TESS finds the first habitable earth twin

TOI-700 d

This is how the life-friendly, earth-sized Exoplanet TOI-700 d could look. (Image: NASA / Goddard Space Flight Center)

He is the new NASA planet hunter: The Transiting Exoplanet Survey Satellite (TESS) is specifically looking for life-friendly exoplanets for nearby stars – and now it has hit the bull’s eye. Because the space telescope has discovered an Earth twin in the habitable zone of a star just 100 light years away. The TOI-700 d baptized planet is around 20 percent larger than Earth and probably has a mild, life-friendly climate, as astronomers report. The home star of this earth twin is also rather calm and shows no explosive outbreaks – this is also beneficial for the development of life.

The Transiting Exoplanet Survey Satellite (TESS) has been orbiting Earth since April 2018. His mission: The space telescope looks at around 200,000 sun-like stars within a radius of around 300 light years around our sun and looks for clues to exoplanets. For this purpose, TESS monitors the brightness of the stars in a sector of its observation area for 27 days each time and can thus detect the slight shadowing that occurs when a planet passes in front of its star. “TESS was specially developed and launched to find Earth-sized planets in orbit around nearby stars,” explains Paul Hertz from NASA. “Because such close exoplanets can then best be examined further with larger telescopes in space and on Earth.” In the past few months, TESS has already made some promising discoveries, including a potentially life-friendly super-earth just 31 light years away.

Outermost planet is Earth-sized and in the habitable zone

What was still missing was the proof of a real Earth twin – an Earth-sized planet in the habitable zone of its star. This is exactly what astronomers have now achieved with the help of TESS. It found what it was looking for at the red dwarf star TOI-700, which was about 100 light years away from us. This star has about 40 percent of the Sun’s mass and is only half as hot as the Sun. The TESS space telescope had repeatedly targeted this star in the first few months of its observation time and detected transits from three planets. However, there was uncertainty about the planet sizes and the conditions that prevail on this planet. Because the star was originally thought to be sun-like, 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 size of the planets,” reports Emily Gilbert of NASA Goddard Space Flight Center and the University of Chicago. Accordingly, 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 part of this system is exciting: “We realized that the outermost planet is roughly 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 around 20 percent larger than Earth and takes around 37 days to orbit its star. It receives about 86 percent of the radiation energy from its star, which the earth receives from the sun, as the researchers report. The probably earth-like planet could therefore have a mild, potentially life-friendly climate.

What are the conditions on TOI-700 d?

Closer observations of the planetary system, including with 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,” said 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 representatives include the seven earth twins around the red dwarf TRAPPIST-1, 40 light years away. In contrast to these, however, the newly discovered planet has a great advantage: In contrast to most other red dwarfs, TOI-700 does not appear to show any explosive outbreaks, so that its planets do not have to constantly withstand bursts of hard radiation. “In the eleven months of observation, we saw no outbursts of rays from the star,” Gilbert reports. “This increases the chance that TOI-700 d is life-friendly and also makes it easier to model the conditions of the atmosphere and the surface.”

However, there is one important difference from Earth: from their data, the astronomers conclude that the three planets around TOI-700 are likely to orbit in orbit around their star. They always turn to the same side so that day is constant in one hemisphere, and night is constant in the other. In a further study, astronomers have investigated how this could affect the climate of the Earth’s twin TOI-700 d. To do this, they simulated the development of this planet as an ocean planet and once as a desert planet dominated by land. According to this data, the planet as an ocean world could have a dense, carbon dioxide-rich atmosphere and dense clouds on the star-facing side – comparable to Mars during its early, still life-friendly period. In any case, it would be warm enough to maintain liquid water in the long term and thus favorable conditions for life, as the researchers report. If the planet were rather poor in water, its temperatures would probably be closer to freezing.

The astronomers hope to find out what conditions actually exist on this Earth twin with the help of future observations through higher-resolution telescopes such as NASA’s James Webb space telescope, which will start in 2021. “If we then have real spectral data from TOI-700 d, we can compare it with our model simulations and find out which scenario fits,” says Gabrielle Englemann-Suissa from NASA Goddard Space Flieght Center. The researchers presented their results at the annual meeting of the American Astronomical Society in Honolulu and have already submitted three specialist publications to specialist journals.

Source: NASA; 235th meeting of the American Astronomical Society

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