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Exoplanet assumptions...


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So something that's been bugging me for a while now when watching documentaries (well, mostly these days they are docutainments, but that's a rant for another day) about planets and exoplanets are the various assumptions being made.

Just the other day I watched something which was trying to speculate on what aliens might look like, going into chemistry etc etc. Amongst various things they concluded aliens would likely have a carbon-water based chemistry and any multicellular organisms would likely evolve on rocky planets, with any industrialised (ie, "bronze-age" and higher) aliens living on land due to needing fire.  All justified in the documentary. They also posited that even though this narrows the number of possible exoplanets exhibiting "intelligent" life significantly, there should still be plenty left in our own galaxy for it to be common.

Anyway, at one stage they asserted that most likely any aliens would evolve under a higher gravity than Earth given that the majority of rocky exoplanets are quite a bit larger than Earth.  Err, hangonaminute!!

This is the bit which, as so often before, threw me.  Yes, the majority of currently discovered rocky exoplanets are more massive than Earth. Simply because our instruments are selectively biased towards discovering more massive planets. Not so long ago, the majority of discovered exoplanets were "hot Jupiters" with documentaries at the time claiming that rocky planets were rare.

Conversely, using our own solar system which we can study in more detail, we discover that most rocky planets are actually less massive than the Earth.

Now I'm just an armchair astronomer, but even I can see that the numbers and types of currently _catalogued_ exoplanets are nowhere near representative of what's really out there at this stage of our exploration of our own galaxy.  How can these supposed experts disregard this very fundamental fact and make these absurd claims?

As I said, this was just the most recent example, but generally speaking pretty much every documentary which talks about exoplanets makes some claim which is only justified when using the currently available data, without any consideration as to the limitations of our current knowledge.

 

Am I missing something here?

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Yeah that’s a common occurrence. 

If it’s a documentary that’s speculating about alien life it’s not that much of a documentary - it’s more likely to be a popular science (not the specific publication, I mean the genre) production. It’s probably meant to be “infotainment” or something along those lines. 

There’s a lot of meddling between what experts/science advisors say and what’s actually shown in a production.

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1 hour ago, micha said:

most likely any aliens would evolve under a higher gravity than Earth

But then again, the vast majority of discovered life did happen in gravity very similar to Earth's!

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1 hour ago, micha said:

Am I missing something here?

Well yes, but actually no. Like all things, it's complicated.

So first off, my one and only defence what you were watching. We can do a pretty good job correcting for observation bias, as its something scientists know about going into things, and they have to work around it on a regular basis. When correcting for the planets we know we can't observe, and using the planets we can see to gauge their likelihood, we still find that most exoplanets are larger than Earth.

That being said, there are still a lot of assumptions and misconceptions that can be unpacked here.

Most of the exoplanets we've found were found by the kepler spacecraft, which was geared towards looking at K, G, and F type stars. These only make up 20-30% of all main sequence stars. The remainder is almost exclusively M type red dwarf stars. So we still lack a good census of what planets are like around these smaller, more abundant stars, which the TESS spacecraft and JWST will hopefully address as time goes on. From what we do know, it seems that planet mass may be loosely correlated with star mass. So worlds around these smaller stars will generally tend to smaller.

In a further miscommunication, there is actually a frustrating amount of ambiguity in the compositions of exoplanets (even those with well defined densities). This is most apparent with mini-neptunes, which could either be rocky worlds with an extended hydrogen atmosphere, or water rich ocean worlds with little or no hydrogen. For smaller planets below about 1.5 Earth radii, where hydrogen is almost certainly not present, their compositions are slightly easier to pinpoint. As we don't know for certain how much iron an exoplanet has, or how much of that iron is oxidized, there is a lot of room for volatiles like water and carbon dioxide to sneak into a "rocky" planet's composition. In the small fraction of rocky planets with known and well constrained densities, most can allow for anywhere from 0 to 10% of their mass in water (for comparison, only 0.02% of Earth's mass is water). This doesn't seem bad, until you realize that an earth mass planet (or larger) only needs to be ~1% water for it to have a global ocean with a seafloor covered in high pressure ices, which would remove the rock-water interface which is thought to be a key requirement for abiogenesis. Also these ocean worlds have much smaller habitable zones, quickly going from frozen iceball to boiling steamhouse, much like the faucets in most buildings.

What makes things worse, is that larger rocky worlds may also be unsuited for life. Planets larger than 2.5 earth masses will likely lack tectonic plates, and possibly magnetic fields as well. While neither are strictly required for life to develop (Earth's magnetic field and plate tectonics likely started after life developed), they are very very nice things to have for a developing biosphere. The thick atmospheres these worlds will have will likely make up for the lack of a magnetic field for surface life, and the larger masses of these planets will prevent atmospheric escape in the habitable zone.

Hope this helps.

Edited by wafflemoder
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IF the life requires tectonic plates and magnetic fields. Earth's biosphere as we know it benefits from those two factors. But we have no way of knowing what could have happened if... Unfortunate effect of having Sample = 1.

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4 hours ago, Scotius said:

IF the life requires tectonic plates and magnetic fields. Earth's biosphere as we know it benefits from those two factors.

Without the magnetic field the upper atmosphere will be under permanent proton bombardment from the Sun, this will likely kill any ozon layer, and the UV will be splitting the water in the upper atmosphere into H (quickly escaping away) and O.

This will result into Venus or Mars with carbon dioxide + nitrgogen atmosphere and no water on surface.

P.S.
Under 2 g the monkeys unlikely could jump in branches and the birds and insects unlikely could fly. They do it with efforts even here. Just look how much prey can a bird lift in air and see the limit of additional gravity,
This in turn would affect the plants reproduction. No flying pollinators.

And under high g the gravitational differentiation would finish faster, so the life would have less time to evolve before the magnetosphere disappears.

The Earth is not perfect, but it's the best from the worse.

Edited by kerbiloid
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When I tell people about exoplanets I always mention what we really know about them, and that is:

-Where they orbit the star, and therefore how much energy they get

-Roughly their mass, sometimes

-Roughly their atmospheric composition, sometimes

Everything else is speculation. There are probably a lot of small, Mars-like planets, and there are probably a lot of icy worlds way outside the so-called "habitable zone" with habitable subsurface oceans full of chemical energy sources, and that's just if water is the only stuff that life can come out of. In short, there are a lot of planets, and each of them is a little bit unique, with many possibilities.

Edited by cubinator
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dont get your science from documentaries, get it from science papers. documentary makers like to tell stories, putting narratives in places they don't belong. i had to stop watching them, they were making me stupid. 

Edited by Nuke
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5 hours ago, Nuke said:

dont get your science from documentaries, get it from science papers. documentary makers like to tell stories, putting narratives in places they don't belong. i had to stop watching them, they were making me stupid. 

I don't; and yes I agree. Modern infotainaries are mostly catering to the LCD and have little actual content. I keep finding myself turning them off part-way through when they get too irritating.

However, like I alluded, this question arose from many different sources. I'm not quite at the reading of actual science papers level, but I do read more than pop articles.

To get this thread back on topic, it was nothing to do with life (I shouldn't have gone into that much backstory); it's the fact that pretty much anything I come across _appears_ to use the current huge selection bias (due to our limited instrument capabilities) when talking about exoplanets in whatever the current topic at hand is.

And I've seen an evolution of this selection bias over years, the overall narrative changing from "multi-planetary systems are rare" to "rocky planets are rare" to "earth-mass sized planets are rare" just as our instruments and techniques have improved. Of course there are many other, more nuanced, biases as well, but ultimately always something which makes me think that it's due to our current detection methods rather than what's actually plausibly out there in the universe.

Now I'm sure in actual science papers this is not the case, but even pieces which are written/presented by (supposedly) respected members of the scientific community have these issues. Hence my question whether (some of) these biases are actually grounded in fact, as @wafflemoder seemed to say for the particular case I mentioned..

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