A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris disks are found around stars with mature planetary systems, including at least one debris disk in orbit around an evolved neutron star. [1] Debris disks can also be produced and maintained as the remnants of collisions between planetesimals, otherwise known as asteroids and comets. [2]
As of 2001, more than 900 candidate stars had been found to possess a debris disk. They are usually discovered by examining the star system in infrared light and looking for an excess of radiation beyond that emitted by the star. This excess is inferred to be radiation from the star that has been absorbed by the dust in the disk, then re-radiated away as infrared energy. [3]
Debris disks are often described as massive analogs to the debris in the Solar System. Most known debris disks have radii of 10–100 astronomical units (AU); they resemble the Kuiper belt in the Solar System, although the Kuiper belt does not have a high enough dust mass to be detected around even the nearest stars. Some debris disks contain a component of warmer dust located within 10 AU from the central star. This dust is sometimes called exozodiacal dust by analogy to zodiacal dust in the Solar System.
In 1984 a debris disk was detected around the star Vega using the IRAS satellite. Initially this was believed to be a protoplanetary disk, but it is now known to be a debris disk due to the lack of gas in the disk and the age of the star. The first four debris disks discovered with IRAS are known as the "fabulous four": Vega, Beta Pictoris, Fomalhaut, and Epsilon Eridani. Subsequently, direct images of the Beta Pictoris disk showed irregularities in the dust, which were attributed to gravitational perturbations by an unseen exoplanet. [5] That explanation was confirmed with the 2008 discovery of the exoplanet Beta Pictoris b. [6]
Other exoplanet-hosting stars, including the first discovered by direct imaging (HR 8799), are known to also host debris disks. The nearby star 55 Cancri, a system that is also known to contain five planets, also was reported to have a debris disk, [7] but that detection could not be confirmed. [8] Structures in the debris disk around Epsilon Eridani suggest perturbations by a planetary body in orbit around that star, which may be used to constrain the mass and orbit of the planet. [9]
On 24 April 2014, NASA reported detecting debris disks in archival images of several young stars, HD 141943 and HD 191089, first viewed between 1999 and 2006 with the Hubble Space Telescope, by using newly improved imaging processes. [10]
In 2021, observations of a star, VVV-WIT-08, that became obscured for a period of 200 days may have been the result of a debris disk passing between the star and observers on Earth. [11] Two other stars, Epsilon Aurigae and TYC 2505-672-1, are reported to be eclipsed regularly and it has been determined that the phenomenon is the result of disks orbiting them in varied periods, suggesting that VVV-WIT-08 may be similar and have a much longer orbital period that just has been experienced by observers on Earth. VVV-WIT-08 is ten times the size of the Sun in the constellation of Sagittarius.
During the formation of a Sun-like star, the object passes through the T-Tauri phase during which it is surrounded by a gas-rich, disk-shaped nebula. Out of this material are formed planetesimals, which can continue accreting other planetesimals and disk material to form planets. The nebula continues to orbit the pre-main-sequence star for a period of 1–20 million years until it is cleared out by radiation pressure and other processes. Second generation dust may then be generated about the star by collisions between the planetesimals, which forms a disk out of the resulting debris. At some point during their lifetime, at least 45% of these stars are surrounded by a debris disk, which then can be detected by the thermal emission of the dust using an infrared telescope. Repeated collisions may cause a disk to persist for much of the lifetime of a star. [12]
Typical debris disks contain small grains 1–100 μm in size. Collisions will grind down these grains to sub-micrometre sizes, which will be removed from the system by radiation pressure from the host star. In very tenuous disks such as the ones in the Solar System, the Poynting–Robertson effect can cause particles to spiral inward instead. Both processes limit the lifetime of the disk to 10 Myr or less. Thus, for a disk to remain intact, a process is needed to continually replenish the disk. This can occur, for example, by means of collisions between larger bodies, followed by a cascade that grinds down the objects to the observed small grains. [13]
For collisions to occur in a debris disk, the bodies must be gravitationally perturbed sufficiently to create relatively large collisional velocities. A planetary system around the star can cause such perturbations, as can a binary star companion or the close approach of another star. [13] The presence of a debris disk may indicate a high likelihood of exoplanets orbiting the star. [14] Furthermore, many debris disks also show structures within the dust (for example, clumps and warps or asymmetries) that point to the presence of one or more exoplanets within the disk. [6] The presence or absence of asymmetries in our own trans-Neptunian belt remains controversial although they might exist. [15]
Belts of dust or debris have been detected around many stars, including the Sun, including the following:
Star | Spectral class [16] | Distance (ly) | Orbit (AU) | Notes |
---|---|---|---|---|
Epsilon Eridani | K2V | 10.5 | 35–75 | [9] |
Tau Ceti | G8V | 11.9 | 35–50 | [17] |
Vega | A0V | 25 | 86–200 | [18] [19] |
Fomalhaut | A3V | 25 | 133–158 | [18] |
AU Microscopii | M1Ve | 33 | 50–150 | [20] |
HD 181327 | F5.5V | 51.8 | 89-110 | [21] |
HD 69830 | K0V | 41 | <1 | [22] |
HD 207129 | G0V | 52 | 148–178 | [23] |
HD 139664 | F5IV–V | 57 | 60–109 | [24] |
Eta Corvi | F2V | 59 | 100–150 | [25] |
HD 53143 | K1V | 60 | ? | [24] |
Beta Pictoris | A6V | 63 | 25–550 | [19] |
Zeta Leporis | A2Vann | 70 | 2–8 | [26] |
HD 92945 | K1V | 72 | 45–175 | [27] |
HD 107146 | G2V | 88 | 130 | [28] |
Gamma Ophiuchi | A0V | 95 | 520 | [29] |
HR 8799 | A5V | 129 | 75 | [30] |
51 Ophiuchi | B9 | 131 | 0.5–1200 | [31] |
HD 12039 | G3–5V | 137 | 5 | [32] |
HD 98800 | K5e (?) | 150 | 1 | [33] |
HD 15115 | F2V | 150 | 315–550 | [34] |
HR 4796 A | A0V | 220 | 200 | [35] [36] |
HD 141569 | B9.5e | 320 | 400 | [36] |
HD 113766 A | F4V | 430 | 0.35–5.8 | [37] |
HD 141943 | [10] | |||
HD 191089 | [10] |
The orbital distance of the belt is an estimated mean distance or range, based either on direct measurement from imaging or derived from the temperature of the belt. The Earth has an average distance from the Sun of 1 AU.
A protoplanetary disk is a rotating circumstellar disc of dense gas and dust surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star. The protoplanetary disk may also be considered an accretion disk for the star itself, because gases or other material may be falling from the inner edge of the disk onto the surface of the star. This process should not be confused with the accretion process thought to build up the planets themselves. Externally illuminated photo-evaporating protoplanetary disks are called proplyds.
A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consist of bodies such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals and circumstellar disks. For example, the Sun together with the planetary system revolving around it, including Earth, forms the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems.
Beta Pictoris is the second brightest star in the constellation Pictor. It is located 63.4 light-years (19.4 pc) from the Solar System, and is 1.75 times as massive and 8.7 times as luminous as the Sun. The Beta Pictoris system is very young, only 20 to 26 million years old, although it is already in the main sequence stage of its evolution. Beta Pictoris is the title member of the Beta Pictoris moving group, an association of young stars which share the same motion through space and have the same age.
AU Microscopii is a young red dwarf star located 31.7 light-years away – about 8 times as far as the closest star after the Sun. The apparent visual magnitude of AU Microscopii is 8.73, which is too dim to be seen with the naked eye. It was given this designation because it is in the southern constellation Microscopium and is a variable star. Like β Pictoris, AU Microscopii has a circumstellar disk of dust known as a debris disk and at least two exoplanets, with the presence of an additional two planets being likely.
HD 107146 is a star in the constellation Coma Berenices that is located about 90 light-years (28 pc) from Earth. The apparent magnitude of 7.028 makes this star too faint to be seen with the unaided eye.
HD 210277 is a single star in the equatorial constellation of Aquarius. It has an apparent visual magnitude of 6.54, which makes it a challenge to view with the naked eye, but it is easily visible in binoculars. The star is located at a distance of 69.6 light years from the Sun based on parallax, but is drifting closer with a radial velocity of −20.9 km/s.
BD+20°307 is a close binary star system approximately 300 light-years away in the constellation Aries. The system is surrounded by a dusty ring, and probably orbited by a 0.48M☉ white dwarf on a wide orbit.
HD 69830 is a yellow dwarf star located 41.0 light-years away in the constellation of Puppis. In 2005, the Spitzer Space Telescope discovered a narrow ring of warm debris orbiting the star. The debris ring contains substantially more dust than the Solar System's asteroid belt. In 2006, three extrasolar planets with minimum masses comparable to Neptune were confirmed in orbit around the star, located interior to the debris ring.
Eta Telescopii is a white-hued star in the southern constellation of Telescopium. This is an A-type main sequence star with an apparent visual magnitude of +5.03. It is approximately 158 light years from Earth and is a member of the Beta Pictoris Moving Group of stars that share a common motion through space. It forms a wide binary system with the star HD 181327 and has a substellar companion orbiting around it, named Eta Telescopii B.
HD 12039, also known as DK Ceti, is a variable star in the constellation of Cetus at a distance of 135 ly (41 pc). It is categorized as a BY Draconis variable because of luminosity changes caused by surface magnetic activity coupled with rotation of the star. The stellar classification G4V is similar to the Sun, indicating this is a main sequence star that is generating energy at its core through the thermonuclear fusion of hydrogen. The effective temperature of 5,585 K gives the star a yellow hue. It has about the same mass as the Sun, but only emits 89% of the Sun's luminosity. This is a young star with age estimates ranging from 7.5−8 million years to 30 million years.
Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the exoplanets reported as of January 2024 have been observed directly, with even fewer being resolved from their host star.
Eta Corvi is an F-type main-sequence star, the sixth-brightest star in the constellation of Corvus. Two debris disks have been detected orbiting this star, one at ~150 AU, and a warmer one within a few astronomical units (AU).
HD 113766 is a binary star system located 424 light years from Earth in the direction of the constellation Centaurus. The star system is approximately 10 million years old and both stars are slightly more massive than the Sun. The two are separated by an angle of 1.3 arcseconds, which, at the distance of this system, corresponds to a projected separation of at least 170 AU.
HD 210277 b is an extrasolar planet orbiting the star HD 210277. It was discovered in September 1998 by the California and Carnegie Planet Search team using the highly successful radial velocity method. The planet is at least 24% more massive than Jupiter. The mean distance of the planet from the star is slightly more than Earth's distance from the Sun. However, the orbit is very eccentric, so at periastron this distance is almost halved, and at apastron it is as distant as Mars is from the Sun.
Beta Pictoris b (abbreviated as β Pic b) is an exoplanet orbiting the young debris disk A-type main sequence star Beta Pictoris located approximately 63 light-years (19.4 parsecs, or 6×1014 km) away from Earth in the constellation of Pictor. It has a mass around 13 Jupiter masses and a radius around 46% larger than Jupiter's. It orbits at 9 AU from Beta Pictoris, which is about 3.5 times farther than the orbit of Beta Pictoris c. It orbits close to the plane of the debris disk orbiting the star, with a low eccentricity and a period of 20–21 years.
HD 172555 is a white-hot Type A7V star located relatively close by, 95 light years from Earth in the direction of the constellation Pavo. Spectrographic evidence indicates a relatively recent collision between two planet-sized bodies that destroyed the smaller of the two, which had been at least the size of the Moon, and severely damaged the larger one, which was at least the size of Mercury. Evidence of the collision was detected by NASA's Spitzer Space Telescope.
Paul Kalas is a Greek American astronomer known for his discoveries of debris disks around stars. Kalas led a team of scientists to obtain the first visible-light images of an extrasolar planet with orbital motion around the star Fomalhaut, at a distance of 25 light years from Earth. The planet is referred to as Fomalhaut b.
An exocomet, or extrasolar comet, is a comet outside the Solar System, which includes rogue comets and comets that orbit stars other than the Sun. The first exocomets were detected in 1987 around Beta Pictoris, a very young A-type main-sequence star. There are now a total of 27 stars around which exocomets have been observed or suspected.
A circumstellar disc is a torus, pancake or ring-shaped accretion disk of matter composed of gas, dust, planetesimals, asteroids, or collision fragments in orbit around a star. Around the youngest stars, they are the reservoirs of material out of which planets may form. Around mature stars, they indicate that planetesimal formation has taken place, and around white dwarfs, they indicate that planetary material survived the whole of stellar evolution. Such a disc can manifest itself in various ways.
A circumplanetary disk is a torus, pancake or ring-shaped accumulation of matter composed of gas, dust, planetesimals, asteroids or collision fragments in orbit around a planet. They are reservoirs of material out of which moons may form. Such a disk can manifest itself in various ways.