Confirmed planetTOI-700 system

TOI-700 c

A sub-Neptune caught between the rocky worlds of TOI-700: too large and gas-wrapped to be an Earth twin.

Transit detection · 16.05-day orbit · 101.5 light-years away

Orbital period
16.05 days
Distance from the star
0.0929 AU · 13.9 million km
Radius
2.60 Earth radii
Mass
Not yet measured
Equilibrium temperature
About 356 K without an atmosphere
Discovery
2020 · transit
Host star
M2.5V red dwarf · about 0.42 solar masses
Distance from the Sun
101.5 light-years

A sub-Neptune in a rocky family

TOI-700 c is about 2.6 times Earth's radius, placing it in the sub-Neptune class: worlds larger than rocky super-Earths but smaller than Neptune, likely wrapped in a thick hydrogen-rich atmosphere. It sits between the system's small rocky planets.

Its mass is unknown, so its density and exact composition are uncertain. But at this size, a substantial gas envelope is the standard picture, meaning it has no solid surface in the way Earth does.

A bridge between two kinds of planet

TOI-700 is a rare laboratory because it pairs a sub-Neptune with rocky, habitable-zone worlds around the same star. That lets astronomers compare how planets of very different sizes form, evolve, and hold onto their atmospheres under identical starlight.

With an equilibrium temperature around 356 K, c is warmer than Earth but cooler than the innermost rock. Future mass measurements would reveal how much gas it carries and help anchor models of sub-Neptune interiors.

What we don't know

Its mass, atmosphere, and interior are all unmeasured. Radial-velocity observations may eventually weigh it, and transit spectroscopy could probe the composition of its gas envelope.

  • How massive is it, and what is its bulk composition?
  • What gases dominate its atmosphere?
  • How did it form among so many smaller, rocky planets?

Key publications

YearPaperVenueLink
2020Gilbert et al., The First Habitable-zone Earth-sized Planet from TESS. I. Validation of the TOI-700 SystemThe Astronomical Journal 160, 116DOI
2020Rodriguez et al., The First Habitable-zone Earth-sized Planet from TESS. II. Spitzer Confirms TOI-700 dThe Astronomical Journal 160, 117DOI

Values compiled from the NASA Exoplanet Archive and the papers listed above.

Worth knowing

  • It is about 2.6 times Earth's radius.

  • It is the only non-rocky planet in the TOI-700 system.

  • Its year lasts about 16 Earth days.