The answer depends on the individual skeptic.
Some skeptics are so devoted to their anti-alien cause, as to be totally unconvincing.
A very insulting name for a believer who is adamant in his belief that aliens are either here, or have visited us in the past. Such as the famous drawing of an alien astronaut in his rocket ship. Or a humanoid wearing what appears to be a space suit, complete with bubble head-dress. Some books have been debunked as being inaccurate to say the least. Other evidence is in the form of UFO reports that have not been convincingly explained.