Finding a second Earth is the grand slam of modern astronomy. The catch: half of the Sun-like stars in our solar neighborhood live in pairs. And current methods can barely search those systems. A NASA mission concept named SHERA proposes to fix that with a 22 cm telescope, a gravitational measuring rod and a measurement precision that borders on science fiction.

Half of the nearby stars, an unexplored desert

Start with a surprising figure: nearly 50% of nearby Sun-like stars are not alone. They orbit in pairs, sometimes more. The closest system to us, Alpha Centauri at 4.3 light-years, is a triple: two Sun-like stars, A and B, plus a distant red dwarf. In other words, the most accessible "Earth 2.0" candidates in the Galaxy may hide inside systems nobody surveys.

The reason for this unexplored desert is technical. Transit, the method that made Kepler famous, requires perfect alignment: an Earth-sized planet blocks only about 100 parts per million of a Sun-like star's light. As for radial velocity, a rocky planet in the habitable zone tugs its star by a mere 9 cm/s, while the best spectrographs in the world stall around 50 cm/s, hampered by the stars' own surface activity.

Relative astrometry: the binary as a measuring rod

SHERA's solution is elegant. Instead of measuring a star's position against a background reference field, the way Gaia does at about 100 microarcseconds, SHERA measures the distance between the two stars of a single binary. Two gravitationally bound stars are a measuring rod that travels with you: optical errors and background noise cancel out, because both lights cross the same instrument at the same time.

If one of the two stars hosts a planet, it wobbles around the common center of mass. That wobble shows up as a periodic modulation of the separation between the two stars. SHERA would track it day after day with a targeted precision of 0.5 to 1.1 microarcseconds depending on the target, roughly 20 times better than Gaia.

Chart of the projected separation between Alpha Centauri A and B over time, showing the annual parallax wobble and the 80-year orbit of the pair.
The projected separation between Alpha Centauri A and B over time: the annual parallax wobble, the pair's 80-year orbit. A planet would imprint its periodic modulation on this curve. Credit: Pierre Kervella, figure from the SHERA paper (arXiv:2608.04250).

A diffractive pupil etched with an electron beam

How does a 22 cm telescope, smaller than an amateur's, reach such precision? The answer lies in an etching. SHERA's primary mirror carries a diffractive pupil: a binary phase pattern etched by electron-beam lithography, the same technique used to make smartphone chips. This mask spreads each star's light into a known pattern on the detector.

That pattern serves three purposes. It acts as an internal ruler, continuously measuring the optics' deformations. It spreads light over multiple detector zones, smoothing out manufacturing defects in the sensor. And it monitors the effective color of the incoming light, telling a planetary signal apart from a starspot that shifts the star's spectrum.

On the left, the phase pattern of the diffractive pupil etched on the 22 cm mirror, on the right the simulated diffraction figure on the detector.
The diffractive pupil etched on the 22 cm mirror (left) and the diffraction pattern it produces on the detector (right), sampled at 0.123 arcseconds. Credit: figure from the SHERA paper (arXiv:2608.04250).

The target list: 7 systems, 14 stars, 4 planets expected

SHERA would target 14 stars in 7 nearby binary systems, all within 17 parsecs, about 55 light-years. All G and K types, our direct stellar cousins. At the top of the list: Alpha Centauri A and B, with a claimed sensitivity of 0.4 Earth masses, the best of the whole sample. Then 61 Cygni, 70 Ophiuchi, 36 Ophiuchi, Xi Bootis, p Eridani and the HR 2667/2668 pair.

Extending the planet occurrence rates measured by Kepler to 3-year periods, the team expects 4 ± 2 small planets of less than 4 Earth masses in the habitable zone. The result counts even if it is a void: if SHERA found 0 or 1, it would statistically prove that binaries form far fewer rocky planets than single stars.

Chart of detectable planet mass sensitivity for the 14 SHERA targets over a 3-year mission, with competing methods' thresholds for comparison.
Planet mass detection limits for the 14 targets over 3 years (cyan curves): from 0.4 Earth masses around Alpha Centauri to about a dozen around the farthest targets. Credit: figure from the SHERA paper (arXiv:2608.04250).

What it changes for the search for life

SHERA is more than a planet hunter. It is also a scout for NASA's future giant. Among the 14 targets, 13 appear on the priority-1 list of the Habitable Worlds Observatory, the 6 to 8 m observatory NASA hopes to launch in the 2040s to photograph Earth twins.

The payoff is economic. A simulation in the paper shows that if SHERA discovers 3 planets around high-priority targets, the observing time HWO needs to characterize a given number of planets drops by 40%. In an observatory where each hour costs tens of thousands of dollars, that is no small change. Knowing in advance that a planet exists, and on what orbit, spares years of pointing at empty stars.

Another dividend: combined with ground-based radial velocity measurements, SHERA's astrometry would reconstruct the full 3D geometry of these systems, inclination, eccentricity, masses. A dynamic laboratory for understanding how double stars shape planet formation, an open question ever since the first exoplanets were found on S-type orbits.

Chart showing the reduction of Habitable Worlds Observatory observing time when SHERA has already found three planets around priority targets.
The Habitable Worlds Observatory observing-time gain when SHERA has already spotted 3 planets around priority targets: up to 40% of time saved. Credit: figure from the SHERA paper (arXiv:2608.04250).

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