What type of government does the U.S. have?

A federalist constitutional Republic with 3 branches: Legislative, Executive, and Judicial.

The Legislative Branch is run by the Senate (4 year election cycles) and the House of Representatives (2 year election cycles). This is where most of the power lies.

The Senate seats are determined by the states. Each state gets 2 senators regardless of population. The entire population of a state in some states, or half the population in others, elects one or both senators. 

The House of Representatives is divided by population. States with more people get more representatives.

The Executive Branch is run by the President. The position mostly has veto, military, and diplomatic power but in the last century presidents have tried leading or pushing legislatures. This has worked up until about 10 years ago when the Legislature largely stopped cooperating with former President Bush, and now President Obama. The President also has the power to create government agencies, with the consent of the legislature and constitutional backing by the Judicial Branch.

The Judicial branch controls nearly all the courts except for immigration courts. They are not elected but appointed. They have significant power because they interpret the US constitution so whatever their interpretation says is the supreme law of the land, and the Supreme Court can change the interpretation whenever they want by a vote. Their decisions cannot be overturned by a democratic vote, by the president, nor by any other one single body except for a future Supreme Court. In order to overturn a decision 2/3rds of the state legislatures must vote to amend the constitution itself to invalidate their decision. This has happened very few times. 

Then there are many other government agencies such as the IRS, Attorney General, NSA CDC, and the FBI, Department of Agriculture, etc which are controlled by the executive branch with the consent of the legislature through House oversight. For example, the House Intelligence Committee is composed of congresspeople who all have Top Secret security clearances. They get reports from the NSA, CIA, Military, etc and read them, and appropriate money to those agencies based partly on their performance.

Then we have states.... States are microcosms of the Federal government but have autonomy and authority within their boundaries. Most states have state versions of things like the Center for Disease Control, and Department of Agriculture, which oversee the things within their own borders.
The us has a constitutional democratic federal republic. It has a constitution which separates and limits the government. Authority is given to the federal government which is composed of representatives that make public decisions. Those head officials are appointed via the democratic process.
What will you do if aliens come to Earth?

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If all the photosynthetic organisms on earth die now, how long can human beings survive?

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If you believe in ghosts and aliens, do you then believe in ghost aliens?

It would be cool.And I'd say it's possible. I do not have enough reliable resources to believe in these particilar