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After decades of hunting, astronomers have catalogued thousands of exoplanets, yet the ultimate prize still eludes them: an Earth-mass, rocky planet orbiting inside the habitable zone of a Sun-like star. Part of the difficulty is simply cosmic geography.

Roughly half of the Sun-like stars near us are not alone; they are accompanied by one or more companion stars that complicate both their own orbits and those of any planets they host. A new preprint on arXiv proposes the Searching for Habitable Exoplanets with Relative Astrometry (SHERA) mission, a NASA Small Explorer concept that aims to turn that gravitational complexity into an advantage in the search for Earth-sized worlds.

The Tracker’s Toolkit

Astronomers currently hunt exoplanets in three main ways, and each has limits:

• Transiting. Missions like Kepler and TESS watch for tiny dips in a star’s light as a planet passes across its face. Hugely productive, but it demands considerable luck, the planet’s orbit must align with Earth’s line of sight.
• Radial velocity (RV). Ground-based spectrographs measure the “wobble” a planet induces in its star. An Earth analogue tugging on a Sun-like star from one astronomical unit (AU) moves it by only about 9 cm/s, while the sensitivity limit of current instruments is around 50 cm/s (limited by surface activity on the star itself). This method is therefore out of reach for finding an Earth twin.
• Astrometry. This measures a star’s precise position against a field of background stars. But sensitivity strikes again: for an Earth twin orbiting a Sun-like star 33 light years away, the induced “wobble” amounts to about 0.3 microarcseconds (µas). GAIA, the most powerful astrometry mission to date, achieves precision near 100 µas, orders of magnitude too coarse.

SHERA’s Twist

SHERA borrows from GAIA but changes the target. Instead of positioning a star against faint background stars, a process that injects hard-to-remove noise, SHERA focuses on bright, closely separated binary pairs. Because binaries are gravitationally bound, they move together and act as co-moving reference points.

Measuring the precise distance between the two members cancels background-star errors and large-scale optical distortions that would otherwise skew the data.

If one of the pair hosts a planet, that planet drives a periodic shift in the relative separation, exactly the signal SHERA is built to catch. Reaching that sub-microarcsecond precision demands technology akin to semiconductor manufacturing: SHERA uses a diffractive pupil, created by electron-beam lithography (the same process used to make computer chips) to imprint a complex phase pattern onto the 22-cm mirror of a space telescope.

The pattern serves three purposes. It acts as an optical ruler, tracking how optical distortions and thermal expansion change over time. It spreads starlight across multiple pixels in a known pattern, letting the instrument average away microscopic detector defects.

And it guards against stellar activity masquerading as planets, by monitoring the star’s spectrum and correcting for changes driven by sunspots rather than by true shifts in the binary separation.

Goals and Stakes

The SHERA team has selected seven nearby binary systems within 17 parsecs of Earth, pursuing three primary goals: finding any Earth-mass planets around the closest targets; testing whether close-in binaries suppress planet formation at temperate distances (known to happen at some distances); and combining its astrometric data with ground-based RV measurements for additional context.

Based on standard detection-rate assumptions, the authors expect the mission to turn up around four small habitable-zone planets across its targets. But even finding fewer would be scientifically valuable, detecting fewer than two would demonstrate that habitable-zone planet occurrence rates around binaries are markedly lower than around single stars.

Why SHERA Matters

Perhaps SHERA’s most important role is enabling another mission: the Habitable Worlds Observatory (HWO). Thirteen of SHERA’s fourteen target stars are classified as Tier 1 targets for HWO. Because that large, costly observatory could spend considerable time merely confirming a planet’s existence, SHERA, a relatively inexpensive SMEX mission, essentially a scaled-up version of the TOLIMAN 16U CubeSat originally aimed at Alpha Centauri, could cut the time HWO needs to characterize planets around those stars by up to 40%, a decisive saving for a very expensive telescope.

To be clear, SHERA has not been adopted as an official mission. HWO is slated to launch late next decade, or more realistically in the early 2040s, so the use case remains open for some time. Given its modest price tag, it might even attract private funding. For now, though, it remains uncertain whether this specialized planet hunter will ever fly.

Reference:

Tomaswick, A. (2026, August 31). Looking for Earth 2.0 in Binary Systems. Universe Today.

About Coram Deo

Coram Deo — before the face of God. This blog reflects on Scripture, world events, science, music, psychology, and the human mind — always through the lens of Christian faith. All of life is lived before the face of God.