Is a US Senate impeachment trial limited to the articles submitted by the House?

Question

Imagine the following scenario: During President Trump's impeachment trial*, video evidence surfaces showing the president committing a clear act of treason, as defined by US law (perhaps discussing a plan of action with Russian president Vladimir Putin to abolish Congress)-- an explicitly impeachable offense, but one that is in no way related to the articles currently being brought against him. Could the Senate vote to remove him from office based solely on this new evidence, or would the House have to draft a new article of impeachment?

_{*: I'm not suggesting this is in any way likely to happen, just using the current situation because it's convenient.}

Answer 1

In theory, the answer is yes. The Senate has to hold a trial on the presented Articles of Impeachment brought by the House. Like judicial trials, the impeachment trial is mostly about the case being argued. Furthermore, nobody really wants to be in there

A Democrat in the back row leaned on his right arm, covered his eyes and stayed that way for nearly a half-hour. Some openly snickered when lead prosecutor Adam Schiff said he'd only speak for 10 minutes. And when one of the freshman House prosecutors stood to speak, many of the senator-jurors bolted for the cloak rooms, where their phones are stored.

In reality, the Senate can ask anything it wants. The Senate sets its own rules and they could, if they wanted to, investigate any charge they see fit, presented or not. Remember, the purpose of the trial is to determine if the person being impeached should be removed from office. If the Senate wants to remove for reasons not presented by the House, I'm not sure anyone could stop them. There is no appellate court to overturn the process here.

If the House wants to add new Articles of Impeachment, it's unclear if they could be presented in an existing trial. That would also depend on the Senate voting to do so.