( CNN ) Are we alone in the cosmos ? Or might there be intelligent life elsewhere ? Last week , NASA scientists discussed in very concrete terms the steps to discovering life elsewhere in the universe over the next decade or two . This year is the 20th anniversary of the discovery of a planet around a star like our own sun , 51 Pegasi . Since then , ground-based surveys and NASA 's Kepler satellite have discovered nearly 2,000 confirmed `` exoplanets , '' and thousands more candidates await confirmation . Many of these planetary systems are quite unlike our own solar system . Some have large planets like Jupiter that orbit their stars far closer than Mercury , the innermost planet in our solar system . But smaller rocky planets like Earth , though harder to find , appear to be even more abundant . Life on Earth developed in its oceans about a billion years after the planet formed . That suggests that rocky planets with liquid water on their surfaces might also have developed primitive forms of life . Life as we know it is carbon-based and requires liquid water . Astronomers define the `` habitable zone '' around a star as the region within which liquid water can exist on a planet 's surface . Any closer to the star , the water will boil into vapor ; any farther and the water freezes into ice . Extrapolating from discoveries to date , astronomers estimate there are perhaps 40 billion Earth-like , habitable-zone planets in our Milky Way galaxy alone . Of course , there is a difference between single-celled organisms -- which developed 3.8 billion years ago and remained the most sophisticated form of life for another billion years or so -- and mammals , which appeared about 200 million years ago . And then the humans , who have existed for only 200,000 years . Intelligent life that can communicate via radio waves with other intelligent life is less than 100 years old here on Earth . So while planets that develop simple forms of life may be a dime a dozen , the number that have sentient beings with whom to converse -- even assuming they evolved as humans did , with ears and spoken language , or eyes and written language -- is likely to be tiny . And life that can use radio waves has existed on Earth for only 0.000002 % of the planet 's history -- 100 years out of 4.5 billion . If the half dozen or so rocky , Earth-like exoplanets now known are similar , the odds of discovering humanlike life on them are about the same as , well , winning your state lottery with one ticket . Of course , if there are 40 billion Earth-like planets out there , the odds improve quite a bit . If they all have histories like the Earth 's , there might be 1,000 planets in the Milky Way that could support communicative beings . But before you start composing your first letter to an alien , think about this : The chance that those beings evolved on exactly the same time scale is minuscule . Another planet 's 100 years of brilliance might have occurred a billion years ago , or it might happen a billion years in the future . A lot depends on how long communication capabilities last . Civilizations that can build huge telescopes and broadcast stations also have the technology to destroy their planet . So the duration of the Communication Age on a planet could be short . Not to mention : The average light-travel-time to such a planet could be tens of thousands of years , so unless humans evolve to be ageless , we 're not exchanging IMs with aliens anytime soon . If advanced civilizations can maintain their capabilities for millions of years or more , the chances of communicating with them are not negligible . But in that case , they are likely to be far more sophisticated than we are ( since they developed the capability far earlier than we did ) -- so if they wanted us to know they exist , would n't they simply tell us ? The SETI project has been listening for such broadcasts for more than 30 years in the search for extraterrestrial intelligence . Probably the most likely form of life on exoplanets is far more primitive . Astronomers have found signatures of organic molecules , the building blocks of life , in the interstellar material that permeates the space between stars in our galaxy . Possible signatures of living organisms on distant exoplanets include an oxygen-rich atmosphere , such as that created by the first bacteria on Earth , or perhaps methane or carbon dioxide . NASA 's James Webb Space Telescope , to be launched in 2018 , will look for evidence of life in the atmospheres of rocky , habitable exoplanets . NASA 's rovers have shown that Mars was once much more habitable . It had fresh-water lakes and streams of water running along its surface . Some water still remains , and there may yet be life discovered on Mars . New missions plan to look at Europa and Ganymede , moons of Jupiter that have liquid water below their icy surfaces . Life elsewhere in the universe , and even elsewhere in our own Milky Way galaxy , is practically inevitable . Signs of life on exoplanets orbiting nearby stars will probably be discovered in the coming decades with advanced telescopes . But the chance of talking to those little green men will probably have to wait for another few hundred million years .
Mercury scientists discuss steps to discover life elsewhere in the universe over the next two decades . Meg Urry : Life elsewhere in the universe , and even elsewhere in our own Milky Way galaxy , is practically inevitable . But the chances that we can communicate with that life are slim , she writes .