Stars are very different from planets. They are generally larger, are made of hot gas, and produce enormous amounts of energy by nuclear fusion, which makes them shine. Planets are smaller, do not shine, and depend upon stars for light and heat. That difference is fundamental, which makes it intriguing that stars and planets are made using the same recipe. The only difference is how much of the cosmic recipe mixture is used.
In every case we start off with a huge cloud of cosmic gas and dust. We see lots of these clouds in our galaxy and others. Sometimes an event, like the explosion of a nearby aging star, makes a cloud become unstable. It then collapses under its own gravity, becoming a selection of lumps of various sizes growing by colliding with and merging with other lumps.
The energy of the impacts is converted to heat, making the lumps become very hot. That is why planets like the Earth were molten in their extreme youth. The cores of the lumps are subjected to higher and higher pressures due to the weight of the increasing amount of overlying material. This compression makes the core even hotter. If the temperature in the middle reaches more than about ten million degrees, and the pressure is high enough to stop the core material expanding and cooling, nuclear fusion starts, liberating energy. The lump has become a star. Typically, if a lump accumulates more than about a tenth of the mass of the Sun, it will graduate to starhood. If the lump has a smaller mass than that, it will radiate its heat into space, gradually cooling off. This could take many millions, even billions of years to happen. These small lumps are what we call planets. Our Earth is a lump that failed to graduate as a star because it is far too small.
However, because the least massive stars achieving nuclear fusion are called “red dwarfs”, those just failing to graduate have become referred to as “brown dwarfs”. How then do we know what to call a planet and what to call a brown dwarf? It’s not easy. Just recently a brown dwarf was discovered that has a surface temperature lower than 100 C, the boiling point of water. There are planets hotter than that.
Jupiter, the biggest planet in our Solar System, is still cooling off, losing the heat it gained when it formed. However, we have decided it is a planet. We have decided, largely arbitrarily, that if a lump has a mass larger than about ten Jupiters, but not achieving nuclear fusion, it is a brown dwarf. Otherwise it is a planet.
Whether we want to make a star, brown dwarf or a planet, we start off with exactly the same mixture. What we succeed in cooking depends totally upon the amount we use. That is really fascinating. However, although we know “what happens” we are still a long way from understanding completely “how it happens”. The interplay of forces in a collapsing cloud of cosmic star and planet mixture is extremely complicated. Thanks to faster computers, we are getting to understand it.
Saturn rises about 7 p.m. The Moon will be Full on the 17th.
Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.