planetesimal formation

Astronomers catch distant planets by the trembling of their stars: an invisible companion tugs the star toward us and away, and its light subtly shifts in color. But that trembling tells only half the truth. If the planet's orbit is tilted to our line of sight, only part of its weight rocks the star, so telescopes reveal not the true mass but its shadow.

On the surface of the Moon and asteroids, you can find pits smaller than a pinhead but with shiny, melted rims. Such marks are left by tiny meteorites, but the exact impact speed at which melting occurs remained unclear.

In the cold depths of space float rocks older than Earth itself. Inside some of them hide tiny glassy spheres — chondrules. They formed when a disk of gas and dust still swirled around the young Sun. Carbonaceous meteorites Asuka 12236, Paris, and Maribo have preserved these spheres almost untouched.

The birth of a moon is a cosmic car crash. Two young planets collide, and a cloud of scorching debris swirls around the surviving world. From this disk, like clay on a potter's wheel, a satellite gradually takes shape. That's probably how our Moon came to be.

Around a just-ignited star swirls a vortex of dust and gas. This vortex, the protoplanetary disk, grows by accreting material from the surrounding cloud.

Edge of the spotlight. Remove the dazzling light of the central star, and a theater of shadows emerges. The James Webb coronagraph, like a dense curtain, hid the star TW Hydrae — and on a stage 120 astronomical units long (four times the distance to Neptune), its protoplanetary disk came alive.

Seventy-two protoplanetary disks, swirling cocoons of cosmic dust and gas, came under the gaze of the MIRI instrument on JWST. Most of them shoot outflows into space: dense conical structures and narrow, high-speed jets. They blast like fountains. This process is not accidental.

Around some young stars where star formation has recently ended, there swirl not just dust rings but actual planetary battlefields — extreme debris disks.

When a giant planet grows, it pulls in gas. Some of that gas swirls around it into a small disk — called a circumplanetary disk. It is in such disks that moons are thought to form. For the planet PDS 70 c, located 370 light-years away, such a disk was directly observed using the ALMA antenna array in the millimeter range.

Two million years away, around the young star CI Tau, a gas-dust disk whirls — a protoplanetary cradle. ALMA observations have already shown rings and gaps in it, but the finest details remained invisible: their width is smaller than the instrument's resolution.

Space is threaded with magnetic fields. They exist around galaxies, stars, and even in nearly empty voids — the vast empty spaces between galaxy clusters. It's usually thought that fields are amplified from weak 'seed' fields by turbulence, much like a dynamo. But the origin of the seed fields themselves has remained a puzzle.

The birth of planets resembles the workings of a cosmic forge. Rocky blocks collide and fuse together. A powerful impact strikes a spark — material heats to white-hot and glows like cooling metal. Computer simulations show: the flash is visible from days to years, peaking at a tenth of the sun's light.

Young Earth was a boiling sphere of liquid rock, while Theia, which crashed into it, had had time to cool and solidify. It’s like dropping a chunk of cold butter into a pot of soup: the soup splashes out, but the butter barely moves. Similarly, Earth’s interior spewed into space, forming a cloud of cosmic dust and debris.

At the center of a distant galaxy, a supermassive black hole is surrounded by a flat, rotating disk of gas and dust — a giant cosmic funnel of matter. A small compact object, such as another stellar-mass black hole, may find itself in this disk.

A thick stew slowly cools in a pot. Suddenly, the flame beneath it flares up, and the stew begins to boil, breaking into many small whirlpools. Much the same happens to the disk of gas and dust around a young star when it experiences a flare — a sudden influx of matter from the disk onto the star.

Putting together the Solar System is like crafting a high-end tasting menu: the recipe isn't always set in stone — sometimes the final ingredient is added while the dish is already cooking.

When a telescope catches the light of a distant white dwarf, the spectrograph sometimes fishes out heavy elements — calcium, iron, magnesium. These metals could not have stayed in the thin atmosphere of the stellar remnant; they are the traces of recently fallen planetary debris.

In the heart of the Eagle Nebula, about 7000 light-years away, three giant pillars of gas and dust rise — the Pillars of Creation. Their shape is familiar to many from Hubble Space Telescope images. Now, Webb's infrared camera has allowed us to peer inside these dense cocoons where stars are born.

The cosmic accounting has hit a snag. The bookkeeper is balancing the books: in one column, all the planets astronomers have counted in the galaxy; in the other, the stockpile of building material in the dusty cocoons of newborn stars. The numbers don't line up. Even if you gathered every last grain of dust and molded it into planets, there wouldn't be enough material.

A massive star collapses, and a black hole is born, surrounded by a disk of incandescent plasma. For narrow jets to burst out of this disk, an ordered magnetic field is needed. But inside the star, it's tangled.

A huge rotating pancake of gas and dust surrounds a young star. In such a "maternity ward" for planets, rings and gaps are often seen, as if an invisible plow had carved furrows. These are usually attributed to already-formed planets that clear paths with their gravity.

After the Big Bang, the Universe resembled a tightly inflated balloon: pressure prevented matter from clumping together. But hypothetical superheavy particles — sort of ‘heavyweights’ of the microcosm — momentarily made the balloon less stiff. Matter easily clumped, giving birth to black holes with masses from a billion to a billion billion tons.

A young star is surrounded by a flat, rotating disk of gas and dust, like pizza dough that's been spun out. Planets are born inside such a disk. But around three stars (Elias 2-24, IM Lup, and DM Tau) astronomers noticed something odd: far beyond the edge of the bright disk stretches a barely visible but enormous dust halo.

In a disk of gas and dust around a young star, dust particles behave like sand grains in a stream of water: the gas flows faster, the dust lags behind, and friction arises between them. This friction can rock the magnetic waves that run through the disk. The galaxy is full of such disks: it is precisely in them that planets are born.

Young planetary systems resemble an orchestra that has just started rehearsing: all the musicians strive to play in the same rhythm.

Around young stars, disks of gas and dust swirl — the building blocks for future planets. Inside these disks, dust grains don't just fly randomly: they align along invisible lines of gas flow. Recently, astronomers noticed that dust grains in such disks behave like badminton shuttlecocks — always turning their long side into the wind.

In the cold outer reaches of the Solar System, water is considered the main building material for ices. But some comets and distant icy bodies have an unusually high ratio of carbon monoxide (CO) to water, as if a scoop of ice cream suddenly had too much syrup. The authors of a new study set out to find where this imbalance comes from.

Planets are born from dust. In a disk around a young star, tiny dust grains collide, stick together, and gradually grow like a snowball rolling down a hill. But there's a catch: the gas in the disk moves faster than the dust and drags on it, preventing it from clumping into large chunks. For dust to stick together, there needs to be enough of it.

In a protoplanetary disk, a cloud of gas and dust around a young star, dust grains don't just fly in circles. They are carried by the gas flow, like sand by the wind, and in turn alter that flow. Layers emerge, sliding past each other, and at their boundaries an instability arises — waves and vortices.

Dust grains in the disk around a young star stick together, grow, and will eventually become planets. But to know how much planet-building material each disk holds, you need to measure its mass and size. You can't weigh a distant cloud directly: you have to infer it from the dust's radiation.

Dough rises faster when it already contains yeast. Similarly, galaxies may have assembled faster if they already had black holes inside. Astrophysicists considered a scenario where the universe contained primordial black holes from the very beginning — objects that could have formed in the first instants after the Big Bang.

The young Solar System resembled a giant construction site: around the newborn Sun swirled a disk of gas and dust, and from this dust gradually assembled stones, boulders, and then planetary embryos — planetesimals. Today, their fragments fall to Earth as meteorites.

4 billion years ago, the young Earth was mercilessly bombarded by meteorites, leaving behind impact craters. Rains filled them with water, forming shallow ponds where lipid bubbles accumulated. These bubbles, like soap bubbles but with a double-layered lipid membrane (just like modern cells), formed spontaneously when water mixed with fat-like substances.

A young planet in a gas disk is like a swimmer trying to swim against the current: friction with the gas slows it down, and it spirals toward the star. For small rocky planets, this fall can take less time than they need to grow. But the disk also has countercurrents — vortices that can push the planet outward. Scientists built a computer model of a disk around a young star.

The icy body Arrokoth looks like a snowman: two spheres pressed together. Many such two-headed objects exist beyond Neptune's orbit, in the Kuiper Belt. For a long time, it was thought they formed from the slow collision of two separate bodies. New simulations show a different path. A cloud of small pebbles around the young Sun collapses under its own gravity.

The young star PDS 66 is surrounded by a flat disk of gas and dust — the cradle of future planets. In the cold outer regions of such a disk, silicon and sulfur are usually hidden inside dust grains, but astronomers spotted a bright patch of gas at about 60 astronomical units from the star.

The dust around the young star CVSO 1942 flared up like a light bulb switched to full power. In 2024, the James Webb Space Telescope (JWST) peered into a transitional disk, a ring of gas and dust where planets are still being built, and saw that in mid-infrared light (wavelengths shorter than 10…

The Murchison meteorite, which fell in Australia in 1969, holds dust grains older than the Solar System. Among them are tiny graphite grains smaller than a thousandth of a millimeter. In five such grains, researchers found the isotope strontium-84: its atoms are slightly lighter than ordinary strontium.

Around young stars, flat clouds of gas and dust swirl: these are the birthplaces of planets. For matter in such a disk to clump together, it must be constantly stirred. But what makes the gas move chaotically when it would otherwise be calm? Part of the answer lies in the fact that the gas in the disk barely conducts electricity: it is weakly ionized.

Around a young star spins a flat pancake of gas and dust — the future building blocks of planets. The star heats it with ultraviolet radiation, and gas from the disk's surface escapes into space, like water from a puddle under the sun.