Today, there are more than 5,000 known exoplanets, ranging from gas giants to small rocky worlds. And perhaps most excitingly, astronomers have now found about a dozen exoplanets that are likely rocky and orbiting within the habitable zones of their respective stars.\n\nAstronomers have even discovered a few systems like TOI 700 (101.4 light years) that have more than one planet orbiting in the habitable zone of their star. We call these keystone systems.\n\nTOI 700 first made headlines when our team announced the discovery of three small planets orbiting the star in early 2020. Using a combination of observations from NASA s Transiting Exoplanet Surveying Satellite mission and the Spitzer Space Telescope we discovered these planets by measuring small dips in the amount of light coming from TOI 700. These dips in light are caused by planets passing in front of the small, cool, red dwarf star at the center of the system.\n\nBy taking precise measurements of the changes in light, we were able to determine that at least three small planets are in the TOI 700 system, with hints of a possible fourth. We could also determine that the third planet from the star, TOI 700 d, orbits within its star s habitable zone, where the temperature of the planet s surface could allow for liquid water.\n\nThe Transiting Exoplanet Surveying Satellite observed TOI 700 for another year, from July 2020 through May 2021, and using these observations our team found the fourth planet, TOI 700 e. TOI 700 e is 95 percent the size of Earth and, much to our surprise, orbits on the inner edge of the star s habitable zone, between planets c and d. Our discovery of this planet makes TOI 700 one of only a few known systems with two Earth sized planets orbiting in the habitable zone of their star. The fact that it is relatively close to Earth also makes it one of the most accessible systems in terms of future characterization.
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