* FAA grounded 171 737 MAX 9 jets after Alaska Airlines incident

* Panel found in backyard of Portland school teacher

* Discovery will help with regulators' probe into cause of incident

* Boeing shares sink in pre-market U.S. trade

SEOUL/LONDON/WASHINGTON, Jan 8 (Reuters) - U.S. officials have recovered a panel that blew off an Alaska Airlines airliner triggering a partial grounding of Boeing's 737 MAX 9 and sending shares in the planemaker tumbling on Monday.

A door plug tore off on Friday following takeoff from Portland, Oregon, en route to Ontario, California, depressurizing the plane and forcing pilots to turn back.

The plane, with 171 passengers and six crew on board, landed safely.

The U.S. Federal Aviation Administration (FAA) on Saturday ordered the temporary grounding of 171 Boeing MAX 9 jets installed with the same panel, which weighs about 60 pounds (27 kg) and covers an optional exit door.

It was recovered on Sunday by a Portland school teacher identified only as "Bob" in the Cedar Hills neighborhood who found it in his backyard, U.S. National Transportation Safety Board (NTSB) Chair Jennifer Homendy said.

She said she was "very relieved" it had been found having called it a "key missing component" to determine why the accident occurred.

"Our structures team will want to look at everything on the door - all of the components on the door to see, to look at, witness marks, to look at any paint transfer, what shape the door was in when found. That can tell them a lot about what occurred," she said.

The force from the loss of the panel was strong enough to blow open the cockpit door during flight, said Homendy, adding that it must have been a "terrifying event" to experience.

"They heard a bang," Homendy said of the pilots, who were interviewed by investigators.

Homendy said the cockpit voice recorder did not capture any data because it had been overwritten. She again called on regulators to mandate retrofitting existing planes with recorders that capture 25 hours of data, up from the two hours required in the U.S. at present.

SHARES SINK

Boeing's shares fell as much as 8% in pre-market U.S. trade on Monday as investors digested the latest setback for the planemaker.

If the losses hold, the company would lose more than $12.5 billion in value, almost the cost of developing a new plane.

The mishap comes as Boeing and supplier Spirit AeroSystems , which made the panel, grapple with ongoing production setbacks that have hampered recovery from an earlier lengthy 737 MAX safety grounding and wider disruption from the pandemic.

Since the 737 MAX was grounded in March 2019, Boeing shares have fallen by more than 40% while Airbus shares are up 25%.

Spirit Aero shares were down 15.9% in U.S. pre-market trading on Monday. Alaska Airlines shares fell 5% while shares in United Airlines, the only other U.S. airline that operates the jets, dropped 2.8%.

PLANES GROUNDED

The FAA said on Sunday the affected fleet of Boeing MAX 9 planes, including those operated by other carriers such as United, would remain grounded until the regulator was satisfied they were safe.

Of the 171 planes covered by the order, 144 are operating in the United States, according to data from aviation analytics firm Cirium. Turkish Airlines, Panama's Copa Airlines and Aeromexico said they were grounding affected jets.

On Monday, Indonesia's transport ministry said it had temporarily grounded three 737 MAX 9 planes, operated by Lion Air, even though they had different configurations from the Alaska Airlines plane.

The FAA initially said on Saturday the required inspections would take four to eight hours, leading many in the industry to assume the planes could very quickly return to service.

But criteria for the checks have yet to be agreed between the FAA and Boeing, meaning airlines have not yet received detailed instructions, people familiar with the matter said.

The FAA must approve Boeing's inspection criteria before the checks can be completed and the planes can resume flying.

Alaska Airlines said late on Sunday it had still not received instructions from Boeing.

The airline canceled 170 flights on Sunday and a further 60 on Monday and said travel disruptions were expected to last through at least mid-week. United, which has grounded its 79 MAX 9s, canceled 230 flights on Sunday, or 8% of its scheduled departures.

(Additional reporting by Tim Hepher in Paris, Valerie Insinna in Washington and Stanley Widianto in Jakarta; writing by Josephine Mason; editing by Jason Neely)