Announced on January 6, 2020, NASA’s TESS mission has just discovered its first Earth-sized, habitable zone planet.

The habitable zone is the range of distances from a star where liquid water might pool on the surface of an orbiting planet. If a planet is too close to its parent star, it will be too hot and water would have evaporated. If a planet is too far from a star it is too cold and water is frozen. Stars come in a wide variety of sizes, masses and temperatures. Stars that are smaller, cooler and lower mass than the Sun (M-dwarfs) have their habitable zone much closer to the star than the Sun (G-dwarf). (NASA/KEPLER MISSION/DANA BERRY)

If it has an Earth-like atmosphere, it could possess liquid water on its surface.

NASA’s TESS satellite surveys the entire sky in chunks that are approximately 12 degrees in radius, ranging from the galactic poles down to near the galactic equator. As a result of this surveying strategy, the polar regions see more observing time, making TESS more sensitive to smaller and more distant planets in those systems. (NASA/MIT/TESS)

TESS works by surveying different slivers of the sky one month at a time in succession.

An illustration of NASA’s TESS satellite and its capabilities of imaging transiting exoplanets. Kepler has given us more exoplanets than any other mission, but TESS has pushed us over the 4,000 mark. We are now using TESS to identify Earth-sized, potentially habitable candidates suitable for direct imaging and transit spectroscopy by James Webb and beyond. (NASA)

The polar areas receive the greatest coverage, and include the region where the star TOI 700 is located.

Owing to the long-term coverage provided by NASA’s TESS satellite, the star TOI 700 was observed to have three independently recurring flux dips, corresponding to the three innermost detectable planets of that system: TOI 700b, c, and d, with d representing an Earth-sized, potentially habitable world. (NASA’S GODDARD SPACE FLIGHT CENTER/CHRIS SMITH (USRA))

Appearing in 11 independently viewed TESS sectors, TOI 700 possesses at least 3 planets orbiting it.

An X-class solar flare erupted from the Sun’s surface in 2012. Around a large number of red dwarf stars like Proxima Centauri, however, flares are far more common, posing a danger of stripping the atmospheres away from any potentially habitable planets. Around TOI 700, however, which is right at the highest-end of the mass range of red dwarfs, flares may not occur with any more frequency than they do around our own Sun. (NASA/SOLAR DYNAMICS OBSERVATORY (SDO) VIA GETTY IMAGES)

Although TOI 700 is an M-class red dwarf, it exhibits no flaring, benefitting potential lifeforms.

The outermost planet in this particular system, TOI 700d, is only 19% larger than Earth, but causes a recurrent flux dip of approximately 0.05% in the star’s light with a ~37 day period. This translates to the planet receiving 86% of the energy that our Earth receives from the Sun. (NASA’S GODDARD SPACE FLIGHT CENTER/CHRIS SMITH (USRA))

The third planet from the star — TOI 700d — is only 19% larger than Earth, receiving 86% of the incident energy on Earth.

The best evidence-based classification scheme of planets is to categorize them as either rocky, Neptune-like, Jupiter-like or stellar-like. With a radius of 1.19 Earth radii, TOI 700d is very likely to be rocky, like Earth, rather than gaseous like a mini-Neptune, but a mass measurement will be required to know for certain. (CHEN AND KIPPING, 2016, VIA HTTPS://ARXIV.ORG/PDF/1603.08614V2.PDF)

Even though the planet is likely to be tidally locked, with the same face always towards its star, it’s still potentially habitable.

All inner planets in a red dwarf system will be tidally locked, with one side always facing the star and one always facing away, but with a ring of potential Earth-like habitability between the night and day sides. Even though these worlds are so different from our own, we have to ask the biggest question of all: could one of them still potentially be harboring life? (NASA/JPL-CALTECH)

Everything depends on the composition of the atmosphere and how energy flows around the world.

If TOI 700d were a wet, ocean-covered world with a rich CO2 atmosphere similar to early Mars, it would have winds that circled the planet and caused temperatures to be quite hot by terrestrial standards, but well below boiling. Life, however, might still be plausible. (ENGELMANN-SUISSA ET AL./NASA’S GODDARD SPACE FLIGHT CENTER)

A CO2-heavy atmosphere creates a uniformly hot world, where life is strongly disfavored.

If TOI 700d were a cloudless, dry-land planet with an atmosphere similar to modern Earth, there would be a ring of potential habitability with Earth-like temperatures and atmospheric pressures near the border between the eternal day/night sides, where the winds always flow from the night side to the day side. (ENGELMANN-SUISSA ET AL./NASA’S GODDARD SPACE FLIGHT CENTER)

For contrast, a cloudless, oceanless world with an Earth-like atmosphere possess sunward-directed winds and various temperature zones.

The Hubble Space Telescope (left) is our greatest flagship observatory in astrophysics history, but is much smaller and less powerful than the upcoming James Webb (center). However, in order to get the resolution and contrast necessary to determine the atmospheric contents of an Earth-sized planet around a M-class star like TOI 700 located ~100 light-years away, a more powerful telescope, such as the proposed LUVOIR observatory, will be necessary. (MATT MOUNTAIN / AURA)

Technologies beyond James Webb will be required to determine its true composition.