As of February 14th, 2020 Coinbase Pro began trading ATOM. Cosmos launched on Mainnet March 14th 2019.
On Monday January 13th customers of Coinbase were able to transfer pre-owned ATOM into their Coinbase Pro account. Then on Tuesday users were able to buy/sell/trade Cosmos (ATOM). This was done in order to have a sufficient supply of ATOM on the platform first.
As of this writing ATOM is at $5.47. It is on a small decline at the moment after having a huge spike. This morning it was valued at $4.33. That is a 81% spike in price which is amazing.
Cosmos does use Proof-of-Stake to validate transactions. However Coinbase has said that there is not plans for staking ATOM right now.