A rare interstellar visitor, comet 3I/ATLAS, is drawing attention as it approaches Earth, growing brighter and taking on a striking green glow. Only the third object from beyond our solar system ever detected, it offers scientists a unique chance to study material formed around distant stars, as its changing color and brightness reveal new insights into its composition and behavior.
A rare interstellar visitor lights up the sky
Comet 3I/ATLAS was discovered on July 1, 2025, by the ATLAS telescope in Chile. From the moment it was detected, astronomers realized it was something special. Its path and speed showed that it did not originate from the Sun’s gravitational family. Instead, it came from far beyond, traveling through space between stars for millions or even billions of years.
Only two interstellar objects had been observed before this one: 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019. Among these rare visitors, 3I/ATLAS stands out. Scientists believe it is the largest and possibly the oldest interstellar object ever detected. Because of this, it carries ancient material that formed around another star system long before our own solar system took shape.
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As the comet passes through our solar system, it has drawn closer to both the Sun and Earth. It will come within about 170 million miles, or 270 million kilometers, of Earth on December 19. While this distance is still very far and poses no danger, it is close enough for telescopes to capture detailed images and data.
Recent observations show that the comet is becoming brighter. This increase in brightness happens as sunlight warms the comet’s surface, causing ice and dust trapped inside it to vaporize. When this material escapes into space, it forms a glowing cloud called a coma around the comet’s nucleus, along with a tail that stretches behind it.
Powerful telescopes reveal a striking green glow
New images of 3I/ATLAS were captured using the Gemini North telescope, an 8.1-meter observatory located atop Hawaii’s dormant Mauna Kea volcano. Scientists used a special instrument called the Gemini Multi-Object Spectrograph to study the comet in detail. These observations were made on November 26, 2025, about four weeks after the comet reached its closest point to the Sun.
To understand what was happening, scientists observed the comet using four different filters: blue, red, orange, and green. By comparing how the comet looked through each filter, they were able to detect changes in both color and brightness.
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Earlier images showed the comet appearing slightly redder. However, the latest observations revealed a faint but clear green glow spreading through the coma. This green color was seen before as well, but it has now become more noticeable as the comet continues its journey through the solar system.
The images also show that the comet is releasing large amounts of dust and gas. This material reflects sunlight and makes the comet appear brighter when seen from Earth. The dust forms a hazy cloud around the comet’s center, while the gas creates glowing colors that can be detected by sensitive instruments.
These detailed observations help scientists understand how interstellar comets react when they encounter the warmth of a new star for the first time.
Scientists explain the reason behind the color change
The striking green color of comet 3I/ATLAS has a clear scientific reason. It comes from diatomic carbon (C₂) molecules in the comet’s coma. When sunlight breaks apart certain carbon-based compounds on the comet, these molecules are released. As they absorb energy from the Sun, they glow at green wavelengths, giving the comet its distinctive hue.
As 3I/ATLAS nears the Sun, the solar radiation heats its surface, causing trapped ice to vaporize. This releases dust and molecules into space, forming a glowing coma and a tail. While green glows are common in many comets, what makes 3I/ATLAS unusual is its shift from a redder appearance to green. This change indicates that fresh material from inside the comet is being exposed as it warms, releasing new molecules that were previously hidden.
Comets often react slowly to heat. The Sun’s warmth takes time to penetrate their interior, so changes in color and brightness can continue even after the comet passes its closest point to the Sun. This delayed reaction allows scientists to observe how interstellar comets respond to solar radiation over time.
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Studying the gas and dust from 3I/ATLAS gives researchers a rare opportunity to examine material formed around another star. By analyzing the light emitted from its coma, scientists can identify its chemical composition and compare it with comets from our solar system. These observations are shedding light on how interstellar objects behave, how they change in new environments, and what they can reveal about the building blocks of other star systems.
This makes 3I/ATLAS a valuable window into the mysteries of the universe beyond our solar system.
