Teegarden\u2019s 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\u2019s 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.<\/p>\n
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.<\/p>\n
A star found in old asteroid images<\/p>\n
Teegarden\u2019s 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\u2019s mass, with a surface temperature around 2,900 kelvin, and at apparent magnitude 15 it needs a large telescope to register at all.<\/p>\n
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.<\/p>\n
What the 2019 detection established<\/p>\n
The planets came later. In June 2019 the CARMENES survey, led by Mathias Zechmeister at the University of G\u00f6ttingen, reported two Earth-mass candidates<\/a> 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<\/a> added a third planet farther out, a cold world beyond the habitable zone, and refined the figures for the inner two.<\/p>\n 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\u2019s 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.<\/p>\n 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.<\/p>\n Why the atmosphere question is stuck<\/p>\n 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.<\/p>\n