A temperate world around a quiet red dwarf
Ross 128 b
Orbits Ross 128 · 11.0 light-years · discovered 2017 (radial velocity)
Ross 128 b is one of the closest known planets of roughly Earth's mass, orbiting a faint red dwarf in the constellation Virgo about 11 light-years away. It circles its star in under ten days, yet because the star is so dim the planet receives only somewhat more energy than Earth does from the Sun.
What makes it stand out is its host. Ross 128 appears to be one of the calmest red dwarfs near the Sun, with far fewer violent flares than stars like Proxima Centauri. That gives any atmosphere a better chance of surviving — although no atmosphere has been detected, and the planet's surface conditions are entirely unknown.
Key facts
- Star
- Ross 128M4V red dwarf
- Distance
- 11.0 light-years
- Radius
- Not measuredIt has not been seen to transit its star
- Mass
- ≥ 1.40 × EarthMinimum mass (radial velocity)
- Orbital period
- 9.87 days
- Equilibrium temperature
- UnknownBefore any greenhouse warming
- Starlight received
- 1.38× Earth's
- Surface gravity
- ≈ 1.17 gRough estimate (rocky composition assumed)
- Discovered
- 2017Radial velocity
- Star
- Ross 128 (M4V red dwarf)
- Distance
- About 11 light-years
- Discovered
- 2017, radial velocity
- Mass
- At least about 1.4 Earth masses (minimum mass)
- Radius
- Unknown — estimated at roughly 1.1–1.6 Earth radii if rocky
- Orbital period
- About 9.9 days
- Starlight received
- Roughly 1.4 times what Earth gets from the Sun
- Equilibrium temperature
- Roughly 213–301 K (−60 °C to 28 °C), depending on how reflective it is
- Constellation
- Virgo
Measured values follow the NASA Exoplanet Archive and discovery papers; many carry large uncertainties. The 3D globe and walkable surface on this page are an artist's impression generated from these numbers.
What we know
Ross 128 b is known only from the regular back-and-forth motion it imprints on its star's light. That signal gives an orbital period of about 9.9 days and a minimum mass of around 1.4 Earth masses. The true mass depends on the unknown tilt of the orbit.
The planet has not been seen to pass in front of its star, so its size has never been measured. A rocky composition is plausible for a planet this light, but it has not been confirmed.
Its star
Ross 128 is a cool red dwarf with roughly a sixth of the Sun's mass. It is thought to be several billion years old and, crucially, relatively inactive: it rotates slowly and flares only rarely compared with many stars of its type. Studies of its chemistry suggest it has a composition fairly similar to the Sun's.
Could it be habitable?
The planet orbits near the inner edge of its star's habitable zone. With about 40% more incoming energy than Earth, whether it could keep liquid water depends heavily on clouds: a reflective cloud cover could keep it temperate, while a thin or absent atmosphere would leave it either scorched by day or frozen by night.
The quiet star is the main argument in its favour. Planets around more active red dwarfs face intense flares that may erode their atmospheres, and Ross 128 b may have been spared the worst of this. Even so, nothing is yet known about its air, water or surface.
Orbit and climate
Ross 128 b orbits about 0.05 astronomical units from its star — roughly 7.5 million kilometres, or about a twentieth of the Earth–Sun distance. At such close range it is probably tidally locked, with one hemisphere in permanent daylight. Climate models suggest that, if it has an atmosphere, a band of cloud could form over the point facing the star and help shield the surface.
Discovery
The planet was announced in November 2017 by a team led by Xavier Bonfils, using more than a decade of measurements from the HARPS spectrograph at ESO's La Silla Observatory in Chile. Later work with other spectrographs has confirmed the signal and refined the minimum mass.
Future observations
Because it does not transit, Ross 128 b cannot be probed by the starlight-filtering method used for TRAPPIST-1. Its relatively wide separation from the star on the sky makes it a candidate for direct imaging with the extremely large telescopes now under construction, which might one day isolate its reflected light and search it for gases such as oxygen.
About the 3D impression
Artist's impression — the real surface has never been seen. The globe and the surface you can walk on are generated from what is measured — size, the starlight it receives and the kind of star it orbits — using the same procedural engine as our imaginary worlds. Seas, ice, clouds and any plant-like colours are possibilities, not observations: around a red dwarf the daylight is redder, so the scene uses warmer skies and darker vegetation.
Earth-like exoplanets compared
| Planet | Distance | Radius | Year | Sunlight |
|---|---|---|---|---|
| Proxima Centauri b | 4.2 ly | — | 11.2 days | 0.65× Earth |
| Ross 128 b | 11.0 ly | — | 9.87 days | 1.38× Earth |
| Teegarden's Star b | 12.5 ly | — | 4.91 days | 1.08× Earth |
| Gliese 667 Cc | 23.6 ly | — | 28.1 days | 0.88× Earth |
| TRAPPIST-1d | 40.5 ly | 0.79 R⊕ | 4.05 days | 1.11× Earth |
| TRAPPIST-1e | 40.5 ly | 0.92 R⊕ | 6.10 days | 0.65× Earth |
| TRAPPIST-1f | 40.5 ly | 1.04 R⊕ | 9.21 days | 0.37× Earth |
| TRAPPIST-1g | 40.5 ly | 1.13 R⊕ | 12.4 days | 0.25× Earth |
| LHS 1140 b | 48.9 ly | 1.73 R⊕ | 24.7 days | 0.43× Earth |
| TOI-700 d | 101 ly | 1.07 R⊕ | 37.4 days | 0.87× Earth |
| Kepler-186f | 580 ly | 1.17 R⊕ | 129.9 days | 0.30× Earth |
| Kepler-22b | 635 ly | 2.10 R⊕ | 289.9 days | 1.01× Earth |
| Kepler-62f | 980 ly | 1.41 R⊕ | 267.3 days | 0.41× Earth |
| Kepler-442b | 1,194 ly | 1.34 R⊕ | 112.3 days | 0.70× Earth |
| Kepler-452b | 1,800 ly | 1.63 R⊕ | 384.8 days | 1.10× Earth |
Surprising facts about Ross 128 b
- Ross 128 is moving towards us and is expected to become the Sun's nearest stellar neighbour in roughly 70,000–80,000 years.
- In 2017 the Arecibo radio telescope recorded odd signals from the direction of Ross 128; astronomers concluded they most likely came from Earth-orbiting satellites.
- Ross 128 b completes an orbit in under ten days, so its 'year' is shorter than two Earth weeks.
- Its star is too faint to see with the naked eye, despite being one of the nearest stars to the Sun.