What's the difference between a star and a planet?

by: Robert Huisman


If you want to examine what conditions are necessary for a planet to support live, an important question will be: When do you call an object a planet? Or what is the difference between a star and a planet? In this article I will try to explain the exact difference between the two and I will look at the atmosphere, the mass and the temperature, to examine if live is possible on these objects.

Star formation

A star forms when a very big cloud of gas contracts under the influence of its own gravitational force. As this contraction takes place the object emits energy. This energy is called fall energy. As a result of this contraction the core gets denser and hotter. When the core reaches a temperature of about 3 million Kelvin, it starts to emit light, because of nuclear fusion  ( Krane, K.S) reactions in the core. At this stage the gas cloud will stop its contraction because now the gravitational force is in equilibrium with the pressure build up by the hot gas. When this starts to happen you can say that a new star is born.

Planet formation

A planet on the other hand is build up out of the dust that surrounds a star. When a star is formed there is still a disk of gas surrounding it. As this gas cools, it condenses and forms solid grains. These grain particles accrete into large bodies called planetesimals, which then collide and accrete to make protoplanets. These protoplanets evolve into planets like the planets in our own solar system.
So the formation of a star is totally different from that of a planet. This is the main difference between a star and a planet. If an object has a mass of 0.084 times the mass of our own sun (85 times the mass of Jupiter) the core reaches a point where it can start the process of nuclear fusion in its core (see fig.1). If the mass is smaller than this, the lowest temperature to support nuclear fusion will never be reached and the object will never shine like a star. But can we call all of these objects planets? No, objects with a mass between 85*Mj (85 times the mass of Jupiter) and 13*Mj can't sustain nuclear fusion of elements like hydrogen (H) and helium (He) but can support the fusion of two protons into deuterium (D), early in their lifetime. These objects are called brown dwarfs. They form the transition between stars and planets.

Fig.1: The temperature in the centre of gas bulbs with a mass between 0.04                        
and 0.10 times the mass of the sun related to their radius.
Fig.2: Saturn (A jovian planet).

Brown dwarfs

Brown dwarfs form like stars so you can't call them planets, but they have a mass that is to small to sustain the nuclear fusion process that takes place inside a star, so they aren't really stars either. As such an object contracts under the influence of it's own gravity, it doesn't reach the temperature that is needed to start the nuclear fusion from H-nuclei into He-nuclei. But it does reach a high enough temperature for the fusion of protons into deuterium. Because of this fusion it emits light during the first period of it's lifetime. It also emits light because of the fall energy that is produced as a result of the contraction of the gas, but this forms only a minor contribution to the total emission of light. Because very little or no fusion takes place, the core of the star can't build up enough pressure to prevent the star from further contraction under the influence of the gravitational forces that are working on the gas. The gas in the center of the star gets so dense that it degenerates. Now the star won't contract any further because of the pressure that is build up by the degenerated matter. This pressure is called electron degeneracy pressure (EDP) (Kulkarni, S.R.). The origin of this pressure is explained by quantum mechanics as arising from oscillations of confined electrons. Because the fusion and contraction have stopped, the only light emission that is left is due to the cooling of the atoms of the star. During the rest of it's lifetime the star will get dimmer and dimmer and eventually and up as a cool object emitting light in the infrared. The chemical composition of the atmosphere of a brown dwarf strongly depends on it's temperature. But for an atmosphere similar to that of the first detected brown dwarf (gl229B), chemical equilibrium calculations indicate that the upper layers of it's atmosphere mainly exist out of methane (CH4), ammonia (NH3), water (H2O) hydrogen sulfide (H2S) and phosphine (PH3). However deep in the atmosphere methane is "replaced" by carbon monoxide (CO) and ammonia is "replaced" by nitrogen (N2) (Marley, M.S.).


Planets can be divided into two different groups. The smaller, solid terrestrial planets (like the earth) and the large, liquid Jovian planets (like Jupiter and Saturn). I will concentrate on the jovian planets because the border between brown dwarfs and planets lies in the mass range of these planets. I already explained how planets are formed, but Jupiter and Saturn may have formed in another way. They may have formed like stars. That is they may have formed out of a gas cloud that contracted under the influence of the gravitational force working on it. In this case the only difference between brown dwarfs and jovian planets is the fusion of protons into deuterium in the core of the brown dwarfs. The jovian planets are large gas bulbs with a small massive core, or in the case of bigger planets, the core may exist out of degenerated material. The very thick atmosphere is build up out of several different layers. The principal constituents of the atmospheres of the jovian planets are molecular hydrogen (H2) and helium (He). The outer most layer of the atmosphere (the photosphere), is build up out of a mixture of these gasses. Underneath this layer is a thick layer of liquid hydrogen. Then you get a layer liquid metallic hydrogen and in the center there possibly exists a rocky core. Although most of the atmosphere consists out of hydrogen and helium there are a lot of other molecules in the atmospheres of the jovian planets. As already mentioned, which elements there are, strongly depends on the temperature of the planets. The temperatures of the extra solar planets that have been discovered until now, differ very much from each other, with values ranging from 100-1500 Kelvin. So the chemical composition of the atmospheres are also very different. Figure 3 gives a rough plot of the chemical species that are likely to condense near the photosphere for a given effective temperature. It is most likely that these kinds of planets can't support life because they aren't solid like terrestrial planets, but for more information on this subject you have to visit Saskia's page.

Fig.3: Chemical species that are likely to condense near the photosphere for a given effective temperature, indicating the nature of the newly discovered extra solar planets.


The border between stars and planets is not very well defined. There is a transition area with objects that show some resemblance to a star and some to a planet. These objects are called brown dwarfs. The mass of a brown dwarf lies between about 13-85 Mj. They resemble jovian planets very well. The only difference between the two lies in there formation. Planets are normally formed out of the dust clouds surrounding the star. Where as brown dwarfs are formed by contraction of a gas cloud. It may be the case that Jupiter and Saturn are formed in the same way that stars are formed. In this case we can't distinguish them from brown dwarfs. Another important difference between the two is the small amount of nuclear fusion that can take place in the center of a brown dwarf. So this is maybe a better way to distinguish them from jovian planets. A problem here is that only the biggest brown dwarfs can sustain this fusion. The structure of brown dwarfs and jovian planets is very much the same. The chemical composition of these objects strongly depends on the temperature of the gasses in there atmospheres. Because jovian planets aren't solid like terrestrial planets it is most likely that they can't support life. Because of the rapid progress there is made on this field of science, it won't be long before we can say with more certainty what the atmospheres of these objects are made of and what there temperatures are. Maybe then it is possible to give a better definition of the difference between a star and a planet.


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