Ross 128 b
Ross 128 b is a terrestrial planet orbiting the M4 star Ross 128 in the constellation Virgo. It lies about 11 light-years from Earth and was discovered in 2017 using the radial velocity method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
Is Ross 128 b in the Habitable Zone?
Ross 128 b orbits within the optimistic habitable zone of Ross 128 — the broader region where liquid water might be possible under favorable atmospheric conditions.
Habitable zone of Ross 128: 0.049–0.130 AU (conservative: 0.063–0.123 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on Ross 128 b
The equilibrium temperature of Ross 128 b is about 301 K (28 °C) — in a range broadly comparable to Earth, whose equilibrium temperature is 255 K. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 1.38 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Ross 128 b — one full orbit around Ross 128 — lasts 9.87 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.050 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.12).
How Was Ross 128 b Discovered?
Ross 128 b was discovered in 2017 using the radial velocity method, with observations from La Silla Observatory.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is Ross 128 b?
Ross 128 b is 11.0 light-years (3.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 2015. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 193,600 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. Ross 128 b scores 0.86, ranking #12 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Ross 128
Ross 128
- Spectral type
- M4
- Surface temperature
- 3,192 K (Sun: 5,772 K)
- Mass
- 0.17 M☉
- Radius
- 0.20 R☉
- Luminosity
- 0.0036 L☉
- Age
- 5.0 billion years (Sun: 4.6)
Planetary System
Ross 128 b is the only planet known to orbit Ross 128 so far.
Ross 128 b — Complete Data
| Mass (best estimate) | 1.40 Earth masses |
|---|---|
| Orbital period | 9.87 days |
| Orbital distance | 0.050 AU |
| Eccentricity | 0.116 |
| Equilibrium temperature | 301 K (28 °C) |
| Stellar irradiation | 1.38× Earth |
| Earth Similarity Index | 0.86 |
| Distance from Earth | 11.0 light-years (3.4 parsecs) |
| Constellation | Virgo |
| Discovery method | Radial Velocity |
| Discovery facility | La Silla Observatory |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2017-11-16. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Ross 128 b
Is Ross 128 b habitable?
Ross 128 b orbits within the habitable zone of Ross 128, the region where liquid water could exist on a planet's surface. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is Ross 128 b?
Ross 128 b is about 11 light-years from Earth in the constellation Virgo. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 193,600 years to get there.
How long is a year on Ross 128 b?
One orbit around Ross 128 takes 9.9 Earth days — short enough that 37 of its years would fit into one Earth year.
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