Is it a Planet or a Star?

The question is often asked “What is the difference between a planet and a star?” The answer will be obvious when we have gained a little knowledge of both types of celestial objects.

Looking up at the sky on any clear night we see apparently countless stars ranging in brightness from those at the limit of our vision to ones which are outstandingly brilliant. Some nights we notice that the stars are twinkling, but there may be one or two among the brighter ones that do not twinkle. Note the positions of these with reference to the pattern of surrounding stars, and watch them carefully for a week or more. It may surprise you to find that, unlike the so-called “fixed” stars, these non-twinklers have moved from the positions first noted. These are the planets.

All the stars which we see on any clear night are members of that vast organization called the Milky Way. The sun is an average member of this group, but as the dominating body in our solar system, is of great importance to us. The Earth and the other planets all move in definite paths about the Sun and consequently the planets move in relation to the background of stars.

Let us outline briefly some other characteristics of the planets and stars to see where they differ, for we shall find that in practically no characteristics are they alike. The stars – like the Sun – are known to be enormous spheres of gaseous material, so hot that they radiate their own light. On the other hand the planets are small, they are cool, and shine only because they reflect some of the sunlight falling upon them. To make a comparison of size, Jupiter, the largest planet, has a diameter of 88,700 miles, while an average star – our sun – has a diameter almost 10 times that amount.

Some of the stars are so enormous that were they hollow, the four inner planets could easily revolve in their paths within one of these giants. Considering temperatures, it is known that the planets nearest the Sun would be uncomfortably hot for us, while the more distant ones would be much too cold. Jupiter, for instance, is about -150 degrees C. However, on stellar standards all the planets are chilly, for a “cool” star like the sun has a surface temperature of about 5,500 degrees C, while “hot” stars have temperatures 9 or 10 times that amount, and perhaps even higher.

Experience has shown that regardless of the telescope used, stars appear as pin points of light, while planets show a sizeable disc. The reason for this is difference in their distances. Pluto, at a mean distance from the Sun of 3, 664, 000, 000 miles, is the most remote solid corps in Solar system, but stellar distances range from 26, 000, 000, 000, 000 miles to many thousands of times that distance. The yardstick used for that measurement is usually the light-year rather than the mile, and on this scale the nearest star is 4, 3 light-years away. A light-year is a distance travelled by light in a year, at the rate of 186, 300 miles per second.

Summing up the main points indicated above we find that:

  • Planets do not twinkle – Stars twinkle
  • Planets move among the stars – Stars appear to be “fixed”
  • Planets revolve about the Sun – Stars do not revolve about the Sun
  • Planets are relatively small – Stars in general are very large
  • Planets are “cool” – Stars are extremely “hot”
  • Planets shine by reflective sunlight – Stars shine by their own light
  • Planets appear as discs – Stars appear as points of light
  • Planets are near to us – Stars are very far away.


Stars and Planets are part of our Univers. Photo: Megan Jorgensen