  All water-covered rocky planets in the inner habitable zones of solar-type
stars will inevitably experience a catastrophic runaway climate due to
increasing stellar luminosity and limits to outgoing infrared radiation from
wet greenhouse atmospheres. Reflectors or scatterers placed near Earth's inner
Lagrange point (L1) have been proposed as a 'geo-engineering" solution to
anthropogenic climate change and an advanced version of this could modulate
incident irradiation over many Gyr or "rescue" a planet from the interior of
the habitable zone. The distance of the starshade from the planet that
minimizes its mass is 1.6 times the Earth-L1 distance. Such a starshade would
have to be similar in size to the planet and the mutual occultations during
planetary transits could produce a characteristic maximum at mid-transit in the
light-curve. Because of a fortuitous ratio of densities, Earth-size planets
around G dwarf stars present the best opportunity to detect such an artifact.
The signal would be persistent and is potentially detectable by a future space
photometry mission to characterize transiting planets. The signal could be
distinguished from natural phenomenon, i.e. starspots or cometary dust clouds,
by its shape, persistence, and transmission spectrum.
