The Astronomy Thread

[Arioch]

Day and night are pretty basic storytelling tools, so to have a system where both are variable really puts a lot of work on the author -- not just to work out the physics, but also a significant amount of expositional effort to explain what's going on to the reader. Until you've tried, you don't realize that you can no longer use basic concepts like "yesterday" and "tomorrow," etc.

However, the reality is that the majority of solar systems have more than one star.

[Mr Bojangles]

Day and night are pretty basic storytelling tools, so to have a system where both are variable really puts a lot of work on the author -- not just to work out the physics, but also a significant amount of expositional effort to explain what's going on to the reader. Until you've tried, you don't realize that you can no longer use basic concepts like "yesterday" and "tomorrow," etc.

However, the reality is that the majority of solar systems have more than one star.

No doubt, Arioch. Trying to write about a multi-star system would be a very large effort. It's probably why we don't encounter it more in novels and such (physics aside). But, it's not too uncommon in sci-fi stories in a visual medium, because it's easier to show people two suns, rather than tell people about two suns. Like I said, it makes for a nice shot.

[discord]

or one of my favorite lines,

Jack: Three suns?
Shazza: Bloody Hell.

[Trantor]

Milky Way in one pic. 9GP, zoomable:

https://www.eso.org/public/images/eso1242a/zoomable/

[Arioch]

Japan's Subaru 8-meter telescope on Mauna Kea in Hawaii has directly imaged a large exoplanet around 168 LY-distant Kappa Andromedae.



The planet (κAnd b) is 12.8 Jupiter masses, putting it right at the edge of being a brown dwarf, and orbits 55 AU from the primary (which is almost twice Neptune distance). The primary is a blue-white B9 subgiant of about 2.5 solar masses.

κAnd b is one of about a dozen exoplanets to be directly imaged, and is the most massive so far (that I'm aware of). In addition to the large size and scale of the system, it's notable that massive stars have very short lifespans, so κAnd is probably only about 30 million years old.

[GeoModder]

Given that I read planetary systems can form in as little as 3 million years, seems 30 million years is plenty of time.

I'm not sure exactly why the planet is designated "b" rather than "a", but those astronomers are a quirky lot.

The "a" annex is kept for the star itself, just in case its necessary.

[Absalom]

Given that I read planetary systems can form in as little as 3 million years, seems 30 million years is plenty of time.

I half-recall reading that gas giants can form in maybe a few hundred years, though admittedly I don't know if that was after things had gotten started.

I'm not sure exactly why the planet is designated "b" rather than "a", but those astronomers are a quirky lot.

The "a" annex is kept for the star itself, just in case its necessary.

How in the world would that come up?

[Mikk]

I'm not sure exactly why the planet is designated "b" rather than "a", but those astronomers are a quirky lot.

The "a" annex is kept for the star itself, just in case its necessary.

How in the world would that come up?

Could be that we end up needing a way to disambiguate between planetary system + star from just referring to the star. :shrug:

[Karst45]

Say anyone found Russell's teapot?

[Trantor]

Say anyone found Russell's teapot?

That´d be devastating...

[Victor_D]

Tau Ceti's planets nearest around single, Sun-like star

Potentially Habitable Planet Detected Around Nearby Star

Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth — a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.

With a minimum mass just 4.3 times that of Earth, this potential planet would be the smallest yet found in the habitable zone of a sun-like star if it's confirmed, researchers said.

---

These periodicities could be interpreted as corresponding to planets on dynamically stable close-circular orbits with periods of 13.9, 35.4, 94, 168, and 640 days and minimum masses of 2.0, 3.1, 3.6, 4.3, and 6.6 M⊕, respectively.

Time to update the info on the 6 worlds of 'humaniti'?

[Arioch]

Unfortunately, a 4.3 Earth mass planet would probably not be habitable by humans; an Earth-composition atmosphere with those gravity levels would probably be dense enough to be toxic.

Luckily, the details of the Earth settlements are largely inconsequential to the Outsider story (being leftovers from the ancestor story that preceded it).

[Victor_D]

Unfortunately, a 4.3 Earth mass planet would probably not be habitable by humans; an Earth-composition atmosphere with those gravity levels would probably be dense enough to be toxic.

Tau Ceti has lower metallicity than our Sun, so it is conceivable that the planet (if it really exists) has a much lower average density (smaller/lighter core) than Earth. It would probably be covered by a global ocean considering how much cometary debris there is in the system, but with some luck, it could be colonizable...?

[discord]

gravity is not linear with mass, nor volume.

the moon is not 1/6th the mass or earth after all.

[Absalom]

"A lot of cometary debris" is likely to mean a lot of impacts to the planet, which in turn means a bad candidate for colonization for a very long time.

[Victor_D]

I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

That's what I'd call 'winning a jackpot'

"A lot of cometary debris" is likely to mean a lot of impacts to the planet, which in turn means a bad candidate for colonization for a very long time.

Not for a civilization capable of interstellar travel. Even if large impacts occurred 10-times as often as here (say, every 5 million years on average), the likelihood of it occurring in a timespan sentients can comprehend is low. And if it were to occur, then they'd simply deflect the comet (or capture it and turn it into an in-orbit refuelling station).

[GeoModder]

I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

Interesting thought.
But with the other planet candidates around Tau Ceti having large masses as well, I'd doubt it.

[Arioch]

Tau Ceti has lower metallicity than our Sun, so it is conceivable that the planet (if it really exists) has a much lower average density (smaller/lighter core) than Earth. It would probably be covered by a global ocean considering how much cometary debris there is in the system, but with some luck, it could be colonizable...?

Yes, this is the other disquieting fact that I have discovered since the creation of the Humanity worlds -- both Tau Ceti and 82 Eridani are ancient stars with very low metallicities. Some had even speculated that such systems might not have terrestrial planets (due to a lack of elements heavier than H and He). The good news about the Tau Ceti finding is that it does appear to have terrestrial-size planets, but it's not clear to me what the compositions of such planets would be like. My idea when I learned about the metallicity issue was that Aldea (at Tau Ceti) might have very deep oceans... like, deeper than the mantle.


But, again, it's mostly irrelevant to the story. Even in a potential sequel, I don't think these particular worlds would be visited.

Science fiction universes have to exist in kind of an "alternate future" from ours, as discoveries and the advance of history will inevitably contradict whatever an author establishes. Bradbury and Heinlein don't have to rewrite Martian Chronicles or Stranger in a Strange Land just because we've discovered that there isn't an alien civilization on Mars, and we to don't have to amend Star Trek's history to compensate for the fact that there wasn't a Eugenics War in the 1990's.

I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

It's a good point that the masses of any such companions would include any moons. I don't see any reason why a planet might not have a very large moon; the collision that formed our moon as with a Mars-sized planet; a slightly different impact might have resulted in a much larger moon. Planets (or at least dwarf planets) do exist with very large moons; Charon is something like 12% the mass of Pluto.

[Victor_D]

But, again, it's mostly irrelevant to the story. Even in a potential sequel, I don't think these particular worlds would be visited.

Science fiction universes have to exist in kind of an "alternate future" from ours, as discoveries and the advance of history will inevitably contradict whatever an author establishes. Bradbury and Heinlein don't have to rewrite Martian Chronicles or Stranger in a Strange Land just because we've discovered that there isn't an alien civilization on Mars, and we to don't have to amend Star Trek's history to compensate for the fact that there wasn't a Eugenics War in the 1990's.

Of course. I just thought that since the human worlds haven't yet appeared 'officially' in your work, it's still time to adjust the 'canon' so that it reflects the reality better I am fine with it either way, it's a minor and unimportant detail as you said

[GeoModder]

I had an idea - what if the 4.3 Earth-mass planet isn't one planet, but a binary super-terrestrial planet? Say 2.5 + 1.8 planets orbiting a common centre of gravity?

It's a good point that the masses of any such companions would include any moons. I don't see any reason why a planet might not have a very large moon; the collision that formed our moon as with a Mars-sized planet; a slightly different impact might have resulted in a much larger moon. Planets (or at least dwarf planets) do exist with very large moons; Charon is something like 12% the mass of Pluto.

You could turn it around, and let Aldea be the "moon" of a 3 Earth-mass ocean world or something.
Besides, this 4.3 E-m number is the minimum mass. The real mass depends on the angle on which those planets orbit around Tau Ceti as seen from Earth. So probably the masses of those planets will turn out to be higher.