Stellar Dust Disks Crumble

Observations show stellar dust disks fragmenting into smaller dust, not growing into planets.

Celestial archaeology:  A triumphant sounding article on PhysOrg announces, “Scientists solve riddle of celestial archaeology.”  Further down in the text, the reader finds out that the “building blocks” around certain white dwarf stars are crumbling, not growing: “the researchers have discovered that many of the stars show signs of contamination by rocky material, the left overs from a planetary system.”  If there ever were planets, in other words, only their leftovers remain.

Destruction in Beta Pictoris:  A couple of decades ago, astronomers were all excited about Beta Pictoris, a star with one of the first dust disks ever seen.  They were sure the dust was clumping into planets, especially when a tilt in the disk hinted at the presence of a perturbing planet.  Now, a paper in Science Magazine is all about destruction, not construction: “Molecular Gas Clumps from the Destruction of Icy Bodies in the β Pictoris Debris Disk.”   Researchers had this to say about an asymmetric clump of carbon monoxide found in the disk: “This gas clump delineates a region of enhanced collisions, either from a mean motion resonance with an unseen giant planet or from the remnants of a collision of Mars-mass planets.”  The paper says nothing about accretion, but rather a “collisional cascade” of debris, perhaps something like that in the blockbuster movie Gravity.  “The CO and compact clump in the β Pic disk indicate that this system is undergoing a period of intense activity driven by planets or planet collisions.”

The debris in low earth orbit in Gravity was not growing into a space station.  Why should we expect circling debris around a star to grow a habitable planet?