Returning Wonders of Lost Space: How Comets Are Born

comets in the starry sky
Photo by Frank Cone on

Comets are small, icy objects that circle the sun. They can be thought of as floating time capsules, preserving a chemical record of the early solar system. Astronomers believe comets materialized more than 4.5 billion years ago from the dust and gas of the protoplanetary disk, a donut-shaped cloud of debris surrounding our newborn star. On the fringes of the disk, far from the sun’s heat, fine grains of dust coated with frozen gases and water ice began clumping together. Over time, clumps of dust assembled into ice-rich rocks, which later evolved into the mile-sized bodies that we observe today traveling among and far beyond the planets.

Where do Comets Come From?

As theorized by astronomer Gerard Kuiper in 1951, a disc-like belt of icy bodies exists beyond Neptune, where a population of dark comets orbits the Sun in the realm of Pluto. These icy objects, occasionally pushed by gravity into orbits bringing them closer to the Sun, become the so-called short-period comets. Taking less than 200 years to orbit the Sun, in many cases their appearance is predictable because they have passed by before. Less predictable are long-period comets, many of which arrive from a region called the Oort Cloud about 100,000 astronomical units (that is, about 100,000 times the distance between Earth and the Sun) from the Sun. These Oort Cloud comets can take as long as 30 million years to complete one trip around the Sun.

Each comet has a tiny frozen part, called a nucleus, often no larger than a few kilometers across. The nucleus contains icy chunks, frozen gases with bits of embedded dust. A comet warms up as it nears the Sun and develops an atmosphere, or coma. The Sun’s heat causes the comet’s ices to change to gases so the coma gets larger. The coma may extend hundreds of thousands of kilometers. The pressure of sunlight and high-speed solar particles (solar wind) can blow the coma dust and gas away from the Sun, sometimes forming a long, bright tail. Comets actually have two tails―a dust tail and an ion (gas) tail.

Most comets travel a safe distance from the Sun―comet Halley comes no closer than 89 million kilometers (55 million miles). However, some comets, called sungrazers, crash straight into the Sun or get so close that they break up and evaporate.

Here is how Comets are Born

  1. First comets grow from microscopic particles to objects the size of mountains. This happens due to gravity.
  2. Particles of dust coated with water ice and other molecules serve as a comet’s building blocks.
  3. Dust particles combine to form icy rocks that join together under the force of gravity.
  4. High-speed collisions result in a loosely bound collection of fused rock and ice.
  5. When a comet passes through the inner solar system, its ice warms, releasing a trail of gas and dust.

Now You Know

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