TOI-700 d

Lore & Background

It has been estimated that the planet receives about 88% the energy that the Earth receives from the Sun. The planet's high density indicates a rocky composition with a higher fraction of iron than Earth and may be explained by high-energy giant impact events during its formation. The host star TOI-700 is a red dwarf of spectral class M that is about 40% the mass and radius, and very roughly 50% of the temperature of the Sun. The star is bright with low levels of stellar activity, showing no single white-light flare over the 11 sectors observed with TESS. The low rotation rate is also an indicator of low stellar activity. TOI-700 d was discovered by a team of astronomers led by Emily Gilbert using TESS in early January 2020. The solar wind ram pressure and intensity of the interplanetary magnetic field are expected to be similar to Earth's, therefore retention of the planetary atmosphere is likely. The presence of an extended hydrogen/helium envelope on TOI-700c indicates the star's high energy emission was insufficient to strip its atmosphere, suggesting TOI-700d may have been able to maintain a secondary high mean molecular weight atmosphere.

Reader's Guide

TOI-700 d is significant as the first Earth-sized exoplanet found in the habitable zone by the Transiting Exoplanet Survey Satellite (TESS), marking a milestone in the search for potentially habitable worlds. Its discovery on 3 January 2020 demonstrated TESS's capability to detect small, rocky planets around nearby red dwarfs. The planet's high density and iron-rich composition provide clues about planetary formation processes, including possible giant impacts. Its location in the habitable zone, combined with low stellar activity from its host star, makes it a prime target for future atmospheric studies. The star's low flare activity and the inferred retention of atmospheres on other planets in the system increase the likelihood that TOI-700 d may retain a secondary atmosphere, offering a valuable analog for understanding exoplanet habitability around M dwarfs.

Did You Know?

• TOI-700 d is the first Earth-sized exoplanet in the habitable zone discovered by TESS.
• The planet has a density of about 8.5 g/cm³, indicating a higher fraction of iron than Earth.
• The host star TOI-700 showed no white-light flares over 11 TESS sectors.

A Milestone in Exoplanet Discovery

On the third of January 2020, a team of astronomers led by Emily Gilbert announced the detection of TOI-700 d, marking a watershed moment in the ongoing search for potentially life-friendly worlds beyond our solar system. The planet was identified using NASA's Transiting Exoplanet Survey Satellite, and it carries the distinction of being the first Earth-sized exoplanet confirmed within a habitable zone by that particular mission. By demonstrating that a world of roughly Earth's dimensions can occupy the temperate band around a red dwarf, this finding opened a new chapter in understanding how common potentially hospitable rocky planets might be throughout our galaxy. The result also served as a strong validation of TESS's broader survey strategy of scanning wide swaths of the sky for small planets around nearby stars, giving the astronomical community renewed confidence that the next generation of discoveries would yield even more candidates for further study.

A Dense, Iron-Rich World

TOI-700 d is a compact yet remarkably massive rocky world. Its radius measures approximately 1.16 to 1.19 times that of Earth, making it only modestly larger than our home planet. Yet its mass is estimated at roughly 2.40 Earth masses, yielding a density of about 8.5 grams per cubic centimeter. This elevated density points to a composition rich in iron, with a higher iron fraction than Earth itself. Researchers have suggested that such a composition could be the legacy of violent high-energy giant impact events during the planet's early formation, which may have stripped away lighter material and left behind a denser, more metal-rich body. If TOI-700 d harbors an atmosphere, its actual surface temperature would likely be warmer than this bare-rock baseline. A small but non-negligible risk of a runaway greenhouse effect has also been noted, though the overall thermal environment remains within a range that does not preclude liquid water on the surface.

Orbiting a Quiet Red Dwarf

This orbital radius is less than half the distance between Mercury and our Sun. The planet completes one full orbit in approximately 37.42 days. Its host star is considerably smaller and cooler than the Sun, possessing about 40 percent of the Sun's mass and radius and roughly 50 percent of its surface temperature. Despite being a red dwarf, TOI-700 is notably quiet. Across eleven observing sectors recorded by TESS, the star did not produce a single white-light flare, and its slow rotation rate further confirms low levels of stellar activity. This calm environment is significant because it means the planet is not subjected to the intense flaring and high-energy radiation that can erode planetary atmospheres. The combination of a stable, low-activity star and a moderate orbital distance allows TOI-700 d to receive about 88 percent of the stellar energy that Earth receives from the Sun, placing it comfortably within the habitable zone.

Prospects for a Retained Atmosphere

One of the most compelling aspects of TOI-700 d is the likelihood that it has been able to hold onto an atmosphere over billions of years. The solar wind ram pressure and the strength of the interplanetary magnetic field around this system are expected to be comparable to those experienced by Earth, which makes long-term atmospheric retention plausible. Further evidence comes from the inner planet TOI-700 c, which still possesses an extended hydrogen and helium envelope. The fact that this outer gaseous layer has survived indicates that the star's high-energy emission has not been powerful enough to strip away even a light, extended atmosphere. Because TOI-700 d receives less than half the insolation that TOI-700 c does, it is even less vulnerable to photoevaporation. This suggests the planet may have been able to maintain a secondary atmosphere of high mean molecular weight, the kind that could support stable surface conditions. While a small probability of a runaway greenhouse scenario cannot be entirely ruled out, the overall picture points toward a world where atmospheric conditions could remain favorable over geological timescales.