"This intriguing, distant world gives us even greater hope that a second Earth lies among the stars, waiting to be found," said an associate administrator at NASA.
"'Are we alone?' is one of the biggest unanswered questions," said study first author Howard Chen.
The exoplanet is 110 light-years from Earth and was discovered by NASA's Kepler spacecraft in 2015.
Build-ups of toxic gases in the atmospheres of many exoplanets in the "habitable zone" could limit the "safe zone" in which life outside our solar system could actually exist.
TESS will scour the skies for signs of undiscovered worlds.
"It's just completely bizarre that we should have two planets the same size but opposite ends of the habitability spectrum."
The ultra-cool dwarf star TRAPPIST-1 has seven roughly Earth-sized planets orbiting it, with three in the star's "habitable zone."