TESS's first Earth-sized habitable-zone planet
TOI-700 d
Orbits TOI-700 · 101 light-years · discovered 2020 (transit)
TOI-700 d is an Earth-sized planet orbiting a small, cool red dwarf about 101 light-years away in the southern constellation Dorado. Announced in January 2020, it was the first Earth-sized planet in a star's habitable zone found by NASA's Transiting Exoplanet Survey Satellite (TESS).
It receives a little less energy from its star than Earth does from the Sun, and its host appears relatively calm for a red dwarf. Those are encouraging signs, but nobody knows whether TOI-700 d has an atmosphere, let alone water. Its mass has not been measured either.
Key facts
- Star
- TOI-700M2V red dwarf
- Distance
- 101 light-years
- Radius
- 1.07 × Earth
- Mass
- Not measured
- Orbital period
- 37.4 days
- Equilibrium temperature
- 269 K (-4 °C)Before any greenhouse warming
- Starlight received
- 0.87× Earth's
- Surface gravity
- ≈ 1.12 gRough estimate (rocky composition assumed)
- Discovered
- 2020Transit
- Star
- TOI-700 (M2V red dwarf)
- Distance
- About 101 light-years
- Discovered
- 2020, transit (TESS)
- Radius
- About 1.1 Earth radii
- Mass
- Not measured — estimated at roughly 1.5–2 Earth masses if rocky
- Orbital period
- About 37.4 days
- Starlight received
- About 87% of what Earth gets from the Sun
- Equilibrium temperature
- About 269 K (−4 °C), a simple estimate that ignores any atmosphere
- Constellation
- Dorado
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
TOI-700 d was found from the small, regular dips in brightness as it passes in front of its star. Those transits give its size — roughly 1.1 times Earth's radius — and its 37-day orbit.
Its mass has not been measured, because the star's wobble is too small for current instruments to detect easily. Planets of this size are usually rocky, so a rocky composition is likely, but not confirmed.
Its star
TOI-700 is a red dwarf with about 40% of the Sun's mass and size, and about 60% of its surface temperature. TESS saw no flares from it during its first year of monitoring, and the star rotates slowly, both signs of a relatively old and quiet star. That calm matters, because strong flares are one of the main threats to atmospheres around red dwarfs.
Could it be habitable?
TOI-700 d lies within the conservative habitable zone of its star. Climate models published alongside its discovery explored many possible versions of the planet — from ocean-covered worlds to dry ones with thick carbon dioxide atmospheres — and found that several could keep at least some surface water liquid.
Those simulations are possibilities, not findings. The planet has no detected atmosphere, and it is too far away and too small for JWST to analyse easily. Its quiet star and Earth-like energy budget make it promising, but habitability is unconfirmed.
Orbit and climate
The planet orbits roughly 0.16 astronomical units from its star — about 24 million kilometres — and completes a year in about 37 days. At that distance it is expected to be tidally locked, with one side in permanent daylight. A thick atmosphere would help spread heat to the night side; without one, the contrast would be severe.
Discovery
TESS observed TOI-700 for most of its first year, because the star lies near the patch of sky the spacecraft watched continuously. Astronomers initially catalogued the star as hotter and larger, which made its planets seem bigger too. After correcting the star's properties, a team led by Emily Gilbert identified three planets, b, c and d. NASA's Spitzer Space Telescope later confirmed TOI-700 d's transit.
TOI-700 e (2023)
Additional TESS data revealed a fourth planet, TOI-700 e, about 95% of Earth's size, orbiting in the optimistic part of the habitable zone on a 28-day orbit.
Future observations
Measuring TOI-700 d's mass with precise radial velocity instruments would reveal its density and help test whether it is rocky. Studying its atmosphere is a much harder task that may have to wait for future telescopes larger than JWST.
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 TOI-700 d
- TOI-700 is one of the few known systems with two roughly Earth-sized planets in its habitable zone.
- The planet's size briefly seemed much larger until astronomers realised its star had been misclassified in the original catalogue.
- TOI-700 d's star lies near the south ecliptic pole, a region TESS watched almost continuously during its first year.
- Its orbit of about 37 days is the longest of TOI-700's four known planets.