- Planet name: TRAPPIST-1 b
- Planet mass (J): 0.0027 (compares to Earth at 0.00315), 86% of the mass of Earth
- Planet semi-major axis (AU): 0.01111 (compares to Earth at 1 AU; compares to Mercury at 0.4 AU)
- Orbital Period (sidereal days): 1.51 (compares to Earth at 365.26)
- Star name: TRAPPIST-1
- Star distance: 12.1 parsecs (39.5 light years)
- Spectral type: M8 (a small, coolish red star, compared to the sun which is a hotter, yellow G class star)
- Star mass (solar masses): 0.08 (compares to the sun at 1 solar mass)
- Star luminosity (solar luminosity): 0.00002 (compares to the sun at 1.0 solar luminosity)
- Theoretical max. time star in main sequence (billion years): 366 (compares to sun at 10)
- Habitable zone inner radius (AU): 0.0044 (compares to the sun’s at 0.95 AU)
- Habitable zone outer radius (AU): 0.0064 (compares to the sun’s at 1.37 AU)
- Planet orbits in star's circumstellar habitable zone: No, in the Cold zone
Comments
This exoplanet is about 86% the mass of Earth, orbiting very close to a small, cool red star about 0.01 the distance that Earth orbits the sun. For reference, Mercury orbits the sun at about 0.4 AU. So, this planet orbits its host star 40x closer than Mercury. And even though this reddish star is less than a tenth as massive as the sun, it would appear very large in the sky, when viewed by the planet – because the planet orbits so closely.
Approximating the star’s history based on the star’s mass (and not its observed apparent magnitude), it will be in its main sequence for 366 billion years. That is an extraordinarily long main sequence. Who can guess if the Universe will last that long? At any rate, this is a tiny star is operating at an extremely low rate of fuel consumption.
Given the planet’s close orbit, it is a prime candidate to become tidally locked – like the moon is tidally locked to Earth. That is, one side of the planet will always face the star, and the opposite side of the planet will always face away from the star.
Given the prospect of tidal locking, this planet probably won't experience a day-night cycle. There won't be warming during daylight, and cooling at night. So, the "day" side will be constantly “warmer” – possibly warm enough for liquid water. This presents a variation that differs from the assumptions for estimating a circumstellar habitable zone – for Earth-like planets. One key assumption for an Earth-like planet is that it is NOT tidally locked to its host star and that it has a day/night cycle. Warming during the day and cooling during the night. So, although the planet is beyond the habitable zone, tidal locking could yield a warm surface on the perpetually “day” side of the planet.
The night side will be constantly very cold -- maybe too cold for liquid water . There is a possible habitable ring at the day-night terminator.
Let’s think more about atmosphere. This planet is 86% the mass of Earth. It might possess enough gravity to accumulate and hold on to gases, possibly sustaining a thin atmosphere. But wait. If the planet is tidally locked to its host star, then atmospheric gases on the perpetually “night” side of the planet could freeze and fall to the planetary surface as solids or liquids. Those gases would later be backfilled by circulation of atmospheric gases from the “day” side of the planet – which would freeze out. The “night” side of the planet would act as a giant gas sink. This could result in a large rocky body with little or no atmosphere, but an interesting accumulation of ice and cold liquids on the dark side of the planet.
How long would the ice on the dark side persist? Wouldn't the ice sublimate (evaporate) rapidly if there was little or no atmosphere? I need to mention sublimation. The physical process of "sublimation" is when a frozen/solid substance undergoes a phase change from solid directly to gas -- without experiencing a liquid phase in the process. Think of the vapor coming off of a cube of "dry ice (frozen CO2). Sublimation rates are dependent on the temperature. For ice in ambient temperatures warmer than -100 degrees C, sublimation would happen quickly, especially in the absence of an atmosphere. But when temperatures drop below -100 degrees C, the rate of sublimation slows considerably. So, the ice on icy bodies subject to extremely cold temperatures -- that surface ice can persist, despite the absence of an atmosphere. That helps explain why Jupiter's moon, Europa, is covered with thick ice, and probably has been so for billions of years.
So, for TRAPPIST-1 b, the tidally-locked dark side probably would be colder than -200 degrees C. If so, any ice deposited there would likely remain, and not sublimate, for billions of years.
Getting there. As this planet is 39.5 light years away, rockets using gravitational slings can attain speeds of over 394,736 miles per hour (NASA Parker solar probe). At that speed, it would take about 67,397 years to get there.
In my imagined view, the star's closest planet orbits very closely and is tidally locked. Although the planet orbits in the "cold" zone beyond the star's circumstellar habitable zone, tidal locking and perpetual "day" on one side of the planet could allow the "day" side to accumulate enough heat to allow for liquid water. The "night" side of the planet would be frozen. Image created by Tom Morris / PlanetaryBiology.com
TRAPPIST-1 b surface as imagined by exoExplorer. Notice that the reddish horizon is NOT a tinted atmosphere. Instead, it is the body of the host star that the planet closely orbits.