At a panel discussion on the search for alien life, held this week at NASA's headquarters in Washington, the agency's top scientists said they're getting close.

NASA scientists were joined by leading figures in the fields of astronomy, physics and planetary sciences.

"We believe we're very, very close in terms of technology and science to actually finding the other Earth and our chance to find signs of life on another world," Sara Seager, a physicist at MIT and recipient of the MacArthur Fellowship, told a packed audience on Monday.

NASA Administrator Charles Bolden said finding alien life was all but inevitable.

"Do we believe there is life beyond Earth?" he asked. "I would venture to say that most of my colleagues here today say it is improbable that in the limitless vastness of the universe we humans stand alone."

Since 2009, NASA's search for an Earth-like exoplanet capable of hosting living organisms has been bolstered by the Kepler Space Telescope. Kepler has discovered thousands of exoplanets, but not definitive signs of alien life. That effort will be strengthened by NASA's super powerful next-generation James Webb Space Telescope -- scheduled to launch in 2018.

"Finding Earth's twin, that's kind of the holy grail," said John Grunsfeld, a former astronaut who did repair work on Hubble Space Telescope in 2009 and now serves as associate administrator of the agency's Science Mission Directorate.