First Habitable Earth Twin Found by NASA's Transiting Exoplanet Survey Satellite (TESS)

NASA's planetary hunter: The Transiting Exoplanet Survey Satellite (TESS) searches for life-friendly exoplanets around nearby stars - and now it has landed a direct hit. 

First Habitable Earth Twin Found.

Because the space telescope has discovered an earth twin in the habitable zone of a star at a distance of only 100 light years. The planet, named TOI-700 d, is about 20 percent larger than Earth and probably has a harmless, life-friendly climate, as astronomers report. Moreover, the parent star of this Earth twin is rather quiet and shows no explosive eruptions - which is another favourable factor for the development of life.

TESS Mission's First Earth-size World in Star's Habitable-zone, picture.
TESS Mission's First Earth-size World in Star's Habitable-zone

Earth twin found

Since April 2018, the Transiting Exoplanet Survey Satellite (TESS) has been orbiting the Earth. Its mission: The space telescope directs its view to around 200,000 sun-like stars in a radius of around 300 light years around our sun and searches for clues about exoplanets.

To do this, TESS monitors the brightness of the stars in a sector of its observation area for 27 days at a time and can thus detect the slight shadows that occur when a planet passes in front of its star. 

"TESS was specifically designed and launched to find Earth-sized planets in orbit around nearby stars," explains Paul Hertz of NASA. "Because such nearby exoplanets are then best studied further with larger telescopes in space and on Earth." 

In recent months, TESS has already made some promising discoveries, including a potentially life-friendly Super Earth only 31 light years away.

TESS Mission's First Earth-size World in Star's Habitable-zone, nasa science.
A final set of transits showed TOI 700 d orbiting even farther out. TESS observed this system for nearly 11 months and saw each planet transit multiple times. 

But what was missing until now was the proof of a real earth twin - an earth-sized planet in the habitable zone of its star. With the help of TESS, astronomers have now succeeded in doing just that. Astronomers found evidence of this by examining the red dwarf star TOI-700, which is about 100 light years away from us, has about 40 percent of the mass of the Sun and is only half as hot as the Sun. 

The space telescope TESS had already repeatedly targeted this star in the first months of its observation period and detected transits of three planets. However, there was some uncertainty about the sizes of the planets and the conditions that exist on these planets. Because the star was originally thought to be similar to the Sun, all three planets were considered too hot to be life-friendly.

But new observations by the TESS telescope have now refuted this and identified the star TOI-700 as a red dwarf. "When we corrected the star parameters, it also reduced the sizes of the planets," says Emily Gilbert of NASA Goddard Space Flight Center and the University of Chicago. 

TOI 700 d is within the star’s habitable zone.

  • All three may be tidally locked, rotating just once each orbit, so the same side always faces the star. But most importantly, TOI 700 d is within the star’s habitable zone.

According to this, the innermost planet, TOI-700 b, is about the size of the Earth, while the middle one is probably a gaseous Sub-Neptune. But the third, outermost planet of this system is interesting: "We realized that the outermost planet is about the size of the Earth and orbits in the habitable zone of the star," says Gilbert. 

According to the new data, this planet, TOI-700 d , is about 20 percent larger than Earth and takes about 37 days to orbit around its star. In the process, it receives about 86 percent of the radiation energy from its star that Earth receives from the Sun, the researchers report. The probably Earth-like planet could thus have a moderate, potentially life-friendly climate.

What conditions prevail on TOI-700 d?

Closer observations of the planetary system, including the NASA Spitzer Space Telescope, confirmed the existence of the planet TOI-700 d and also its orbit. "Given the importance of this discovery - it is the first Earth-sized life-friendly planet discovered by TESS - we wanted to know as much as possible about this system," explains Joseph Rodriguez of the Harvard Smithsonian Center for Astrophysics. The exoplanet TOI-700 d is only the tenth known Earth-sized planet in the habitable zone of its star, as astronomers explain. 

Other examples include the seven Earth twins around the red dwarf TRAPPIST-1, 40 light years away, but unlike them, the newly discovered planet has one major advantage: Unlike most other red dwarfs, TOI-700 does not appear to have any explosive eruptions, so its planets do not have to withstand constant bursts of hard radiation. 

"In the eleven months of observation, we have not seen any radiation bursts from the star," reports Gilbert. "This increases the chance that TOI-700 d is life-friendly and also makes it easier to model atmospheric and surface conditions."

However, there is an important difference compared to Earth: From their data, astronomers conclude that the three planets around TOI-700 probably orbit their star in bound rotation. They always turn the same side towards it, so that on one hemisphere it is always daytime and on the other night. In another study, astronomers have investigated how this could affect the climate of the Earth twin TOI-700d. 

They simulated the evolution of this planet as an ocean planet and as a desert planet dominated by land. According to these data, the planet as an ocean world could have a dense atmosphere rich in carbon dioxide and dense clouds on the side facing the star - roughly comparable to Mars during its early, still life-friendly period. In any case, it would be warm enough to sustain liquid water in the long term and thus provide favourable conditions for life, the researchers report. But if the planet were rather dry, its temperatures would probably be closer to freezing point.

Astronomers hope to find out what conditions actually prevail on this twin Earth with the help of future observations through high-resolution telescopes such as NASA's James Webb Space Telescope, which will be launched in 2021. 

"Once we have real spectral data from TOI-700 d, we can compare it with our model simulations and find out which scenario fits," says Gabrielle Engelmann-Suissa of NASA Goddard Space Flight Center. 

The researchers presented their results at the annual meeting of the American Astronomical Society in Honolulu and have already submitted three papers to professional journals.

All three may be tidally locked, rotating just once each orbit, so the same side always faces the star. But most importantly, TOI 700 d is within the star’s habitable zone.

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