One of the most exciting exoplanetary systems discovered so far, and one we’ve talked about many times on Exoplanet Radio, is TRAPPIST-1, a system of of seven planets, all of them rocky and more or less Earth sized, and about three of them are in the habitable zone of the star. But new observations from an amazing space telescope have just discovered a system that rivals TRAPPIST-1.
Habitable zones and Earth-sized are words we hear a lot when we talk about other star systems because those words outline a hope that life might exist on their planets. Life could possibly arise under many scenarios, and it’s entirely possible that life may well survive without it. But based on our experience -here, on Earth - life needs liquid water and if a planet might have it, then we are naturally interested.
TRAPPIST-1 has held our attention for a long time because it has so many rocky worlds orbiting a star that may allow some of them to have liquid water. The promise of life is too great to ignore, so we turn our most powerful telescopes to this system whenever possible.
Lately, however, there’s been a new system on the block. The Transiting Exoplanet Survey Satellite, or TESS, has brought us the discovery of a system that is every bit as interesting as TRAPPIST-1.
Known as TOI 700, named because it is the 700th TESS Object of Interest cataloged by the mission, is a system that contains four worlds, all presumed to be rocky based on the parameters measured from the planets as they transit the star. The four planets are imaginatively named b, c, d, and e with d and e being the most interesting.
So what’s the system like? Well like TRAPPIST-1 all of these planets are very close to their star. They all have orbits measured in days, the longest being TOI 700 e at 28 days. The reason we’re seeing so many systems like this is that our transit measuring space telescopes don’t stare at the system long enough to get planets that take longer to go around their star. Those types of planets are probably out there, but we haven’t seen them yet because we don’t look at them long enough. TESS for example only looks at one region of the sky for 27 days before it moves onto another sector.
The star TOI 700 is a small, cool M dwarf star located around 100 light-years away in the southern constellation Dorado.
The innermost planet, TOI 700 b, is about 90% Earth’s size and orbits the star every 10 days. TOI 700 c is over 2.5 times bigger than Earth and completes an orbit every 16 days. These planets are probably tidally locked, which means they spin only once per orbit such that one side always faces the star, just as one side of the Moon is always turned toward Earth.
In fact all the planets are so close they may all be tidally locked.
TOI 700 d and e reside in what astronomers are calling the optimistic habitable zone. This is a term we’ve not mentioned before but refers to a range of distances from a star where liquid surface water could have existed at some point in a planet’s history. Compare this to what we usually refer to as a habitable zone where liquid water could have existed over most of the planet’s lifetime.
TOI 700 d and e orbits in the optimistic habitable zone and are a little farther out, with TOI 700e, the most promising for habitability, orbiting once over 28 days.
Like TRAPPIST-1 this system is full of promise Follow-up study of the TOI 700 system with space- and ground-based observatories is ongoing, and with each new observation, our anticipation of what we might learn about habitable exoplanets, and life elsewhere, grows.