The Milky Way galaxy is so big, that there's no doubt, statistically speaking, that ETs exist.

That's what several prominent scientists say on the month-long series "Are We Alone?," airing on the Science Channel.

But what do these otherworldly denizens look like? Are they smart or dumb? Will we ever come face to face with them?

On the first installment of "Aliens: The Definitive Guide," premiering tonight, famed theoretical physicist Michio Kaku is among those who take a strong stand on the biggest questions about the search for intelligent life in the universe.

"Some scientists say that perhaps we are the only life forms in the universe. Give me a break! I mean, how many stars are there out there in the universe, anyway? The Hubble Space Telescope can see about a hundred billion galaxies -- that's the visible universe," Kaku says on the alien TV special.

"Each galaxy consists of a hundred billion stars. Do the math. A hundred billion times a hundred billion is 10 sextillion. That's one with 22 zeros after it. There definitely are aliens in outer space -- they're out there!"

Recent planetary discoveries by NASA's Kepler Space Telescope confirm the existence of huge numbers of planets in our nearby galactic neighborhood, and the number of Earth-like worlds is on the increase, raising expectations that life has evolved on many of them.

"There's an exotic zoo of planets out there. Many of them aren't anything like the kind of planets we find in our own solar system," according to astrobiologist Lewis Dartnell. "One kind of planet common in the galaxy is a water world -- a planet with a very deep ocean, perhaps hundreds of kilometers deep. The life in these water worlds might look pretty recognizable."

But while scientists still don't know exactly what life may look like on worlds hundreds of trillions of miles away (see illustration above), Dartnell theorizes about possible similarities found between Earth and conditions that may have developed on another planet.

"The propulsion mechanisms that are adapted and evolved by alien fish would be pretty similar to the propulsion mechanisms that you find on Earth simply because they're solving the same survival situation with the means available to them."

Because of the advances in our ability to locate other planets in the Milky Way galaxy, scientists now estimate there could be hundreds of billions of planets that may harbor life. But how intelligent might that life be?

"There are three basic ingredients to become an advanced civilization," says Kaku, one of the leading physicists in the world.

"First, you have to have stereo eyes -- eyes of a hunter -- because predators are smarter than prey. That's why we say 'sly fox' and 'dumb as a bunny.' If you are a predator, that means you have to have camouflage and stealth. You have to be able to outsmart the prey.

"Second, you have to have an opposable thumb, a hand, a claw, a tentacle -- something by which you can manipulate the environment to create machines and, eventually, starships.

"And third, language. You have to be able to accumulate knowledge between generations. The knowledge you get from this generation has to be handed down in order to create vast civilizations capable of taking you to the stars."

"Aliens: The Definitive Guide" premieres tonight on the Science Channel. Check your local listings for channel and time.