NASA’s TESS Detects Second Potentially Habitable Planet Around TOI 700

NASA’s Transiting Exoplanet Survey Satellite (TESS) returned data that led to the discovery of a second planet in the habitable zone of TOI 700. This star is an M-class red dwarf star about 100 light-years from Earth in the southern constellation Dorado.

The newly discovered exoplanet is about 95% Earth’s size, which demonstrates scientists’ increasing ability to discover smaller exoplanets with newer instruments like TESS. Even with the more sensitive instruments, it proved a challenge to confirm the existence of the exoplanet now officially called TOI 700 e.

“If the star was a little closer or the planet a little bigger, we might have been able to spot TOI 700 e in the first year of TESS data,” said Ben Hord, a doctoral candidate at the University of Maryland, College Park and a graduate researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “But the signal was so faint that we needed the additional year of transit observations to identify it.”

TOI 700 e may be tidally locked to its host star, meaning that the same side of the planet always faces its host star. The gravitational pull from the planet’s star causes the planet to rotate at the same rate that it orbits the star. Tidal locking happens most often to planets that orbit close enough to their host star, like planets that orbit within the habitable zone of red dwarfs and other cool stars.

Tidal locking can also happen when one object orbits a bigger object that isn’t a star. One familiar example of tidal locking is the Moon, which is tidally locked in its orbit around Earth.

TOI 700 also has three other known planets orbiting it, including TOI 700 d, which also orbits in the habitable zone.

“This is one of only a few systems with multiple, small, habitable-zone planets that we know of,” said Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Southern California who led the work. “That makes the TOI 700 system an exciting prospect for additional follow up. Planet e is about 10% smaller than planet d, so the system also shows how additional TESS observations help us find smaller and smaller worlds.”

The habitable zone, also known as the “Goldilocks zone,” is the region around a star within which a planet can have temperatures that allow liquid water to exist on its surface. The presence of liquid water is critical for carbon-based life.

The location of a star’s habitable zone depends on the star’s luminosity and temperature. For our Sun, the habitable zone is located roughly between 0.95 and 1.4 astronomical units (AU). Earth is the only planet in the solar system that is in the habitable zone of our Sun. Mars averages about 1.5 AUs from the sun but does come as close as 1.38 AUs.

An M-class dwarf star is a type of red dwarf, which is the most common type of star in our galaxy. These stars are also known as “red dwarfs” due to the reddish color of their light. The classification “M” is used in the system of spectral classification and it indicate the stars with a surface temperature of less than 4,000 K (3,730 °C; 6,700 °F) and which emit most of their radiation in the infrared part of the spectrum. Due to its cool temperature, the habitable zone for a red dwarf like TOI 700 is estimated to be located between about 0.02 and 0.08 AU from the star.

They are relatively cool and faint compared to other types of stars. M-class dwarfs typically have temperatures between 2,500 and 3,500 K, and luminosity that is thousands to millions of times less than that of the Sun. Due to their relatively cool temperatures, M-class dwarfs have low levels of ionizing radiation, which makes it more likely that a planet orbiting in the habitable zone around an M-class dwarf could have conditions that are conducive to the development of life. M-class dwarf stars tend to have expected lifespans that are much longer than the Sun’s, potentially providing enough time for complex life to evolve.

This makes the discovery of a roughly Earth-sized planet in TOI-700’s habitable zone especially exciting because it might have life if it has liquid water. Emily Gilbert presented her research team’s findings at the 241st meeting of the American Astronomical Society in Seattle. A paper on the research team’s newly discovered exoplanet was accepted by The Astrophysical Journal Letters.