Dusty Disc Found Around Nearby Star


The star Fomalhaut is about 25 light years away and twice as massive as our Sun. It is considered by astronomers to be a few hundred million years old. Images from the IRAS infrared satellite in the 1980s indicated that the star was surrounded by large amounts of dust. ESA's Herschel Space Observatory has provided an even higher resolution image of the star and its dust disk. It shows the star to be surrounded by hot dust and gas, but al so has a dusty belt of material at the outer fringes of the star system

(Credits: ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium)

The image of the infrared emission of the star Fomalhaut and its dust disc surrounding it taken by ESA's Herschel Space Observatory at a wavelength of 70 micron.

The belt is more than 100 times the distance between the Earth and the Sun makes it very cold, with a temperature of about -200 Celsius. About of this material is water ice. Fomalhaut icy dust belt forms a relatively narrow off-centre ring with respect to Fomalhaut suggesting the presence of planets orbiting close to it or as suggested by the 2004-2006 Hubble imaging of a planet orbiting on the edge of the ring.

(Credit: NASA/ESA/STScI)

This image is a comparison of the Fomalhaut star system with our own Solar System.

Based on the emitting, absorbing and scattering of the starlight by the disk the grains have been found to be as small grains a few thousandths of a millimeter across. However Hubble images had suggested larger particles. This indicates fluffy dust grains consisting of small particles stuck loosely together forming the larger ones. However these fluffy grains should have been blown away by the light pressure from Fomalhaut. The explanation proposed by astronomers is that continuous collisions and disintegration of larger asteroid-sized comets re-supplies the small particles.

However re-supplying the large amount of dust observed by Herschel would require the destruction of a whopping 2,000 1km across comets every day. Maintaining that many collisions per day requires trillions of comets to be orbiting inside the ring. That many commits would have a combined mass of over 100 Earths.

The flaw in this conclusion is that there seems to be no indication of these trillions of comets in the data. The claim that this dust is the result of thousands of dally comet collisions is based on two assumptions.

The first is the age if the star. If Fomalhaut is actually considerably younger then generally thought then this dust would not require re-supplying.

 

(Credit: NASA/ESA/STScI)

2004-2006 Hubble image

The second assumption is that this dust is a result of the stars formation from a larger dust cloud. If this dust came from a planet breaking up or evaporating from internal heat; a model supported by the 2004-2006 Hubble image of Fomalhaut showing a planet on the edge of the ring..

The point is that astronomers started with these two assumptions and there fore did not consider the other options. The result is a need for a minimum of a thousand of dally collisions of unobserved comets. That come to about 42  collisions an hour  or about one collision every 87 second. This is an amazingly high collision rate. Even if this theory is true is presents a problem for planet formation theory since it shows that larger peaces material in these dust clouds tend to get broken up as opposed to forming new planets


Reference

Dusty Disc of Crushed Comets Around a Nearby Star Caused by Collisions With Thousands of Comets a Day

 

 

 

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