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How did Earth, alone among the Solar System's rocky planets, become the home for life? How, among all this frigid lifelessness, did our planet become warm, hospitable, and life-sustaining? The answer to these questions is complex and multi-faceted, and part of the answer comes from cosmochemistry, an interdisciplinary field that examines how chemical elements are distributed.

The Solar System is a busy place where everything is in motion. It was even more chaotic 4.5 billion years ago, with planets still forming and planetesimals and planetary embryos whizzing around and crashing into one another. Somehow, in all that chaos, Earth received more than its share of carbonaceous chondrites and the amino acids and other life-enabling chemicals that came with them.

Cosmochemistry studies have shown that between 5% and 10% of Earth's mass came from carbonaceous chondrites that crashed into the young planet. Studies also show that a large chunk of that came from the Theia impactor that created the Moon. To test these ideas more rigorously, a trio of researchers used dynamical simulations of the Solar System's formation to see if they could replicate it.

The research is titled "Dynamical origin of Theia, the last giant impactor on Earth." The lead author is Duarte Branco from the Institute of Astrophysics and Space Sciences at the Lisbon Astronomical Observatory in Portugal. The research will be published in the journal Icarus.

One of the critical distinctions in cosmochemistry is the difference between carbonaceous chondrites (CCs) and non-carbonaceous meteorites (NCs). It divides the Solar System's meteor population into two groups and suggests that the Solar System contains two distinct reservoirs of material. CCs formed further from the Sun, likely beyond Jupiter, and carry more volatiles like water and organic compounds with them. NCs include things like iron meteorites, and contain fewer volatiles.

In order to test the idea that Theia delivered CCs and volatiles to Earth, the researchers ran detailed simulations of the Solar System. These were N-body simulations of the later stages of the growth of terrestrial planets.

The simulations began in the late stages of planetary growth after the Solar System's gaseous disk was dispersed. The available solid mass was divided into planetesimals and planetary embryos. The simulation included CCs that were scattered inward as Jupiter and Saturn were still growing and accreting matter. Because of the size distinction between planetesimal and planetary embryos, embryos have a higher possibility of interacting with the terrestrial planets and delivering CC material.

The researchers ran three types of simulations. The first they call small only and includes only small CC objects, or planetesimals. The second they call large only and includes only large CC objects, planetary embryos. The third includes both CC planetesimals and embryos and is called the mixed scenario.

For a subset of 10 simulations from each of those scenarios, they included the effect of the giant planet dynamical instability. This is known as the "Nice model" in astronomy and describes how the giant planets shifted their orbits from where they initially formed.

The goal was to determine how CCs and NCs were distributed in the Solar System and to understand how Earth ended up with more CCs than the other rocky planets, especially Mars. The researchers also wanted to understand if the Theia impact could be responsible for delivering a large amount of Earth's CC material.

One clear result is that the role of giant planet instability, especially Jupiter's shift to a different orbit, had a pronounced effect on Earth's accretion of CC material.

This figure shows snapshots from the mixed simulation scenario without giant planet dynamical instability. In early times, CC objects and NC bodies mix together where the terrestrial planets are forming. Some CCs remained orbiting between planets or were still too far to collide. By the simulation's end, four terrestrial planets existed, including good analogues for Earth and Mars.

Image Credit: Branco et al. 2025. Icarus

When the researchers added giant planet dynamical instability, things looked even more interesting. "The giant planet instability dramatically changed the evolution of the system causing a strong pulse of eccentricity excitement, which lead to a wave of collisions and ejections," the authors write. However, the final state of the system didn't change much.

This figure shows eccentricity and position snapshots over the time of the simulation, including giant planet dynamical instability. The final snapshot is the real Solar System.

Image Credit: Branco et al. 2025. Icarus

A critical part of the simulations concerns the Theia impactor. Previous research suggests that Theia may have been a carbonaceous object. If that's true, much of Earth's life-giving habitability may have resulted from that collision.

"In the mixed scenario with no giant planet instability, Earth’s final impactor included a CC component in more than half of all simulations. In 38.5% of simulations, the final impactor was a pure CC embryo, and in another 13.5%, the impactor was an NC embryo that had previously accreted a CC embryo," the researchers write.

Overall, the simulations paint a picture of the early Solar System where two distinct rings of planetesimals. An inner ring consisting of rocky planetesimals and an outer ring of carbonaceous chondrites. Later, as the ice giants migrated inward, they propelled CC material into the inner Solar System. Some of these were trapped in the asteroid belt, while more massive ones were preferentially scattered into the orbits of the rocky planets. "The late-stage accretion of the terrestrial planets involved a series of giant impacts between NC embryos and planetesimals, with occasional impacts of CC objects," the authors explain.

This scenario explains several things about the Solar System. It explains the masses and orbits of the terrestrial planets, and the orbital distribution of asteroids. It also matches the CC mass fraction of Earth and Mars, where Mars lacks the same concentrations of CC material as Earth. If the small only simulation were correct, where CC material was only in the form of planetesimals, the CC mass fraction of Mars and Earth would be roughly the same.

This figure compares the timing of the last giant impacts in 10 mixed simulations that were run both with and without the giant planet instability. The black line represents the point where both values are equal. Each point has two halves with the left half representing the impactor type in the simulation without the giant planet instability and the right half representing the simulation with the giant planet instability. Dry NC impactors are black, CC embryos are blue and CC+NC mixed embryos are green.

Image Credit: Branco et al. 2025. Icarus

The researchers set out to show that, in line with other research, Theia could've been Earth's final large impactor and that it contained ample CC material. They appear to have succeeded.

In the simulations, Earth's final giant impact was with Theia, and that object had higher concentrations of CC material which helped make Earth habitable. That result is in line with scientific thinking. The work shows that the last impact was after between 5 to 150 million years after gas dispersal. A large fraction of those were within 20 to 70 million years. There are uncertainties in the timing of the Theia impact and these results work within those.

The simulations also support other conclusions showing that CC embryos and planetesimals could've been accreted throughout Earth's growth, but were concentrated in later phases of growth.

"Within the context of this scenario, the last giant impactor on Earth contained a CC component in roughly half of all of the mixed simulations," the authors write. "In the majority of these (38% of simulations), Theia was a pristine CC embryo, and in the remainder of cases Theia was an NC embryo that had previously accreted a CC embryo."

The research also shows that Jupiter played an important role in the Solar System's architecture. It not only truncates the asteroid belt, but played an important role in determining the final composition of the terrestrial planets by scattering CC material from the outer Solar System into the path of the rocky planets, especially Earth.

A million things had to be just right for Earth to become the life-sustaining world it is today. How likely it is that there are other worlds out there like it is unknown. It may take more than being in a habitable zone for an exoplanet to support life. There may be a bewildering number of variables that have to go right, including outer giant planets that migrate and deliver carbon to rocky worlds in habitable zones.

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One of the most iconic cosmic scenes in the Universe lies nearly 3.8 billion light-years away from us in the direction of the constellation Carina. This is where two massive clusters of galaxies have collided. The resulting combined galaxies and other material are now called the Bullet Cluster, after one of the two members that interacted over several billion years. It's one of the hottest-known galaxy clusters, thanks to clouds of gas that were heated by shockwaves during the event. Astronomers have observed this scene with several different telescopes in multiple wavelengths of light, including X-ray and infrared. Those observations and others show that the dark matter makes up the majority of the cluster's mass. Its gravitational effect distorts light from more distant objects and makes it an ideal gravitational lens.

Astronomers pointed the infrared-sensitive James Webb Space Telescope (Webb) to view the Cluster in part to help refine its mass. The Bullet is actually two clusters, a smaller sub-cluster called the Bullet, and the larger one it collided with in the past. The observations provided extremely detailed images of the cluster's galaxy members, as well as a view of hundreds of other faint ones that lie beyond. They also mapped the distribution of hot gas, which appears to be in separate "blobs". Those gaseous regions helped them learn more about the distribution of dark matter in the cluster. “With Webb’s observations, we carefully measured the mass of the Bullet Cluster with the largest lensing dataset to date, from the galaxy clusters’ cores all the way out to their outskirts,” said Sangjun Cha, the lead author of a paper published in The Astrophysical Journal Letters. Not only that, but the Webb view also allows scientists to study the distant galaxies "behind" the cluster in great detail. Their distorted images also give clues to the distribution of dark matter in the lens.

This image shows the different wavelengths at which scientists studied the Bullet Cluster using JWST's NIRCam instrument. The circles show the two clusters (in blue with their hot gas clouds in red). The one on the left shows an elongated shape, which suggests it's been through more than one collision.

Credit: NASA, ESA, CSA, STScI, CXC

“Webb’s images dramatically improve what we can measure in this scene — including pinpointing the position of invisible particles known as dark matter,” said Kyle Finner, a co-author and an assistant scientist at IPAC at Caltech in Pasadena, California. Dark matter plays a role, not just in the Bullet Cluster's hot gas clouds, but also in the light from distant galaxies passing through and around the cluster.

What Happened with the Bullet?

When you look at the combined infrared and X-ray views of the Bullet Cluster, among other things, you see those blobs of hot gas. One is in the form of a bow shock whipped up when the smaller sub-cluster member passed through the larger galaxy cluster. That sent the temperature of the gaseous regions up to millions of degrees, which released X-ray emissions detectable by Chandra.

A Chandra X-ray view of hot gas clouds in the Bullet Cluster. This one gives the cluster its distinctive name. It lies entirely separated from the dark matter in the cluster. This indicates something about how dark matter behaved in the collision.

Credit: X-ray: NASA/CXC/SAO

To understand why astronomers find the Bullet Cluster so fascinating, it helps to understand how it got the way it appears in Chandra and Webb observations. Well more than four billion years ago, these two galaxy clusters began a close approach. Both clusters were rich in stars, gas, and dust. Like the rest of the Universe, they were permeated with dark matter. Eventually, the two clusters collided. The stars were largely "unhurt" by this, other than perhaps having their velocities through space slightly altered. The collision basically caused a separation of the hot gas and dark matter. The gas, being affected by ram pressure (caused by something moving through the interstellar/intergalactic medium), slowed down due to the collision. The dark matter, which interacts primarily through gravity, passed through without any problem. This separation provided key evidence for the existence of dark matter. "As the galaxy clusters collided, their gas was dragged out and left behind, which the X-rays confirm,” Finner said. Webb’s observations show that dark matter still lines up with the galaxies — and was not dragged away.

What the Cluster's Gravitational Lens Reveals

While we can't see the dark matter at all, its presence around and within the Bullet Cluster's galaxies turns it into a giant gravitational lens. Think of it as a cosmic magnifying glass that shows otherwise unseen things. It also does something remarkable: “Gravitational lensing allows us to infer the distribution of dark matter,” said James Jee, a co-author, professor at Yonsei University, and research associate at UC Davis in California. Jee suggests that we think of this gravitational lensing as working the same way that water in a pond magnifies the view of things in the pond. “You cannot see the water unless there is wind, which causes ripples,” Jee explained. “Those ripples distort the shapes of the pebbles below, causing the water to act like a lens.”

That lens reveals thousands of distant galaxies whose light is "smeared" and distorted by the gravitational effect of the dark matter lens. The distribution of those galaxies across the lens also helps astronomers map the distribution of the dark matter that makes it up.

The Webb NIRCam view of the Bullet Cluster, showing an infrared look at distant galaxies, with their images deformed by the gravitational effect of the dark matter.

Credit: Near-infrared: NASA/ESA/CSA/STScI;

Image processing: NASA/STScI/J. DePasquale

Now that astronomers know where that dark matter is distributed in the cluster, the images and data also show that the particles (no matter what they're made of) don't affect each other beyond whatever gravitational attraction they have toward each other. It implies that they act independently of each other. Now the trick is to figure out what kind of particles act as dark matter has been observed to do. Webb’s observations also show that dark matter still lines up with the galaxies — and was not dragged away during the chaos of the cluster collisions. These new observations place stronger limits on the behavior of dark matter particles.