A Ring of Powdered Comet

The image is an infrared view of the Fomalhaut dust disk taken by the Herschel Space Observatory. The new information reveals that comets are constantly colliding around the star, creating its dust disk. Image credit: ESA

You may have already seen this story, but here is some more to chew on. On April 11 the European Space Agency (ESA) announced that a study using their Herschel Space Observatory revealed the dust belt surrounding the nearby star Fomalhaut to likely be the result of the daily collisions of thousands of icy comets.

The white dwarf star Fomalhaut is the brightest star in the constellation Piscis Austrinus (the Southern Fish) and one of the brightest stars in our sky. Fomalhaut is a young star, just a few hundred million years old, and it has twice the mass of our Sun. The dust belt around Fomalhaut was discovered in the 1980s by the Infrared Astronomical Satellite (IRAS), but Herschel’s new images of the belt show it in much more detail at far-infrared wavelengths than ever before.

Bram Acke, at the University of Leuven in Belgium, and his team analyzed the Herschel observations and found the dust temperatures in the belt to be between –382 and –274F (–230 and –170C). However, because Fomalhaut is slightly off-center and closer to the southern side of the belt, the southern side is warmer and brighter than the northern side.

Both the narrowness and asymmetry of the dust belt are thought to be caused by the gravity of a possible planet in orbit around the star, as suggested by earlier Hubble Space Telescope images.
The Herschel data show that the dust in the belt has the thermal properties of small solid particles, with sizes of only a few millionths of a meter across (a few micrometers across, or a few microns across. For reference, the diameter of a hair is 40 to 50 microns.).

But this observation created a paradox because the Hubble Space Telescope observations suggested solid grains more than ten times larger. Those observations collected starlight scattering off the grains in the belt and showed it to be very faint at Hubble’s visible wavelengths, suggesting that the dust particles are relatively large. But that appears to be incompatible with the temperature of the belt as measured by Herschel in the far-infrared.

To resolve the paradox, the team suggests that the dust grains must be large fluffy aggregates, similar to dust particles released from comets in our own Solar System.

This possibility of fluffy aggregates would have both the correct thermal and scattering properties. However, this leads to another problem.

The bright starlight from Fomalhaut should blow small dust particles out of the belt very rapidly, yet such grains appear to remain abundant there. The only way to overcome this contradiction is to re-supply the belt through continuous collisions between larger objects in orbit around Fomalhaut, creating new dust.

To sustain the belt, the rate of collisions must be impressive: each day, the equivalent of either two 10 km-sized comets or 2,000 1 km-sized comets must be completely crushed into small fluffy, dust particles.

To keep the collision rate so high, there must be between 260 billion and 83 trillion comets in the belt, depending on their size. Our own Solar System has a similar number of comets in its Oort Cloud, which formed from objects scattered from a disc surrounding the Sun when it was as young as Fomalhaut.

The Herschel Space Observatory is the largest infrared space observatory launched to date. Equipped with a 3.5 meter diameter reflecting telescope and instruments cooled to close to absolute zero, Herschel observes at wavelengths that have never previously been explored. Launched May 2009, and after a roughly 50-day journey from Earth, Herschel entered its operational orbit around the second Lagrange point of the Sun-Earth system (L2), for a nominal mission lifetime of three years. Herschel is an ESA mission with important contributions by the National Aeronautics and Space Administration (NASA). To learn more, please follow these links.

ESA Science and Technology site for the Herschel Space Observatory:
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=16

Caltech site for the Herschel Space Observatory:
http://www.herschel.caltech.edu

The NASA news and information site for the Herschel Space Observatory:
http://www.nasa.gov/herschel

The ESA site for the Herschel Space Observatory:
http://www.esa.int/SPECIALS/Herschel

Infrared Astronomical Satellite (IRAS):
http://lambda.gsfc.nasa.gov/product/iras/

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