No! No one can prove this either way!!!!

There are views but they are conjectures. For example, consider a PRO argument.

There are 300 billion to a trillion galaxies, and on average each galaxy has circa 100-300 million stars. So if we take 500 billion galaxies each with 300 billion stars, then the number of stars in the Cosmos is 10²³! More stars than there are grains of sand on all Earth beaches (circa 10¹⁸).

Now, if only 1% of stars have planets in the habitable zone and only one out of a million of these supports life and only one out of a thousand of those support ‘advanced’ life, then the number of planets with advanced life is ONE TRILLION!

Closer to home: just in the Milky Way Galaxy the numbers are pretty large. With 300 billion stars in Milky Way, and only 1% supporting planets in habitable zone, that means there are 3 billion possibilities where life could have gotten a hold. So the argument for life elsewhere is the nothing special argument: if there is nothing special about the conditions for life to get going, then there are LOTS of possibilities.

The CON argument is:
Intelligent life is very contingent on a bunch of things happening and this concatenation of events is of very low, low, low probability such that any life is rare and intelligent life is very, very, very, rare.

So no one knows. Perhaps in 50 years, or 100 years, or maybe ten years we will have a better idea.

One thing we can say: If we find an independent origin of life on Mars or a Moon of Jupiter or Saturn or in the clouds of VENUS or elsewhere IN OUR SOLAR SYSTEM, then this would mean that life arose INDEPENDENTLY TWICE in ONE SOLAR SYSTEM. This would argue that life is very abundant in the Milky Way and Universe.

Dan, I'll help you out. Simply, no evidence. One day there may be evidence that UFOs from other planets do exist. Currently there is none. Your friends are excited and hopeful for intelligent life! I love that! However, science takes solid evidence to form conclusions. Science is a system to investigate the world around us. Right now solid evidence of UFOs from other planets doesn't exist.

Here's an interesting explanation. The US government recently released a report about UFOs. While many believers in aliens took this report as proof of extraterrestrial life, the government scientifically concluded otherwise. These objects likely were birds, balloons, drones, or debris like plastic bags. There are natural ice crystals, moisture and temperature fluctuations that may register on radar and infrared. Overall Dan, we don't know if intelligent life exists off Earth. It's reassuring to believe in that. The scientific consensus is that no evidence of UFOs from other planets exists.

I like how interested in scientific arguments you are! Look up "Drake Equation" if you're interested in the calculated probability that intelligent life does exist.

Well, we don't know for a fact that alien spacecraft from other planets don't exist. It's just very improbable.

At the moment, we know of no ways that an object (such as a spacecraft) in space could make itself invisible. This is because any object that has a temperature will emit light, and we can see that light and tell what the temperature of the object is. Additionally, if a spacecraft is going to make a maneuver, then it will have to produce exhaust, which again we would be able to detect.

It's always possible that our understanding of physics is wrong, and that the aliens would understand physics better than we do. This means that your friends could always argue that "the reason why we think cloaking devices are impossible is just that we don't know how to make one yet". Your friends are probably wrong - our theories of physics exist for a reason: they are based on what we have observed to be reality - but there's still a chance, however small, that they're right.

The only UFOs are objects in the sky that haven't been identified. Generally, objects in the sky are not UFOs, because they eventually get identified. But some objects don't get identified. That makes sense. It doesn't mean that those are aliens from space!

The U.S. Congress told the Pentagon to release a report this year about the objects that haven't been identified. The report about the 'UFOs' doesn't give any evidence that any of them are from aliens in space. Here's what scientists have been saying for a long time: "...the media has lavished too much attention on sensational claims that vague lights in the sky are actually extraterrestrial spacecraft."

You're not likely to convince anyone who doesn't want to be convinced about this. My daughter and I once watched a crazy UFO program from Australia. My daughter had a boyfriend who was also watching the program, in a different place. In one ear, my daughter was hearing from her boyfriend about all this evidence for UFO's. In the other ear, she was hearing from me about how crazy and weak all the 'evidence' was.

Here are 2 links to this info, from Scientific American and the New York Times:
scientificamerican ufo

nytimes ufo