# TOI-700

TOI-700 is a red dwarf 101.4 light-years away from Earth located in the Dorado constellation that hosts TOI-700 d, the first Earth-sized exoplanet in the habitable zone discovered by the Transiting Exoplanet Survey Satellite (TESS).

## Nomenclature and history

The acronym "TOI" refers to stars and exoplanets studied by TESS, and is short for: "Transiting Exoplanet Survey Satellite Object of Interest".

## Stellar characteristics

TOI-700 is a red dwarf of spectral class M (much redder, cooler, and dimmer than the sun) that is 40% the mass, 40% the radius and 55% of the temperature of the Sun. The star is bright with low levels of stellar activity. Over the 11 sectors observed with TESS, the star does not show a single white-light flare. The low rotation rate is also an indicator of low stellar activity.

## Planetary system

The TOI-700 planetary system :
| Companion (in order from star) | Mass | Semimajor axis (AU) | Orbital period (days) | Eccentricity | Inclination | Radius |
| b | <0.74 M$_🜨$ | 0.0677±0.0011 | 9.977203$^{+0.000019}$ $_{−0.000016}$ | 0.040+0.44 −0.025 | 89.61+0.27 −0.31° | 0.944+0.050 −0.049 R$_🜨$ |
| c | 2.50±0.34 M$_🜨$ | 0.0916+0.0055 −0.0057 | 16.0510951$^{+0.0000075}$ $_{−0.0000082}$ | 0.041+0.041 −0.026 | 88.873+0.062 −0.083° | 2.65+0.14 −0.15 R$_🜨$ |
| e | <1.16 M$_🜨$ | 0.1321+0.0079 −0.0082 | 27.80977$^{+0.00018}$ $_{−0.00017}$ | 0.051+0.055 −0.033 | 89.60+0.15 −0.11° | 0.931±0.066 R$_🜨$ |
| d | 2.40+0.49 −0.52 M$_🜨$ | 0.1610+0.0097 −0.0099 | 37.42343$^{+0.00021}$ $_{−0.00013}$ | 0.047+0.054 −0.030 | 89.82+0.12 −0.13° | 1.156+0.064 −0.063 R$_🜨$ |
Four exoplanets have been detected by TESS to be orbiting the host star TOI-700. All four exoplanets may be tidally locked to TOI-700.
Three papers describe the validation of the planetary system, the follow-up observations of TOI-700 d with the Spitzer Space Telescope and the characterization of TOI-700 d.
The composition of planets b and d is more likely rocky and the composition of planet c is more likely similar to that of Neptune.
The two inner planets might have grown faster and accreted significant gaseous envelopes, but the outer planet formed more slowly and accreted less gas. The innermost planet may later have lost its envelope due to photoevaporation. Another scenario that could explain the arrangement of densities in this system is long-term planetary migration. Planet c might have migrated inwards, but this scenario is more plausible if future studies show that planet c is significantly more massive than planet b or d.

### Potentially habitable planets
#### TOI-700 d

TOI-700 d lies in the habitable zone. It receives 35 times more EUV photons than Earth, but also 50 times less than TRAPPIST-1 e. The host star has low stellar activity. The atmosphere of a planet with an Earth-like pressure would survive for longer than 1 Gyr. Simulations of the planet have shown that TOI-700 d is a robust candidate for a habitable world. The simulated spectral feature depths from transmission spectra and the peak flux and variations from synthesized phase curves do not exceed 10 ppm. This will likely prohibit JWST from characterizing the atmosphere of TOI-700 d.

#### TOI-700 e

In November 2021, a fourth possible planet, Earth-sized and receiving approximately 30% more flux from TOI-700 than Earth does from the Sun, was found at the inner edge of the habitable zone of TOI-700. In January 2023 the existence of this planet, designated 700 e, was confirmed.
Discovered in 2023, TOI-700 e is a terrestrial exoplanet that NASA claims to be an "earth-like" planet, with 95 percent of the Earth’s radius. Discovered by NASA's TESS (Transitioning Exoplanet Survey Satellite), TOI-700 e has a mass of about 0.818 Earths and takes 27.8 days to orbit once around its star. The planet is in a habitable zone distance from the M-type star TOI-700 it orbits, leading NASA scientists to believe that there is potential for liquid water on its surface. Ten percent smaller than its neighboring planet TOI-700 d, both are at a distance from their sun to be considered habitable, however, TESS requires an additional year to acquire more data about the exoplanets. Being one in only about a dozen habitable zone planets known, further research and data collection of the TOI-700 solar system is important for understanding Earth-like planets.

### Near orbital resonance

The system is near (but not in) orbital resonance: from planets b to d, period ratios are approximately 5:8, 4:7, 3:4.