TESS discovers planets during a month of stargazing, and during this month small fluctuations in brightness can indicate the transit of the planet between the star and the telescope. Based on these light fluctuations, astronomers can estimate the size of the planet and its orbit.
In 2020, Gilbert and his colleagues reported the discovery of three planets around a small star called TOI 700 (TOI stands for TESS Object of Interest), located 100 light-years from Earth. This star is of the red dwarf type, but unlike many of its counterparts, it is very quiet and shows no sign of the sudden and intense fluctuations in activity that destroy life on nearby planets. In a complete two-year data set from TESS, no trace of starbursts was seen, Gilbert said.
Two of the three planets discovered by TESS in orbit close to the star TOI 700 are very similar to Earth, but the world known as TOI 700 d is also very interesting. This world is 20% larger than the Earth and completes its star orbit every 37 Earth days, thus placing it in the life belt. The life belt is a point in the star’s orbit where surface liquid water can flow.
Scientists compare the TOI 700 system with the TRAPPIST-1 system, which has seven Earth-sized planets 39.5 light-years away. According to Gilbert, the comparison is interesting, but further investigation of TOI 700 will be much easier because the star TRAPPIST-1 is more active and fainter. Also, this system is very compact and all its planets are located at a close distance from each other.
Now Gilbert and his colleagues say TOI 700 d has a sibling. The planet, called TOI 700 e, is not entirely in the habitable zone, but the definition of the habitable zone is variable. For example, the planets Venus and Mars both once hosted surface liquid water, which is why some astronomers have proposed an optimistic habitable zone for TOI 700 e.
According to the researchers’ estimation, the size of the planet TOI 700 e is about 95% of the size of Earth, so it is probably a rocky planet and completes its orbit around its star every 28 Earth days. As a result, it falls between the TOI 700 c and d planets. This new world is likely to be fatally locked, meaning that one half of it is always facing its star. These features are the most information gleaned from TESS data alone, Gilbert says, and we’ll hear exciting news in future observing campaigns.