is one of the fun parts of amateur astronomy, and it is a point of pride, showing that you are learning. They are very different. But give it a try…see if you can identify some of the planets that are visible in the night sky!