To date, however, nobody has come up with credible proof that aliens do exist. But now we have Gliese 581g.
The planet is far away, but it might be able to support life, scientists say. We don't know if intelligent beings like us live there, much less if they have the technology to visit us. But someone does seem to be visiting earth, if we're to believe frequent reports of UFO sightings. Early last month, many people in Hong Kong claimed to have seen UFOs.
The Hong Kong Observatory said it received eight reports of UFO sightings on September 8 in the two days after.
Among the reported cases, says Woo Wang-chun, a scientific officer at the observatory, "three involved a ring of light hovering at a bus stop near Tai Lam Tunnel". The simple explanation, Woo says, is the heavy rain that night caused witnesses to mistake the refracted light of a street lamp for a UFO.
September 8 was the night when a record number of lightning strikes hit Hong Kong during a storm. The observatory reported that in the hour after midnight more than 13,000 bolts of lightning hit the ground.
Woo adds that most reported UFOs turn out to be weather balloons, lights from aircraft, or even kites. When the observatory receives reports of UFO sightings, its scientific officers will look for natural causes as an explanation.
"On many occasions sightings can be explained by simple astronomic phenomena," Woo says. Yet he admits that "the observatory is not able to give scientific explanations for all the cases in our records".
In Hong Kong, UFO sightings have been recorded since 1961. More than 90 sightings were reported in the past two years alone, with 25 cases reported this year until last month.
Asked about the observatory's guidelines for dealing with unexplained cases, Woo notes the term UFO "refers to any flying object that cannot be identified. It does not necessarily have to have a link to aliens".
In July, Hangzhou's Xiaoshan Airport was shut down owing to UFO sightings. Another unidentified flying object was spotted hovering in the sky over Chongqing.
No official explanation has been given in either case. Yet UFO watchers believe the closure of the airport indicates something important happened.
On the mainland and in Hong Kong, UFOs do not seem to be on top of the government agenda. Yet popular interest worldwide in the subject remains as strong as ever.
On September 26, The Sunday Times in England and other newspapers reported that the UN was about to appoint a Malaysian astrophysicist, Dr Mazlan Othman, to the new post of Ambassador to Theoretical Extraterrestrials.
This means Othman, who is head of the UN's Office for Outer Space Affairs, would become the world's first officially appointed representative to greet aliens, should they come to visit.
A day later Othman denied such plans, dismissing the reports as a hoax. Yet some people think such an official position has long been overdue.
One prominent advocate of the need for increased attention to UFOs is Neil Gould, an author of books on UFOs and director of a Hong Kong-based alien research institute.
Gould is critical of the Hong Kong government's approach to downplay UFO sightings and ignore alien-related issues.
"The government is ignorant of and uninterested in these issues which require attention," he says. "Their thinking is limited. I hope to have a chance to talk to the government and share my knowledge of extraterrestrials with them."