Mohegan — Long before they christened their second hotel tower a couple of months ago, Mohegan Sun officials were engaged in planning another relatively modest expansion, this one an $80 million conference center.

Modest, at least, by Mohegan Sun standards.

“It’s not definite that it’s a go, but we’re in the advanced planning stages,” Bobby Soper, president and chief executive officer of the Mohegan Tribal Gaming Authority, said Thursday. He said the planning began a year ago.

In November, Mohegan Sun opened its $130 million Earth Tower, a 13-story, 400-room hotel that addressed the casino’s longstanding need for more lodging. Mohegan Sun’s original Sky Tower has 1,200 rooms.

Mohegan Sun officials first revealed details of the proposed conference center in late November during a gaming authority presentation to investors.

The presentation highlights a proposed “Mohegan Sun Expo Center” that Mohegan Sun officials believe would enable the casino to capture most of the 85,000 “room nights” a year it now turns away because of a lack of meeting space.

Last June’s “highly successful” Barrett-Jackson collector car auction, held at Mohegan Sun Arena, “validates the proposition,” the presentation says.

Soper confirmed that future Barrett-Jackson events — Mohegan Sun is committed to hosting one in each of the next four years — would take place in the proposed conference center, if it materializes. This year’s event is scheduled for June 21-24.

“That’s an example of the type of events we want there,” he said. “We can’t take the arena out of commission for every event like that.”

Workers had to scramble to prepare Mohegan Sun Arena for Barrett-Jackson’s inaugural Northeast auction, which started just days after a Connecticut Sun basketball game had been played there.

The auction drew more than 90,000 people over three days, setting single- and multi-day arena attendance records.

“We actually had a very successful Barrett-Jackson event,” Soper said. “In our first year, we got the kind of numbers they get at Mandalay Bay in Las Vegas, which is in its eighth year. ... We anticipate growing. We need more space — for cars and for people.”

Soper said that if the gaming authority decides to move forward with the conference center, it will do so this year. That would mean an opening in 2018 at the earliest, he said.

Mohegan Sun’s existing convention center, which includes the Uncas Ballroom, simply isn’t big enough to accommodate all of the convention business the casino attracts, Soper said.

“It’s the same as with hotel rooms,” he said. “Over the years, there’s been business we’ve had to turn away because we didn’t have the capacity to accommodate it. This (the conference center) would make us one of the premier convention destinations in all of the Northeast. We could host trade shows that are currently being held in Boston and New York.”

The proposed conference center would be located in a vacant area next to the casino’s Winter Garage and would link the garage, the Earth Casino and the Earth Tower.

“Everything will be under one roof,” Soper said. “Whether you’re staying at Earth, or Sky or visiting the conference center, it’s going to be all under one roof.”

b.hallenbeck@theday.com