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Recent direct imaging discoveries suggest a new class of massive, distantplanets around A stars. These widely separated giants have been interpreted assigns of planet formation driven by gravitational instability, but theviability of this mechanism is not clear cut. In this paper, we first discussthe local requirements for fragmentation and the initial fragment mass scales.We then consider whether the fragment's subsequent growth can be terminatedwithin the planetary mass regime. Finally, we place disks in the larger contextof star formation and disk evolution models. We find that in order forgravitational instability to produce planets, disks must be atypically cold inorder to reduce the initial fragment mass. In addition, fragmentation mustoccur during a narrow window of disk evolution, after infall has mostly ceased,but while the disk is still sufficiently massive to undergo gravitationalinstability. Under more typical conditions, disk-born objects will likely growwell above the deuterium burning planetary mass limit. We conclude that ifplanets are formed by gravitational instability, they must be the low mass tailof the distribution of disk-born companions. To validate this theory, on-goingdirect imaging surveys must find a greater abundance of brown dwarf and M-starcompanions to A-stars. Their absence would suggest planet formation by adifferent mechanism such as core accretion, which is consistent with the debrisdisks detected in these systems.

The Runts of the Litter: Why planets formed through gravitational instability can only be failed binary stars