200 light years away, “super earth” exoplanet K2-141b orbits a star so closely that its “year” is only 7 hours long. Not its day…its YEAR! K2-141b orbits a mere million kilometers from the fiery surface of its star. Earth is 150 million km from our Sun. Even Mercury, the planet closest to our Sun, is never less than 47 million km. Standing on the surface of K2-141b you’d look up at an orange star that filled fifty degrees of the sky appearing a hundred times wider than our Sun appears in Earth’s sky. It would be a giant blazing orb so bright that its light shines two thirds of the way around the entire planet unlike Earth’s two day/night halves. Of course, the surface you’re standing on wouldn’t be much of a surface at all – it would be an ocean of liquid hot magma.
Artist’s impression of a close orbiting exoplanet around a star. c. ESO
We’ve discovered a number of “lava planets” or “magma worlds” throughout the Milky Way. These planets are in such close proximity to their stars that their surface is literally melted into a perpetual ocean of lava. The surface is also partially vaporized by the constant bombardment of solar radiation and is blasted around the planet by super sonic winds creating an atmosphere that is literally just vaporized rock. The planet, the oceans, the atmosphere – just rock and minerals in different states of solid, liquid, and gas. No water. No air. Certainly no life. Just a planet of “living” rock.
Of all these lava worlds observed, K2-141b is ideal to study. Discovered in 2018, this world has one of the closest and shortest orbital periods of any known rocky exoplanet. It also has the best signal-to-noise ratios of all these worlds providing the best quality data to scientists. Lead author Giang Nguyen – PhD student at York University; with a team of researchers from York, McGill University, and the Indian Institute of Science Education modeled the possible conditions of this world using computer simulations.
In the sunlight, the surface temperature of K2-141b is 3000 degrees Kelvin which evaporates minerals to create a thin atmosphere clinging to the planet. The research team modeled three possible atmospheric compositions based on the presence of minerals and elements common to Earth’s rocky crust – Sodium (Na), Silicon Oxide (SiO), and Silicon Dioxide (SiO2). With each model, the atmospheres predicted were thinner than Earth’s. The thickest variant, created by Sodium, produced an atmosphere of 13.9 kilopascals (kPa) pressure (Earth’s atmosphere at sea level is 103.325 kPa). The thinnest was SiO2 at just 240 Pa. But in all predicted atmospheres, the windspeeds created by the proximity to the star were astonishingly fast. A supersonic maelstrom. A Sodium atmosphere featured wind speeds of 2.3km/s. Per SECOND! That’s more than 8000 km/h.
These winds drive toward the far side of the planet – this side in perpetual darkness. Being so close to its parent star, K2-141b is tidally locked. The same side of the planet always faces its star while the other side always faces the darkness of outer space. By contrast, the dark side of the planet is a frigid -200 Kelvin. As minerals approach the boundary between day and night, they cool and rain or “snow” down to the planet’s magma ocean where they flow back into the direct sunlight. This means the atmosphere is mostly limited to the day side of the planet.
An Ocean of Magma
The whole cycle is similar to the water cycle on Earth where sea water evaporates, rains onto land, and flows back into the ocean…just with magma hot rocks. Sodium, being most volatile of the possible atmospheres, is carried farther into the night side of the planet meaning it may rain down onto the solid rock past the shores of the magma ocean which is itself also limited to the day side of the planet. This sodium would then be transported on glacier like slabs of solid surface that eventually make their way back to the magma ocean. The research team, using the planet’s gravity, heat, and density were able to predict the maximum depth of the magma ocean. With K2-141b’s gravity double that of Earth’s due to it being five times more massive, the planet could produce a magma ocean over 100km deep. The deepest regions of Earth’s water oceans are only 11km. Models predict that, in some scenarios, the rate of mineral flow back to the daylight may be too slow resulting in more mass on one side of the planet than the other. The planet would become unbalanced resulting in “reorientation of the planet’s spin.” Basically, K2-141b could be shoved off its axis by the perpetual blasting of its star.
Future Super-Scope Observations
Presently, telescopes don’t have the resolving power to see the exoplanet’s atmosphere, but the next generation of scopes, such as the James Webb Space Telescope, will. There’s another challenge. The atmosphere is on the wrong side of the planet. Because most of 141b’s atmosphere rains down before reaching the dark side, there’s concern that we won’t be able to see the atmosphere from our vantage point. By the time the planet turns around during its year presenting day side, it has traveled behind the star. But given the extreme viewing angles created by orbiting its star so closely, and the amount of light which bleeds around the curve of the planet, the research team determined that a telescope should be able to see up to 26 degrees past the planet’s hemisphere just as it appears or disappears behind its star. A telescope could then peer into the region where the supersonic atmosphere resides and confirm not only the composition of the atmosphere but also the predicted wind speeds.
Artist’s rendition of the Earth’s Hadean period, it’s earliest geological eon when first formed. During this time Earth would have also featured magma oceans and intense volcanic activity c. Wikimedia Tim Bertelink CC BY-SA 4.0
Looking out to other worlds helps us understand not only the wider Universe, but ultimately our own planet. Studying K2-141b provides a glimpse into our history. On at least two occasions, planet Earth was molten. Our first molten period was at the very beginning, when our planet was fresh off the solar system assembly line heated by the accretion of all the stuff that made us. The second period followed the massive impact of a Mars-sized object that resulted in the formation of the Moon and literally melted the entire Earth. So whether it’s Mars, Venus, or K2-141b all these worlds are a glimpse into the past of our own planet and provide insight into where, in the future, we may find a planet that resembles ours in the present.