The Weather Patterns of HD 209458b:


If any of our loyal fans are like me (and most other sane people), winter is the worst time of year. Besides being cooped up inside the house for several months, with about ten blankets, flipping through the channels with nothing on tv, I don't fancy the cold very much. (I won't even get started on the Christmas music) However, I typically do enjoy about one good snow each year. Waking up and seeing a foot of freshly fallen snow lying out on the roads and my front lawn brings out the little child in me, and most other adults, for at least a few hours (until it turns to mush and freezes the roads).


Anyhow, despite snow being relatively normal here on Earth, it is quite abnormal on other planets. It's either too hot for water to retain on the surface of a planet (like Mercury and Venus), or much too cold (basically all of the moons of the gas-giants in our solar system) to not freeze up entirely. Exoplanets are another thing though. Some of them are so alien to us; it's hard to picture exactly what the planet would look like if we were able to visit it. HD 209458b is one of those worlds. Let’s just say, there is no 'white Christmas' there.


HD 209458b, also known as Osiris, lies some 150 light-years from Earth in the constellation of Pegasus. In one of our previous articles about this hot-Jupiter exoplanet, we discussed some of the basics of this planet, including its distance from its parent star, atmospheric composition, surface temperatures, and the fact that it is dying.


All of which, can be read about here:


I want to delve deeper into the abnormal weather patterns here, on this bipolar exoplanet, nick-named as such (by me), because this planet may as well be two completely places, due to it being tidally locked to its parent star. The temperatures on the sun-facing side can exceed 2000 degrees Celsius, while the perpetual night-side of the planet is much cooler, at about 500 degrees C, which is still far too hot for liquid water to exist on the planet... let alone snow, which is composed of water.


However, since this planet has drastically different temperatures on each side, atmospheric currents are produced that swirl materials from the day side to the night sky. Essentially, certain gaseous substances are created on the day side and transferred to the night side, where it can condense into a solid state before falling as precipitation. On Earth, we have water. On Venus, there is sulfuric acid. Jupiter rains neon. And on the Saturnian moon Titan, there is petroleum precipitation.


Osiris, it has titanium oxide snow. Astronomers use computer models to input the known data about certain exoplanets to decipher which characteristics would account for some of the observations we've noted about exoplanets. In this instance, Osiris contains only a small amount of oxides of titanium in its atmosphere, but still enough to condense on the night-facing side and fall to the surface as snow, where it would remain on the surface forever (or at least until this planet sheds the remainder of its mass out into space via solar winds)


This 'snow' is not snow in the same way 'snow' is 'snow' on Earth (yes, that's totally a mouth-full). There are no snowballs, snow-angels, snow-men (unless you are talking about the abominable snowman). There’s no snow glistening sunlight, reflecting from the trees and the plants. Instead, the silica oxides found in Osirius' atmosphere would turn the snow pitch black and it would stink of (and probably feel like) a forest fire. (Gloves and a toboggan would do very little for you.)


Of course, all of this is moot, as you and I would only survive on this planet for zero minutes. In fact, because of its proximity to its parent star, you would be incinerated before you ever reached the planet. It’s still cool to contemplate though…and suddenly, I'm much more appreciative of our home rock.


- Jaime


For Further Reading:


" 3D mixing in hot Jupiter atmospheres I: application to the day/night cold trap in HD 209458b:"


"Dying Planet Leaks Carbon-Oxygen:"


Image Source:


"Hubble finds first ever carbon dioxide on exoplanet:"



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