Fomalhaut

On May 7, 2007, astronomers modelling the vertical thickness and dust production in Fomalhaut's circumstellar debris disk based on Hubble observations submitted a paper arguing that its disk likely contains planetary embryos as large as Pluto that are undergoing runaway growth into larger bodies (Quillen et al, 2007 -- more below). On June 22, 2007, one of the astronomers studying the dust disk announced that its off-center position around Fomalhaut and sharp inside edge is probably due to the presence of a Neptune-sized planet in orbit just inside the disk (University of Rochester press release -- more below).

Also known as Alpha Piscis Austrini (or Australis), this bright star is located within 25.1 light-years (ly) away from our Sun, Sol. Fomalhaut lies at the western edge (22:57:39.05-29:37:20.05, ICRS 2000.0) of Constellation Piscis Austrinus (or Australis), the Southern Fish (chart and photo). Visible with the naked eye as the constellation's brightest star, Fomalhaut (pronounced like "foam a lot") is derived from "Fum al Hut" in Arabic, which means Fish's Mouth. While the nearby orange-red (K4-5) dwarf star TW Piscis -- reported in 1897 by Thomas Jefferson Jackson See (1866-1962) -- has been determined to be a distant physical companion (D. Barrado y Navascues, 1998), another K5 dwarf (LTT 8273) later observed in proper motion studies is believed to be an optical companion although it also may be a remaining member of a low-density star cluster including Fomalhaut, Vega, and Castor that has gradually dispersed over hundreds of millions of years. (See an animation of the planetary, dust disk, and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)

Ann Feild, NASA, ESA



Larger illustration.



Fomalhaut is the brightest star
in Constellation Piscis Austrinus
(or Australis), the Southern Fish.

The Star

Fomalhaut is a white main sequence dwarf star of spectral and luminosity type A3 V. According to various estimates, the star has about 2.3 times Sol's mass (Chiang et al, 2008; and Quillen et al, 2007), 1.7 to 1.82 times its diameter (Extrasolar Planets Encyclopaedia; the Yale Bright Star Catalogue, 1991 5th Revised Edition notes entry for HR 8728 provides a diameter estimate of 0.00198 to 0.00210"), and about 14 to 17.6 times its luminosity. Although the Fomalhaut may be only about 200 million years old (D. Barrado y Navascues, 1998), it is so much bigger and hotter than Sol that it should exhaust its core hydrogen in about a billion years and turn into a red giant or Cepheid variable before puffing away its outer layers to reveal a remnant core as a white dwarf. The star is a New Suspected Variable designated NSV 14372. Useful star catalogue numbers for Fomalhaut include: Alp or Alf PsA, 24 PsA, HR 8728, Gl 881, Hip 113368, HD 216956, CD-30 19370, CP(D)-30 6685, SAO 191524, FK5 867, and LTT 9292.

Dust Disk

In 1983, an orbiting satellite called IRAS discovered far more infrared radiation -- which has waves longer than red light -- coming from the Fomalhaut than expected for small interstellar dust grains found around young, early-type stars. The radiation is coming from a huge disk of matter of around 370 AUs in diameter (nearly five times the dimension of the Solar System) that surrounds the star, much like those that encompass Vega, Beta Pictoris, and Denebola (S. A. Stern, 1993) but is inclined 20 degrees from an edge-on view. The disk is thought to be made of icy dust particles that have been warmed by the star which, according to Holweger et al (1999), tends to develop after most of the surrounding nebula of gas has been absorbed or expelled from the developing star.

Submillimetre Common-User Bolometer Array, James Clerk Maxwell Telescope, JAC
(Cold dust disk around Fomalhaut)

In 1998, British and American astronomers at the Joint Astronomy Center (JAC press release) in Hawaii, the University of California in Los Angeles (UCLA), and the Royal Observatory of Edinburgh obtained the first pictures of huge disk-like structures of dust around Fomalhaut (Holland et al, 1998, in postscript). The "sub-millimeter" image, above, shows emissions from tiny dust particles (that are only a fraction of a millimeter in size) in orbit around Fomalhaut. Yellow to red areas of the image indicate the highest concentrations of cold dust, while blue to black areas suggest very little dust. In JAC's image the brightest emission area, which should have the most dust, is located far from the star, which appears to be surrounded by a huge disk of dust with a hollow central cavity. Looking like a doughnut sliced in half and tilted upright, the dust disk has a hole in the center comparable to the size of Sol's planetary system (including its Edgeworth-Kuiper Belt).

K. Stapelfeldt, SIRTF/Spitzer,
CalTech, JPL, JCMT, NASA



Larger and jumbo infrared images.



New infrared images show that
the southern lobe is one-third
brighter and that warmer dust
within the apparent "doughnut
hole" extends to within 10 AUs
of the star (more).

The dust is densest at a distance from Fomalhaut around the Kuiper Belt's location in the Solar System. Indeed, the structures resemble a younger version of the Kuiper Belt of icy objects (dormant comets and larger planetary bodies) that surround the Solar System beyond the orbit of Neptune. Although the pictures taken do not show the icy bodies directly, the dust that is shown is believed to be debris that is forming, or being fragmented from, these bodies. The dust may have disappeared from Fomalhaut's inner region because it has coalesced into planets. If an astronomer could have seen what our Solar System looked like four billion years ago, then it would have looked very much as Fomalhaut looks today.

A Planetary System?

Ann Feild, NASA, ESA




Larger and jumbo illustrations.




A comparison of Fomalhaut's
dust disk and planetary orbits
with the Solar System's
planetary orbits and Main
Asteroid and Edgeworth-Kuiper
belts highlights the Fomalhaut
System's greater size and mass.

After the initial discovery of the dust disk, new data at short-submillimetre wavelengths of Fomalhaut suggested the presence of a "warp" in the observed, "half-donut" slice image of the dust disk (shown at the top of the page, see: the Royal Observatory Edinburgh 2002 press release; and a 2001 discussion). At a telescope beam-size resolution of about 50 AU at the distance of Fomalhaut, the image of the dust disk is confirmed to be an almost edge-on dust disk (ring or torus), with an inner cavitying lack similar dust from about 100 AUs orbital distance to the star itself. Numerical simulations suggest that the dust is concentrated in mean-motion resonances with a planet, a Saturn-sized object with about 30 percent of Jupiter's mass that is hidden in a gap in the torus just with the inner edge of the dust disk (Ozernoy et al, 2000). While other possibilities have not been ruled out, simulation results are in good agreement with the observation that concentrations of dust that have been collected into librating orbits about the L4 and L5 Lagrangian points, and the presence of small and large gaps and short and long arcs. Like the case of Epsilon Eridani, the observation of severe perturbations in the dust disk appears to be the result of multiple planetary companions at 60 to 100 AUs from Fomalhaut, larger than the orbital distances observed in the Solar System (more discussion).

David Hardy, ROE, ATC, NSF, NASA


Larger and jumbo images.


A Saturn-sized planet may be creating
a wake or trail of dust, creating severe
perturbations in the dust disk around
Fomalhaut (more discussion from the
Royal Observatory of Edinburgh and
Astronomy Picture of the Day).

In 2005, astronomers released a Hubble Space Telescope image indicating that Fomalhaut's dust disk looks more like a narrow circumstellar ring in visible light (Kalas et al, 2005, in pdf). The new image directly confirms that the center of the dust ring is located about 15 AUs from the star, as was already inferred from previous and longer wavelength observations using submillimeter telescopes. Hubble's new observations provide strong evidence that at least one unseen planetary mass object is orbiting Fomalhaut at a distance around 50 to 70 AUs, as Hubble failed to detect a larger substellar object, such as a brown dwarf. Fomalhaut's ring is similar to the dust disk generated by the Solar System's Edgeworth-Kuiper Belt where icy objects collide to generate a much less dense dust disk, but it is about four times larger than the Solar System's. The new observations indicate that Fomalhaut's ring is around 280 AUs in diameter with a narrow width of around 25 AUs, a relatively sharp inner edge centered around 133 AUs from the star, and an outer edge extending out to around 158 AUs (more illustrations at hubblesite.org and links at APOD).

Paul Kalas, James Graham,
Mark Clampin, NASA, ESA




Larger image.




Subsequent imaging in visible light
supports the hypothesis that a large
planetary object is tugging on the
dust around Fomalhaut (more).

On May 7, 2007, astronomers modelling the vertical thickness and dust production in Fomalhaut's disk based on Hubble observations submitted a paper arguing that its disk likely contains planetary embryos as large as Pluto that are undergoing runaway growth into larger bodies (Quillen et al, 2007). Their model predicts how large the planetary bodies in a dust or debris disk must be to puff it up to a certain thickness. Although a developing star system's dust disk should thin as the system ages, more dust grains may be "knocked" into eccentric orbits that "puff out" the disk if dust agglomeration has proceeded enough for planetesimals to form increasingly large embryonic planets. In the case of Fomalhaut, the model indicates that a Pluto-sized object may have already formed at an orbital distance of around 133 AUs from the bluish white dwarf star.

NASA



Larger image.



A Pluto-sized planetary
embryo may have already
formed at an orbital
distance of around 133
AUs from Fomalhaut (which
may look like Neptune's
icy moon, Triton).

On June 22, 2007, one of the astronomers (Alice C. Quillen) studying the dust disk announced that its off-center position around Fomalhaut and sharp inside edge is probably due to the presence of a Neptune-sized planet in orbit just inside the disk (University of Rochester press release). Fomalhaut's dusk disk is offset by 15 AUs, which suggests that a hidden planet with a similarly skewed orbit is present. This is considered to be unusual in such a young system, since when stars form from their could gas and dust, the angular momentum of the cloud carries over to the planetary objects formed from the cloud. While newly formed objects may initially have circular orbits, subsequent collisions can send them into eccentric orbits, if not into their host star or out of their star system altogether.

NASA


Larger and jumbo images.



A Neptune-like gas giant
may orbit Fomalhaut just
inside its dust disk (more).

The distance from Fomalhaut where an Earth-type planet would be "comfortable" with liquid water is centered around 4.1 AU -- between the Main Asteroid Belt and Jupiter in the Solar System. At that distance from the star, such a planet would have an orbital period of about 5.5 Earth years. If there is life on any Earth-type planet orbiting youthful Fomalhaut, it is likely to be primitive single-cell, anaerobic (non-oxygen producing) bacteria under constant bombardment by meteorites and comets as Earth was for the first billion years. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O₃) although Fomalhaut puts out a lot more hard radiation than Sol. (See an animation of the planetary, dust disk, and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)