Ross 128 b

An Earth-sized world

1.11 times Earth's radius, squarely in the terrestrial size range. With 1.4 Earth masses packed into that size, its density points to rock and iron rather than gas.

The orbit

Its year, one full orbit, takes just 9.9 Earth days. Orbiting this tightly around a cool star, it is very likely tidally locked: one hemisphere in permanent day, the other in permanent night.

Temperature and starlight

Its equilibrium temperature (the airless-world estimate) is about 28 °C. In the range where, with the right atmosphere, liquid water is conceivable; a real atmosphere could shift the true surface figure a lot (Earth's equilibrium value is −18 °C, yet we average +15 °C). It receives 1.38 times Earth's starlight, the same order as our own world.

Its sun

Its star is a red dwarf (3,192 K surface), type M4: small, dim, flare-prone, and destined to outlive the Sun many times over.

How far is that, really?

At 11 light-years, tonight's light from this system left it around the year 2014. That is close by galactic standards, which makes it a prime target for follow-up telescopes. A Voyager-speed probe would need roughly 194,730 years to get there.

How we found it

It was found by the radial-velocity method: the planet's gravity swings its star in a small circle, and that wobble shows up as a rhythmic Doppler shift in the starlight. The wobble's size gives the planet's minimum mass. The discovery was announced in 2017 from La Silla Observatory.

Everything above is computed from the archive’s measured values for this planet. Where a quantity is missing, the statement is omitted rather than guessed.

01 How Ross 128 b Was Found

The discovery rested on a decade's worth of radial velocity measurements collected by the HARPS spectrograph — the High Accuracy Radial velocity Planet Searcher — mounted at the European Southern Observatory's La Silla Observatory in Chile. The radial velocity technique works by detecting the tiny Doppler wobble a planet induces in its host star as both objects orbit their common center of mass. Because Ross 128 b never passes in front of its star as seen from Earth, the transit method was unavailable, so HARPS's exquisite precision was essential. The planet's signal appeared as a repeating shift in the star's spectrum with a period of about 9.9 days, matching one full orbit. This method, however, delivers only a minimum possible mass rather than a true mass, because the unknown tilt of the orbit means astronomers cannot tell how much of the star's wobble is directed toward Earth.

02 What We Know — and Don't — About Its Mass

The radial velocity method sets a floor, not a ceiling, on Ross 128 b's mass. The planet is at least 1.35 times the mass of Earth, equivalent to roughly 8.06×10²⁴ kilograms. That makes it slightly more massive than its famous neighbor Proxima Centauri b, which has a minimum mass of 1.27 Earth masses. Because only a minimum mass is pinned down, a 2019 study attempted to statistically account for the unknown orbital inclination and predicted a true mass of about 1.8 times that of Earth — but with large margins of error. Those error bars are a honest reminder of how much remains uncertain. The low minimum mass still strongly suggests a rocky, solid-surface world rather than a gas or ice giant, because planets much more massive than Earth tend to accumulate thick volatile envelopes.

03 Estimating a Radius Without Seeing a Transit Deeper

Because Ross 128 b never transits its star, its physical size cannot be measured directly. Instead, scientists bracket the radius using compositional models. At one extreme, a planet made entirely of iron would compress to just 0.5 Earth radii. At the other, a pure hydrogen-helium world would balloon to 3.0 Earth radii. Both extremes are considered implausible. For a composition similar to Earth's mix of rock and metal, the planet would measure approximately 1.10 Earth radii — about 7,008 kilometers in radius — making it modestly larger than our own world. At that size and with that rocky composition, the planet would actually be slightly denser than Earth, because rocky material compresses more under the greater gravity of a larger body. Surface gravity would reach roughly 10.945 m/s², or about 1.12 times Earth's pull. The 2019 study pushed the radius estimate further, to about 1.6 Earth radii, again with large uncertainties.

04 Temperature Ranges Across Three Albedo Scenarios Deeper

The discovery team modelled Ross 128 b's equilibrium temperature — the temperature at which incoming and outgoing energy balance — across three different albedo values, where albedo measures the fraction of starlight reflected rather than absorbed. With a low, dark albedo of 0.100, the planet would absorb most incoming energy and reach 294 K (21 °C; 70 °F), close to a warm spring day on Earth. A moderate albedo of 0.367 drops the temperature to 269 K (−4 °C; 25 °F), near freezing. A highly reflective albedo of 0.750 — like a world covered in fresh snow or thick bright clouds — pushes it down to 213 K (−60 °C; −76 °F). For an Earth-like albedo of 0.3, the equilibrium temperature lands at 280 K (7 °C; 44 °F), about 8 Kelvins cooler than Earth's average. All of these figures ignore the unknown greenhouse effect of any atmosphere, which could raise the actual surface temperature considerably.

05 The Ancient, Quiet Star at the Center

Ross 128 is a red dwarf of striking age and calm. It is 17% the mass and 20% the radius of the Sun, and burns at a surface temperature of just 3,192 K — barely more than half the Sun's 5,772 K. Its luminosity is a mere 0.00362 times that of the Sun, making it far dimmer than our star. At an estimated age of 9.45±0.60 billion years, it is more than twice as old as the Sun's 4.5 billion years, placing it among the elderly members of the solar neighborhood. Despite being only 11.03 light-years away — close enough to rank among the 20 nearest known stars — it is far too faint to see without a telescope. A 2018 analysis using high-resolution near-infrared APOGEE spectra measured the chemical abundances of carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron in the star, finding that Ross 128 has near-solar metallicity, suggesting its planetary system formed from material compositionally similar to our own.

06 An Orbit Tighter Than Mercury's

Ross 128 b completes a full orbit every 9.9 days at a semi-major axis of just 0.0496 AU, equivalent to 7.42 million kilometers. Earth sits 149 million kilometers from the Sun on average, meaning Ross 128 b orbits roughly 20 times closer to its star than Earth does to ours. Because of that proximity, the planet is almost certainly tidally locked — one hemisphere bakes under permanent starlight while the opposite hemisphere endures perpetual darkness. The orbital shape, however, remains disputed. Some models favor a nearly circular orbit with an eccentricity of about 0.03, but a 2024 study of the same radial velocity data found an eccentricity closer to 0.21, comparable to Mercury's elongated path around the Sun. Both estimates carry large error ranges, and the true shape of the orbit has meaningful consequences for the planet's habitability prospects.

07 Eccentricity and What It Means for Life Deeper

Orbital eccentricity — how egg-shaped rather than circular a planet's path is — matters enormously for habitability because it controls how much the planet's distance from its star varies over the course of a year. Ross 128 b already sits near the inner edge of its star's habitable zone, receiving about 38% more starlight than Earth gets from the Sun. An eccentricity of 0.21, as the 2024 study suggests, would swing the planet significantly closer to its star at each closest approach, delivering intense bursts of additional energy. The discovery team's study author Xavier Bonfils noted that this moderately high stellar flux already makes the planet more prone to water loss, particularly on the star-facing side. A higher eccentricity compounds this concern. The question remains open: the earlier, lower eccentricity estimates and the newer, higher one all carry large error ranges, so the orbital shape of Ross 128 b is genuinely unresolved as of the most recent published analysis.

08 Why Ross 128's Calm Nature Matters

Many red dwarf stars are magnetically hyperactive, unleashing frequent and powerful stellar flares that can strip planetary atmospheres over billions of years and flood surfaces with dangerous radiation. Proxima Centauri and TRAPPIST-1 are prominent examples of such flare stars, and the threat they pose to their planets' habitability is a serious ongoing debate. Ross 128 is different. While it is known to produce flares, they are currently far less common and less powerful than those of Proxima Centauri or TRAPPIST-1. The star's advanced age of roughly 9.45 billion years may help explain this — older red dwarfs tend to spin more slowly and generate weaker magnetic fields, reducing flare activity. This relative stellar quiet is a significant part of why Ross 128 b is considered one of the best candidates for habitability among all currently known nearby exoplanets.

09 Clouds and Atmospheres: Open Questions

Whether Ross 128 b has an atmosphere at all is unknown, and the planet's refusal to transit its star makes the usual technique for probing planetary atmospheres — transmission spectroscopy, in which starlight filtered through a planet's atmosphere reveals its chemical composition — unavailable. Study author Xavier Bonfils raised the intriguing possibility that significant cloud cover could develop on the permanently sunlit side of the tidally locked planet, reflecting away a substantial fraction of incoming stellar energy and keeping temperatures low enough for liquid water. As of 2017, no instruments had the capability to detect an atmosphere without a transit. That may change with the James Webb Space Telescope and next-generation ground-based observatories such as the Thirty Meter Telescope and the Extremely Large Telescope, which could search for biosignatures including oxygen, ozone, and methane — chemicals whose presence in combination would suggest biological processes.

10 Ross 128 b in the Neighborhood of Earth

At 11.007 light-years (3.375 parsecs) from Earth, Ross 128 b sits in the constellation of Virgo and holds the distinction of being the nearest known exoplanet orbiting a quiet red dwarf star. It is not the nearest exoplanet overall — Proxima Centauri b holds that title at roughly 4.2 light-years — but the calm nature of its host star makes Ross 128 b arguably a more promising target for detailed study. The proximity also makes it a prime candidate for future long-baseline observations and, in the very speculative long term, any interstellar mission concepts. The star itself, at 11.03 light-years, ranks among the 20 closest stars to the Sun. Light leaving Earth right now would take just over eleven years to reach it — a cosmic stone's throw in a galaxy spanning roughly 100,000 light-years.

11 Stellar Chemistry and Planet Composition Clues Deeper

A planet's bulk composition is shaped in part by the chemical makeup of the nebula from which both it and its star formed. The 2018 APOGEE study used near-infrared, high-resolution spectra to measure the abundances of eight elements in Ross 128: carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium, and iron. The finding that Ross 128 has near-solar metallicity is informative. A star with solar-like proportions of rock-forming elements such as magnesium, iron, and silicon is more likely to have produced rocky planets with compositions similar to Earth. This chemical evidence lends additional support — though not proof — to the hypothesis that Ross 128 b is a rocky, Earth-like world rather than some exotic composition. It also suggests that the planet's interior could support geochemical cycles similar to those that help regulate Earth's climate over geological timescales, though this remains entirely speculative without direct measurement.

12 How Ross 128 b Compares to Proxima Centauri b

Proxima Centauri b and Ross 128 b are natural rivals for the title of most compelling nearby potentially habitable world. Proxima b is closer, at about 4.2 light-years versus Ross 128 b's 11.007 light-years, and has a slightly lower minimum mass of 1.27 Earth masses compared to Ross 128 b's 1.35. However, Ross 128 b benefits from orbiting a star that is far less prone to dangerous flares. Proxima Centauri is a notoriously active flare star, and repeated energetic outbursts raise serious questions about whether Proxima b could retain an atmosphere or support surface life. Ross 128, by contrast, is currently much quieter. Both planets are tidally locked to their stars and both lack confirmed atmospheric detections, leaving their habitability genuinely unresolved. The relative calm of Ross 128 tips many researchers' informal rankings in favor of Ross 128 b as the more promising nearby target.

About this record

The measured values come from the NASA Exoplanet Archive, and any missing value has not been determined yet; the Atlas never fills gaps with guesses. More standout worlds: Proxima b, TRAPPIST-1e, K2-18 b, 55 Cancri e, and the detection methods that found them all.