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midtskogen
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04 Jan 2022 15:59

Has there been any more evidence of Planet 9? How many other planets could be in our system without us really knowing about it?
I think that is more a question about how we define a planet.  There probably wont be a planet 9, because we wont define it as a planet.
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04 Jan 2022 18:09

Has there been any more evidence of Planet 9? How many other planets could be in our system without us really knowing about it?
I think that is more a question about how we define a planet.  There probably wont be a planet 9, because we wont define it as a planet.
How can something multiple Earth masses not be a planet? Not cleared its orbit? That doesn't make any sense. We are going to have to change that definition I would think. I would definitely strongly disagree with the notion of such a world not being considered a planet, really. 

Unless of course...it is a small black hole. Like some have said. 
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midtskogen
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05 Jan 2022 01:55

How can something multiple Earth masses not be a planet? Not cleared its orbit? That doesn't make any sense
Yes, that requirement is troublesome, and might not even make sense.  If a Planet Nine of that size is found, how can we even know for sure that it has cleared its orbit, under whatever definition of "cleared"?
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05 Jan 2022 13:57

How can something multiple Earth masses not be a planet? Not cleared its orbit? That doesn't make any sense
Yes, that requirement is troublesome, and might not even make sense.  If a Planet Nine of that size is found, how can we even know for sure that it has cleared its orbit, under whatever definition of "cleared"?
Then we need to change that definition. 
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midtskogen
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06 Jan 2022 13:03

Then we need to change that definition. 
Definitions are not easily changed, nor was the current definition.  Maybe if a "Planet Nine" is found, that discovery could trigger a new definition.
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07 Jan 2022 21:26

There probably wont be a planet 9, because we wont define it as a planet.
If it exists and has the expected mass and orbit (between about 5 and 9 Earth masses and between 360 and 620 AU according to dynamical estimates and constraints from lack of prior detection in surveys), then we probably would define it as a planet, because it would meet the criteria for orbital clearing timescales (e.g. "Margot's Pi"):

Image

Margot's Pi is a useful definition for this case because it only requires knowledge of the object's mass, orbital distance, and the star's mass, yet is completely consistent with a definition that involves observing all the objects in that orbit. (You can think of Margot's Pi as the physics that predicts why the other definitions work, like Soter's mu.)

Extending this plot out to 1000 AU and adding in a point for Planet 9 with its uncertainties in mass and orbital distance, it lands here:

Image

This is an interesting position. Even with the uncertainties, it would be massive enough to clear out its neighborhood on relevant timescales. We would expect it to be a unique object in its neighborhood, and not a member of a belt of similar objects, like Ceres in the asteroid belt, or Pluto and Eris in the Kuiper belt. 

However, it would also be the closest object to that boundary between planets and dwarf planets known in the solar system. That, and especially if it turns out to actually lie even closer to it, could pose a challenge to how we define a planet. It challenges the notion that there are two distinct populations of objects in this mass-distance phase space with an empty region in between. We expect there to be this separation not just because it fits solar system objects, but because if an object is accreting and reaches this threshold, then it very rapidly begins accreting more from the neighborhood. So it is unlikely for an object's growth to stop close to that boundary. The most likely explanation for how one could exist like that is if it formed closer to the star and then was scattered there (which is in fact one of the leading explanations for Planet 9's origins.)

A much more difficult challenge for planetary classification would be if Planet 9 is confirmed, lies much closer to the planet / dwarf-planet boundary, and is somehow shown beyond reasonable doubt to have formed there in situ. 
 
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10 Jan 2022 21:38

Then we need to change that definition. 
Definitions are not easily changed, nor was the current definition.  Maybe if a "Planet Nine" is found, that discovery could trigger a new definition.
What if it fulfill's Bode's "Law"-- how far away would the next body after Neptune have to be to obey Bode's "Law" anyway?
 
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11 Jan 2022 05:22

how far away would the next body after Neptune have to be to obey Bode's "Law" anyway?
Well, Bode's "Law" appears to stop working beyond Uranus anyway.
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11 Jan 2022 23:25

how far away would the next body after Neptune have to be to obey Bode's "Law" anyway?
Well, Bode's "Law" appears to stop working beyond Uranus anyway.
But it seems to work for exoplanetary systems?
https://en.wikipedia.org/wiki/Titius%E2 ... ry_systems
Recent astronomical research suggests that planetary systems around some other stars may follow Titius–Bode-like laws.[sup][27][/sup][sup][28][/sup] Bovaird and Lineweaver[sup][29][/sup] applied a generalized Titius–Bode relation to 68 exoplanet systems that contain four or more planets. They showed that 96% of these exoplanet systems adhere to a generalized Titius–Bode relation to a similar or greater extent than the Solar System does. The locations of potentially undetected exoplanets are predicted in each system.
Subsequent research detected five planet candidates from the 97 planets predicted for the 68 planetary systems. The study showed that the actual number of planets could be larger. The occurrence rates of Mars- and Mercury-sized planets are currently unknown, so many planets could be missed due to their small size. Other possible reasons that may account for apparent discrepancies include planets that do not transit the star or circumstances in which the predicted space is occupied by circumstellar disks. Despite these types of allowances, the number of planets found with Titius–Bode law predictions was lower than expected.[sup][30][/sup]
In a 2018 paper, the idea of a hypothetical eighth planet around TRAPPIST-1 named "TRAPPIST-1i," was proposed by using the Titius–Bode law. TRAPPIST-1i had a prediction based exclusively on the Titius–Bode law with an orbital period of 27.53 ± 0.83 days.[sup][31][/sup]
Finally, raw statistics from exoplanetary orbits strongly point to a general fulfillment of Titius–Bode-like laws (with exponential increase of semi-major axes as a function of planetary index) in all the exoplanetary systems; when making a blind histogram of orbital semi-major axes for all the known exoplanets for which this magnitude is known, and comparing it with what should be expected if planets distribute according to Titius–Bode-like laws, a significant degree of agreement (i.e., 78%)[sup][32][/sup] is obtained
 
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12 Jan 2022 07:37

But it seems to work for exoplanetary systems?
But the question is, how predictive is the law for planets far from their star?
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13 Jan 2022 01:08

But it seems to work for exoplanetary systems?
But the question is, how predictive is the law for planets far from their star?
Probably not much in its original form, I noted that the formula changes when talking about different systems....it was reformulated to account for the outer planets below
Titius-Bode Law: Blagg formulation
Note how her new formulation also worked for the satellite systems of the outer planets

Mod edit: Removed a bunch of glaring, copy-pasted text from the above wikipedia article. If you feel more explanation is needed then write your own, which is nicer for readers. Otherwise, let the link and your short description do the job. Sometimes less is more. :) --Wats
Last edited by A-L-E-X on 13 Jan 2022 01:13, edited 3 times in total.
 
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01 Feb 2022 22:20

Are there any regions on the Moon Io that are generally stable / could support artificial structures? Is the Radiation on the moon less on the side that faces away from Jupiter? 
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Re: Science and Astronomy Questions

24 Apr 2022 00:25

If I shone a a powerful, non-burning laser light into into a telescope and pointed it at a star, would anyone there be able to detect it?
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Re: Science and Astronomy Questions

24 Apr 2022 15:54

If I shone a a powerful, non-burning laser light into into a telescope and pointed it at a star, would anyone there be able to detect it?
I'm not sure a telescope works if you use it backwards.
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Re: Science and Astronomy Questions

25 Apr 2022 21:05

If I shone a a powerful, non-burning laser light into into a telescope and pointed it at a star, would anyone there be able to detect it?
No, but let's see why. What we'll discover is that it would be slightly better (but still quite hopeless) to ditch the telescope and simply shine the laser directly at the star.

I'm not sure a telescope works if you use it backwards.
It does! A telescope works by taking in parallel beams of light through the aperture, focusing them, and projecting them out of the eyepiece to form a magnified and brightened image for your eye. If we instead shine parallel beams of light (a laser) into the eyepiece, they will be expanded and projected out of the aperture. 

You can try this experiment yourself with a (preferably weak) laser pointer and aiming the telescope at a wall. You'll see an expanded laser dot, similar in size to the size of the telescope's aperture. The spot will have a lower surface brightness because the same amount of light was spread over a larger area.

So why can't we just aim this bigger spot at a distant star and have someone there see it? The problem is not the telescope, but the nature of light. Light has a wave-like behavior, and the waves spread apart a little bit as they travel. So even though we usually think of a laser as a perfectly parallel beam of light, it still gets wider as it travels. This is the real problem. The minimum possible divergence angle depends on the wavelength of the light and the radius of the laser beam at its narrowest point. The narrower the beam and the larger the wavelength of light, the more the beam must diverge. For typical visible-light handheld laser pointers this angle is about 1.2 milliradians or 0.069 degrees. 

Let's imagine taking a 5mW green (532nm) handheld laser pointer (beam radius a little less than 1mm) and shine it through an 8" (20cm) aperture telescope. This immediately expands the laser beam to a width of 20cm. Let's aim the telescope to shine it at the nearest star system, Alpha Centuari, 4.2 light years away. The divergence of the beam remains 1.2 milliradians, which means that for every kilometer it travels, the beam spreads out by another 1.2 meters.

When the beam reaches Alpha Centuari, it will be more than 600 AU across.

Let's do a little calculation. The intensity of our laser beam at the source is 5mW spread over a spot about 1mm in radius, which is about 500 watts per square meter. This is about half the intensity of sunlight at Earth's surface. After being expanded by the telescope, the intensity is about 0.16 watts per square meter.  Still bright enough to see if in the dark. But when arriving at Alpha Centuari and being spread over the whole star system, the intensity is a mere 10^-31 watts per square meter.  There is little hope in anyone being able to detect it, even if the astronomers there could filter out the light of Sol, or even the rest of the light coming from Earth. If we imagine you leave the laser shining at Alpha Centuari forever, and someone there keeps a similar 20cm diameter telescope aimed at Earth, they need to wait on average half a million years to receive a single photon of your laser! (Which would be swamped by all sorts of other photons in the meantime. For perspective the laser altogether is emitting about 10^16 photons per second!) 

Ditching the telescope and pointing the laser directly at Alpha Centuari would give us roughly the same result. The difference is that it would take out the first 20cm of beam expansion by the telescope, equivalent to removing about 200 meters of beam divergence out of the 4.2 light years to Alpha Centuari. (Woohoo?) We'd also prevent a little bit of the laser light from being absorbed by the mirrors or lenses, since no mirror or lens is perfect.

So, little hope of signaling other star systems with a handheld laser, and if we want to do it anyway then we may as well not use telescopes. But is signaling with lasers completely hopeless? 


What if we use a better laser?  


To optimize our chances, we should maximize the amount of light we throw out, and minimize how rapidly the beam spreads out. So we should use a more powerful laser, a shorter wavelength of light (maybe violet, since anything shorter will be scattered or absorbed too much by the atmosphere), and a larger initial beam diameter. The shorter wavelength of light and larger beam diameter both allow us to reduce the beam divergence, achieving a smaller spot at the target star. (This is an unintuitive thing about lasers: a larger beam can be made to spread apart more slowly, which over large distances allows us achieve a smaller spot size!)

If we were to shine a 1 megawatt violet (400nm) beam with a 1 meter initial diameter, then we could theoretically achieve a spot size at Alpha Centuari of just 10 million kilometers, or about 33 light seconds diameter. A huge improvement. We also have more light in the beam altogether, and the intensity of the beam at Alpha Centauri would be about 10^-15 watts per square meter.  Very weak, but at least now it's in the realm of "detectable with a lot of effort". A 10m wide telescope would expect to receive hundreds of thousands of photons of the laser beam per second. Detectable if the light of Sol were excluded (could be blocked with an occulting disk in their telescope), and it would be a few times stronger signal than the light of Earth itself.

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