NASA's TESS space observatory has located its first planet the size of the Earth in the habitable zone of its star, the range of distances compatible with liquid surface water. The scientists confirmed the finding, called TOI 700 d, using NASA's Spitzer Space Telescope and modeled the potential environments of the planet to help inform future observations. TOI 700 d is one of the few planets the size of the Earth discovered so far in the habitable zone of a star. Others include several planets in the TRAPPIST-1 system and other worlds discovered by NASA's Kepler space telescope. TESS (Transiting Exoplanet Survey Satellite) monitors large swaths of the sky, called sectors, for 27 days at a time. This long look allows the satellite to track changes in stellar brightness caused by an orbiting planet that crosses in front of its star from our perspective, an event called transit. A HUNDRED YEARS LIGHT TOI 700 is a small and cold M dwarf star located just over 100 light years away in the southern Golden constellation. It is approximately 40% of the mass and size of the Sun and approximately half of its surface temperature. The star appears in 11 of the 13 sectors that TESS observed during the first year of the mission, and the scientists took multiple transits on their three planets. The star was originally mistakenly classified in the TESS database as more similar to our Sun, which meant that the planets seemed larger and hotter than they really are. Several investigators, including Alton Spencer, a high school student who works with members of the TESS team, identified the error. "When we corrected the parameters of the star, the sizes of its planets fell, and we realized that the outermost one was about the size of the Earth and in the habitable zone," said Emily Gilbert, a graduate student of the University of Chicago. "In addition, in 11 months of data we did not see flashes of the star, which improves the chances that TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions." Gilbert and other researchers presented the findings at the 235th meeting of the American Astronomical Society in Honolulu, and three documents, one of which Gilbert directed, have been sent to scientific journals. The innermost planet, called TOI 700 b, is almost exactly the size of the Earth, is probably rocky and completes an orbit every 10 days. The central planet, TOI 700 c, is 2.6 times larger than the Earth, between the sizes of the Earth and Neptune, orbits every 16 days and is likely to be a world dominated by gases. TOI 700 d, the outermost planet known in the system and the only one in the habitable zone, measures 20% larger than Earth, orbits every 37 days and receives from its star 86% of the energy that the Sun provides to the earth. It is believed that all planets are mareally blocked by their star, which means that they rotate once through orbit so that one side is constantly bathed in daylight. Because TOI 700 is bright, close and shows no signs of stellar flashes, the system is an ideal candidate for accurate mass measurements by current ground observatories. These measurements could confirm the scientists' estimates that the inner and outer planets are rocky and that the central planet is made of gas.