Could comets be the source of life on Earth?

Humans have observed comets for thousands of years as their orbits have brought them within visible distance of Earth. Appearing throughout historical records, these mysterious lights that came out of nowhere and disappeared after a short while were thought to be ill omens of war and famine, or the wrath of gods. But recent research has revealed that comets may be even more deeply connected to humanity and our presence on Earth than any of these mythical explanations suggested. When you think of our Solar System, you probably imagine the nine, sorry eight, planets orbiting the Sun.

But beyond Neptune, far from the heat of the Sun, there is a sparse ring found formed by icy chunks ranging from the size of marbles to that of small planets. And thousands of times farther at the outer reaches of the Solar System lies a spherical cloud of small fragments and gases. Many of these ancient clumps of stardust are leftovers from the formation of the Solar System 4.6 billion years ago, while some of the most distant may even come from a neighboring system. But sometimes the gravity from passing planets or stars pulls them toward our sun, beginning a journey that can take up to millions of years.

As the frozen object travels further into the Solar System, the sun grows from a distant spark to an inferno, melting the ice for the first time in billions of years. Gas and steam eject dust into space, forming a bright surrounding cloud, called a coma, that can grow even larger than the sun itself. Meanwhile, the intense stream of high-energy particles constantly emitted by the Sun, known as the solar wind, blows particles away from the comet’s core, forming a trail of debris up to millions of miles long. The ice, gas and dust reflect light glowing brightly. A comet is born, now orbiting the sun along with the rest of the objects in our Solar System.

But as the comet travels through the Solar System, the solar wind tears apart and recombines molecules into various compounds. In some of the compounds that scientists found, first in the rubble left by a meteorite that disintegrated above northern Canada, and then in samples collected by a space craft from a passing comet’s tail, were nothing less important than amino acids. Coming together to form proteins according to the instructs encoded in DNA, these are the main active components in all living cells, from bacteria to blue whales.

If comets are where these building blocks of life were first formed, then they are the ultimate source of life on Earth, and, perhaps, some of the other places they visited as well. We know that planets orbit nearly every star in the night sky, with one in five having a planet similar to Earth in size and temperature.

If Earth-like planets and the molecules found in DNA are not anomalies, we may be only one example of what’s possible when a planet under the right conditions is seeded with organic molecules by a passing comet. So, rather than an omen of death, the comet that first brought amino acids to Earth could have been a portent of life, a prediction of a distant future, where creatures of stardust would return to space to find the mysteries of where they came from.

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