- Planet name: K2-18 b
- Planet mass (J): 0.02807 (compares to Earth at 0.00315), nine times the mass of Earth
- Planet semi-major axis (AU): 0.1429 (compares to Earth at 1 AU; compares to Mercury at 0.4 AU)
- Orbital Period (sidereal days): 32.9 (compares to Earth at 365.26)
- Star name: K2-18
- Star distance: 38.07 parsecs (124 light years)
- Spectral type: M2.5 (a small, coolish red dwarf, compared to the sun which is a hotter, yellow G class star)
- Star mass (solar masses): 0.36 (compares to the sun at 1 solar mass)
- Star luminosity (solar luminosity): 0.028 (compares to the sun at 1.0 solar luminosity)
- Theoretical max. time star in main sequence (billion years): 125 (compares to sun at 10)
- Habitable zone inner radius (AU): 0.16 (compares to the sun’s at 0.95 AU)
- Habitable zone outer radius (AU): 0.23 (compares to the sun’s at 1.37 AU)
- Planet orbits in star's circumstellar habitable zone: No, in the Hot zone. But very close to the inner boundary of the habitable zone.
- Theoretical max. time planet will reside in habitable zone (billion years): - my method indicates – not possible (compares to Earth at 5.5 billion years) (approximated based on the star’s mass). This means that the star’s habitable zone will continue to migrate outward – away from the planet.
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This exoplanet is about nine times the mass of Earth, orbiting a small, cool red star about three times closer to its star than mercury’s orbit to the sun. And even though this reddish star is about a third 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 125 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 small star operating at an extremely low rate of fuel consumption.
Given the planet’s close orbit, my method estimates the planet would become tidally locked within 32 million years of the origination of its current orbit. 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 too warm 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. Since this planet is at the outer edge of the “hot zone”, starward to the habitable zone, tidal locking could yield a very hot surface on the perpetually “day” side of the planet.
Depending on the thickness and chemical composition of the atmosphere, the night side could 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 nine times the mass of Earth. Its significant gravity could accumulate and hold on to gases, possibly sustaining a very thick atmosphere. The presence of such a thick atmosphere could result in a scenario far different from that of a small planet with a very thin atmosphere. Think of Venus.
Venus has a very hot surface temperature. But the heat on the surface is mostly geologic heat. As that heat emerges from the planet’s core, the heat is trapped by the thick, CO2-rich atmosphere. And this results in surface temperatures as high as 900 degrees (F), or 475 (C).
Also, it turns out that the highly reflective Venusian atmosphere reflects about 70% of the sunlight that falls on it. That’s why Venus is so bright in Earth’s morning or evening sky. So, the sun contributes to the Venusian surface temperatures in a small way. It could be a similar situation for K2-18 b.
Back to tidal locking. Even if the corpus of the planet is tidally locked to its host star, its thick atmosphere may continue to circulate and rotate around the locked planet, transferring massive amounts of gas and heat from the “day” side of the planet to the “night” side of the planet – and back again. As a result, the high temperatures expected by: 1) being in the hot zone; and 2) being tidally locked, could be mitigated. Much of the weak star’s light could be reflected, and what heat the atmosphere absorbed could be delivered to the “night” side of the planet, which might not necessarily be cold enough to liquify or solidify those gases. But it might provide a way for “dayside” heat to radiate off into the night sky.
Dimethyl Sulfide. There have been preliminary reports of perhaps detecting the substance, dimethyl sulfide (DMS) in the planet's atmosphere. On Earth, DMS is produced mainly by marine plankton, Including coccolithophores. The release of dimethyl sulfide from marine plankton on Earth is suspected of contributing to cloud formation over the oceans. As clouds tend to reflect sunlight, it has been proposed that modulation of DMS releases could result in feedback processes where plankton inadvertently influence global temperature.
Getting there. As this planet is 124 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 only take about 211,574 years to get there.
My visualization of planet K2-18 b. Image created by Tom Morris / PlanetaryBiology.com
Surface of K2-18 b as visualized by exoExplorer
Another one of my visualizations from the surface of K2-18 b. Image created by Tom Morris / PlanetaryBiology.com
Link to short animation
Notice in this animation that the atmosphere is circulating while the planet body is NOT rotating.