"Types cast" (2005)

This vicious creature, with more teeth than a bandsaw, would never survive in the galactic gene pool. Its eggs sit around on worthless planets, hoping a passing spaceship will land and investigate. The eggs explode in any intruder's face, implanting an alien embryo. After a hasty gestation, the creature arranges its own caesarean delivery, bursting from the host's chest. This is similar to the repulsive reproductive cycle of the ichneumon wasp. Imagine if making human babies required a biologically compatible visitor from another world. It's far more efficient to find a partner for reproduction at the pub. Alien's scheme is going to have a very low yield.

Daleks

Americans like anthropomorphic aliens; the British prefer Daleks, who resemble oversized salt shakers with smallpox. These aggressive creatures, the mutated remnants of a nuclear war on their home planet, roll around in tank-like epidermises. But if they're advanced enough to manage interstellar contact, why can't they do some genetic engineering and unburden themselves of those debilitating mutations? Their metal mantles seem unsuited to locomotion on anything rougher than a studio floor. Imagine Daleks at the beach. Fitting an extra motor and a dust bag would at least make it useful for vacuuming the living room.

ET

ET and his buddies journey here to augment their collection of plants, a field trip that sounds plausible until you note they come from the Andromeda galaxy - a tedious 2m light years away. Are the hundred billion planets of their own galaxy cursed with uninteresting flora? Unintentionally abandoned, ET chills out with some latchkey kids from suburbia, appropriate given that he is a kid himself: big eyes, short nose, and a small, cuddly body. Improbably, his biochemistry is so similar to ours, he gets drunk on supermarket beer. This isn't an alien - it's every child's dream of the perfect pet.

Roswell aliens

Some folks think that in 1947 alien rocket jockeys made a last-minute navigation error and crashed in the desert. The putative victims are generally portrayed as humanoid, if somewhat shorter than us, and with bigger heads - like children. While not impossible, this is unlikely. The fact that we have two eyes is good engineering, but having four appendages is an evolutionary accident. Most Earthlings have six (they're called insects). The Roswell aliens resemble us because we relate better to anthropomorphic creatures. Real aliens won't be so similar. They probably won't crash, either.

The Man from Mars by Frank R Paul (1939)

This Red Planet resident and comic book cover art, a cross between a 7ft-high bat and Pinocchio, was a valiant attempt to design a realistic alien. Mars gravity is only 38% of Earth's, ergo this Martian's imposing stature. Barrel-size lungs suck in the Red Planet's thin air. Sadly, these adaptations are inadequate. The landscape of Mars is dry as dust, cold as Antarctica, desperately short of oxygen, and stung by lethal ultraviolet light. If we ever meet real Martians, we'll need a microscope to see them, and a drilling rig to find them. The Red Planet's surface is surely as sterile as a mule.

Tribbles

These pesky aliens from Star Trek do only one thing: make more Tribbles. How this happens is discreetly obscure, as are all anatomical features of these furry lumps. But modesty aside, reproduction - whatever the dirty, nasty mechanics - eventually relies on food to bulk up the offspring. What is it that Tribbles eat? Air? Without legs, and too large to float, the Tribbles seemed destined to stay in one place and mound up. Given their fleecy physique, it's possible that such creatures are bred by enterprising extraterrestrials for the garment trade.

· Seth Shostak is senior astronomer at the Seti Institute, California