Jayce plays promising future, reveals a Dazzling Aurora. I reveal a brittle steels. Can i target the Aurora being played?

No — by the time you pick targets for Brittle Steel, the Aurora isn't a gear on the board yet.

Since Jayce is the caster, you're the "next player," so your banished card is put on the chain first and finalizes first. Brittle Steel's text is "Kill a gear," which chooses a specific gear on the board (Rule 355.9.a / 355.10.d), and targets are locked in during that card's own finalization (Rule 355.8). At that point the Aurora is still just a pending chain item — it only enters the board when its play is finalized (Rule 337.2, older items finalize first). So your only legal choices are gears already on the board.

Note the contrast with Acceptable Losses in the classic Promising Future ruling: that card says "each player kills one of their gear," so it doesn't target and does catch the Aurora at resolution. Brittle Steel doesn't get that out. If you have no other gear on the board to target, Brittle Steel can't legally be played and stays banished for the rest of the game (FAQ 6208).