On November 15th, 2017 we learned about

Climate models that explain colder conditions on dwarf and exoplanets

As the Earth struggles with atmospheric warming, scientists are finding mechanisms that are actually cooling more distant worlds. Unfortunately, they’re not systems that we could reproduce to deal with our greenhouse gas problem on Earth, but understanding what chills the atmospheres of these distant bodies will likely help with the search for habitable worlds in other solar systems.

Pluto is colder than we calculated

Everybody knows that Pluto is cold, but the dwarf planet still surprised scientists with just how cold it really was. Remote measurements of the elements in Pluto’s atmosphere suggested that the Sun should warm its surface to around -343° Fahrenheit, but that isn’t what the New Horizons spacecraft measured when it flew by in 2015. Instead, more direct measurements found that the atmosphere was a frosty -397° Fahrenheit. Over the last two years, researchers have been looking through the collected data to see what could account for this temperature difference.

The current hypothesis is built around the size of the particles in Pluto’s atmosphere. Most atmospheres are made of gas, meaning the atoms and molecules suspended in the sky are tiny and fairly energetic. Pluto seems to have some larger hydrocarbon particles in the mix though, suspended as solid crystal structures instead of gases. Even though these particles are small enough to be measured in nanometers, they’re big enough to interact with heat from the Sun in an unusual manner. Instead of trapping heat against the dwarf planet’s surface like a greenhouse gas, they’re likely absorbing heat from the Sun and surrounding atmosphere, then radiating that heat back into space. This means that the heat is essentially reflected before it has a chance to warm Pluto up.

Heavy snowfall on a hot planet

While Pluto’s hydrocarbon haze may be an effective heat shield, the exoplanet Kepler-13Ab uses similar mechanisms in even more extreme circumstances. The giant planet has a close orbit around its star, Kepler-13A, and thus doesn’t doesn’t rotate like the Earth does on a daily basis. This leaves one side of the so-called “hot Jupiter” in constant daylight, raising temperatures as high as 5,000° Fahrenheit. The night side is then left in permanent darkness, with temperatures dropping appropriately.

Kepler-13Ab was found to have titanium oxide (TiO) in it’s atmosphere, which by itself isn’t unusual for a gas giant planet. Usually this leads to higher temperatures in a planet’s upper atmosphere, as the TiO acts like a greenhouse gas more or less, collecting and emitting heat. Kepler-13Ab changes that formula thanks to its immense gravity, which is six-times greater than Jupiter’s. So instead of remaining in the sky as gases, the planet is pulling some TiO down on its cold, night side as larger particles, almost like a form of snow. As the snow gets pulled down, it takes some heat away with it, lowering the maximum temperature in the upper atmosphere. Without Kepler-13Ab’s intense gravity, the particles would be more likely to recirculate in the atmosphere again, redistributing heat.

Nobody is planning to visit Pluto or Kepler-13Ab any time soon, but these extreme climates will help us as we search for more temperate planets. Being able to understand all the variables that can influence a planet’s climate, including less obvious factors like gravity and particulate size, will make it easier to identify the more subtle conditions that could make some planets pleasant place for life to take root.

Source: Astronomers discover sunscreen snow falling on hot exoplanet, Phys.org

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