TOI-421 b
TOI-421 b is a mini-Neptune orbiting the G9 V star TOI-421 in the constellation Lepus. It lies about 244 light-years from Earth and was discovered in 2020 using the transit method.
How Big Is TOI-421 b?
TOI-421 b has a radius of 2.64 times that of Earth. Its mass is 6.7 times that of Earth, giving it a density of 2.04 g/cm³ — between that of rocky and gaseous planets.
Is TOI-421 b in the Habitable Zone?
TOI-421 b orbits inside the inner edge of the habitable zone of TOI-421. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of TOI-421: 0.560–1.340 AU (conservative: 0.710–1.271 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on TOI-421 b
The equilibrium temperature of TOI-421 b is about 922 K (649 °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 TOI-421 b — one full orbit around TOI-421 — lasts 5.20 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.055 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.13).
How Was TOI-421 b Discovered?
TOI-421 b was discovered in 2020 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 TOI-421 b?
TOI-421 b is 244.0 light-years (74.8 parsecs) from Earth. Light arriving here tonight left the planet around the year 1782. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,294,400 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. TOI-421 b scores 0.23, ranking #3,510 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: TOI-421
TOI-421 b belongs to a system of 2 stars; it orbits TOI-421.
TOI-421
- Spectral type
- G9 V
- Surface temperature
- 5,291 K (Sun: 5,772 K)
- Mass
- 0.83 M☉
- Radius
- 0.87 R☉
- Age
- 10.9 billion years (Sun: 4.6)
The TOI-421 Planetary System
TOI-421 b is one of 2 known planets in the TOI-421 system. Its siblings:
- TOI-421 c (Neptune-like)
TOI-421 b — Complete Data
| Radius | 2.640 Earth radii (0.236 Jupiter radii) |
|---|---|
| Mass | 6.70 Earth masses (0.021 Jupiter masses) |
| Density | 2.04 g/cm³ (Earth: 5.51) |
| Orbital period | 5.20 days |
| Orbital distance | 0.055 AU |
| Eccentricity | 0.130 |
| Equilibrium temperature | 922 K (649 °C) |
| Earth Similarity Index | 0.23 |
| Distance from Earth | 244.0 light-years (74.8 parsecs) |
| Constellation | Lepus |
| Discovery method | Transit |
| Discovery facility | Transiting Exoplanet Survey Satellite (TESS) |
| Discovery year | 2020 |
Data: NASA Exoplanet Archive, last updated 2024-08-07. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About TOI-421 b
Is TOI-421 b habitable?
No — TOI-421 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is TOI-421 b?
TOI-421 b is about 244 light-years from Earth in the constellation Lepus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,294,400 years to get there.
How big is TOI-421 b compared to Earth?
TOI-421 b has 2.64 times the radius of Earth and about 6.7 times its mass.
How long is a year on TOI-421 b?
One orbit around TOI-421 takes 5.2 Earth days — short enough that 70 of its years would fit into one Earth year.
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