The James Webb Observatory is being hailed as the successor to the Hubble Space Telescope. Although the space-based observatory has not yet launched, astronomers are already starting to talk about what could replace that eyepiece to the heavens.

This theoretical telescope could have a diameter of 12 meters (nearly 40 feet) across and could be used to search alien stars for small exoplanets similar to the Earth.

The Association of Universities for Research in Astronomy (AURA) has ignited a discussion some astronomers are referring to as a "call to arms" to develop the next large space-based telescope.

The James Webb telescope is currently scheduled for launch sometime in the year 2018. This will replace the Hubble Space Telescope (HST), which has been in orbit since 1990. Webb will be followed by the Wide-field Infrared Survey Telescope, which should lift off sometime around the year 2025. The significant amount of time it takes to design, build and launch a space observatory leads astronomers to start considering future designs well ahead of their planned launch dates.

"If we think about what we want in the sky after the James Webb Space Telescope, we need to start thinking about it now. These are decades-long projects. No mission happens accidentally. AURA thought that it was time to start looking ahead to find a path forward that is scientifically transformative but also technologically possible," Julianne Dalcanton, co-chair of the AURA committee that produced the proposal, said.

The High-Definition Space Telescope (HDST) would search planetary systems far from our own sun, looking for planets roughly the size of our own world. The observatory would carry out this mission as it orbited 1 million miles from Earth, far beyond the orbit of the moon.

The light reflected from tiny worlds is drowned out by light from their parent stars. To get around this problem, the HDST will be capable of blocking light from the star, allowing the planet to be imaged. By breaking the light from the planets into its constituent colors, like using a prism, astronomers will be able to analyze the composition of the alien atmosphere, potentially revealing the presence of gases likely to be produced by life. The atmosphere of Earth contains far more oxygen and methane than would be expected on a lifeless world. Such a mixture, seen surrounding an exoplanet, could be a calling card of alien life.

"When we imagine the landscape of astronomy in the decade of 2030, we realize it is at last within our grasp to make a monumental discovery that will change mankind forever. We hope to learn whether or not we are alone in the universe," Matt Mountain, president of AURA, said.

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