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Project Progress

We have discovered evidence for grain growth in protoplanetary disks surrounding young stars embedded within the Orion Nebula. Modeling of the disk properties indicates that the mean particle sizes are at least several micro-meters and a fraction of the mass in these disks may be locked-up up in particles larger than 1.3 mm. Grains may grow to such large sizes within less than 100,000 years.

The majority of stars appear to form in Orion Nebula-like environments in OB associations where young planetary systems face many hazards. Proto-planet formation must be prompt, or exo-planets will be rare. Within the past 10 Myr, about 20,000 to 50,000 stars were born in OB associations within 500 pc of the Sun. Within the same region, only about 5,000 to 10,000 stars were born in shielded dark clouds. Thus between 70% to over 90% of all stars are born in OB associations.

He have started to constrain the possible architectures of exo-planetary systems. Protoplanetary systems forming in Orion Nebula type environments rapidly lose the volatile (H, He, CO, etc.) and small grain components of their disks due to photo-ablation. Thus, gas giants are not likely to form in Orion-like environments unless they do so within the few hundred thousand years prior to the onset of external UV irradiation. But enough material may already be locked into large bodies so that rocky planets may eventually form around these stars.

Hazards to planet formation include radiation fields which photo-ablate protoplanetary disks, and three-body stellar encounters in the dense proto-star clusters and nascent binary/triple star systems. Dynamical encounters with close companions truncate or destroy proto-planetary
disks.