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6 Minutes
Why finding Earth’s twins is so difficult
Detecting an Earth-sized planet orbiting a Sun-like star is fundamentally a problem of contrast and resolution. A star outshines its planet by factors of millions to billions depending on wavelength. When the star and planet are not spatially resolved, the faint planetary signal is lost in the star’s glare. Optical physics sets the limits: angular resolution scales with the observing wavelength divided by the telescope aperture. For planets that might host liquid water, thermal emission peaks near 10 microns in the mid-infrared. At that wavelength, achieving the angular separation necessary to distinguish an Earth analogue from its star at a distance of ~30 light-years requires a collecting dimension on the order of 20 meters.
Space-based observation is required because Earth’s atmosphere blurs mid-infrared images and emits its own thermal background. The James Webb Space Telescope (JWST), our largest operational space infrared observatory, has a 6.5-meter primary mirror—well short of the 20-meter scale needed for routine direct imaging of Earth-like planets at these distances. Launching a monolithic 20-meter class telescope presents prohibitive challenges with present-day rockets and deployment systems.
Existing alternatives and their limits
Astronomers have proposed several strategies to get around the size problem. Interferometry stitches signals from multiple smaller telescopes to emulate a much larger aperture, but that requires formation flying with nanometer-scale precision over large baselines—techniques that are still experimentally demanding. Observing at shorter (visible) wavelengths improves angular resolution for a given aperture, yet contrast becomes worse: in visible light a Sun-like star can be more than ten billion times brighter than an Earth twin, pushing coronagraphs and starlight suppression methods beyond current capabilities.
The starshade concept—an external occulter flown tens of thousands of kilometers ahead of a space telescope to block starlight—can deliver excellent contrast, but it demands two costly spacecraft and significant fuel for retargeting. Moving a starshade between target stars consumes mission-limiting propellant, complicating surveys of many nearby systems.
A pragmatic alternative: a long, narrow mirror
A recently proposed design rethinks mirror geometry rather than simply scaling up a circular aperture. Instead of a large round mirror, imagine a 1-by-20-meter rectangular primary operating in the mid-infrared (~10 microns). Along its long axis the rectangle provides the angular resolution equivalent to a 20-meter telescope, allowing the instrument to separate a star from a nearby planet in that direction. By rotating the telescope (or its mirror) through different angles, the system can sample all position angles around a target star and thus search for planets located anywhere in the stellar system.
This rectangular configuration—illustrated in concept studies like the Diffractive Interfero Coronagraph Exoplanet Resolver (DICER) model—promises a practical route to surveying the roughly 60 Sun-like stars within 30 light-years. Modeling suggests such a telescope, with sensitivity similar to JWST but with the elongated aperture, could detect roughly half of the Earth-size planets in habitable zones around those nearby Sun-like stars in a survey lasting under three years. Importantly, the proposal does not demand fundamentally new physics or unattainable engineering breakthroughs; it trades a challenging increase in diameter for a change in shape and operational approach.
Mission and observational approach
Operating at 10 microns, the rectangular mirror would combine high angular resolution in one dimension with coronagraphic or diffractive starlight suppression techniques to reveal faint planetary thermal emission. A survey strategy would rotate the long axis while integrating at each orientation, building up two-dimensional detections of candidate planets. Confirmed detections could be followed up with spectroscopy to search for atmospheric biosignatures such as oxygen, ozone, methane, or water vapor.
Expert Insight
"A 1-by-20-meter architecture is an elegant compromise," says Dr. Maya R. Singh, an astrophysicist specializing in exoplanet instrumentation. "It leverages familiar infrared detector technology and deployment experience from missions like JWST while delivering the resolution we need at 10 microns. Engineering challenges remain—thermal control, mirror stability, and precise rotation mechanics—but none require breakthroughs beyond current engineering practice. This design could realistically accelerate the search for Earth analogues within our stellar neighborhood."
Implications and next steps
If the occurrence rate of Earth-like planets around Sun-like stars is near unity, a rectangular mid-infrared telescope could identify on the order of dozens of promising worlds within 30 light-years. Those targets would be prioritized for atmospheric characterization to look for potential signs of life. For the most compelling candidates, far-future robotic probes or advanced imaging missions could enable direct surface imaging. The rectangular-mirror concept offers a cost-and-complexity-efficient path toward these scientific goals and complements other approaches such as interferometry and starshades.
Conclusion
Reimagining telescope geometry—moving from circular to elongated rectangular mirrors—provides a feasible method to attain the angular resolution required for direct imaging of nearby Earth-like planets in the mid-infrared. Operating at ~10 microns and leveraging rotation to survey full orbital angles, a 1-by-20-meter-class instrument could survey dozens of nearby Sun-like systems within a few years, delivering a prioritized set of targets for biosignature searches. While further engineering, optimization and mission studies are essential, the rectangular telescope concept represents a promising, practical step closer to finding an "Earth 2.0."
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