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Nemesis hypothesis: Is there a small hidden star orbiting the sun?

Every 27 million years, there is a much higher likelihood of a mass extinction. Some astronomers have proposed that these extinctions are caused by a small comet-disturbing star orbiting our sun, the so-called Nemesis hypothesis.

The Nemesis hypothesis proposes that the mass extinctions that occur on Earth every 27 million years or so are caused by a dim star that orbits about one light year away from sun. This hypothetical star, nicknamed Nemesis, passes through a cloud of comets, sending them scattering toward the inner planets.


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Some astronomers believe a hidden mini star nicknamed Nemesis is orbiting the sun, but a new analysis of life extinction cycles on Earth suggests this dark companion may not exist.

Nemesis was first proposed in 1984 to explain perplexing cycles in mass extinctions on Earth. About every 27 million years, almost like clockwork, there is a significantly higher likelihood that a mass extinction will take place on our planet – akin to the apocalypse that killed off the dinosaurs (and much of the rest of Earth's life) about 65.5 million years ago.

The scale of this cycle – on the order of tens of millions of years – suggests some celestial phenomenon might be affecting it.

"If you see something with a very regular cycle that's like 10 million or 100 million years, you tend to think of things astronomical," said physicist Adrian Melott of the University of Kansas in Lawrence, Kan. "For example, the galaxy takes about 250 million years to orbit once."

Unseen companion

Astronomers suggested that a low-mass star – such as a dim red dwarf or white dwarf – called Nemesis might be orbiting our sun at a large distance, nearly a light-year away, out beyond the orbits of the planets. Such a body could have an orbit that might take about 27 million years to make a complete circuit.


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