APOD: In Ganymede's Shadow (2022 Oct 07)

[APOD Robot]

In Ganymede's Shadow

Explanation: At opposition, opposite the Sun in Earth's sky, late last month Jupiter is also approaching perihelion, the closest point to the Sun in its elliptical orbit, early next year. That makes Jupiter exceptionally close to our fair planet, currently resulting in excellent views of the Solar System's ruling gas giant. On September 27, this sharp image of Jupiter was recorded with a small telescope from a backyard in Florence, Arizona. The stacked video frames reveal the massive world bounded by planet girdling winds. Dark belts and light zones span the gas giant, along with rotating oval storms and its signature Great Red Spot. Galilean moon Ganymede is below and right in the frame. The Solar System's largest moon and its shadow are in transit across the southern Jovian cloud tops.

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[beryllium732]

Does gas giants always form in the outer colder parts of a star system or do they wander in as the time passes by? I guess it's due to gas not being able to be captured if the body is near the star?

[Ann]

Does gas giants always form in the outer colder parts of a star system or do they wander in as the time passes by? I guess it's due to gas not being able to be captured if the body is near the star?

Our own solar system is unusual in many ways, and Jupiter is one of the planets that makes it unusual.

Ross Pomeroy of Real Clear Science wrote:

To us, our solar system, situated in the suburbs of the Milky Way, seems normal, almost mundane. It's home after all, the only one we've ever known. But when astronomers began gazing earnestly at our stellar neighbors a few decades ago, they soon realized that, in many respects, our solar system contrasted sharply with others out there. In fact, it's downright strange. Here are four reasons why.

1....

2....

3....

4. What's Up With Jupiter? Jupiter is the juggernaut of our solar system, with a mass more than two and a half times that of all the other planets combined. The simple fact that our solar system hosts a gas giant of this size sets us apart, says astrophysicist Sean Raymond. Just 10-15% of Sun-like stars have one. Moreover, he writes, most gas giants in other solar systems have tight, eccentric orbits around their stars, while Jupiter's is wide and nearly circular. "Only about one in a hundred stars like the Sun has a Jupiter like ours," he wrote.

So it is more common with Jupiters near their sun than it is with Jupiters in large orbits much farther away from their sun, like our own giant planet.

That said, many astronomers do believe that Jupiter has indeed "moved around" in the solar system until it came to a halt in its current position. More precisely, the leading hypothesis is that Jupiter formed at 3.5 AU (closer to the Sun than today), then moved inwards to a position of 1.5 AU (halfway between the current positions of the Earth and Mars), where it was caught in a resonance with Saturn, which made it "move back out" to where it is now, at 5.2 AU.

This is called the Grand tack hypothesis.

Ann

[Case]

Ross Pomeroy of Real Clear Science wrote:

4. What's Up With Jupiter? […] Moreover, he writes, most gas giants in other solar systems have tight, eccentric orbits around their stars, while Jupiter's is wide and nearly circular. "Only about one in a hundred stars like the Sun has a Jupiter like ours," he wrote.

I’d say that has all the signs of discovery bias. Exoplanet discovery methods finds close orbits more easily. That says very little about the amount of planets out there in wide 12-earth-years orbits.
[Just because you can see better in a lamp’s glow, that doesn’t mean that is were lost items are (Streetlight effect).]

[orin stepanek]

Don't you wish there were a cutaway view that accurately showed
through the clouds to Jupiter's surface? Does Jupiter even have a
surface?

[Chris Peterson]

JovianEclipse1024c.jpg
Don't you wish there were a cutaway view that accurately showed
through the clouds to Jupiter's surface? :shock: Does Jupiter even have a
surface?

Define "surface". Jupiter almost certainly has a solid core, deep in its interior, which you would reach by descending through the gases and then exotic fluid layers. Whether that should be called its "surface" or not is debatable.

[orin stepanek]

JovianEclipse1024c.jpg
Don't you wish there were a cutaway view that accurately showed
through the clouds to Jupiter's surface? :shock Does Jupiter even have a
surface?

Define "surface". Jupiter almost certainly has a solid core, deep in its interior, which you would reach by descending through the gases and then exotic fluid layers. Whether that should be called its "surface" or not is debatable.

You're most certainly correct and a lot of the liquid is probably solid with gasses being liquid! Still!!!

[De58te]

Sometimes the physics of astronomy simply astonishes you. I am referring to a comment by Ann that Jupiter once moved inward in the solar system to 1.5 AU, but then was pulled out by Saturn to its present position. That is even though the mass of Jupiter is over 3 times the mass of Saturn! It defies what I learned in school about Newtonian physics. It is like the wrestler Hulk Hogan getting into a tug-of-war match with 90 pound Twiggy - and Twiggy wins! You wonder why the much heavier Jupiter didn't pull Saturn in?

[heehaw]

Yes! I was flabbergasted when the first extra-solar planets turned out to be "Jupiters in Mercury orbits". But apparently under gravitation, planets can and do migrate. Whoda thunkit?

[Chris Peterson]

Sometimes the physics of astronomy simply astonishes you. I am referring to a comment by Ann that Jupiter once moved inward in the solar system to 1.5 AU, but then was pulled out by Saturn to its present position. That is even though the mass of Jupiter is over 3 times the mass of Saturn! It defies what I learned in school about Newtonian physics. It is like the wrestler Hulk Hogan getting into a tug-of-war match with 90 pound Twiggy - and Twiggy wins! You wonder why the much heavier Jupiter didn't pull Saturn in?

It did. The movements were caused by resonances. Small forces applied at the right time can make big changes in orbits (not unlike pushing someone on a swing). Jupiter was the dominant player in all of the shifting of orbits.

[stevie]

The shadow makes Ganymede looks so close to Jupiter. Perspective is crazy fun. Great pic!

[Ann]

Ross Pomeroy of Real Clear Science wrote:

4. What's Up With Jupiter? […] Moreover, he writes, most gas giants in other solar systems have tight, eccentric orbits around their stars, while Jupiter's is wide and nearly circular. "Only about one in a hundred stars like the Sun has a Jupiter like ours," he wrote.

I’d say that has all the signs of discovery bias. Exoplanet discovery methods finds close orbits more easily. That says very little about the amount of planets out there in wide 12-earth-years orbits.
[Just because you can see better in a lamp’s glow, that doesn’t mean that is were lost items are (Streetlight effect).]

Ethan Siegel has written an interesting article on observation bias in our search for exoplanets:

Ethan Siegel wrote:

In a surprising find, the most abundant type of planet discovered so far is neither a gas giant nor a rocky planet, but rather a new class of planet in between the two: best known as super-Earths. But are super-Earths really the most common type of planet in the Universe, or is this simply a way that our current data and capabilities are fooling us?
...

In other words, we disproportionately detected the easiest types of planets that we could detect with the specific method we were using.
...

When we take into account all of the nearly 5000 exoplanets known at the start of 2022, we can see that the greatest number of planets can be found in between the sizes of Earth (at -1.0 on the x-axis) and Neptune (at -0.5 on the x-axis). However, that does not mean that those worlds are the most abundant, nor that they’re even, as we’ve been calling them, super-Earth worlds. (Credit: Open Exoplanet Catalogue)

---

I highly recommend the rest of Ethan Siegel's text. Anyway, you are right: Finding Jupiter-like planets (or any planets) in Jupiter-like orbits is sufficiently hard that we are going to find disproportionately fewer of them than we find planets in tighter orbits. If a planet is in a large orbit, even if its orbit is perfectly aligned with its sun from our perspective, we are likely to have to wait for many years to catch it crossing the face of its sun. If we use the stellar wobble method, we will have to wait for a planet to make three orbits before we can know if the star is wobbling because of a planet tugging at it - and for a planet moving at the speed of Jupiter in an orbit the size of Jupiter's orbit, we would have to wait - well, would it be 36 years?

I also think, although I may certainly wrong about that, that a planet in a tight orbit has a greater chance of being seen transiting the disk of its sun than a planet in a large orbit, simply because it seems to me that the effects of the "misalignment" of the orbit would grow, the larger the size of the orbit is.

Transit of Mercury. Image: NASA/GSFC/TRACE/SMEX

---

I also very much recommend what Ethan Siegel has to say about so called "super-Earths". Very interesting!

Ann

[VictorBorun]

JovianEclipse1024c.jpg
Don't you wish there were a cutaway view that accurately showed
through the clouds to Jupiter's surface? Does Jupiter even have a
surface?

Define "surface". Jupiter almost certainly has a solid core, deep in its interior, which you would reach by descending through the gases and then exotic fluid layers. Whether that should be called its "surface" or not is debatable.

An evident way to define surface of a galaxy or a galaxy cluster is to mark all the points of maximum free fall acceleration.
However for a planet it gives such absurd as proclaiming Earth's core/mantle interface as surface


I think we humans readily sympathise with birds, squirrels, fish and moles. For us a surface is a relief that exists for years while the air and water masses come and go in a week or sooner.

The Red Spot, rather orange in this APOD, is there for a hundred years; therefore it is an island drifting in the ocean

[johnnydeep]

JovianEclipse1024c.jpg
Don't you wish there were a cutaway view that accurately showed
through the clouds to Jupiter's surface? Does Jupiter even have a
surface?

Define "surface". Jupiter almost certainly has a solid core, deep in its interior, which you would reach by descending through the gases and then exotic fluid layers. Whether that should be called its "surface" or not is debatable.

An evident way to define surface of a galaxy or a galaxy cluster is to mark all the points of maximum free fall acceleration.
However for a planet it gives such absurd as proclaiming Earth's core/mantle interface as surface


I think we humans readily sympathise with birds, squirrels, fish and moles. For us a surface is a relief that exists for years while the air and water masses come and go in a week or sooner.

The Red Spot, rather orange in this APOD, is there for a hundred years; therefore it is an island drifting in the ocean

Very interesting definition of "surface"! I found that graph and corresponding discussion with math here - https://en.wikipedia.org/wiki/Gravity_of_Earth#Depth