The hunt for alien life has been given a boost after scientists discovered a habitable planet almost the same size as Earth.
Astronomer Thomas Barclay from Nasa's Ames Research Centre in California made the discovery using data collected by the Kepler space telescope.
The unnamed planet was found orbiting an unidentified star in its so-called Goldilocks zone - a region around the star that emits just enough energy, light and temperature for liquid surface water to appear.
At 72, he said, he feels a moral obligation to step up his activism in his remaining years.
"If we burn even a substantial fraction of the fossil fuels, we guarantee there's going to be unstoppable changes" in the climate of the earth, he said. "We're going to leave a situation for young people and future generations that they may have no way to deal with."