Science News

The Dusty Disc Disappearance

Sun, 8th Jul 2012

Ben Valsler

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A huge, hot dusty disc around a Sun-like star seems to have vanished before our very eyes, and its disappearance cannot be explained by any current scientific theory.

The star TYC 8241 2652 1 has been observed by a number of telescopes, and a bright infra-red glow indicated the presence of a large amount of warm dust absorbing visible light from the star and re-emitting it in the infra-red range. 

Carl Melis and colleagues at the University of California, San Diego report in Nature that this dust seemed to be the result of collisions of rocky objects and fell in the region analogous to where rocky planets formed in our solar system.  Estimation of the size of dust grains hinted that this could be a system undergoing an active stage of terrestrial planet formation.  This made it an ideal candidate for future observation; as calculation of likely grain size suggested our theories of planetary formation suggest that rocky planets like the Earth will eventually form from these discs of dust and ice.

However, since 2008, the infra-red emission from this star has reduced by a factor of about 30, suggesting that the star is now only host to a meagre amount of far cooler dust.  This is an unprecedented observation, changes like this have never been seen or even predicted before, so what could have caused it?

No major changes have occurred in the star in this time, so it’s unlikely that a violent stellar event blew the dust away.  Models of planet formation predict the loss of dust over at least many thousands of years, as the gravitational field of newly formed planets disturb the dust cloud.

Melis argues that only two hypotheses could fit the results.  They are: 

  • The collisional avalanche hypothesis, where dust grains become gradually smaller through collisions, until they’re below a certain threshold and are blown out of the system by radiation pressure from the star.
  • Runaway accretion, where gas in the cloud places aerodynamic drag on the dust, causing it to fall in to the star.

However both ideas have problems, so the jury is still out.

Even though we lack a clear explanation for what has happened in this system, it is a unique observation and certain to push our understanding of how planets like the Earth first form.



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We had only observed the disk of dust for 25 years, but we assumed it had been there for millions or billions of years. We also assumed that no planets had already formed around the star. So what if the disk was the result of a recent collision of pre-existing planets? Perhaps one or more large parts of the planets that collided were still orbiting within the dust cloud. This could have been similar to the collision between proto Earth and Theia, believed to have resulted in formation of our Moon. Phractality, Sun, 8th Jul 2012

A guess: The collision of particles dramatically transitions from elastic to inelastic due to the particles' state changing from solid to liquid/semi-liquid due to the star's radiation or other source, with the kinetic energy converting to thermal energy which accelerates/maintains the coalescence into a planet. Lmnre, Mon, 9th Jul 2012

I suspect that Dyson guy had something to do with it. Geezer, Tue, 10th Jul 2012

Too bad that it doesn't sound as if there was a series of repeated observations. 

I would think that planetary formation would take far more than a decade or so.  If the dust cloud had a stable orbital velocity, then it should take a long time for all of it to coalesce into a planet.  If it had zero orbital velocity, then it might rapidly fall back to the star.  Consider that it takes Jupiter a decade to just circle the sun once, let alone vacuuming up stuff in its path (or in British....  Hoovering up the dust).

Is it possible that there was a HUGE Coronal Mass Ejection (CME), that would either fall back into the star, or continue moving out into very cool (and rarefied) regions of space.  What would happen if a large planet collided with a star? CliffordK, Tue, 10th Jul 2012

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