Our solar system consists of one star and 8 planets, both rocky and gaseous.The Sun contains 99.9% of the total mass of the solar system.Even the gas giant planet, Jupiter, is ONLY 1/1000th or 0.1% of the mass of the Sun.
Given this disparity, you might think that planets and stars are vastly different.However, there are types of stars and types of planets that are much closer in physical characteristics.
Rocky planets, like Earth and Mars, form when solid particles (dust) coalesce, forming larger objects, perhaps large enough to be called a planet.
Gas giants, such as Jupiter and Saturn, form from cores of rock and ice first (up to 10 earth masses), then the gravity of those cores draws in hydrogen and helium gas.The mass of the gas giants is so great that over time, they shrink under their own gravity.Jupiter shrinks about 1 mm per year.This contraction of gas increases the temperature which results in the emission of energy across the electromagnetic spectrum, but 99.9% is in the infrared range. In that sense, it is similar to a star.Amazingly Jupiter emits more energy than it receives from the Sun, but very little of that energy is visible to the eye.
Hydrogen/helium objects that are 13-80 times the mass of Jupiter can collapse only to the point where further contraction cannot occur due to ‘electron degeneracy,’ i.e. the repulsive force of negatively charged electrons.The temperature cannot increase sufficiently to support hydrogen fusion; hence they become ‘failed stars,’ AKA ‘brown dwarfs.’A brown dwarf with a mass of 80 Jupiters will have a mass of only about 8% of the Sun’s mass.As with Jupiter, brown dwarfs emit infrared energy due to contraction.
However, the object will also contain a minute amount of deuterium, which is an isotope of hydrogen containing a neutron.Deuterium will initiate fusion at a much lower temperature than hydrogen, releasing energy.Hence, a brown dwarf emits enormously more energy than a hypothetical gas giant planet 80 times the mass of Jupiter. However, it will never reach the very high temperatures we associate with a typical star.Brown dwarfs emit very little in the visible range and detection requires infra-red sensors.
TYPICAL stars form when clouds of hydrogen with sufficient mass collapse.The cloud may contain other matter, but hydrogen is the key ingredient. At a certain point, the increasing temperature is sufficient to initiate hydrogen fusion in the core and a star is born.The subsequent life of the star is determined based on its initial mass.We measure the mass of a star relative to the mass of our Sun.If the mass is less than 8% of the Sun, then it is unable to initiate hydrogen fusion, but can initiate deuterium fusion, that is, a brown dwarf.
Hence, we see that the difference between a gas giant planet and a brown dwarf ‘star’ is just a matter of threshold related primarily to the mass of hydrogen.Some people might even consider Jupiter a failed star.However, its mass is too low to fuse deuterium.
Music written and produced by Kenny Mihelich (ma-HELL-itch). Western Slope Skies is produced by the Colorado Mesa University Astronomy Club, the Western Slope Dark Sky Coalition, and KVNF Community Radio. This feature was written and voiced by Bryan Cashion.