The differences seen here could be throwing off how we study planetary atmospheres.
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JWST maps the weather on a hot gas giant 700 light-years away
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You might mention that this planet has an orbital period (and therefore day length, since it's tidally locked) of 3.95 earth days.
Because when I think of "tidally locked", I think of our moon, which has a period of ~28 earth days, and I assume a gas giant orbiting a star would have a much longer orbit than that. And so reading about "morning" and "evening" just derailed me completely.
But this thing is ridiculously close to its star - 0.055 AU, so it actually has a "day" short enough to make terms like "morning" pretty reasonable.
Oh, one more interesting fact, the companion star also has at least one planet: WASP-94 B b.
(Which reminds me, typo at "team determined WASP-94 b" - there's a missing A before the b. )
Because when I think of "tidally locked", I think of our moon, which has a period of ~28 earth days, and I assume a gas giant orbiting a star would have a much longer orbit than that. And so reading about "morning" and "evening" just derailed me completely.
But this thing is ridiculously close to its star - 0.055 AU, so it actually has a "day" short enough to make terms like "morning" pretty reasonable.
Oh, one more interesting fact, the companion star also has at least one planet: WASP-94 B b.
(Which reminds me, typo at "team determined WASP-94 b" - there's a missing A before the b. )
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Where's George Carlin's Hippy Dippy Weatherman when you need him,
Tonight's forecast: Dark!
Tonight's forecast: Dark!
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Based on the article. the morning limb is where the winds are blowing the atmosphere into the sunside of the planet, hence it is just being exposed to light after having been on the dark half. Likewise the evening limb is the atmosphere rotating into the dark half of the planet after having spent time on the sunlit side.
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Mostly Ignorant
Ars Centurion
Initially I thought "wow, they were lucky to catch the planet while it was at an orbit that went in front of it's sun."
Then I read that the orbital period is 4 days, so it seems there are many opportunities for such measurements.
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Speaking just of the distance we're able to study these planets, it's pretty impressive. In a human lifetime, 700 years is a long time, BUT for planetary science, it's practically as good as current. As an example, while the atmosphere/weather/etc of Earth may well have not been identical to today (leaving out climate change, I mean just natural weather cycles), anyone looking at Earth 700 years ago would still see clear evidence of advanced life.
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Advanced life from atmospheric spectrum data? I could see making the argument around life or not based on an oxidative atmosphere, but what would be the differentiator for advanced life?
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*For varying definitions of "advanced".
In Earth's context, one could argue that photosynthesis-capable life is significantly (structurally, and biochemically) more advanced than the more ancient and ancestral, Archean life. Whether the same sort of evolutionary progression is universally applicable to other planets, is an interesting question that probably doesn't have a firm answer right now.
But other than detecting (or not) a large amount of free oxygen in the atmosphere, modern science would struggle to tease out any further unambiguous signs of "advancement" from spectroscopic data.
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I think I'm sharing some of your confusion. The article mentions the coriolis effect, which I think makes sense because it takes a slow rotation to keep a single face aimed at the sun as it orbits about that sun.
I think you're right that the terms are being used as locations not transient times of day, and it's the fact that there's a constant easterly wind that we're able to see 'morning side atmosphere' as it passes into the 'evening' side, and we can see the 'evening side atmosphere' flow around to the morning side.
When backlit by the sun as it transits across from JWST's perspective, we see the differences in those atmospheric regions or 'limbs' that you mentioned.
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Wow! This article destroys my entire mental model of the atmosphere of a rocky planet in a tidally locked orbit. I pictured an atmosphere essentially frozen and precipitated onto the night side with the day side baked and barren. At best I thought librations might lead to minor sublimation near the limbs, but I never once considered coriolis forces. That might change everything.
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Tidally locked? How do they figure out what's morning an evening when there's only one "day" a "year" and always on the same side of the planet?
Ah, okay, I can picture that.
A TIL moment.
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And even free oxygen is only a possible indicator, not a certain one. When you only have one sample, it is really, really hard to generalize.
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Anywhere you've got atmosphere, and a hot and a cold zone, you're going to get some winds, at the very least at the limbs. Interesting that they can be planet wide though.
So, they don't want a bigger telescope?! I guess you could read that as they will inevitably conclude they want a bigger one
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The orbital period may be 3.95 Earth days, but day length on a planet that's tidally locked to its star is infinite. If you're on a fixed position on the planet, the star stays in the same position in the sky forever, except maybe for minor wiggling back and forth if the planet's orbit isn't perfectly circular.
The tidally locked situation is just an extreme example of the common off-by-one error:
- How long is the day length on Earth? 24 hours.
- How long does it take the Earth to make one complete rotation about its axis? 23 hours 56 minutes 4 seconds
- How many days are there in one Earth orbit around the Sun? 365.24
- How many times does the Earth rotate about its axis in one Earth orbit around the Sun? 366.24
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Nora Lenderbee
Ars Tribunus Militum
I'm trying to understand how, with a gas giant, you can tell if it's tidally locked or not. Do they assume it's tidally locked because it's so near its primary, or is it observed in some fashion?
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Mainly because it is near. However, it is worth noting that we thought Mercury was tidally locked until we discovered it wasn't.
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Wait, if it's tidally locked doesn't that mean the same side of the planet is always facing the sun?
My understanding of the morning limb and evening limb was that it related purely to the leading side and trailing side of the planet. I know it's a gas giant so there's not exactly a beach to sit on and enjoy the sunset, but even if there was there would be no sunset to enjoy. The star would always be in the same spot in the sky. Unless I've badly misunderstood something.
The moon only has a 28-day day because it is tidally locked to Earth, but it's source of light is the sun.
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You and Higgsforce are quite right, that part of my post was quite wrong. I can't seem to edit my post now, sorry.
I guess I was still fixated on Earth's moon. That has the same side always facing the Earth, which, as you note, means different parts of the Moon's surface are exposed to the Sun at different times (i.e., monthly). But WASP-94-A-b is locked to its sun, so by definition, even though it does rotate, the same part of the surface always faces its sun. "Morning" and "Evening" are fixed locations on the surface.
(Can I wriggle out of this? WASP-94 is a binary, so WASP-94-A-b technically has two suns, which means it sort of does have a 4-Earth-day "day" caused by the companion star WASP-94-B. But given that star B is at least 49,000 times farther from the planet than star A is (and the two stars are similar in luminosity) .... I have a feeling I'm going to get the math wrong, but I guess star B would look (from the top of planet A-b's atmosphere) more like a very bright star than like another sun (ie. less bright than a moon would be) so it can't really cause much of a "day".) Oh well.)
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Nora Lenderbee
Ars Tribunus Militum
Well... locked into a 3:2 spin–orbit resonance. It's an interesting digression, however.
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Albino_Boo
Ars Tribunus Angusticlavius
Roman lead mining and smelting effected the atmosphere globally. Coal use for iron smelting produces sulphides. The Norman aristocracy tired to ban the use of coal for blacksmithing and heating in 12th century London because it was already causing smog in certain weather conditions.
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Yep and I thought about those (and CFCs), and whether I thought it would be possible to detect those signals from dozens to hundreds of light years away. I could not think of a possible way given the low abundance of those markers in the atmosphere.
The only one I could reasonably think of was detecting O2 in the atmosphere which is already a well known potential signal. I questioned whether that meant advanced life, but as Boris pointed out I guess it depends on the definition of advanced life.
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I know it's been mentioned several times, but here's another view from a layman that may use less technical terms to aid in other laymen understanding:
One thing that greatly helped my own understanding of this article was to think of the limbs as locked-in parts of the planet, with the atmospheric shell being a separate entity.
The limbs are like goalposts for the atmosphere to pass through: one on the leading side of the planet (therefore morning) and one on the trailing side (therefore evening).
If you think of the atmospheric shell as rotating instead of the planet, there is indeed a morning and evening to the atmosphere itself as specific parts of the shell rotate through the limbs/goalposts.
One thing that greatly helped my own understanding of this article was to think of the limbs as locked-in parts of the planet, with the atmospheric shell being a separate entity.
The limbs are like goalposts for the atmosphere to pass through: one on the leading side of the planet (therefore morning) and one on the trailing side (therefore evening).
If you think of the atmospheric shell as rotating instead of the planet, there is indeed a morning and evening to the atmosphere itself as specific parts of the shell rotate through the limbs/goalposts.
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