An artist's impression of Gliese 581d, an exoplanet discovered by the Kepler spacecraft that is thought to be in its star's habitable zone, and a speculative moon.

At a June 27 press conference, Russian astronomer Andrei Finkelstein said that extraterrestrials definitely exist, and that we're likely to find them within two decades.

"The genesis of life is as inevitable as the formation of atoms," said Finkelstein, the director of the Russian Academy of Sciences' Applied Astronomy Institute in St. Petersburg. He was speaking at the opening of an international symposium on the search for extraterrestrial civilizations that was being held at the institute.

"There are fundamental laws which apply to the entire universe," Finkelstein was quoted as saying by the Interfax news agency. Because those fundamental laws allowed intelligent life to develop on Earth, they ought to engender intelligent life elsewhere, too, he reasoned.

Finkelstein pointed out that in recent years, astronomers have found more than 1,000 exoplanets —planets orbiting stars other than our own — some of which lie within their stars' "habitable zones," or the regions in which the temperature is right for water to exist as a liquid. Finkelstein said there will be life on such planets if there is water.

Furthermore, he conjectures that the Search for Extraterrestrial Intelligence (SETI), a worldwide effort to detect radio and optical signals sent our way by extraterrestrials, will find examples of that life within two decades. (Some media sources have interpreted Finkelstein's words to mean that we will communicate back and forth with aliens within that time. Actually, he only said that we will detect their signals.) [10 Alien Encounters Debunked]

For Finkelstein's striking prediction to come true, certain conditions must be met: There must be a planet with an alien civilization that is capable of transmitting high-power radio or optical signals our way. That civilization needs to exist within 20 light-years of Earth and have been broadcasting those signals starting today, or earlier, in order to reach Earth within 20 light-years of today. (Or, if the civilization is farther away, then it needs to have broadcast long enough ago that it could reach here within 20 years.)

Though detecting life elsewhere in the cosmos sounds difficult, as it turns out, several astronomers believe that Finkelstein's 20-year prediction is realistic. In fact, they have an equation that takes into account all the conditions that must be met in order to find life on other planets, and according to the equation, 20 years is a pretty good estimate for when we'll find it.

Seth Shostak, the senior astronomer at the SETI Institute in Mountain View, Calif., the most well-known SETI program, estimated that intelligent life would be found in 25 years in a paper he wrote five years ago. "Maybe Finkelstein read my paper," Shostak told Life's Little Mysteries, a sister site to SPACE. He agrees that we'll detect alien signals within two decades. [Earth's 10 Wildest Attempts to Contact Aliens]

One in a million

Shostak explained that the SETI Institute has aimed its radio telescopes at a few thousand star systems over the past 50 years. (They didn't detect any "deliberate signals" sent by aliens.) Assuming that technology will continue to improve, he thinks we will be able to check out 1 million stars over the next two decades, and that one in that million will have a habitable planet that has intelligent life capable of transmitting signals that are strong enough for us to detect.

While other planets in that million may have had life that was broadcasting radio or optical waves sometime in the past (but which has since been wiped out by an asteroid or some other cataclysm), or will broadcast signals in the future, approximately one of them will be doing so at just the right moment for us to hear or see them.

Shostak's number, 1 in a million, follows from what is known as the Drake Equation. It is a formula created by Frank Drake (also of the SETI Institute) that takes various factors into account to determine the number of intelligent and signal-transmitting civilizations in our galaxy. Drake and Shostak both calculate that there are 10,000 such civilizations transmitting signals at any given moment. Because there are 100 billion stars in the galaxy, the math says 1 in 10 million stars will be sending radio signals our way. "Because you can throw out a lot of stars," Shostak said, making smart choices about which ones are likely to have life, we should be able to find someone or something by searching just 1 million stars.

"If we haven't succeeded once we've done 10 million or 100 million stars by around 2050, then we've grossly overestimated the strength of their transmitters, or some other factor," Shostak said. "One reason we could fail is that there's nobody out there, but I would consider that a last resort."

Candidate planets

SETI can narrow down its search by directing its attention to stars that astronomers discover to have planets in their habitable zones. So far, 1,235 exoplanets have been found by NASA's Kepler spacecraft, a probe that surveys regions of space, collecting data from stars and their planets which scientists then analyze. [How Do Astronomers Find Alien Planets?]

According to Bill Borucki, a planetary astronomer at NASA Ames Research Center and the principal investigator of the Kepler mission, about 50 of the exoplanets that have been found so far are in their stars' habitable zones, and five of those could be rocky rather than gaseous. ("To have life, you probably have to have a solid surface to walk on," Borucki said.)

By induction — what's true of a subset of stars is likely to be true of the rest — "there must be on the order of a billion planets in our galaxy in the habitable zones of their stars," Borucki said. When he and his team identify habitable planets, they tell SETI to point its radio telescopes their way.

Borucki isn't as bold when it comes to specifying when life will be found, but he is optimistic: "I think there's a good chance that there's life in our galaxy. With so many habitable planets, it's hard to imagine that there wouldn't be. And I think that at some point we'll probably discover it. I hope that SETI does that soon."

Even more likely than finding intelligent life far off in the galaxy, Borucki thinks we'll find much simpler, bacterial life much closer to home. "NASA has a number of missions to Mars, and there may be primitive life there. They're talking about missions to Enceladus [a moon of Saturn] and Europa [a moon of Jupiter], both of which probably have subsurface oceans," he said. "I think those are wonderful places to look. I think we might find life in our solar system first."

The Europa Jupiter System Mission (EJSM), a mission to Jupiter and its icy moon, is proposed for a launch in 2020.

Little green men?

Finklestein made one other suggestion — that intelligent alien life forms would be humanoid. He reasoned that, because the laws of nature led life on Earth to evolve in the way it did, alien life forms would develop similarly. Like humans, they probably have two arms, two legs and a head, he said, adding that "they may have different color skin, but even we have that."

Are aliens really little green — or blue or red — men, as Hollywood and Finkelstein suggest? Shostak doesn't think so. "All you have to do is go down to the zoo and look around. There aren't too many critters there that look a lot like us," he said. "The fact that we have two arms and two legs is a consequence of our evolutionary past: We happened to evolve from a four-lobed fish. Among critters on Earth, the most popular number of appendages is six, not four; they're called bugs."

Intelligent aliens probably do have heads and appendages, though. "Having a head seems to be a good thing. Lots of organisms have heads and it seems to be a very efficient model. Having appendages is also important," Shostak said. "If they were dolphins then they wouldn't build radio transmitters."

This article was provided by Life's Little Mysteries, a sister site to SPACE.com. Follow Life's Little Mysteries on Twitter @llmysteries, then join us on Facebook. Follow Natalie Wolchover on Twitter @nattyover.