Astronomers think rectangular telescopes are key to finding Earth-like planets

Life on Earth runs on liquid water. Single-celled organisms appeared almost as soon as our planet could sustain them, but multicellular life took around three billion years to emerge. Humans, in comparison, have existed for only the briefest flicker of time – less than one ten-thousandth of Earth’s age.

That timeline hints at a sobering possibility: watery worlds may be common, but civilizations capable of studying the cosmos – and traveling through it – might be rare. If we want proof of life elsewhere, we may have to go looking for it ourselves.

Stars that host stable environments

Physics sets unforgiving limits. Nothing can outrun light, so messages and machines crawl across interstellar space on human timescales.

Only the closest stars are even plausible targets for robotic probes, and only a subset of those stars are likely to host long-lived, stable environments.

That’s why astronomers focus on roughly 60 Sun-like stars within about 30 light-years. Around such stars, the most promising planets will be Earth-sized and temperate – places where rock, liquid water, and chemistry can mingle for eons.

Finding Earth-like planets is hard

Even if an inviting world is there, seeing it is like trying to pick out a firefly beside a searchlight. At the mid-infrared wavelengths where a watery Earth shines most (around 10 microns), its host star is still about a million times brighter. If the planet’s glow blurs together with the star’s, it disappears.

Sharpening the view requires a telescope whose resolving power is set by two things: the wavelength of light and the size of the aperture that gathers it.

At 10 microns, cleanly separating an Earth–Sun pair from 30 light-years away demands a collector spanning about 20 meters – and it has to be in space, because Earth’s atmosphere smears the image.

The problem: our biggest space telescope, the James Webb Space Telescope (JWST), is 6.5 meters across. Getting something three times larger into orbit, fully deployed and rock-solid, is beyond today’s launch and engineering playbook.

Workarounds that run into walls

Ingenious alternatives exist on paper, but each hits a brutal practical snag. You can fly multiple small spacecraft in formation and combine their light like a single giant telescope (an interferometer).

However, the distances between them must be controlled and measured with near-molecular precision. We’re not there yet.

You can shift to shorter, visible wavelengths so a smaller telescope has the same resolving power. Unfortunately, in visible light the brightness contrast gets far worse: a Sun-like star can outshine an Earth by factors of ten billion or more.

Current starlight-suppression optics can’t punch that much glare down to reveal the planet.

You can block the star directly with a separate “starshade,” a petal-edged screen tens of meters across that would fly tens of thousands of miles ahead of a space telescope, casting a precise artificial eclipse.

But every time you want to target a different star, the shade must retarget over vast distances, guzzling fuel and time. Worse, you need to build, launch, and choreograph two spacecraft instead of one.

Earth-like planets need sharper vision

A simpler path may be hiding in plain sight: keep the mid-infrared wavelength where Earth-like planets are brightest, keep the overall collecting area comparable to JWST’s, but stretch the telescope’s mirror.

The new proposal argues that a space telescope with a rectangular primary – about one meter by twenty meters – could deliver the critical 20-meter resolution in one direction. It would also remain much more launchable than a 20-meter circle.

Here’s the trick. Angular resolution depends on the dimension of the aperture along the direction you’re trying to separate things.

With a long, skinny mirror, you get razor-sharp vision along its long axis. Rotate the telescope and you sweep that sharp axis around the star, catching any planet whose position angle lines up.

Over time, a rotating rectangular mirror samples all directions, turning a geometric compromise into a search strategy.

Telescope design meets planet search

Run the numbers, and the design is potent. Operating near 10 microns, a one-by-twenty-meter space telescope could, in principle, separate a Sun-like star from an Earth-warm planet out to roughly 30 light-years.

Pointed methodically at the approximately 60 Sun-like stars in that volume and rotating as needed, such an observatory could detect roughly half of the Earth-like planets that actually exist in those systems within three years of observing time.

Crucially, the concept doesn’t hinge on complex formation flying, billion-to-one starlight suppression, or two-spacecraft choreography. It still needs serious engineering and optimization, but it avoids the most daunting leaps other ideas require.

What this planet survey might reveal

Assume nature is moderately generous – on average, one Earth-like planet per Sun-like star. A successful survey would hand back on the order of thirty nearby worlds that are the right size and temperature.

With follow-up spectroscopy, we could search their atmospheres for gases that hint at biology, such as oxygen produced faster than geology alone can supply.

We could watch for seasonal swings, cloud patterns, and thermal signatures that reveal oceans and continents.

And a shortlist of true standouts would reshape our ambitions. A world with the strongest biosignatures, close enough and compelling enough, would become the focus of bold planning.

Concepts for fast robotic flybys, long-baseline laser communications, and eventually close-up imaging could move from science fiction toward a century-scale engineering roadmap.

Searching for Earth-like planets

No telescope can shrink the planet-to-planet distances between stars. But a cleverly designed mid-infrared observatory with a long rectangular mirror could finally push us past the contrast and resolution barrier that’s kept “another Earth” just out of reach.

It is, at heart, an argument for pragmatism. Rather than wait for technologies we don’t yet have, build the sharpest, most feasible eye we can at the wavelength where Earth-like planets glow brightest.

If it works, the payoff is profound. We would move from guessing about life in the universe to checking the neighborhood – system by system, world by world – until one of them answers back in the only language we all share: the faint heat of a small, wet planet under a quiet, yellow sun.

The study is published in the journal Frontiers in Astronomy and Space Sciences.

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