TOI 700 e has been confirmed to orbit within the habitable zone of its star, TOI 700. This is a region of space on the surface of which a significant amount of water will be at temperatures suitable for liquid form. Too hot for an ice cap but still cold enough for steam to condense, such planets are considered “best fit” for life as we know it. We can thank NASA’s Transiting Exoplanet Survey Satellite, or TESS, for finding and naming TOI 700 e (TOI stands for TESS Object of Interest). It is the second habitable zone planet in this system, joining the TOI 700 d observed in 2020.

“This is one of the few systems we know of with more than one small habitable zone planet,” says planetary scientist Emily Gilbert of NASA’s Jet Propulsion Laboratory (JPL) in California.

“This makes the TOI 700 system an exciting possibility for further observations. Planet e is about 10 percent smaller than planet d, so the system also shows how additional TESS observations are helping us find increasingly smaller worlds.

TOI 700 is a small, cold star (known as the M dwarf star) located about 100 light-years away in the constellation Dorados. These stars are not as massive or hot as our Sun, so the planets need to be closer to them for the water to be hot enough to keep it from freezing.

TOI 700 e is believed to be 95 percent the size of Earth and is mostly rocky. In the “optimistic” residential area – the zone where water can exist at any time. TOI 700 d is in the narrower “conservative” habitable zone, where astronomers believe liquid water could exist for most of the planet’s existence.

Telescopes see these exoplanets (planets outside our solar system) as regular flashes of light from their parent stars as they pass in front of it during so-called transits. With more surface area blocking the star’s light, larger planets are easier to see than small rocky worlds, making discoveries like this one rare on Earth.

TOI 700 e takes 28 days to complete one orbit, while TOI 700 d, a little further away from its neighbor, takes 37 days. Since TOI 700 e is smaller than TOI 700 d, more data was needed to confirm that the silhouette did indeed represent a new planet.

“If the star were a little closer or the planet was a little larger, we could see TOI 700 e in the first year of the TESS data,” says astrophysicist Ben Gord of the University of Maryland. “But the signal was so weak that we needed another year of observation of the transition to detect it.”

TESS observes about 100 million stars, so any way we can narrow down the search for life would be helpful. Finding exoplanets in their habitable zones is one of the best ways to do this.

Both TOI 700 e and TOI 700 d are considered tidally locked: in other words, one side of the planet always faces the star (just as the same side of the Moon is always visible from Earth). Of course, the continued scorching of one side of the planet by the sun makes it less likely that complex life could begin smoothly.

Even if these “correct” planets aren’t exactly perfect for life, they do tell us something about how to find solar systems that might be better suited for life. By studying star systems like ours, astronomers can better understand the evolution of our home and how neighboring planets enter their current orbits.

“Even among the more than 5,000 exoplanets discovered to date, TOI 700 e is a prime example of how much we still have to learn,” says Michigan State University astronomer Joey Rodriguez. Source