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Debris disk

A debris disk is a ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a neutron star. They can constitute a phase in the formation of a planetary system following the protoplanetary disk phase. They can also be produced and maintained as the remnants of collisions between planetismals.

By 2001, over 900 candidate stars have been found to possess a debris disk. They are usually located 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 disk, then radiated away as infrared energy.

In certain cases the debris disks can be observed directly by occulting the primary star and then imaging the system.

Observation history

In 1984 a debris disk was located in orbit around the star Vega using the IRAS satellite. Initially this was believed to be a protoplanetary disk, but it is now thought to be a debris disk due to the lack of gas in the disk. Subsequently irregularities have been found in the disk, which may be indicative of the presence of planetary bodies. Similar discoveries of debris disks were made around the stars Fomalhaut and Beta Pictoris.

By 1998 a debris disk had been located around the nearby star 55 Cancri, a system that is also known to contain a planet. Structures in the debris disk around Epsilon Eridani also suggest perturbations by a planetary body in orbit around that star, which may be used to constrain the mass and orbit of the planet.

Origin

Typical debris disks contain small grains 1–100 μm in size. Radiation from the host star can cause these particles to spiral inward because of the Poynting-Robertson effect, so the lifetime of the disk will be on the order of 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. This can occur on a continual basis as collisions grind objects down into ever smaller bodies.

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.

Known belts

Belts of dust or debris have also been detected around stars other than the Sun, including the following:

Star	Spectral class	Distance (ly)	Orbit (AU)
Epsilon Eridani	K2V	10.5	35–75
Tau Ceti	G8V	11.9	35–50
Vega	A0V	25	86–200
AU Microscopii	M1Ve	33	50–150
HD 69830	K0V	41	<1
55 Cancri A	G8V	41	27–50
Pi¹ Ursae Majoris	G1.5Vb	46.5	?
HD 139664	F5IV-V	57	60–109
Eta Corvi	F2V	59	100–150
HD 53143	K1V	60	?
Beta Pictoris	A5V	63	25–550
Zeta Leporis	A2Vann	70	2–8
HD 92945	K1V	72	45–175
HD 107146	G2V	88	130
Fomalhaut	A3V	133	25
HD 12039	G3-5V	137	5
HD 15115	F2V	150	315–550
HR 4796 A	A0V	220	200
HD 141569	B9.5e	320	400
HD 113766 A	F4V	430	0.35–5.8

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.