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 celestial object TOI 700 e to this 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 where significant amounts of water on its surface would be at a temperature suitable for liquid form. Too warm for an ice blanket but still cool enough for steam to condense, these kinds of planets are considered “suitable” for life as we know it.
We can thank NASA’s Transiting Exoplanet Survey Satellite (TESS) for finding and naming TOI 700 e (TOI stands for TESS Object of Interest). It is the second planet in the habitable zone of this system, joining TOI 700 d which was spotted in 2020.
“It’s one of the few systems we know of with many small habitable planets,” says planetary scientist Emily Gilbert of NASA’s Jet Propulsion Laboratory (JPL) in California.
“This makes the TOI 700 system an exciting prospect 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, cool star (known as an M dwarf) located about 100 light years away in the constellation Dorado. These stars are nowhere near as big or as hot as our sun, so the planets must be closer to them for conditions to be warm enough for water to avoid freezing.
As for TOI 700 e, it is believed to be 95 percent the size of Earth and mostly rocky. It is located in the “optimistic” habitable zone – a zone where water may have existed at some point. TOI 700 d is in the narrower “conservative” habitable zone, which is 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 spots in the light of their host stars as they pass in front of it in what is known as transit. Because the larger surface area blocks the star’s light, larger planets offer greater opportunities to see than small, rocky worlds, making Earth-like discoveries like this a rare treat.
TOI 700 e completes a single orbit in 28 days, 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 really represents a new planet.
“If the star was a bit closer or the planet a bit larger, we might have been able to spot TOI 700 e in the first year of the TESS data,” says astrophysicist Ben Hord of the University of Maryland. “But the signal was so weak that we needed an extra year of transit observation to identify it.”
TESS monitors about 100 million stars, so any way to narrow down the search for life will be useful. Finding exoplanets in the right habitable zones is one of the best ways we do that.
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 the planet constantly burning in the sun reduces the likelihood of a smooth start to a complex life.
Even if these “perfect” planets aren’t ideal for life, they do tell us something about finding solar systems that might be more suitable for it. By studying star systems like the one we live in, astronomers can also gain a better understanding of how our home evolved and how neighboring planets ended up in their current orbits.
“Even with more than 5,000 exoplanets discovered so far, TOI 700 e is a key example of how much more we have to discover,” says astronomer Joey Rodriguez of Michigan State University.
The research was accepted for publication in the journal Letters from an astrophysics journaland is currently available for viewing on arXiv.