Using the expertise of a number of renowned scientists, an exhibition that originated in London is set to open April 10 at the Montreal Science Center explores ideas on what aliens might look like, taking into consideration biology, astronomy, and the laws of physics and chemistry. The main attraction of the exhibition are alien forms envisioned by the scientists to fit the specific characteristics of two planets, such as carbon content, the temperature, the type of atmosphere.

The planets are about the same size as Earth, says Simon Conway Morris, a Cambridge evolutionary biologist who took a lead role in the exhibition, first presented in London, England, in 2006.

The planet  supports life even though the star it orbits is much cooler than our sun. It does so by orbiting very closely, so close that the star's gravitational pull prevents Aurelia from spinning. One-half of the planet is always dark, the other always day.

The top predator on Aurelia is the bipedal gulphog, which has a long neck and a claw-like beak, and stands 4.5 meters tall. It might feed on six-legged mudpods that scurry on the ground, hide in burrows, and swim like crocodiles. There are also stinger fans, which look like plants but are actually animals that use tentacles to capture a weak star's energy.

"Blue Moon" is a planet more like Earth, but with much more oxygen and carbon dioxide in the atmosphere, allowing both plants and animals to grow bigger than on Earth. The air is extremely dense, allowing all sorts of animals to fly, such as so-called sky whales, which have nine-metre wingspans and fly using echolocation. They hover above thick "pagoda" forests, which stand at an astonishing 1,000 meters.

There are distinct similarities to creatures on Earth. Conway Morris says that's no mistake. "My constraint," he explains, "is that I seem to think that life could only evolve in a number of limited directions" -a theory known as "convergent evolution," whereby similar physical attributes evolve from completely unrelated ancestors.

"From different starting points, you end up with very similar biological solutions," he says. The eye, for instance, is found in all sorts of unrelated creatures. The octopus and the human have each developed an eye that's similar in construction.

"What I find so fantastic, if we did meet an alien, we might first (express disgust). But those differences would turn out to be skin deep. While they might look quite different, in the details of its organization, we'd be impressed by terrestrial similarities."

The scientists believe that extraterrestrials would also be subject to the laws of evolution that have shaped life on this planet.

"If you don't have evolution, you're not going to get from a simple replicating thing to bacteria to complex life," says NASA's Michael Meyer. "I can't think of another way." Other assumptions were made, such as the fact both Aurelia and Blue Moon have oxygen to support life. "There is reasonable scientific speculation that you need oxygen to get complex life forms because the energy-usage rate is so much higher to maintain different kinds of cells working in cooperation," Meyer says.

Meyer notes that we didn't have an aerobic world until 2.8 billion years ago, roughly the time we began getting more complex organisms.

As scientific as these researchers' approach was, it doesn't deny some of Hollywood scific visions. For example, the terrifying queen in Ridley Scott's scifi classic, Alien, with the acid blood. Neither Meyer nor Conway Morris discount the possibility. Here on Earth, they point out, the African bombardier beetle can spray boiling hydrogen peroxide at enemies.

The final zone of the exhibition looks at our attempts to communicate with aliens, largely through the SETI project, listening to radio waves, hoping for contact.

There is no hint at positing the existence of intelligent life, just the existence of "something."
Meyer's bet is based on simple math: "There are 100 billion stars in our galaxy. And there are 100 billion galaxies," he says. "That's a pretty large number – so there's got to be something out there."

Real forms of life that exist on Earth in extreme environments – under extreme pressure at the bottom of the ocean, or in extremely high temperatures, or acidic conditions – offer clues to the real forms that alien life might take. A few years ago, for example, scientists discovered a single-celled extremophile at a volcanic vent two kilometers under the sea, at 121 degrees C. At those depths, they need no sunlight. Researchers suggest they could survive on a planet that is habitually dark.