In 2015, Hawking co-launched an initiative called Breakthrough Listen, which will search for alien communication signals out in the cosmos, and eventually broadcast signals from the human race, with the goal of enabling communication across the universe.

"We don't know much about aliens, but we know about humans," Hawking said at the Breakthrough announcement. "If you look at history, contact between humans and less intelligent organisms have often been disastrous from their point of view, and encounters between civilizations with advanced versus primitive technologies have gone badly for the less advanced. A civilization reading one of our messages could be billions of years ahead of us. If so, they will be vastly more powerful, and may not see us as any more valuable than we see bacteria."

Are the aliens benign?

Alien invasions of Earth are nothing new in sci-fi. One of the most famous examples is "War of the Worlds," an H.G. Wells 19th-century classic that was adapted into a Tom Cruise movie in 2005. In that film, aliens inexplicably begin bursting from underneath city pavement to incinerate humans.  

But some films portray aliens as benign, such as 1997's "Contact," based on a 1980s book written by Carl Sagan. A signal begins spouting prime numbers, which Jodie Foster's SETI (search for extraterrestrial intelligence) team tries to decode. Their efforts at talking with the aliens turn a little strange, but there definitely is no invasion. 

This hopeful view is something that science communicator Ann Druyan, Sagan's widow, shared at the Breakthrough event in 2015. (It's also the belief held by astronomer Jill Tarter, former director of the Center for SETI Research, whom Foster's character in "Contact" was based on.) Druyan is also involved in the Breakthrough initiative.

"We may get to a period in our future where we outgrow our evolutionary baggage and evolve to become less violent and shortsighted," Druyan said. "My hope is that extraterrestrial civilizations are not only more technologically proficient than we are but more aware of the rarity and preciousness of life in the cosmos."

Active SETI debate

Three spacecraft currently heading out of the solar system — the Pioneer probe and the two Voyager probes — contain maps that point the way back to Earth. And there continue to be initiatives to put messages for aliens on spacecraft. When the NASA New Horizons probe finishes its Pluto and possible Kuiper Belt work, some people are hoping to create a crowdsourced message for aliens to upload to its hard drive.

Some researchers, however, aren't sure it's such a good idea to shift SETI efforts from just listening to actually sending out signals — a method some call "active SETI."

"Active SETI advocates broke with the conventional wisdom of the SETI pioneers, which was to listen but not transmit. This change may have been driven by the impatience of younger SETI people after 40 years of unsuccessful searches," Michael Michaud, author of the book "Contact with Alien Civilizations: Our Hopes and Fears About Encountering Extraterrestrials" (2007, Copernicus), said in a 2014 interview with Space.com. 

"But active SETI is not science," added Michaud, a former U.S. Foreign Service Officer for the U.S. Department of State. "It is an attempt to provoke a response from an alien society whose capabilities and intentions are not known to us." 

But Earth's sheer distance from other civilizations might serve as a shield against aliens that don't exactly want us around. 

Douglas Vakoch, director of interstellar-message composition at the SETI Institute, said in 2010, "Even if they tend to be hateful, awful folks, can they do us any harm at interstellar distances?"