Difference Between Planet and Brown Dwarf

What does a brown dwarf? What is a planet? Where is the line between the two?An astronomer of the University Johns Hopkins (United States) offers a new way to classify these objects that abound in the universe.

These questions have given trouble to scientists for years. If the rules are established, the astronomer Kevin Schlaufman, in an article published in the Astrophysical Journal, nevertheless advocates a new classification system. Schlaufman observed 146 solar systems, which enabled him to fill some gaps in our understanding of the formation of brown dwarfs and planets. And the mass of the object here won't be the only factor at play.

A brown dwarf, we are told, is a star aborted, not massive enough for nuclear reactions can be maintained in his heart. It's also bigger than a planet, and varies on average between 10 and 80 times the mass of Jupiter. The range is wide. But what is the lower limit? Schlaufman would like to set this limit to between 4 and 10 times the mass of the largest planet in our solar system, Jupiter. "A limit on the masses of planets is one of the most important details that were missing", noted astronomer. Above this limit, the object would be thus classified as "brown dwarf".This new classification has also to take into account how the object formed. So let's talk about "metallicity.

Indeed, the stars have a metallicity content. In Astrophysics, this means that the fraction of the mass of a star who's not hydrogen or helium, is - in fact - of the metal. It should be noted, however, that the early universe was that of hydrogen and helium, and the almost insignificant amounts of the two following, lithium and beryllium. The first stars were therefore not metallicity, or even almost no. Today, 13.5 billion years after the Big Bang, young stars like our Sun contain metal. The latter was formed there are about 5 billion years, and is still largely composed of hydrogen and helium. But about 2% of its mass is made up of other items, mainly oxygen, carbon, neon and iron. This is because generations of stars have lived and died, expelling heavy elements in space, later other stars have been generated. It is the study of Schlaufman comes into play.

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Comparison of the sizes of the Sun, a star of low mass, a red dwarf, the Jupiter and the Earth.

Credits: NASA / JPL-Caltech / UCB

According to him, we could distinguish the giants - like Jupiter - brown dwarfs by the nature of the star around which they orbit. The types of planets that form around stars reflect the metallicity of the star itself. The gas giants like Jupiter are generally orbiting stars whose metallicity is equal or superior to that of our Sun. Brown dwarfs are less difficult: they form around almost any Star. What for?

The planets like Jupiter are formed by accretion, which means that they come from a process that makes them soft: a rocky core is formed, then the gas accumulates around. But for that to happen, you need metal. If they are present for the training of these rocky cores, their presence will be reflected in the metallicity of the host star. However brown dwarfs are not formed by accretion as are the planets, since they are formed in the same way as the stars, namely by gravitational collapse. The metallicity is therefore not a determining factor. What astronomer proposing in his study, is to determine the part mass that an object does take into account of the metallicity of a star. He then concluded that the objects above 10 times the mass of Jupiter not cared, these not forming from Rocky cores. Therefore, they are not planets similar to Jupiter, are brown dwarfs that are formed by gravitational collapse.

What does this mean? The current definition of what is a planet is based on these three criteria: a planet is orbiting a star, has sufficient mass to assume a hydrostatic equilibrium (a nearly round shape), and did clean up around its orbit. For example, Pluto no longer met these three characteristics. In these criteria determining the mass, but not the upper limit that would make these planets of brown dwarfs. Our naming conventions for astronomical objects are important, because they help to define how everything fits. So we need to understand what makes different objects, and how the as reflects this difference.

This principle applies to brown dwarfs and the gas giants. It is not enough to give them names based only on their mass. This new definition takes here meaning to the extent where it determines an object is based more on its mass, but also on the formation of this object. But still, everyone will not agree. And the debate has only just begun.