What are comets? The secret lives of space snowballs.

No one yet knows how our planet evolved into an inhabited world, with the chemistry to spawn a variety of lifeforms. But scientists believe comets hurtling through space may hold that wisdom.

Like an asteroid, a comet orbits the sun. The latter is a ball of ice, dust, and rock that formed in the outer solar system. These icy bodies are left over from the early days of planet formation, about 4.6 billion years ago. Their ice starts to disintegrate as they get closer and closer to the sun, converting from a solid to a gas in one swift process, skipping over the liquid phase.

Laypeople sometimes confuse shooting stars, aka meteors, for comets because they each create a glowing streak that humans can see. But comets are known for their telltale tails, millions-of-miles-long debris trails of vaporizing ice, dust, and carbon dioxide.

In a NASA video, Ryan Park, a near-Earth asteroid expert, sums up the differences in space rocks: “Asteroids are rocky, comets are icy, and meteors are much smaller and are the shooting stars that you see up in the sky.”

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Hundreds of years ago, comets were considered bad omens. Legend holds that Pope Callixtus III excommunicated Halley’s Comet in 1456 as an “instrument of the devil,” though a Catholic priest disputed that as myth in a 1908 article published in Popular Astronomy.

Scientists today know these icy objects as ancient relics of the solar system. Astronomers often think of them as time capsules.

“Comets may have brought water and organic compounds, the building blocks of life, to the early Earth and other parts of the solar system,” according to NASA.

How many comets are in the solar system?

As 2023 came to a close, there were more than 3,900 known comets. Astronomers say countless others are likely orbiting the sun beyond Neptune in a disk known as the Kuiper Belt or on the outer edge of the solar system, in the so-called Oort Cloud, about 50 times farther from the sun. The Oort Cloud is thought to be a sphere of ancient, icy objects surrounding the system. NASA says the cloud remains a theory because the comets there have been too faint and distant to be directly observed.

Comets are sometimes dislodged from their home in the outer solar system by the gravity of another object, redirecting them toward the sun. As the comet gains speed, it swings around the sun. If it survives this close encounter, it heads back toward home.

Depending on where a comet originated, it could take hundreds to millions of years to complete a single orbit. Comet Bernardinelli-Bernstein, for example, follows a 3-million-year-long oval-shaped orbit, taking it about half a light-year away from the sun.

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What is the largest comet found so far?

Bernardinelli-Bernstein made news in 2022 when scientists confirmed it as the largest comet observed so far, spanning the length of three marathons end-to-end. A team of scientists used the Hubble Space Telescope, an Earth-orbiting observatory shared by NASA and the European Space Agency, to determine that its nucleus — the solid, frozen core — is about 85 miles wide.

That’s about 50 times larger than the average known comet and 40 percent larger than the runner up. The previous record-holder for largest comet was C/2002 VQ94, with a nucleus estimated to be 60 miles across.

Researchers say the scale of Bernardinelli-Bernstein, which has a mass of 500 trillion tons, is significant because it provides a clue about the size range of other comets orbiting in the distant outskirts of our solar system.

“This comet is literally the tip of the iceberg for many thousands of comets that are too faint to see in the more distant parts of the solar system,” David Jewitt, a UCLA astronomer, said in a statement.

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Why do some comets appear green?

In January 2023, a comet dubbed C/2022 E3 (ZTF) captured the attention of people around the world because of its color, receiving the nickname “the green comet” for its emerald hue. The color comes from carbon in the gas cloud surrounding its nucleus, known as the coma.

The comet wasn’t nearly as spectacular as NEOWISE was back in 2020, the brightest comet to come this way since Hale-Bopp in the 1990s. But it was still a rare opportunity to see a space phenomenon that won’t return for another 50 millenia or more.

“Asteroids are rocky, comets are icy, and meteors are much smaller and are the shooting stars that you see up in the sky.”

In recent years, several spacecraft have studied comets up close. NASA’s Stardust mission brought back samples from Comet Wild-2 to Earth in 2006. The particles were chock full of hydrocarbons — chemicals considered the “building blocks” of life as we know it.

The European Space Agency’s Rosetta mission, which included several onboard NASA instruments, dropped a lander on Comet 67P Churyumov-Gerasimenko‘s nucleus, then orbited it for two years. The European mission also detected building blocks of life.

Astronomers are also using the James Webb Space Telescope, a partnership of NASA, ESA, and the Canadian Space Agency, to research comets in infrared wavelengths of light that are invisible to human eyes. The telescope is orbiting the sun about 1 million miles from Earth.

At those wavelengths, scientists will study the chemical composition of the dust and gas that comes off a comet’s nucleus and discern what it is, said Heidi Hammel, a planetary scientist, in a NASA statement. They hope to get to the bottom of whether all solar system comets start out the same but evolve differently or if they are chemically distinct from the start.

“If this is primitive material, it will give us some clues to the makeup of the early solar system,” she said.