When it comes to finding life outside our solar system, planets that closely resemble Earth seem like a good place to start. We can now welcome TOI celestial body 700 e to that group of promising leads.
TOI 700 e has been confirmed to be orbiting within the habitable zone of its star, TOI 700. This is a region of space in which large amounts of water are on its surface at a temperature suitable for liquid form. Quite warm for a blanket of ice, these types of planets are ‘just right’ for life as we know it, yet still cold enough for steam to condense.
We can thank NASA’s Transiting Exoplanet Survey Satellite, or TESS, for finding TOI 700 e, and for giving it its name (TOI stands for TESS Object of Interest). It is the second planet in the habitable zone of this system, joining TOI 700 d observed in 2020.
“This is only one of the few systems with multiple, small habitable planets that we know of,” says planetary scientist Emily Gilbert, of NASA’s Jet Propulsion Laboratory (JPL) in California.
“This makes the TOI 700 system an exciting opportunity for additional follow-up. Planet e is about 10 percent smaller than Planet d, so the system also shows how additional TESS observations are helping us find smaller and smaller worlds.”
TOI 700 is a cool little star (known as an M dwarf star), located about 100 light-years away from us in the constellation Dorado. These stars are not near or anywhere near our Sun, so the planets need to be closer to them for conditions to be warm enough for water not to freeze.
For TOI 700 e, it is believed to be 95 percent of Earth’s volume and is mainly rocky. It is located in the “optimistic” habitable zone – an area where water may have existed at some point. TOI 700 d lies in the narrow “conservative” habitable zone, where astronomers believe liquid water may have existed for the majority 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 them, in what is known as a transit. With more surfaces blocking the star’s light, larger planets offer easier viewing opportunities than small, rocky worlds, making Earth-like finds like this a rare treat.
TOI 700 e takes 28 days to make one orbit, while TOI 700 d – which is a little further than its neighbor – takes 37 days. Since TOI 700 e is smaller than TOI 700 d, it took more data to confirm that the silhouette really represented a new planet.
“If the star was a little closer or the planet a little bigger, we might have been able to identify TOI 700 e in the first year of TESS data,” says astrophysicist Ben Hord of the University of Maryland. “But the signal was so weak that we needed an extra year of observing the transit to determine it.”
TESS monitors about 100 million stars, so any way we can find to 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 thought to be tidally locked: in other words, one side of the planet is always facing its star (in the same way that the same side of the Moon is always visible from Earth). Admittedly, having one side of a planet constantly baking in sunlight makes it less likely that complex life will start smoothly.
Even if these “just right” planets aren’t exactly ideal for life, they do tell us a thing or two about finding solar systems that might be a better fit for them. By studying star systems like the one we’re in, astronomers can also better understand the evolution of our home and how neighboring planets reached their current orbits.
“Even with more than 5,000 exoplanets discovered so far, TOI 700 e is a prime example that we have a lot to learn,” says astronomer Joy Rodriguez of Michigan State University.
The research has been accepted for publication in Astrophysical Journal Lettersand is currently available for viewing on arXiv.