1071: "Exoplanets"

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jewish_scientist
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Re: 1071: "Exoplanets"

Postby jewish_scientist » Fri Dec 01, 2017 12:16 am UTC

Soupspoon wrote:
jewish_scientist wrote:Wait, Project Orion allows interstellar travel? Just how fast can these ships suppose to go!

Do you mean "allows interstellar travel within a reasonable length of voyage time?"? We already got some Voyagers/Pioneers escaping the Solar system gravity well, and they're just drifting slowly now, but they'll probably get somewhere, sometime, if there's not too many hazards like Oumuamua or bored Klingons out there to destroy them.

(But putting a nominal 1G ac(then de)celeration and a stellar distance of your choice into this might be useful. Alpha Centauri within 6 years, or 3.5 in ship-time, assuming you can pack the bomb-fuel necessary to do that.)

The maximum kinetic energy for that trip would be ~2 * 10^11 mega-joules/ kilogram. For comparison, the strongest nuclear bomb ever detonated, the Tzar Bomba, had a yield of ~2 * 10^11 mega-joules/ kilogram. How much fuel were these ships planning to take with them?

P.S. I would define the size of a solar system, galaxy, etc. by the size of its gravity well in some way.
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Re: 1071: "Exoplanets"

Postby Soupspoon » Fri Dec 01, 2017 1:01 am UTC

jewish_scientist wrote:How much fuel were these ships planning to take with them?
A handwavium amount, obviously.
:P

P.S. I would define the size of a solar system, galaxy, etc. by the size of its gravity well in some way.

https://en.wikipedia.org/wiki/Hill_sphere would probably be where to start that defining. Applied to stellar systems, a 'foam' of packed bubbles would define the regions of interstellar space that are most influenced by a given star, though that'd be dynamic due to the movement of the stars themselves, certainly at current interstellar cruising speeds.

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Re: 1071: "Exoplanets"

Postby gmalivuk » Fri Dec 01, 2017 4:44 pm UTC

jewish_scientist wrote:The maximum kinetic energy for that trip would be ~2 * 10^11 mega-joules/ kilogram. For comparison, the strongest nuclear bomb ever detonated, the Tzar Bomba, had a yield of ~2 * 10^11 mega-joules/ kilogram. How much fuel were these ships planning to take with them?
Tsar Bomba had a yield of 2e11 MJ total, not per kilogram. But in any case, that's the energy you'd need to impart all at once from Earth if you wanted it to go that fast, but rockets can be more efficient because adding a specific amount of momentum (which is what rocket exhaust does) when the vehicle is already at high velocity adds more kinetic energy than the same amount of momentum when it's at low velocity.
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Re: 1071: "Exoplanets"

Postby Soupspoon » Thu Dec 14, 2017 11:53 pm UTC

We've now actually found a planetary system as populated as ours. Like our system with eight planets but what about Pluto? *grumble grumble*, there's now officially eight planets for Kepler 90's and I'm not holding my breath to detect their Pluto.…

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Re: 1071: "Exoplanets"

Postby orthogon » Fri Dec 15, 2017 10:15 am UTC

Soupspoon wrote:We've now actually found a planetary system as populated as ours. Like our system with eight planets but what about Pluto? *grumble grumble*, there's now officially eight planets for Kepler 90's and I'm not holding my breath to detect their Pluto.…


Maybe the elusive Kepler-90a is their Pluto?

(BTW I love the way we're saying "their" rather than "its"! >impying there's a "they").
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1071: "Exoplanets"

Postby Raidri » Fri Dec 15, 2017 11:44 am UTC

orthogon wrote:
Soupspoon wrote:We've now actually found a planetary system as populated as ours. Like our system with eight planets but what about Pluto? *grumble grumble*, there's now officially eight planets for Kepler 90's and I'm not holding my breath to detect their Pluto.…


Maybe the elusive Kepler-90a is their Pluto?

Kepler-90a is the not-so-elusive star itself (see https://en.wikipedia.org/wiki/Exoplanet ... convention).

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Re: 1071: "Exoplanets"

Postby speising » Fri Dec 15, 2017 11:54 am UTC

yeah, it's a stupid convention.
just about every sf book since ever has numbered planets with roman numerals from I (Sol III), why didn't astromomers adopt that sensible notation?

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Re: 1071: "Exoplanets"

Postby Lothario O'Leary » Fri Dec 15, 2017 1:00 pm UTC

speising wrote:yeah, it's a stupid convention.
just about every sf book since ever has numbered planets with roman numerals from I (Sol III), why didn't astromomers adopt that sensible notation?
Now I'm wondering what would Earth be called under the astronomical exoplanet convention, assuming Sol for the star.

If all the planets are considered to be discovered at the same time, probably Sol d; in realistic order of discovery... probably either Sol f or Sol g, after Jupiter, Saturn, Neptune, Uranus, and possibly Venus, probably in that order.

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Re: 1071: "Exoplanets"

Postby orthogon » Fri Dec 15, 2017 1:13 pm UTC

Lothario O'Leary wrote:
speising wrote:yeah, it's a stupid convention.
just about every sf book since ever has numbered planets with roman numerals from I (Sol III), why didn't astromomers adopt that sensible notation?
Now I'm wondering what would Earth be called under the astronomical exoplanet convention, assuming Sol for the star.

If all the planets are considered to be discovered at the same time, probably Sol d; in realistic order of discovery... probably either Sol f or Sol g, after Jupiter, Saturn, Neptune, Uranus, and possibly Venus, probably in that order.

I was trying to work out your order, but I now realise you probably mean "realistic order of discovery by observers from another solar system". I was trying to figure out the actual historical order of discovery by humans -- I thought you were positing that in a sense Venus and Mars were discovered before Earth, in that they were identified as celestial bodies by proto-astronomers before Earth itself was recognised as being the same type of thing as them.
xtifr wrote:... and orthogon merely sounds undecided.

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Re: 1071: "Exoplanets"

Postby Soupspoon » Fri Dec 15, 2017 6:24 pm UTC

I was thinking that "Sol b" would have been the Moon (later deprecated), at first (c-g being one order or other of the five other pre-George's Star 'discoveries').

But then I realised that it would maybe be "Terra c" (assuming b being Sol, maybe verse-vica if otherwise) until shown otherwise.

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Re: 1071: "Exoplanets"

Postby Lothario O'Leary » Fri Dec 15, 2017 8:48 pm UTC

orthogon wrote:
Lothario O'Leary wrote:
speising wrote:yeah, it's a stupid convention.
just about every sf book since ever has numbered planets with roman numerals from I (Sol III), why didn't astromomers adopt that sensible notation?
Now I'm wondering what would Earth be called under the astronomical exoplanet convention, assuming Sol for the star.

If all the planets are considered to be discovered at the same time, probably Sol d; in realistic order of discovery... probably either Sol f or Sol g, after Jupiter, Saturn, Neptune, Uranus, and possibly Venus, probably in that order.

I was trying to work out your order, but I now realise you probably mean "realistic order of discovery by observers from another solar system". I was trying to figure out the actual historical order of discovery by humans -- I thought you were positing that in a sense Venus and Mars were discovered before Earth, in that they were identified as celestial bodies by proto-astronomers before Earth itself was recognised as being the same type of thing as them.
I didn't even think of that possibility. I was, indeed, considering a "realistic" order of discovery if Earth was an exoplanet (i.e. from another solar system).
Soupspoon wrote:I was thinking that "Sol b" would have been the Moon (later deprecated), at first (c-g being one order or other of the five other pre-George's Star 'discoveries').

But then I realised that it would maybe be "Terra c" (assuming b being Sol, maybe verse-vica if otherwise) until shown otherwise.
I'd say it probably went like this: Sun and Moon (likely, but not certainly, in that order), Mars (obvious color), Jupiter, Venus (it took a while to figure out it wasn't two celestial bodies), Mercury (inconvenient to observe); then eventually Earth joined the list, and it stayed stable until the discovery of Georgium Sidus.

This means that, as of just before Earth would have joined the list, the numbering would've been Terra b-g for Sun and Moon, Mars, Jupiter, Venus and Mercury respectively.
When Earth does join the list, things depend on which description we use (Ptolemaic, Copernician, Tychonic, or one of the weird versions from the early heliocentrists).

Assuming a straight switch from Ptolemaic to Copernician, we probably would've, theoretically, ended up with either Sol b-e (renumbered) or Sol d-g (keeping the numbers) for Mars, Jupiter, Venus and Mercury.
In the former case Earth is straightforwardly Sol f (Georgium Sidus/Uranus is Sol g, Ceres is Sol h, and it gets messy after that); in the latter case, Earth is probably Sol h (but could in principle also end up as Sol b).

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Re: 1071: "Exoplanets"

Postby Soupspoon » Fri Dec 15, 2017 9:20 pm UTC

(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

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Re: 1071: "Exoplanets"

Postby rmsgrey » Sat Dec 16, 2017 2:01 am UTC

Soupspoon wrote:(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

Jupiter is rather more visible than Saturn - even at its dimmest, Jupiter is brighter than any star (except the Sun); Saturn has two to five stars brighter than it, depending on relative position in orbit, and moves rather slower.

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Re: 1071: "Exoplanets"

Postby Soupspoon » Sat Dec 16, 2017 1:05 pm UTC

rmsgrey wrote:
Soupspoon wrote:(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

Jupiter is rather more visible than Saturn - even at its dimmest, Jupiter is brighter than any star (except the Sun); Saturn has two to five stars brighter than it, depending on relative position in orbit, and moves rather slower.

I thought about that afterwards (except that a Saturn's visibility is variable according to rings-inclination, as well as relative position in solar orbit for best bodily albedo, and hadn't gotten round to finding out what degree of magnitudes shift that might contribute). So probably last of the five (non-Earth) antiquity planets, then. But still there in the original "catalogue".

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Re: 1071: "Exoplanets"

Postby rmsgrey » Sat Dec 16, 2017 8:51 pm UTC

Soupspoon wrote:
rmsgrey wrote:
Soupspoon wrote:(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

Jupiter is rather more visible than Saturn - even at its dimmest, Jupiter is brighter than any star (except the Sun); Saturn has two to five stars brighter than it, depending on relative position in orbit, and moves rather slower.

I thought about that afterwards (except that a Saturn's visibility is variable according to rings-inclination, as well as relative position in solar orbit for best bodily albedo, and hadn't gotten round to finding out what degree of magnitudes shift that might contribute). So probably last of the five (non-Earth) antiquity planets, then. But still there in the original "catalogue".


Mercury's also tricky - its brightest is brighter than any actual star except the Sun, but its dimmest is dimmer than Uranus' brightest, and it spends all its time pretty close to the Sun, which makes it tricky to pick out...

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Re: 1071: "Exoplanets"

Postby pogrmman » Sun Dec 17, 2017 5:49 pm UTC

rmsgrey wrote:
Soupspoon wrote:
rmsgrey wrote:
Soupspoon wrote:(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

Jupiter is rather more visible than Saturn - even at its dimmest, Jupiter is brighter than any star (except the Sun); Saturn has two to five stars brighter than it, depending on relative position in orbit, and moves rather slower.

I thought about that afterwards (except that a Saturn's visibility is variable according to rings-inclination, as well as relative position in solar orbit for best bodily albedo, and hadn't gotten round to finding out what degree of magnitudes shift that might contribute). So probably last of the five (non-Earth) antiquity planets, then. But still there in the original "catalogue".


Mercury's also tricky - its brightest is brighter than any actual star except the Sun, but its dimmest is dimmer than Uranus' brightest, and it spends all its time pretty close to the Sun, which makes it tricky to pick out...


Yeah. I’d say Saturn probably was discovered before Mercury. Sure, it doesn’t move as fast, but you usually need a mostly unobstructed horizon to see Mercury. I guess it might have been spotted by people looking at sunsets, but how easy is it to recognize that a spot is moving over time when you can’t really the other spots near it (because of the sun).

Saturn, on the other hand, is bright and can be seen to change positions somewhat year to year. It’s not as obvious as Jupiter and Venus, but it’s still kind of clearly not a star.

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Re: 1071: "Exoplanets"

Postby Lothario O'Leary » Wed Dec 20, 2017 6:36 pm UTC

pogrmman wrote:
rmsgrey wrote:
Soupspoon wrote:
rmsgrey wrote:
Soupspoon wrote:(You seem to have missed Cronus, but which (theoretically discovered) way round with Jove I wouldn't know.)

Jupiter is rather more visible than Saturn - even at its dimmest, Jupiter is brighter than any star (except the Sun); Saturn has two to five stars brighter than it, depending on relative position in orbit, and moves rather slower.

I thought about that afterwards (except that a Saturn's visibility is variable according to rings-inclination, as well as relative position in solar orbit for best bodily albedo, and hadn't gotten round to finding out what degree of magnitudes shift that might contribute). So probably last of the five (non-Earth) antiquity planets, then. But still there in the original "catalogue".


Mercury's also tricky - its brightest is brighter than any actual star except the Sun, but its dimmest is dimmer than Uranus' brightest, and it spends all its time pretty close to the Sun, which makes it tricky to pick out...


Yeah. I’d say Saturn probably was discovered before Mercury. Sure, it doesn’t move as fast, but you usually need a mostly unobstructed horizon to see Mercury. I guess it might have been spotted by people looking at sunsets, but how easy is it to recognize that a spot is moving over time when you can’t really the other spots near it (because of the sun).

Saturn, on the other hand, is bright and can be seen to change positions somewhat year to year. It’s not as obvious as Jupiter and Venus, but it’s still kind of clearly not a star.

I did miss Saturn; I'm not sure how exactly. I'd guess that I probably remembered that the list was supposed to have seven, and forgot that Earth wasn't one of those seven.
I agree that Saturn came before Mercury (but well after Mars and Jupiter).

The problem with Venus, as I alluded to in my previous comment, is essentially that I suspect that at some point the "catalogue" would've probably contained Hesperus and Phosphorus separately (perhaps even briefly in two copies each, corresponding to Venus and Mercury), and I have no idea where might the realization of their identity fall with respect to the initial "discoveries" of Saturn and Mercury (especially since said realization seems to have happened at different times in different civilizations) - or what does this mean for the numbering, for that matter.
In fact, the initial identification of Hesperus!Venus and Phosphorus!Venus as (at that point separate) non-stellar objects might well have predated the identification as such of Jupiter (not sure about Mars).


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