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, ranging from a few molecules to 0.1 millimeters (100 micrometers) in size. Examples include micrometeoroids (less than 30 micrometers) and meteoroids (more than 30 micrometers). Cosmic dust can be categorized based on where it is found: intergalactic dust, interstellar dust, interplanetary dust (like the zodiacal cloud), and circumplanetary dust (like in a planetary ring). Scientists study dust outside the Solar System using tools like photometry, polarimetry, and infrared spectroscopy. Within the Solar System, direct methods, such as spacecraft missions, are used to collect and analyze space dust. For example, the Stardust spacecraft gathered cometary dust and found some particles likely from outside the Solar System, returning samples to Earth in 2006.
In the Solar System, interplanetary dust creates the zodiacal light. Other types of dust in the Solar System 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 weighing between 0.1 picograms and 0.1 grams. The density of the dust cloud Earth travels through is about 10 dust grains per cubic meter.
A large portion of space dust is "stardust," made of strong minerals like silicates, graphite, and amorphous carbon. These materials form around evolved stars, such as red giants, carbon stars, novae, and supernovae, and are carried into space by stellar winds. The dust also includes organic compounds, 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 began, scientists discovered 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, contributes to the early stages of star formation, and helps form planets. In the Solar System, dust plays a key role in the zodiacal light, Saturn's B Ring spokes, the outer diffuse rings around 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 a shared idea: cosmic dust changes over time in chemical, physical, and dynamic ways. The way dust evolves shows how the universe recycles materials, similar to daily recycling steps people know: creating, storing, processing, collecting, using, and discarding.
Observing cosmic dust in different places helps scientists understand how the universe recycles materials. These places include the diffuse interstellar medium, molecular clouds, dust around young stars, and planetary systems like the Solar System, where dust is considered to be in its most recycled state. Astronomers collect "snapshots" of dust at different life stages and, over time, create a clearer picture of the universe's complex recycling process.
Factors like a particle's starting movement, its material, surrounding plasma, and magnetic fields determine where the dust particle ends up. Small changes in these factors can lead to very different behaviors of the dust. This helps scientists learn where the dust came from and what is present in the space between objects.
Detection methods
A wide range of methods is used to study cosmic dust. Cosmic dust can be studied using remote sensing techniques that rely on how dust particles reflect or emit light, such as measuring the Zodiacal light.
Cosmic dust can also be studied directly, called "in-situ," through various collection methods and locations. Scientists estimate that between 5 and 300 tonnes of extraterrestrial material enter Earth's atmosphere each day.
NASA collects dust samples from Earth's atmosphere using plate collectors attached to airplanes that fly high in the stratosphere. Dust is also collected from Earth's ice-covered regions, such as Antarctica and Greenland, as well as from deep-sea sediments.
In the late 1970s, Don Brownlee from the University of Washington in Seattle first confirmed that some collected dust particles came from space. Another source of cosmic dust is meteorites, which contain stardust from ancient stars. Stardust grains are solid, heat-resistant pieces of stars that formed before the stars mixed with space. These grains are identified by their unique chemical compositions, which only occur in stars before they interacted with the space between stars. They formed as the material from stars cooled and solidified.
In space, dust detectors on spacecraft have been used to study cosmic dust. These detectors measure effects caused by dust particles, such as light flashes, sound waves, or electrical changes, and use laboratory tests to determine the dust's size and speed. Some spacecraft, like Stardust, have captured dust particles intact using special materials like aerogel.
Dust detectors have been used on many space missions, including HEOS 2, Helios, Pioneer 10 and 11, Giotto, Galileo, Ulysses, Cassini, and others. Currently, dust detectors are active on Ulysses, PROBA, Rosetta, Stardust, and New Horizons. Dust collected from Earth or space is studied in laboratories worldwide. A major storage facility for cosmic dust is located at NASA's Johnson Space Center in Houston.
Infrared light can pass through cosmic dust clouds, allowing scientists to observe star-forming regions and galaxy centers. NASA's Spitzer Space Telescope was the largest infrared telescope until the James Webb Space Telescope launched. Spitzer studied space objects by detecting heat from 3 to 180 micrometers, a range blocked by Earth's atmosphere. Spitzer's findings helped scientists learn more about cosmic dust, including evidence that some dust forms near supermassive black holes.
Another method to study cosmic dust is polarimetry. Dust grains are not perfectly round and often align with magnetic fields in space, which changes the light from stars passing through dust clouds. This technique has helped scientists study dust structures in nearby space.
In 2019, scientists discovered interstellar dust in Antarctica. They identified the dust by detecting radioactive elements, such as iron-60 and manganese-53, using a technique called accelerator mass spectrometry.
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 their refractive index. The way a single particle emits radiation is called its emissivity, which depends on how efficiently it interacts with light. Additional details about this process include extinction (blocking or dimming of light), scattering (redirecting light), absorption (taking in light), or polarization (changing the direction of light waves). Radiation emission curves often show specific patterns that help identify the composition of dust particles.
Dust particles can scatter light unevenly. Forward scattered light is light that is slightly redirected by diffraction, while back-scattered light is light that reflects off the particle.
The way light is scattered and dimmed by dust provides 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 particles are about one micrometer in diameter.
In long-exposure visible photographs, the scattering of light from dust grains is clearly visible in reflection nebulae. This scattering helps scientists understand the light-scattering properties of individual dust particles. In X-ray wavelengths, scientists are studying how interstellar dust scatters X-rays. Some researchers suggest that this scattering might create faint, spread-out halos around X-ray sources.
Presolar grains
Presolar grains are found inside meteorites, and scientists study them in laboratories on Earth. The term "stardust" or "presolar stardust" is sometimes used to describe grains from a single star, compared to dust made of many particles from space. However, this term is not always used. Presolar material was part of the dust in space before it became part of meteorites. Meteorites have kept these 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 when gases from individual stars cooled and condensed. These particles later became part of the cloud that formed our solar system.
Scientists have identified many types of presolar grains by studying the unusual chemical makeup of the elements in each grain. These hard, heat-resistant mineral grains may have once been covered with materials that easily evaporate, but those materials are removed when meteorites are dissolved in acid, leaving only the hard minerals. Scientists have found ways to study the inner parts of these grains without dissolving most of the meteorite, but the process is very difficult and time-consuming.
Presolar grains have helped scientists learn new things about how elements are created in stars. A key feature of presolar grains is their ability to withstand extreme heat. 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 a supernova. They are very different from materials that form in the cold space between stars.
Another important feature of presolar grains is their unusual chemical makeup, which is not found in space dust. This suggests the grains formed from the gases of individual stars before mixing with space dust diluted their chemical makeup. These unique chemical signatures help scientists identify the types of stars that created the grains. For example, the heavy elements in silicon carbide grains are mostly made from elements created in a specific process called the S-process, which happens in stars known as AGB stars. These stars are known to produce these elements, and their atmospheres are rich in them.
Another example is material formed in supernovae, often called SUNOCONs (short for SUperNOva CONdensate). These materials contain unusually high amounts of calcium, showing they formed when radioactive titanium was still active. Titanium has a short half-life of 65 years, so it would have stopped being radioactive after mixing with space gas. Finding these materials proved a prediction from 1975 that scientists could identify them this way. Silicon carbide grains from supernovae are much less common than those from AGB stars.
Stardust, including both SUNOCONs and grains from AGB stars, makes up less than 0.1% of all solid material in space. Scientists are very interested in presolar grains because they provide new information about how stars evolve and create elements.
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 material left. The discovery of presolar grains showed this belief was wrong.
Some bulk properties
Cosmic dust is made of tiny dust grains that come together to form dust particles. These particles have irregular shapes and can have different amounts of air pockets, ranging from very fluffy to tightly packed. The type of materials, size, and other features of the dust depend on where it is found. By studying the materials in 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 unique characteristics. For example, dust in dense clouds often has a layer of ice and is usually larger than dust in the space between stars. Interplanetary dust particles (IDPs) are generally even larger.
Most of the material from space that reaches Earth comes from meteoroids with sizes between 50 and 500 micrometers. These meteoroids have an average density of 2.0 grams per cubic centimeter and contain about 40% air pockets. The total amount of IDPs collected in Earth's stratosphere ranges from 1 to 3 grams per cubic centimeter, with an average density of about 2.0 grams per cubic centimeter.
Other features of cosmic dust include the presence of molecules like carbon monoxide, silicon carbide, amorphous silicate, polycyclic aromatic hydrocarbons, water ice, and polyformaldehyde in circumstellar dust. In the space between stars, evidence shows silicate and carbon grains. Cometary dust is different from asteroidal dust, though there is some overlap. Asteroidal dust is similar to carbon-rich meteorites, while cometary dust is similar to interstellar grains, which may include silicates, polycyclic aromatic hydrocarbons, and water ice. Studies of Comet 67P/Churyumov–Gerasimenko show that cometary dust particles can vary in structure, from tightly packed grains to loosely packed clusters, with different sizes and strengths.
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 dust depletion shows that the composition of cosmic dust can change over time as the availability of elements in space changes.
Cosmic dust grains can have porous structures, which affect chemical reactions in space. This shows the importance of considering porosity when studying dust in space models.
Dust grain formation
Large dust grains found in space are likely complex, with hard cores that formed inside the gas released by stars. These cores are covered by layers that form when the dust enters cold, dense areas of space. This process of growing and breaking apart happens outside of these clouds and has been studied to show that the cores last much longer than the typical lifespan of dust. These cores usually begin as silicate particles that form in the atmospheres of cool, oxygen-rich red giant stars or carbon-based particles that form in the atmospheres of cool carbon stars. Red giants are stars that have left the main stage of their life cycle and are now in the giant phase. They are the main source of these hard dust cores in galaxies. These cores are also called stardust, a term used to describe the small part of cosmic dust that formed when gases from stars cooled and solidified. A few percent of these hard cores form inside the expanding gas of supernovae, which are powerful explosions that release energy and materials into space. Studies suggest that dust can also form when materials from Type Ia supernovae interact with surrounding space materials. Scientists who study stardust found in meteorites often call it 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 areas are very cold, usually less than 50 degrees above absolute zero, allowing ices to form on dust grains. However, these ices can later be destroyed by radiation or heat, turning into gas. Later, when the Solar System formed, many dust grains were changed by joining together and reacting chemically in the area where planets formed. The history of different types of dust in the early Solar System is complex and not fully understood.
Astronomers know that dust forms around stars based on observations. Infrared light shows a signal at 9.7 micrometres, which indicates silicate dust in cool, oxygen-rich giant stars. A signal at 11.5 micrometres shows silicon carbide dust in cool, carbon-rich giant stars. These signals help prove that small silicate particles in space came from the outer layers of these stars.
Conditions in space are usually not suitable for forming silicate cores because it would take longer than the age of the Universe to create them. However, dust is seen forming near nearby stars, in the material ejected by novae, supernovae, and in R Coronae Borealis stars, which release clouds of gas and dust. This shows that dust from stars is the main source of these hard cores.
Most dust in the Solar System has been changed many times, formed from the material that created the Solar System and later collected in planetesimals, comets, and asteroids. During the Solar System's formation, hydrogen gas was the most common element. Metals like magnesium, silicon, and iron formed solid materials at high temperatures in the area where planets formed. Some gases, like carbon monoxide and ammonia, remained as gas, while others, like graphite and silicon carbide, formed solid grains. However, carbon and silicon carbide grains found in meteorites are from before the Solar System formed, as shown by their chemical makeup. Some gases formed complex organic compounds or froze into ice layers that covered the hard grain cores. Stardust is an exception because it formed directly from the cooling of gases inside stars and remains unprocessed. Graphite forms inside supernovae even when there is more oxygen than carbon, due to the intense radioactive environment of these explosions. This unique process has been studied closely.
The formation of materials in the early Solar System depended on the temperature of the space around the young Sun. Scientists can determine where dust came from by studying the materials in dust grains. Some materials could only form at high temperatures, while others formed at much lower temperatures. A single dust grain from space may have parts that formed in different places and at different times in the early Solar System. Most of the material from the original space around the young Sun has since been used to form the Sun, planets, or expelled into space.
Interplanetary dust particles (IDPs) are finely broken mixtures of many tiny mineral grains and amorphous materials. They act like a "matrix" containing pieces formed in different parts of the early Solar System and even before the Solar System formed. Examples of these pieces include GEMS, chondrules, and CAIs.
Recent observations from the James Webb Space Telescope have found carbon-based dust in galaxies that formed less than a billion years after the Universe began. 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 can see the interplanetary dust particles that contain the most volatile and primitive elements. This trail begins with interplanetary dust particles and moves to chondritic interplanetary dust particles. Scientists classify chondritic IDPs based on how much they are oxidized, which divides them into three main groups: carbonaceous, ordinary, and enstatite chondrites. Carbonaceous chondrites contain a lot of carbon and often have unusual patterns in the amounts of hydrogen, carbon, nitrogen, and oxygen isotopes. From carbonaceous chondrites, the trail leads to the most primitive materials. These materials are almost fully oxidized and include 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 higher than about 500 K, so the IDP grain "matrix" contains some of the most primitive material from the early Solar System. This is true for comet dust. The small part of IDPs that is stardust comes from different origins. These refractory interstellar minerals form inside stars, become part of interstellar matter, and remain in the presolar planetary disk. Nuclear damage tracks are created by ions from solar flares. Solar wind ions hitting the particle’s surface cause amorphous, 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 many more tracks, thicker amorphous layers, and higher total radiation exposure than a particle from the main-asteroid belt.
Based on 2012 computer model studies, complex organic molecules needed for life (extraterrestrial organic molecules) may have formed in the protoplanetary disk of dust surrounding the Sun before Earth formed. Recent research suggests that cosmic dust particles may have helped create the chemical conditions necessary for life on Earth by supporting reactions in early planetary environments. According to the computer models, this process may also 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 compounds. These changes are a step toward creating amino acids and nucleotides, which are the building blocks of proteins and DNA. As a result, PAHs lose their unique spectroscopic signature, which may explain why they are rarely detected in interstellar ice grains, especially in the outer parts of cold, dense clouds or the upper layers of protoplanetary disks.
In February 2014, NASA announced an improved database for identifying 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 are believed to have 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 the laboratory under space-like conditions using chemicals such as pyrimidine, which is found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), is the most carbon-rich chemical in the universe. Scientists suggest that pyrimidine may have formed in red giant stars or in interstellar dust and gas clouds.
Some "dusty" clouds in the universe
The Solar System has its own interplanetary dust cloud, and so do systems outside our Solar System. There are several types of nebulae, each formed by different 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 kinds of light and energy they produce. For example, HII regions, such as the Orion Nebula, where many stars are forming, are called thermal emission nebulae. Supernova remnants, like the Crab Nebula, are called nonthermal emission nebulae because they produce synchrotron radiation.
Some of the best-known dusty areas in space are the diffuse nebulae listed in the Messier catalog, such as M1, M8, M16, M17, M20, M42, and M43.
Some larger collections of dust information include Sharpless's 1959 Catalogue of HII Regions, Lynds's 1965 Catalogue of Bright Nebulae, Lynds's 1962 Catalogue of Dark Nebulae, van den Bergh's 1966 Catalogue of Reflection Nebulae, Green's 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 NASA's Discovery program, was launched on February 7, 1999. Its goal was to gather samples from the part of comet Wild 2 that releases gas and dust, as well as collect cosmic dust. The mission brought its 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 urban areas on Earth. The team collected 500 micrometeorites from rooftops in Oslo and Paris. These particles are mostly made of silicate minerals and have round, melted shapes. They formed when space rocks entered Earth's atmosphere and cooled quickly, creating glass with crystals of magnesian olivine, forsterite, and iron-bearing 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.