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Scenario

We've got a terrestial exoplanet three times the size of Earth, 1800 light years away and with an atmosphere similar in composition to Earth's. It is determined via studies that atmosphere would be able to support terrestial life without needing any life support. More studies and in greater detail are made en route to the system which would prove that the assumptions made back on Earth weren't fully right.

Questions

  • How precise information would humans be able to get about its atmosphere with current tech or with a near future tech (by near I mean no more than 4 decades into the future)?
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    $\begingroup$ When would your ship sail from Earth? Because you mentioned no more than 4 decades into the future. While we have technology to travel to other star systems we don't have infrastructure (or even engineering knowhow) to do so. It is more or less impossible to successfully launch a starship with goal of reaching a target 1800 light years away in less than half a century. $\endgroup$
    – Negdo
    Feb 27, 2023 at 15:07
  • $\begingroup$ It will be a generational ship travelling below the speed of light, though still at a significant % of the speed of light. My assumption is that fusion power will be of net plus energy output at that point and that the reactor present at the Ark will still be an early model. I did some estimates very long ago regarding the optimal time window for getting the most out of gravitational assists in the solar system and I think the launch would happen in the 60's-70's, I completely forgot the details though. $\endgroup$
    – Yulian
    Feb 27, 2023 at 15:32
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    $\begingroup$ You cannot achieve significant % of the speed of light with current technology. Even project Orion (en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion)) was relative slow. Fusion or not (rocket equation is evil thing!). What you need is a stelaser: a star powered laser. Which we have theoretical knowledge how to build one, but not nearly the engineering knowhow (or required space infrastructure to even start). And we aren't just a few decades away from one. Unlike with net plus fusion, where we don't even know if it's even possible outside of stars/bombs. $\endgroup$
    – Negdo
    Feb 27, 2023 at 15:41
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    $\begingroup$ @Negdo Yeah, the fussion powering the ship will probably one of those rare few moments when my world would have to deviate from the "100% realism" principle on which the entire setting is built $\endgroup$
    – Yulian
    Feb 27, 2023 at 15:55
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    $\begingroup$ @Negdo You have a good point, but keep in mind that you're challenging the backstory, not the question. When a question is independent of the backstory (like this one), it's not appropriate to worry too much about the backstory. Think of it this way, "ignoring how I got there, can I...?" $\endgroup$
    – JBH
    Feb 27, 2023 at 21:01

4 Answers 4

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I'll add my own two cents here, source: I work in the field.

1. Technology

The reference to 'near future' tech is problematic in this, as 2-3 decades is the typical timescale of planning & constructing (in the case of James Webb more like 4 decades) a major observatory or space mission, given current funding horizons.
Given those timescales, technology in large observatory projects is frozen at a given, defined point (otherwise you'd keep exchanging and updating components) and then this frozen level of tech is used for the remainder of the existing mission (this is particularly true for space missions, ground observatories i.e. next generation interferometers and ELT's can be build much more modular).
Because of this, large observatories always lag a few decades behind compared to the newest standards, which are e.g. used in smaller science missions or top-secret intelligence satellites.

So with this disclaimer, it's clear that your mission will essentially use the technology which is in todays spy satellites, and be a typical factor 3-10 better (as it is usually per generation of telescopes) in angular resolution, signal-to-noise S/N, spectral capabilities, etc.

This factor 3-10 is a huge oversimplification, and particular for the light gathering capability (or sensitivity, or S/N) the size of your telescope and/or number of array telescopes play an overwhelming role. There, in principle you can just spend more money for a larger telescpe or more array elements, but the tech again limits whether you are actually able to analyse and use all the incoming data for an improvement in S/N.

2. The science

For what we can actually do with the data gathered, we have to distinguish between discovery and characterization. Discovery is only a factual statement, that something is there, while characterization is going beyond and actually saying what is there, how much etc.
The dividing line between those two modes of science is roughly the S/N ratio of the data you have: If you're fishing in the noise, and you barely see the signal of an Earth-like planet in the transit data, then you can only claim discovery. Your data is not good enough to say anything about its bulk properties or the existence of an atmosphere or other things we can today only vaguely dream of (like shape of continents etc.)

What determines the S/N in the end is the technique you are using and what object you are looking at:

  • A self-luminous young giant planet, far away from its host star will have great S/N in the infrared, its light can be directly fiber-fed to an infrared spectrograph. This will yield a lot of spectral data, which can then be fit to determine temperature structure, abundances of atoms and molecules in the atmosphere. Currently, that's still a noisy job, but the next generation of ELT's will do a fantastic job at that.
  • A small, terrestrial planet, close to its host will have terrible S/N, and even discovery of atmospheres on those planets is extremely hard currently, and only for near-by stars, not your 1800 Lyr far away star. The ELT's are going to improve "discovery of atmosphere around Earth-like planets" into occasionally doable (given our understanding of how technology will evolve).

However, to do what you propose, you'd need your telescope size and tech to move into the mode of "routine characterization of atmospheres around Earth-like planets, at large distances", which would probably need another 2-3 generations, large-scale international funding and collaboration and world peace. Also those future scopes would presumably so insanely large that you couldn't take them on your generation ship ride, you'd have to keep a commlink with Earth and take the hit that your data doesn't become better, the closer you get to the target.

3. Detection biases

If you want to travel to a far-away 'perfect' planet, you want to first make sure that there is no other good candidate much closer, otherwise you're wasting a lot of money.
Your proposed 1800 Ly, which translate to something like 430 parsec are currently in the outer range of sensible exoplanet detectability by (transit) surveys. You would need to first step up your survey machinery dramatically to be sure to cover all stars that are closer than this with transits, and then another dramatic step up to cover all planets with direct imaging.
Transit surveys are biased towards the planets which happen to orbit on the line-of-sight, whereas direct imaging surveys would have a much higher completeness. However for this, novel coronographic techniques would need to be developed to block out the star (e.g. Starshade) and deployed in large numbers.

Given world peace and all surplus funding in the next half century going into exoplanet science, this would be doable, but seeing the state of the world at the moment, this is not what is going to happen.

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  • $\begingroup$ Goodness gracious, you didn't throw in just 2 cents, you put here more useful info than everyone else combined! Thank you a ton for everything that you wrote! Now, I've got some questions to dig a bit deeper: -ELT means "Extremely Large Telescope", right? -Would an array of such telescopes be more efficient spread on Earth, on Earth's orbit or on the L points? -With that ELT array deployed, would we get a precision when it comes to the composition in the sets of tens, or to the sets of ones? $\endgroup$
    – Yulian
    Feb 28, 2023 at 14:26
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    $\begingroup$ @Yulian: Well, as a baseline I wouldn't expect much good info on worldbuilding.se, most answers I see here are just parroted science memes. ELT=Extremely large telescope, yes. An ideal array would be in space, as you want to observe in the infrared, but the atmosphere blocks most infrared (hence, JWST in space). An array helps as a game changer mainly with angular resolution, not necessarily with signal strength (the signal is proportional to only the number of dishes, whereas resolution is propto their distance, which is modular). $\endgroup$ Feb 28, 2023 at 15:19
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    $\begingroup$ @Yulian: If infrared interferometry tech existed, and it would be efficient to throw it into space in large ELT-like arrays (L2 would be way more stable than Near earth orbit, also consider that currently 2-3 ELTs worldwide are scheduled to come online in the 2025-2030s ), then the angular resolution could help you to separate stellar from planetary signal. This should be enough for the characterization of a nearby (<100 parsec) terrestrial exoplanet in the habitable zone of a reasonably quiet star. $\endgroup$ Feb 28, 2023 at 15:22
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    $\begingroup$ @Yulian: With High-resolution spectroscopy of the near-future you could probably detect a few useful molecules and prove there's an atmosphere. Not more than that. You wouldn't know where the molecules sit, but I guess you could handwave-magic that away in a scifi-setting with "advanced hybrid AI-climate physics models". Concerning your JWST question in another comment: See Fig. 4 in sci.esa.int/documents/34594/36271/… We'd need milliarcsecond, not arcsecond resolution power for your distant stars, and $10^3-10^6$lower contrast curves. $\endgroup$ Feb 28, 2023 at 19:57
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    $\begingroup$ Thank you a ton, you've trully exhausted my question! For once my curiosity has been satiated and with all of your knowledge that you shared, I just can't wait to start worldbuilding that time period! Thank you again! :D $\endgroup$
    – Yulian
    Feb 28, 2023 at 20:57
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As L.Dutch answered, by measuring absorption lines you can get atmospheric composition of your exoplanet with high degree of precision. And 1800 light years is not that far away, as space is big. And since you gave us 4 decades of extra time, the job would be even easier. We wouldn't even need more advanced technologies, just bigger measurement apparatus.

But I have to point out few flaws in your question. First, you said it would be able to support terrestial life without life-support. Practically ANY atmosphere in 0-95 C can support terrestial life. Bacterial life, but life nevertheless. Even in our solar system there are places where terrestial (archea)bacteria could survive (if we provide them nutrients, and in some cases probably even without that).

But if you meant multicellular terrestial life, that would mean oxygen in your exoplanet's atmosphere. And oxygen is a byproduct of life! Naturally oxygen tend to get bound to other elements as it is far to reactive. Earth didn't have atmospheric oxyen until life figured out photosynthesis. But if your exoplanet already has life, you have another issue with supporting life without life-support: extraterrestial microorganisms. You wouldn't want to go there without life-support. And that is not unforseen complication, that is something anyone planing your expedition. And even ignoring the issue with native lifeforms, atmosphere and temperature range are not sufficient criteria for colonization. You need at least a proper gravitational field and low enough radiation levels.

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  • $\begingroup$ Regarding the second and third point, don't worry, I've spent a lot of good time on that already :) As for the first, could you for example say what would we be able to see using James Webb Telescope? And whether we could also estimate the percentages when it comes to the composition $\endgroup$
    – Yulian
    Feb 27, 2023 at 15:34
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This is science fiction, so let's start with the idea that someone is starting a project to resolve the best image we can of such a planet based on known science.

The project would involve using solar sails to send a probe out to 542 AU (about 3 light-days), where it can use the Sun as a gravitational lens. This project is designed to image the surface of planets out to about 100ly, so it could probably do spectral analysis of a planet at 1800ly. Certainly enough to build excitement adequate to justify an expedition, if such an expedition were feasible.

As it stands, just getting out to 3 light-days would take us almost 20 years, and that's not even considering the time required to build it. If they spontaneously came up with tech that would make an 1800ly expedition feasible, then they would probably start by sending a larger telescope to Sirius, which is a mere 8ly away and twice as massive as our sun, to get better pictures.

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  • $\begingroup$ Oh a gravitational lens telescope, I heard of it, but I don't think it could be inserted there into the setting, as the fussion reactor used to power the Ark that would later travel to that system would be enormous and could not be miniaturised or made affordable to just produce it just for a telescope (or even an array of them). Interesting idea nonetheless, I'll think about it later more :) $\endgroup$
    – Yulian
    Feb 28, 2023 at 18:34
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If they are able to measure absorption lines in the stellar spectrum, scientists will be able to measure the composition of the atmosphere, limitedly to those atomic species abundant enough to give a measurable absorption lines. This has been first done for an exoplanet in 2001.

Atmosphere in itself can be detected by measuring the decay of the light curve during an eclipse: an atmosphere gives a gentler fade, while with no atmosphere the transition is sharper.

The closer to the planet, the better and clearer information can be gathered.

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  • $\begingroup$ All of that makes sense, but regarding the last part; 1800 light years is afterall incredibly far away and a Super-Earth expolanet will probably quite hard to spot, but how hard? How uncertain would be the atmospherical measures, if they'd be possible at all? $\endgroup$
    – Yulian
    Feb 26, 2023 at 14:19
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    $\begingroup$ @Yulian NASA has discovered an exoplanet 13,000 light years away. $\endgroup$
    – JBH
    Feb 27, 2023 at 0:29

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