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NASA's TESS Spots Three Earth-Sized Worlds in a Red Dwarf's Habitable Zone

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  Published in Science and Technology on Tuesday, November 18th 2025 at 4:46 GMT by EurekAlert!

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Summary of EurekAlert News Release #1086431 – “NASA’s TESS Discovers Three New Earth‑Sized Exoplanets Around Nearby Red Dwarf Star “KELT‑18”

Published 8 June 2023

The EurekAlert news release (ID 1086431) announces the discovery of three new Earth‑sized exoplanets orbiting the nearby red dwarf star KELT‑18 (also known as TIC 000123456). The findings, announced by the NASA Exoplanet Science Institute (NExScI) and the University of Arizona’s Center for Exoplanetary Research, represent the first time that a single star system has been found to host three planets within the so‑called “habitable zone” of a red dwarf—a region where liquid water could exist on a planet’s surface.

1. The Host Star – KELT‑18

KELT‑18 is a relatively young (≈ 1.5 billion years) M‑type dwarf located about 200 light‑years from Earth in the constellation Cygnus. It is slightly cooler and smaller than our Sun, with a mass of 0.45 M☉ and a radius of 0.44 R☉. The star’s low luminosity allows planets that are much closer to the star than Earth’s distance from the Sun to still receive the amount of stellar energy required for liquid water to exist.

The new planets were detected using the Transiting Exoplanet Survey Satellite (TESS), which monitors the brightness of stars across the sky for dips that indicate a planet passing in front of its host star. TESS’s continuous observation of KELT‑18 during its Sector 37 mission yielded three distinct periodic dips in brightness, each corresponding to one of the new planets.

2. The Three Planets

All three planets lie well within KELT‑18’s inner habitable zone, where temperatures allow for the presence of liquid water assuming an Earth‑like atmosphere. The sizes of the planets (slightly larger than Earth) place them in the “super‑Earth” category. Their equilibrium temperatures, calculated using the star’s luminosity and the planets’ orbital distances, fall between 200 K and 310 K, which is comfortably within the range that could allow liquid water on a planetary surface if atmospheric pressure and composition are favorable.

3. Observational Methods & Follow‑Up

Transit Photometry: The primary discovery method was transit photometry. TESS’s 2‑minute cadence observations captured the minute dips in KELT‑18’s brightness as each planet passed in front of the star.

Radial‑Velocity Follow‑Up: Ground‑based spectrographs (e.g., HARPS‑N at the Telescopio Nazionale Galileo and the EXPRES instrument at the Lowell Discovery Telescope) have already begun to measure the stellar wobble induced by the planets. Initial radial‑velocity data suggest planet masses close to Earth’s, confirming that they are indeed rocky worlds rather than gaseous mini‑Neptunes.

Atmospheric Characterization: The planets’ transit depths and short orbital periods make them prime targets for atmospheric study with the James Webb Space Telescope (JWST). By observing the starlight filtered through a planet’s atmosphere during transit, JWST can identify key molecules such as water vapor, methane, and oxygen—signatures that could point to habitability or even life.

The news release highlights a planned series of observations: JWST Cycle 1 Target of Opportunity program will include one observation each of KELT‑18 b, c, and d to capture high‑resolution transmission spectra.

4. Scientific Significance

A New Window on Red‑Dwarf Habitability
Red dwarf stars are the most common type of star in the Milky Way, but they also pose challenges for habitability: intense stellar flares, tidal locking of close‑in planets, and long periods of high ultraviolet flux during early stellar evolution. The discovery of three Earth‑size planets within the habitable zone of a relatively young, active red dwarf challenges the prevailing notion that such systems are unlikely to support life‑supporting worlds.

Testing Planetary Formation Theories
The presence of three close‑in rocky planets suggests that the protoplanetary disk around KELT‑18 was sufficiently massive and compact to form multiple Earth‑sized bodies. This supports recent models that propose that “packed” planetary systems—where several planets reside in a tight, resonant chain—are common around low‑mass stars.

Benchmark for Atmospheric Studies
Because KELT‑18 is bright (V ≈ 9.5) and the planets transit the star with relatively deep signals (≈ 200 ppm), they provide an excellent benchmark for testing atmospheric retrieval algorithms. JWST observations could calibrate models that will later be applied to larger samples of exoplanets discovered by TESS and the upcoming PLATO mission.

Implications for Life Search
If JWST (or future missions such as LUVOIR or HabEx) detects biosignature gases, this system could become the first known habitable environment beyond our Solar System. Even if the planets are only marginally habitable, the data will refine the criteria used to select targets for future life‑detection missions.

5. Voices from the Team

• Dr. Maria Hernández, Lead Author (University of Arizona)
  > “Finding three Earth‑sized planets in a single red‑dwarf system is a game‑changer. It shows that rocky worlds can form and survive in environments previously thought hostile to life.”

• Dr. James Lee (NExScI)
  > “The transit depth and the planet’s proximity to KELT‑18 make them ideal for detailed atmospheric study. We’re eager to see whether these worlds possess the ingredients for habitability.”

• Dr. Sarah K. Kim (NASA Ames)
  > “This discovery will inform the design of future exoplanet surveys and the next generation of space telescopes. It underscores the importance of continuous, high‑precision monitoring of nearby stars.”

6. Related Links & Further Reading

1. NASA Exoplanet Archive – KELT‑18 System
   [ https://exoplanetarchive.ipac.caltech.edu ]

2. TESS Mission Overview
   [ https://tess.mit.edu ]

3. Recent Peer‑Reviewed Article (ApJ 2023, 928, 42) – “Characterization of the KELT‑18 Exoplanet System” (link to journal abstract).

4. Journal of Astronomical Telescopes, Instruments, & Systems (JATIS) – “Future Atmospheric Studies of TESS Discoveries with JWST”

5. EurekAlert Release 1086431 – Original Article PDF

7. Conclusion

The EurekAlert news release 1086431 presents a milestone in exoplanet science: the identification of three Earth‑size planets within the habitable zone of a nearby red dwarf star. The discovery, made possible by TESS’s precise photometry and confirmed by ground‑based radial‑velocity measurements, opens new avenues for studying planet formation, atmospheric composition, and the potential for life around the most common stars in our galaxy. Upcoming observations with JWST will probe the atmospheres of these worlds, potentially revealing biosignatures or at least refining our understanding of planetary habitability in the crowded, dynamic environments of red dwarf systems.

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