Cosmic dust, also called extraterrestrial dust, space dust, or star dust, is tiny particles found in outer space or that have fallen to Earth. Most cosmic dust particles are very small, measuring between a few molecules and 0.1 mm (100 μm), such as micrometeoroids (less than 30 μm) and meteoroids (more than 30 μm). Cosmic dust can be classified based on where it is found: intergalactic dust (between galaxies), interstellar dust (between stars), interplanetary dust (like the zodiacal cloud between planets), and circumplanetary dust (found in planetary rings). Scientists study dust outside the Solar System using methods like photometry, polarimetry, and infrared spectroscopy. Within the Solar System, direct methods are used, such as the Stardust spacecraft, which collected cometary dust and some particles likely from outside the Solar System. These samples were brought back to Earth in 2006.
In the Solar System, interplanetary dust creates the zodiacal light. Other types of Solar System dust include cometary dust, planetary dust (such as from Mars), lunar regolith (Moon dust), asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. It is estimated that thousands of tons of cosmic dust reach Earth each year, with most particles having a mass between 10 kg (0.1 pg) and 10 kg (0.1 g). The density of the dust cloud Earth travels through is about 10 dust grains per cubic meter.
A large part of space dust is called "stardust," made of strong minerals like silicates, graphite, and amorphous carbon. These materials form around older stars, such as red giants, carbon stars, novae, and supernovae, and are pushed into space by the stars' winds and outflows. The dust also includes organic compounds, such as amorphous organic solids with a mix of aromatic and aliphatic structures, which may form naturally in the outflows of carbon stars or in the space between stars.
Study and importance
Cosmic dust was once a problem for astronomers because it hides objects they wanted to study. When infrared astronomy started, scientists found that dust particles are important parts of space processes. Studying dust can show information about events like the formation of the Solar System. For example, cosmic dust helps stars lose mass as they near the end of their lives, plays a role in the early stages of star formation, and helps form planets. In the Solar System, dust is important in the zodiacal light, Saturn's B Ring spokes, the outer rings of Jupiter, Saturn, Uranus, and Neptune, and in comets.
The study of dust connects many scientific areas: physics (solid-state, electromagnetic theory, surface physics, statistical physics, thermal physics), fractal mathematics, surface chemistry on dust grains, meteoritics, and all areas of astronomy and astrophysics. These different fields are connected by one idea: cosmic dust changes in cycles, both chemically, physically, and in movement. The way dust changes shows how the Universe recycles materials, similar to how people recycle things daily: making, storing, using, collecting, and throwing away.
Observing dust in different places helps scientists understand how the Universe recycles materials. This includes areas like the diffuse interstellar medium, molecular clouds, the dust around young stars, and planetary systems like the Solar System, where dust is seen as being most recycled. Astronomers collect "snapshots" of dust at different life stages and, over time, create a clearer picture of the Universe's recycling steps.
Factors like the dust particle's starting movement, its material, the plasma and magnetic fields around it, determine where the particle ends up. Small changes in these factors can lead to very different ways the dust moves. This helps scientists learn where the dust came from and what is in the space around it.
Detection methods
Many methods are used to study cosmic dust. Scientists can find cosmic dust by using remote sensing, which looks at how dust particles reflect or emit light. One example is measuring the Zodiacal light, which is a faint glow in the sky caused by sunlight reflecting off cosmic dust.
Cosmic dust can also be studied directly by collecting it in different places. Scientists estimate that between 5 and 300 tonnes of space material fall into Earth's atmosphere each day. NASA collects dust samples from Earth's atmosphere using special plates attached to airplanes that fly high in the stratosphere. Dust is also collected from ice-covered areas like Antarctica and Greenland, as well as from deep ocean sediments.
In the late 1970s, Don Brownlee from the University of Washington in Seattle first confirmed that some collected dust came from space. Another source of cosmic dust is meteorites, which contain tiny pieces of dust from stars. These dust grains are solid and extremely hard to melt. They are identified by their unique chemical makeup, which can only form in old stars before mixing with space material. These grains formed when the material from stars cooled as it left the star.
In space, dust detectors on spacecraft have been used to study cosmic dust. These detectors measure the effects of dust particles hitting them at very high speeds (usually 10–40 km/s), which makes capturing whole particles difficult. Instead, scientists use laboratory tests to understand the dust's properties, such as its mass and speed. Over time, detectors have measured light flashes, sound waves, and electrical changes caused by impacts. Recently, the Stardust mission captured dust particles intact using a special material called aerogel.
Dust detectors have been used on many spacecraft, including HEOS 2, Helios, Pioneer 10 and 11, Giotto, Galileo, Ulysses, and Cassini. They have also been on Earth-orbiting satellites like LDEF, EURECA, and Gorid. Some scientists have used the Voyager 1 and 2 spacecraft to study dust. Today, dust detectors are on Ulysses, PROBA, Rosetta, Stardust, and New Horizons. Dust collected from Earth or space is studied in labs worldwide. A major storage facility for cosmic dust is at NASA's Johnson Space Center in Houston.
Infrared light can pass through cosmic dust clouds, allowing scientists to see star-forming regions and the centers of galaxies. NASA's Spitzer Space Telescope was the largest infrared telescope before the James Webb Space Telescope was launched. Spitzer studied objects by detecting heat they emit at wavelengths between 3 and 180 micrometers. Most of this light cannot be seen from Earth because the atmosphere blocks it. Spitzer's findings helped scientists learn more about cosmic dust. One study suggested that dust may form near supermassive black holes.
Another way to study dust is through polarimetry, which measures how light is polarized. Dust grains are not round and often line up with magnetic fields in space, causing starlight passing through dust clouds to become polarized. In nearby space, where light is not too dim, scientists use precise polarimetry to study dust in the Local Bubble.
In 2019, scientists found interstellar dust in Antarctica. They linked it to the Local Interstellar Cloud by using a technique called accelerator mass spectrometry to detect radioactive elements like iron-60 and manganese-53.
Radiation properties
Dust particles interact with electromagnetic radiation based on their size and shape, the wavelength of the radiation, and the material properties of the particles, such as how light bends through them. The way a single particle emits radiation is called its emissivity, which depends on how effectively it interacts with light. Other factors that describe this interaction include how much light is blocked (extinction), how light is redirected (scattering), how much light is absorbed, or how light waves are aligned (polarisation). In graphs showing radiation emission, specific patterns help scientists determine the composition of the dust particles involved.
Dust particles can scatter light unevenly. Forward scattered light is light that is slightly redirected from its original path due to diffraction, while back-scattered light is light that reflects directly back toward its source.
The way light is scattered and blocked by dust provides important information about the size of the dust particles. For example, if an object appears much brighter in forward-scattered visible light than in back-scattered visible light, it suggests that many of the dust particles are about one micrometer in diameter.
In long exposure visible light photographs, the scattering of light by dust particles is clearly visible in reflection nebulae, offering clues about how individual dust particles scatter light. In X-ray wavelengths, scientists are studying how interstellar dust scatters X-rays, and some have proposed that astronomical X-ray sources may appear with faint, extended haloes caused by this scattering.
Presolar grains
Presolar grains are found inside meteorites and are studied in laboratories on Earth. Sometimes, the term "stardust" or "presolar stardust" is used to describe grains from a single star, compared to dust made up of many particles from space. However, this distinction is not always used. Presolar material was part of the dust in space before it became part of meteorites. Meteorites have kept these presolar grains since they formed more than four billion years ago in the early solar system. Carbonaceous chondrites are especially rich in presolar material. Presolar grains existed before Earth was formed. The scientific term for these grains is "presolar grain," which refers to hard, heat-resistant dust particles that formed from gases released by stars before the solar system existed.
Scientists have identified many types of presolar grains by studying the unusual chemical makeup of their elements. These grains may have once been covered with materials that easily evaporate, but those materials are removed when meteorites are dissolved in acids, leaving only the hard, heat-resistant parts. Finding the grain cores without dissolving most of the meteorite is possible but difficult.
Presolar grains have helped scientists learn new things about how elements are created in stars. These grains are extremely hard and formed at very high temperatures. Examples include silicon carbide, graphite, aluminum oxide, and aluminum spinel. These materials form when gases cool in environments like the winds from stars or inside exploding stars called supernovas. They are different from materials formed at lower temperatures in space.
The chemical makeup of presolar grains is also very unusual and not found in space dust. This suggests the grains formed before mixing with space dust diluted their chemical makeup. These unique chemical patterns help scientists identify the types of stars that created the grains. For example, silicon carbide grains contain heavy elements that are mostly from a process called the S-process, which happens in a type of star called an AGB star. These stars are known to produce these elements.
Another example is material formed in supernovas, called SUNOCONs. These grains have a large amount of calcium, showing they formed when radioactive titanium was still active. Titanium has a short half-life of 65 years, so it was still radioactive when the grains formed inside a supernova. After mixing with space gas, the titanium would have become inactive. This discovery confirmed a prediction from 1975. Silicon carbide grains from supernovas are much less common than those from AGB stars.
Presolar grains, including SUNOCONs and grains from specific stars, make up less than 0.1% of all space dust. Scientists are very interested in these grains because they provide new information about how stars and elements form.
Laboratories have studied materials that existed before Earth formed. This was once thought impossible, especially in the 1970s, when scientists believed the solar system began as a hot gas with no solid materials. Presolar grains proved this idea was wrong.
Some bulk properties
Cosmic dust is made of tiny dust grains that come together to form dust particles. These particles are not shaped like perfect spheres; instead, they have irregular shapes and can be either fluffy or tightly packed. Their composition, size, and other features depend on where they are found. By studying the makeup of a dust particle, scientists can learn about where it came from. Different types of dust, such as dust in the general space between stars, dust in dense clouds, dust in planetary rings, and dust around stars, have different characteristics. For example, dust grains in dense clouds often have a layer of ice and are larger than dust in the space between stars. Interplanetary dust particles (IDPs) are usually even larger.
Most of the material from space that falls to Earth is made up of small meteoroids, which are about 50 to 500 micrometers in size. These meteoroids have an average density of 2.0 grams per cubic centimeter and are about 40% empty space. The total amount of IDPs that fall into Earth's stratosphere each year is between 1 and 3 grams per square centimeter, with an average density of about 2.0 grams per cubic centimeter.
Other properties of dust include the presence of specific molecules. In circumstellar dust, scientists have found signs of molecules like carbon monoxide, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, and polyformaldehyde. In the space between stars, there is evidence of silicate and carbon-based grains. Cometary dust is different from asteroidal dust, though they share some similarities. Asteroidal dust is similar to a type of meteorite called carbonaceous chondrites. Cometary dust is similar to interstellar grains, which may include silicates, polycyclic aromatic hydrocarbons, and water ice. Studies of a comet called 67P/Churyumov–Gerasimenko show that cometary dust can vary in structure, ranging from tightly packed grains to loosely held groups of particles, with differences in strength and size.
In September 2020, scientists found evidence of solid water in the space between stars, specifically water ice mixed with silicate grains in cosmic dust.
Research on how dust changes over time shows that the composition of cosmic dust can vary depending on the amount of metal and other elements available in different regions of space.
Cosmic dust grains can have porous structures, which affect chemical reactions in space. These structures are important to consider when studying dust in space models.
Dust grain formation
Large dust grains found in space are likely made of complex materials. These grains have hard, resistant cores that formed when material from stars cooled and solidified. These cores were later covered by layers that formed when the grains entered cold, dense areas of space. Scientists have studied how these grains grow and break down over time. They found that the cores last much longer than the average dust grain. These cores often begin as silicate particles that form in the atmospheres of cool, oxygen-rich red giant stars or as carbon grains that form in the atmospheres of cool carbon stars. Red giants are stars that have left the main stage of their life and are now in the giant phase. They are the main source of these hard, resistant dust grain cores in galaxies. These cores are also called stardust, which is a scientific term for the small part of cosmic dust that formed when material from stars cooled and solidified as it was ejected. Some of these cores formed inside the expanding areas of supernovae, which are like large cosmic chambers that expand rapidly. Studies suggest that during supernova explosions, dust can form when material from these explosions interacts with surrounding space materials. Scientists who study stardust found in meteorites often call these grains presolar grains, but only a small part of all presolar dust is found in meteorites. Stardust forms inside stars through different chemical processes than the rest of cosmic dust, which forms when materials cool and attach to existing dust in cold, dark areas of space. These cold areas, called molecular clouds, are very cold, usually less than 50 degrees Kelvin. In these areas, ices can form on dust grains, but they may later be destroyed by radiation or heat and turn into gas. Later, when the Solar System formed, many interstellar dust grains were changed further by combining with other materials and through chemical reactions in the area where planets formed. The history of different types of dust grains in the early Solar System is complex and not fully understood.
Astronomers know that dust forms around stars that are in the late stages of their life based on specific signs. In infrared light, a glow at 9.7 micrometers shows silicate dust in cool, oxygen-rich giant stars. A glow at 11.5 micrometers shows silicon carbide dust in cool, carbon-rich giant stars. These signs help prove that small silicate particles in space came from the outer layers of these stars.
Conditions in space are not usually good for forming silicate cores. It would take too long, even if it were possible. Scientists argue that, given the typical size of a grain and the temperature of space gas, it would take longer than the age of the Universe for dust grains to form in space. However, dust grains are seen forming near nearby stars, in material ejected from novae, supernovae, and in R Coronae Borealis stars, which release clouds of gas and dust. This shows that dust grains form from material lost by stars.
Most dust in the Solar System has been changed many times. It was once part of the material that formed the Solar System, later collected in planetesimals, and then found in leftover materials like comets and asteroids. These materials were reshaped over time. During the Solar System’s formation, hydrogen was the most common element. Metals like magnesium, silicon, and iron, which are important for forming rocky planets, solidified in the hottest parts of the planetary disk. Some molecules, like CO, N₂, NH₃, and free oxygen, remained as gas. Other molecules, such as graphite (carbon) and silicon carbide, formed solid grains in the planetary disk. However, carbon and silicon carbide grains found in meteorites are not from the planetary disk, as their chemical makeup shows they formed before the Solar System. Some molecules formed complex organic compounds, while others formed frozen layers of ice that could cover the hard, metal-rich grain cores. Stardust is an exception because it appears unchanged since it formed as crystalline minerals inside stars. Graphite forms inside supernovae as they expand and cool, even in gas with more oxygen than carbon, which is possible because of the intense radioactive environment in supernovae. This special way of forming dust has been studied closely.
The formation of materials in the planetary disk was influenced by the temperature of the solar nebula. Since the solar nebula was hotter closer to the Sun and colder farther away, scientists can determine where dust grains formed based on their materials. Some materials could only form at high temperatures, while others could only form at very low temperatures. A single dust particle from space may contain materials that formed in different places and at different times in the solar nebula. Most of the material from the original solar nebula no longer exists; it was either pulled into the Sun, sent into space, or used to form planets, asteroids, or comets.
Interplanetary dust particles (IDPs) are finely broken mixtures of thousands to millions of tiny mineral grains and amorphous materials. They can be thought of as a "matrix" made of different materials that formed at different times and places in the solar nebula and even before the solar nebula existed. Examples of these materials in space dust include GEMS, chondrules, and CAIs.
Recent observations using the James Webb Space Telescope have found carbon-based dust grains in galaxies that existed less than a billion years after the Big Bang. This shows that dust was created very early in the history of the Universe.
From the solar nebula to Earth
The arrows in the diagram show one possible path from a collected interplanetary dust particle back to the early stages of the solar nebula.
By following the trail to the right in the diagram, we reach interplanetary dust particles (IDPs) that contain the most volatile and primitive elements. The trail begins with interplanetary dust particles and leads to chondritic interplanetary dust particles. Scientists classify chondritic IDPs based on their level of oxidation, which divides them into three main groups: carbonaceous, ordinary, and enstatite chondrites. Carbonaceous chondrites are rich in carbon and often show unusual patterns in the amounts of hydrogen, carbon, nitrogen, and oxygen isotopes. From carbonaceous chondrites, the trail continues to the most primitive materials. These materials are nearly fully oxidized and contain elements that condense at the lowest temperatures ("volatile" elements) and the highest amounts of organic compounds. Dust particles with these elements are believed to have formed in the early history of the Solar System. Volatile elements have never been exposed to temperatures above about 500 K, so the IDP grain "matrix" includes very primitive material from the early Solar System. This is true for comet dust. The small portion of IDPs that are stardust comes from different origins. These refractory interstellar minerals form within stars, become part of interstellar matter, and remain in the presolar planetary disk. Nuclear damage tracks are caused by ions from solar flares. Solar wind ions hitting the particle’s surface create amorphous, radiation-damaged layers on the particle’s surface. Spallogenic nuclei are formed by galactic and solar cosmic rays. A dust particle from the Kuiper Belt at 40 AU would have far more track density, thicker amorphous layers, and higher total radiation exposure than a particle from the main-asteroid belt.
According to 2012 computer model studies, complex organic molecules necessary for life (extraterrestrial organic molecules) may have formed in the protoplanetary disk of dust grains surrounding the Sun before Earth formed. Recent research suggests that cosmic dust particles may have contributed to the geochemical origins of life on Earth by enabling complex chemical reactions in early planetary environments on Earth’s surface. These studies also suggest that similar processes may occur around other stars with planets.
In September 2012, NASA scientists reported that polycyclic aromatic hydrocarbons (PAHs), when exposed to conditions in the interstellar medium (ISM), change through processes like hydrogenation, oxygenation, and hydroxylation to form more complex organic molecules—steps toward creating amino acids and nucleotides, which are the building blocks of proteins and DNA, respectively. These transformations cause PAHs to lose their unique spectroscopic signatures, which may explain why PAHs are rarely detected in interstellar ice grains, especially in cold, dense clouds or the outer regions of protoplanetary disks.
In February 2014, NASA announced an updated database for detecting and tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. NASA scientists estimate that over 20% of the carbon in the universe may be linked to PAHs, which could be the starting materials for life. PAHs likely formed shortly after the Big Bang, are widespread in the universe, and are found near new stars and exoplanets.
In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds, including uracil, cytosine, and thymine, were created in a laboratory under conditions similar to outer space. These experiments used starting chemicals like pyrimidine, which is found in meteorites. Pyrimidine, like PAHs, is a carbon-rich chemical that may have formed in red giant stars or in interstellar dust and gas clouds, according to scientists.
Some "dusty" clouds in the universe
The Solar System has an interplanetary dust cloud, as do systems outside our Solar System. There are several types of nebulae, each with different causes and processes: diffuse nebula, infrared (IR) reflection nebula, supernova remnant, molecular cloud, HII regions, photodissociation regions, and dark nebula.
The differences between these types of nebulae are based on the types of radiation they produce. For example, HII regions, such as the Orion Nebula, where many stars are forming, are known as thermal emission nebulae. Supernova remnants, such as the Crab Nebula, are known as nonthermal emission, which produces synchrotron radiation.
Some well-known dusty areas in the Universe are the diffuse nebulae listed in the Messier catalog, such as M1, M8, M16, M17, M20, M42, and M43.
Some larger collections of dust data include the Sharpless (1959) Catalogue of HII Regions, Lynds (1965) Catalogue of Bright Nebulae, Lynds (1962) Catalogue of Dark Nebulae, van den Bergh (1966) Catalogue of Reflection Nebulae, Green (1988) Rev. Reference Cat. of Galactic SNRs, The National Space Sciences Data Center (NSSDC), and CDS Online Catalogs.
Dust sample return
The Stardust mission, part of the Discovery program, was launched on February 7, 1999. Its goal was to gather samples from the dust surrounding comet Wild 2 and collect cosmic dust. The mission brought these samples back to Earth on January 15, 2006. In 2007, scientists announced that they had found particles of interstellar dust in the collected samples.
Dust particles on Earth
In 2017, Genge and other scientists published a study about collecting dust particles from cities on Earth. The researchers gathered 500 micrometeorites from rooftops in Oslo and Paris. These dust particles are mostly made of silicate minerals and are shaped like small, round spheres. They form when tiny space rocks melt as they enter Earth's atmosphere and cool quickly, creating glass with small crystals of magnesian olivine, older forsterite crystals, and iron-rich olivine. In the UK, scientists search for micrometeorites on rooftops of cathedrals, such as Canterbury Cathedral and Rochester Cathedral. Every year, about 40,000 tons of cosmic dust fall to Earth.