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[Music throughout] Narrator: NASA’s TESS mission has found its first

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Earth-size world in its star’s habitable zone. This means 

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the planet, called TOI 700 d, has the potential for liquid 

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water on its surface. TESS stares at patches of sky

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for long stretches, recording light from thousands of stars.

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Some of these stars have planets that cross, or transit, in front of them.

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TESS sees these events as tiny, regular dimmings of the

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host stars. One star where TESS saw transits is

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TOI 700. It’s a red dwarf about 40% the mass

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and size of our Sun and roughly half its temperature.

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One set of transits announced the presence of a planet close to the star,

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called TOI 700 b. Another set

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revealed a second planet, named TOI 700 c, a little farther out.

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The deeper, shorter transit means the planet is larger than the first,

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and the plane of its orbit is slightly tipped. A final set

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of transits showed TOI 700 d orbiting even farther out.

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TESS observed this system for nearly 11 months

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and saw each planet transit multiple times. Scientists

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determined that the inner and outer planets are almost Earth-size and may be

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rocky. The middle world is more than twice as large and most likely

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made of gas. All three may be tidally locked, rotating

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just once each orbit, so the same side always faces the star.

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But most importantly, TOI 700 d is 

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within the star’s habitable zone. Scientists wanted

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independent confirmation of TOI 700 d, so they monitored

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its star with NASA’s Spitzer Space Telescope. Spitzer saw a clear

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transit from the outer planet, affirming its existence and improving

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scientists’ certainty of the planet’s size. 

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TOI 700 d is one of only a few Earth-size planets found in

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potential habitable zones. Others include discoveries by Kepler

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and several planets in the TRAPPIST-1 system.

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Because TOI 700 is bright and nearby, the planets are good

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candidates for precise mass measurements by ground-based telescopes.

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Future missions may also tell us if the worlds have atmospheres.

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But scientists need to know what kinds of signals to look for.

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Researchers at NASA’s Goddard Space Flight Center created models of the planet

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to explore its potential conditions. One version is a 

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water-covered world with an atmosphere similar to early Mars but denser.

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Another looks like a completely dry version of today’s Earth.

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Both models have vastly different surface temperatures.

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Light passing through their atmospheres creates distinct signals

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because different molecules are present. By simulating

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these data now, scientists can make predictions for real future observations

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and narrow the range of TOI 700 d’s possible

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conditions. We still have much to learn about the TOI

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700 system, but thanks to TESS, Spitzer, and the work of

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many scientists, we’re beginning to form a picture of its exciting

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new worlds. [Explore: Solar system & beyond]

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[NASA]