HD 21749 c
HD 21749 c is a terrestrial planet orbiting the K4.5 star GJ 143 in the constellation Reticulum. It lies about 53 light-years from Earth and was discovered in 2019 using the transit method.
How Big Is HD 21749 c?
HD 21749 c has a radius of 0.89 times that of Earth. Its mass is 3.7 times that of Earth, giving it a density of 31.93 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is HD 21749 c in the Habitable Zone?
HD 21749 c orbits inside the inner edge of the habitable zone of GJ 143. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of GJ 143: 0.361–0.886 AU (conservative: 0.457–0.840 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 21749 c
The equilibrium temperature of HD 21749 c is about 701 K (428 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on HD 21749 c — one full orbit around GJ 143 — lasts 7.79 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.070 AU — closer to its star than Mercury is to the Sun.
How Was HD 21749 c Discovered?
HD 21749 c was discovered in 2019 using the transit method, with observations from Transiting Exoplanet Survey Satellite (TESS).
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is HD 21749 c?
HD 21749 c is 53.2 light-years (16.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1973. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 936,320 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. HD 21749 c scores 0.32, ranking #980 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: GJ 143
GJ 143
- Spectral type
- K4.5
- Surface temperature
- 4,640 K (Sun: 5,772 K)
- Mass
- 0.73 M☉
- Radius
- 0.70 R☉
- Luminosity
- 0.2060 L☉
- Age
- 3.8 billion years (Sun: 4.6)
The GJ 143 Planetary System
HD 21749 c is one of 2 known planets in the GJ 143 system. Its siblings:
- GJ 143 b (Super Earth)
HD 21749 c — Complete Data
| Radius | 0.892 Earth radii (0.080 Jupiter radii) |
|---|---|
| Mass | 3.70 Earth masses (0.012 Jupiter masses) |
| Density | 31.93 g/cm³ (Earth: 5.51) |
| Orbital period | 7.79 days |
| Orbital distance | 0.070 AU |
| Equilibrium temperature | 701 K (428 °C) |
| Earth Similarity Index | 0.32 |
| Distance from Earth | 53.2 light-years (16.3 parsecs) |
| Constellation | Reticulum |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2019 |
Data: NASA Exoplanet Archive, last updated 2019-04-22. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 21749 c
Is HD 21749 c habitable?
No — HD 21749 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 21749 c?
HD 21749 c is about 53 light-years from Earth in the constellation Reticulum. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 936,320 years to get there.
How big is HD 21749 c compared to Earth?
HD 21749 c has 0.89 times the radius of Earth and about 3.7 times its mass.
How long is a year on HD 21749 c?
One orbit around GJ 143 takes 7.8 Earth days — short enough that 47 of its years would fit into one Earth year.
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