The hunt for “earth twins”
Astronomers repeatedly report discoveries of potentially habitable exoplanets, sometimes called “Earth twins.” These are worlds with gravity similar to Earth, a solid rocky surface, and temperatures that allow liquid water. Studying Earth twins helps scientists understand not only the dream of space colonization but also the origins of life in the galaxy and how unique our own existence might be.
In reality, no one has seen an Earth-like planet directly. Modern telescopes lack the capability to image such worlds clearly. Researchers infer their existence from indirect clues by analyzing starlight. The method often involves watching for regular dips in brightness as a planet transits the star, revealing the planet’s orbit and giving hints about atmospheric conditions based on how starlight changes as the planet passes in front of the star.
Still, planets can be detected around distant stars. A notable example involves the Keck Observatory, which captured images showing the motion of giant planets around HR 8799, a star about 129 light-years away. Keck combines two ten-meter telescopes into an interferometer, dramatically increasing resolution. Perhaps a telescope built even larger could reveal smaller worlds as well?
The challenge isn’t only about optics. Planets are faint and sit close to bright stars that can overwhelm the instrument. To see them, the light from the star must be blocked, using a coronagraph. In images of HR 8799, the coronagraph appears as a central black dot. But current coronagraphs, like those used at Keck, typically reveal only very large planets far from their stars, roughly near the orbit of Uranus.
Astronomers also draw parallels with solar observations. The solar corona — the hot plasma surrounding the Sun — is visible during solar eclipses when the Moon acts as a natural coronagraph. Ground-based instruments with built-in coronagraphs can observe the corona year-round, but image quality remains poorer than during an eclipse. For truly sharp images of Earth-like exoplanets, a dedicated device positioned away from the telescope is often required. Even a telescope as capable as the James Webb Space Telescope would struggle to map continents; it would likely see only a point of light. Yet that would still mark a major advance in understanding distant worlds.
Experts discuss how future instrumentation might tease out atmospheric composition, including oxygen levels, which could signal biological activity. As one senior researcher noted, deciphering an exoplanet’s atmosphere is as important as the raw observations themselves, because it helps researchers assess habitability and potential life signals more reliably .
Bigger is better?
Multiple telescope projects are proposed to hunt Earth twins. Among the most ambitious is HabEx, the Habitable Exoplanet Observatory, along with its updated concepts like HWO. The plan envisions a four-meter primary mirror and an external coronagraph positioned tens of thousands of kilometers away from the main telescope. This external device would block starlight with exquisite precision, allowing the faint glow of a planet to emerge without losing sight of its starry companion. The external coronagraph concept draws on ideas akin to a sunflower, with a large, delicate structure that must be deployed and aligned from a separate rocket to shadow the star while leaving planets visible .
These concepts remain in development and have not yet received approval; if funded, they would not launch before 2030. Even with HabEx, resolving Earth-like geography would prove difficult; the planets would appear as mere points of light. Nevertheless, spectroscopy could reveal atmospheric composition and the presence of oxygen, offering strong clues about possible life .
Researchers emphasize that interpretation matters as much as observation. Scientists continue to refine methods for detecting planetary features from brightness variations across daily and yearly cycles, striving to deduce surface patterns from light regimes .
Some specialists describe two theoretical paths to an ultra-large telescope. One relies on gravitational lensing, where the Sun’s gravity bends light like a gigantic lens, though the telescope would need to be far from the outer solar system to harness this effect. The other option envisions a space-based interferometer made of multiple HabEx-scale telescopes working in concert across kilometers, creating a single, powerful observational instrument .
The edge of the universe and black holes
A guiding principle in astronomy is that a larger effective aperture yields better resolution, with few theoretical limits on what could be observed. Paradoxically, it can be easier to image a supermassive black hole than a tiny Earth-like planet, and in some cases a gigantic telescope may offer little advantage. For instance, in 2019 and 2022, images of the black hole at the center of M87 and of Sagittarius A* in our Milky Way provided strong confirmations of general relativity and enhanced our understanding of gravity [Cited observations].
One observer noted that even enormous telescopes do not show the black hole itself, but rather the surrounding hot gas. The same holds across optical and radio wavelengths, with differences in how the details appear but not in the fundamental physics .
When considering the edge of the observable universe, James Webb has already expanded what can be seen. Light travel means distant objects we observe are seen as they were billions of years ago, offering a look back in time. The universe’s observable reach is limited by the age of the cosmos itself, about 13.8 billion years, yet peering farther can reveal the first light emitted after the Big Bang [Observational summaries].
The telescope has already captured galaxies formed roughly 500 million years after the Big Bang, surprising researchers who expected older signals given standard dating methods. While some galaxies appear older than anticipated, this challenges prevailing estimates and spurs new approaches [Cited findings].
Looking toward the very edge of the visible universe, some scientists speculate about Population III stars—the first generation of stars composed almost entirely of hydrogen without heavy elements. A future thirty-meter space telescope could unlock spectra from individual star clusters in the earliest galaxies, helping to determine whether those stars contained heavy elements. If not, they would likely be the universe’s first stars .
Of course, the cost of mega-telescopes is enormous. The James Webb Space Telescope represents a substantial share of the US budget, and a thirty-meter instrument would be even more costly. With shifting global priorities, large astronomical megaprojects face long timelines and tight funding windows, making practical realization a major challenge [Budgetary analyses].