When it comes to finding life outside our solar system, planets that look a lot like Earth seem like a good place to start. We can now welcome the celestial object TOI 700 e to this group of promising prospects.
TOI 700 e is confirmed to be orbiting in the habitable zone of its star, TOI 700. This is the region of space where significant amounts of water on its surface would be at a temperature suitable for liquid form. Too warm for an ice sheet but still cool enough to condense vapor, these types of planets are thought to be “just right” for life as we know it.
We can thank NASA’s Transiting Exoplanet Survey Satellite, or TESS, for finding TOI 700 e and giving it its name (TOI stands for TESS Object of Interest). It is the second habitable zone planet in this system, joining TOI 700 d, which was spotted in 2020.
Illustration showing TOI 700 e in the foreground and TOI 700 d in the distance. (NASA/JPL-Caltech/Robert Hurt)
“This is one of the few systems with multiple minor habitable zone planets that we know of,” said planetary scientist Emily Gilbert of NASA’s Jet Propulsion Laboratory (JPL) in California.
“This makes the TOI 700 system an exciting prospect for further tracking. 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 small, cool star (known as an M dwarf star) located about 100 light-years away in the constellation Dorado. These stars are nowhere near as big or as hot as our Sun, so planets must be closer to them to be warm enough for water to avoid freezing.
As for TOI 700 e, it is thought to be 95 percent the size of Earth and is mostly rocky. It is located in the “optimistic” habitable zone – an area where water may have existed at some point in time. TOI 700 d is in the narrower “conservative” habitable zone where astronomers think liquid water could exist for most of a 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 surface area blocking the star’s light, larger planets provide easier opportunities to be seen than small, rocky worlds, making terrestrial discoveries like this a rare treat.
TOI 700 e takes 28 days to make one orbit, while TOI 700 d – which is slightly further away than its neighbor – takes 37 days. Because TOI 700 e is smaller than TOI 700 d, more data was needed to confirm that the silhouette was indeed a new planet.
“If the star had been a little closer or the planet a little bigger, we might have noticed TOI 700 e in the first year of TESS data,” says University of Maryland astrophysicist Ben Hord. “But the signal was so weak that we needed an additional year of transit observations to identify it.”
TESS observes about 100 million stars, and any way we can find to narrow down the search for life will be helpful. Finding exoplanets in their respective 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 always faces its star (in the same way that the same side of the Moon is always visible from Earth). Having one side of a planet constantly baking in sunlight really makes it less likely that complex life will get off to a smooth start.
Even if these “just right” planets aren’t perfect for life, they tell us a thing or two about finding solar systems that might be better suited for it. By studying star systems like the one we’re in, astronomers can also better understand the evolution of our home and how neighboring planets got to their current orbits.
“Even with more than 5,000 exoplanets discovered to date, TOI 700 e is a prime example that we still have a lot to learn,” says Michigan State University astronomer Joey Rodriguez.
The research has been accepted for publication in the Astrophysical Journal Letters and is currently available for review on arXiv.
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