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than ~20 mas; this would allow us to distinguish objects such as Earth and Venus in solar system analogues at a distance of 15 pc from Earth. Our turbulent atmosphere limits ground-based telescopes to resolutions no better than 50 mas—even with the best available adaptive optics. Furthermore, the Hubble Space Telescope, with its 2.4 m mirror also has a resolving power no better than 50 mas. New advanced space telescopes are needed to image planetary systems similar to our own.

Beyond angular resolution limitations, a more difficult challenge is that planets are extremely faint as compared to the stars around which they orbit. An Earth-like planet would be about 10 billion times fainter than a Sun-like star when viewed at optical wavelengths, albeit somewhat brighter at infrared wavelengths—then only a factor of 10 million fainter. Because of this, scattered starlight within a telescope, caused by what would otherwise be negligible imperfections in mirror surfaces, can completely overwhelm the light from a planet. Telescopes must be significantly oversized compared to the required diffraction limited resolution so that planets could be seen beyond the glare of scattered starlight. Space telescopes with diameters of 8 m or more are needed to look for terrestrial planets around just the nearest dozen or so stars.

Building an 8-m optical space telescope is a formidable technical and engineering challenge. The largest telescopes on Earth are only slightly larger; namely the twin 10-m telescopes of the W. M. Keck Observatory. The largest telescope that can fit easily inside a launch vehicle is much smaller: only about 3.5 m in diameter. Innovative approaches to telescope design and packaging are therefore needed. In addition the telescope must have optics capable of suppressing starlight by a factor of 10 million to 10 billion—which is yet beyond the state of the art. Although this approach is certainly feasible with sufficient investment, it would provide images of only a handful of nearby planetary systems. Other innovative approaches have also been under study.

A potentially simpler approach might be to use a starshade to block starlight even before it enters the telescope, and have it an appropriate size and distance so that planet light could yet be seen. A starshade would need to be several 10’s of meters in diameter and situated at several 10,000 km away from the telescope. This approach may greatly relax the engineering requirements on the telescope itself, but at the same time introduces other logistical challenges. It also would not significantly increase the number of planetary systems that could be imaged.



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