Scientific theories are constantly changing as we learn new information about the world around us. It can be strange to look back at some of the things people believed before modern science and technology came around. No doubt in the future someone will look back on what we currently believe and think its obsolete. With that in mind, here are several scientific ideas that have become obsolete, or were just proven wrong.

Planet Vulcan – Originally proposed to explain Mercury’s strange orbit, for years witnesses had claimed to see the mysterious planet orbiting the sun. When Einstein’s Theory of General Relativity came out, and helped to explain Mercury’s orbit, the search was abandoned. It did help inspire Star Trek creators, however.

Ever Expanding Earth – Before the idea of plate tectonics, the behavior of the Earth was widely attributed to the theory of the expanding Earth. It was explained that as the Earth gradually grew larger the space between continents would expand, as would the Earth’s crust, which would create mountains. Although the theory has never officially been proven wrong, it has been replaced by the theory of plate tectonics.

Blank Slate – This theory stated that people are born with no personality traits or instincts. Everything is learned through experience or education. It is now believed that genes and innate instincts play a critical role in a developing personality. Of course, personal experience also has a huge impact.

Miasma Theory of Disease – This theory explained that diseases were caused by “bad air,” called miasmas. It was supposedly a mist or vapor that contained particles from decomposed matter. It was abandoned in favor of germ theory.

Transmutation of Species – Followers of this school of thought believed that simple life is created spontaneously (spontaneous generation) and was driven by some life force to become more complex. The species would then adapt to its environment over time, giving rise to the various types of life we have today. This theory was eventually superseded by Darwin’s Theory of Evolution, although aspects of it remain relevant.

Emission Theory – This is the theory that vision is accomplished by emitting rays of light from the eyes. This has been replaced by what is basically the opposite theory that vision comes from light entering the eyes.

Hollow Earth Theory – This is a hypothesis that states the center of the Earth is either completely hollow, or contains a large amount of empty space. Although dismissed by the scientific community and the public, it has inspired folklore, fiction, and conspiracy theories.