Just 12.5 light years away, around a star so faint it was only discovered in 2003, sits one of the most Earth-like rocky worlds ever found in a habitable zone — and yet we still cannot answer the single question that decides everything: whether it has any atmosphere at all

Teegarden’s Star b is a planet of at least about 1.16 Earth masses, orbiting a red dwarf about 12.5 light-years away in the constellation Aries. It lies inside its star’s habitable zone and is, by one common measure, among the most Earth-like worlds yet catalogued. Astronomers have known it exists since 2019. They still cannot say whether it holds any atmosphere, and that one unanswered question is the difference between a possibly habitable planet and a bare rock.

The obstacle is not a lack of attention. It is geometry. The planet does not pass in front of its star from our line of sight, which removes the main tool we have for reading the air of a distant world.

A star found in old asteroid images

Teegarden’s Star was not found by pointing a telescope at it. It was identified in 2003 by the NASA astronomer Bonnard Teegarden and colleagues, who combed through archival images taken years earlier by the Near-Earth Asteroid Tracking programme. Despite being one of the two dozen or so nearest stars to the Sun, it had gone unnoticed because it is extraordinarily faint. It is an M7 dwarf, less than a tenth of the Sun’s mass, with a surface temperature around 2,900 kelvin, and at apparent magnitude 15 it needs a large telescope to register at all.

What gave it away was movement. The star crosses the sky at roughly five arcseconds a year, among the largest proper motions of any star known, the kind of shift that stands out across images separated by time.

What the 2019 detection established

The planets came later. In June 2019 the CARMENES survey, led by Mathias Zechmeister at the University of Göttingen, reported two Earth-mass candidates in Astronomy & Astrophysics, planets b and c, with orbital periods of 4.9 and 11.4 days and no sign that either transits. A 2024 study led by Stefan Dreizler added a third planet farther out, a cold world beyond the habitable zone, and refined the figures for the inner two.

It is worth being precise about what these measurements are. The detections come from the radial-velocity method, which registers the small tug a planet exerts on its star. That yields a minimum mass, not a true one, because the tilt of the orbit is unknown. Teegarden’s Star b is at least about 1.16 Earth masses and may be somewhat heavier. The description of it as rocky follows from that low minimum mass, not from a measured size, because the planet has never been seen to transit and so its radius is not known directly.

For a time, b held the highest Earth Similarity Index of any known exoplanet, near 0.95. The 2024 revision lowered that figure to about 0.90. The index is a similarity score built from estimated size and temperature, and it is not a measure of habitability.

Why the atmosphere question is stuck

The decisive limit is the lack of transits. When a planet crosses its star, a thin sliver of starlight passes through any atmosphere on the way to us, and the gases there leave fingerprints in the spectrum. This is how the James Webb Space Telescope probes the air of other rocky worlds. It cannot be applied to a planet that never crosses its star from our vantage point, and the Teegarden planets do not.

Some more basic numbers are still soft as well. Even small differences in the estimated light and heat the planet receives matter, because Teegarden’s Star b appears to sit close to the inner edge of habitability. A 2025 modelling study of its climate found that one estimate of its irradiation keeps it below the runaway-greenhouse threshold, while a slightly higher one places it past it. Whether an atmosphere would have survived the star’s youth is a separate open question, and for red-dwarf planets the concern is real, since these stars flare fiercely for their first billions of years and can strip a planet’s air. Webb has found little sign of thick atmospheres on the inner TRAPPIST-1 planets, b and c. But that is a different system, and the rest is modelling with assumed inputs. None of it is a measurement of Teegarden b.

One detail works in the planet’s favour. Teegarden’s Star is unusually quiet for its type, less magnetically active than most late-M dwarfs, which gives any atmosphere a better chance of having lasted across the system’s eight-billion-year history.

What it would take to answer it

With transmission spectroscopy off the table, the answer waits on instruments that can study the planet directly, through reflected light or thermal emission rather than a transit. The literature names two. One is the Planetary Camera and Spectrograph planned for the Extremely Large Telescope, now under construction in Chile. The other is LIFE, a proposed space interferometer designed to detect the infrared glow of nearby temperate planets. The same modelling study states the position plainly: detailed characterisation has not been done, and it will require the next generation of observatories.

So Teegarden’s Star b sits in an awkward place. It is close enough, and Earth-like enough by mass and irradiation, to rank among the best nearby targets in the search for habitable worlds, and at the same time invisible to the technique that has driven most of the field’s recent progress. The next real news about it will not come from the planet. It will come from the telescopes being built to look at it.