2025 was a year of cosmic discoveries. Here's a look back at 6

The High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA’s Mars Reconnaissance Orbiter captured this image of interstellar comet 3I/ATLAS on Oct. 2, 2025. At the time it was imaged, the comet was about 19 million miles from the spacecraft. The comet didn't come nearly as close to Earth, when it reached a distance of 170 million miles from our planet on Dec. 19.

This image shows the 3I/ATLAS interstellar comet as a bright, fuzzy orb in the center. Traveling through our solar system at 130,000 miles per hour, 3I/ATLAS was made visible by using a series of colorized stacked images from Sept. 11-25, using the Heliocentric Imager-1 (H1) instrument, a visible-light imager on the STEREO-A (Solar Terrestrial Relations Observatory) spacecraft. The colorization was applied to differentiate the image from other observing spacecraft images.

Because it's big enough to be deemed a "city killer," asteroid 2024 YR4 became a source of alarm due to the uncommonly high risk it had of colliding with Earth on Dec. 22, 2032. For a time, it was the only object among more than 37,000 known large space rocks with any chance of hitting Earth anytime soon – with its probability of impact even rising to a record level of 3.1%.That began to change in late February as more precise observations allowed scientists to effectively winnow down the asteroid's odds of impact to a number so low, it might as well be zero.

An exoplanet known as K2-18b achieved a degree of fame in April when a team of astronomers claimed to have found in its atmosphere "the strongest evidence yet" that life exists anywhere else besides Earth. Other scientists have since cast doubt on the findings – putting a damper on the notion that humanity finally had proof that we aren't alone in the cosmos.

This artist's concept shows what exoplanet K2-18b could look like based on scientific data. NASA's James Webb Space Telescope has observed K2-18b, an exoplanet 8.6 times as massive as Earth, revealing conditions that could support life on the exoplanet.

NASA’s Perseverance Mars rover is seen in a "selfie" that it took over on Sept. 10, 2021. Perseverance rover, along with Curiosity, is one of the agency's two car-sized robots exploring the Martian surface for signs that the planet was once habitable. And in September, NASA officials confirmed that one of the rovers’ finds contained a potential biosignature.

A reddish rock nicknamed "Cheyava Falls", with features resembling leopard spots was discovered by NASA's Perseverance rover in Mars’ Jezero Crater in July 2024, in this handout photograph released on September 10, 2025.

The northern lights, also known as the aurora borealis, light up the night sky Nov. 11 east of Denver, Colorado. A powerful geomagnetic solar storm in November blasted Earth and created the conditions necessary to reveal the auroras much further south in the United States than is typical.

A group of friends take photos of the northern lights Nov. 11 as they appear over Clinton Lake in Lawrence, Kansas. After NOAA's Space Weather Prediction Center issued a "severe" geomagnetic storm watch in November, many people in the Northern Hemisphere, which includes the U.S., had an extraordinary opportunity to gaze upon some breathtaking red and green auroras in their own backyard.