Earth-sized exoplanet ‘habitable zone’ | Hacker News Reader

Earth-sized exoplanet ‘habitable zone’

nature.com

80 pointspseudolus77 comments

>Astronomers have discovered a world only a little bit bigger than Earth, whirling around a bright star about 31 parsecs from our planet.

Parsec is one of those units that is difficult to remember if you just look at the conversion (1 parsec = 3.26 light years). If you remember how it's defined, however, it somehow becomes easier (for me anyway). One parsec is the distance at which one astronomical unit (AU, mean distance between earth and sun) spans one arcsecond in the sky. One arcsecond, in turn, is 1/3600th of a degree, and is approximately the width of a human hair from 20 meters away.

On a side note, if you think that's small, many stars in the night sky have apparent diameters measured in thousandths of an arcsecond.

I’m not sure if the definition helped.

Haha fair point. I don't know why it helped me, but it seemed to.

How about a picture?

https://socratic.org/questions/how-do-you-convert-parsecs-to...

I mean, that helps explain the definition. But in practical use I just know to multiply by three to get lightyears and I couldn't really care less about the projection of an arc subtended by a certain angle at a certain distance.

The point of a parsec is that if you're stuck on earth a very long way from the stars and you're trying to work out where they are relative to each other, you can take advantage of the fact that the earth itself moves around the sun. So if you take a photo of a star in summer and then take another photo of it in winter, your vantage point has conveniently moved 2AU because during that time you've moved to the other side of the sun (and 1AU is by definition your distance from the sun).

So then if the star appears to move against the stars so far away that they didn't appear to move, you can do a spot of geometry and work out how much the subject star has moved in "arcseconds".

A star that is exactly far enough away to "move" 2 arcseconds over this 2AU distance (or, if you prefer, one which has "moved" 1 arcsecond when we move 1AU) is one parsec away. By definition.

When discussing relative distances from earth for stars, it's therefore a useful unit (we can't just compare arcseconds because further stars move less against the background, not more).

As with so many puzzling units the key is to find out why they're used. This one was used by astronomers who didn't have a better way to determine how far away stars were.

The wikipedia article, incidentally, is much better than that old Encyclopaedia Britannica article that I read in high school. I envy kids today.

https://en.wikipedia.org/wiki/Parsec

Oh and the actual distance is kind of irrelevant and I don't even try to remember it :)

That's a great way to compute the distance to a star from yourself stuck on Earth for half a year.

It's not a great way to understand the magnitude of that distance when planning a trip.

But if you're planning a trip, why would you plan it in Parsecs rather than Light Years (or some other more convenient unit) when you're not stuck on earth?

That was a lot of fascinating information in one comment.

Also fun to note that you mentioned "mean distance" and not just "distance" from the Sun because the Earth's orbit isn't perfectly circular.

Thanks for the explanation.

Many people say distance when they mean distance.

mean people quibble over a millionth of the distance

Of course, that definition is confusing, because it's an inverse thing - from the name you'd expect something 31 parsecs away to have a parallax of 31 arcseconds, but instead it's 1/31 of an arcsecond.

I just remember that a parsec is the same as a light year. By astronomical standards of precision, that's about right.

If you're measuring galactic distances (nearest galaxy is 2 million light years away) then that's fair, but for "nearby" objects, a 3x the distance means a lot.

I remember it as 1/12th of a Kessel Run, easy peasy

The Kessel Run as performed by the Millennium Falcon; not just any ship can do that!

Is that the same as discarding constants in big O notation?

  > I just remember that a parsec is the same as a light
  > year. By astronomical standards of precision, that's about right.

I just multiply them by π. Close enough, and probably on par with our measuring precision anyway.

> 1 parsec = 3.26 light years

I mentally just estimate 1 parsec as equal to π light years.

Or that 1 apc (attoparsec) is ~3cm...

Pretty close to 1.2in, hence why I just prefer to remember a parsec as quite close to 0.1 eft (exafoot).

I just imagine 337453805774280 football fields end-to-end when I want to visualize a parsec

Finally, a measure that speaks to me, as an American.

I imagine 18131846280409076 George Smoots standing on top one another.

They still painting those on the bridge? Smoots, I mean. Been outta the US for a while...

I think of a parsec as 1/12th of a Falcon Kessel Run

Perhaps it is difficult to remember, but the light years are more useful because it directly tells you the communication delay or lower bound for travel time. Arcsecond parallax in the sky doesn't really help much.

> 1 parsec = 3.26 light years

So that means that even if we could travel at the speed of light, it would still take 100 years to get there, so probably not a candidate for near-term colonization (if something bad happens to this planet, for example).

I've been pondering this for a while: once we have artificial gravity in space (from rotation, for instance), would we ever want to shackle ourselves to the gravity of a foreign planet? It would be so much easier just to create the ideal environment in a large, rotating cylinder instead of having adjust a planet's environment to our needs.

Take this new discovery for instance. Imagine that this planet is 400m years younger than earth. Would it have the necessary composition in its atmosphere to make it breathable? Does it share our gravity? Does it deflect enough stellar radiation?

I doubt we'll find earth 2, but we can recreate a comfortable environment in our own solar system. We just have to let go of the notion that we will colonize another planet for more than just curiosity-sake

To give some idea of what I mean by rotating cylinder, check out the O'Neill Cylinder https://en.wikipedia.org/wiki/O'Neill_cylinder

Should it ever become practical to do so, the reasons for doing so will look very similar to the reasons Europeans colonized America. The thing about densely populated environments is it requires lots of governance, and populations have a habit of increasing exponentially, which would be difficult for artificial environments to keep up with indefinitely. When you're on the frontier, you have a lot more freedom.

How so? You've already become adept at living in space, working in space, and building things in space out of materials you found in space or else it wouldn't be practical at all to travel to a new star system anyway. At that point, just build new space habitats.

Unfortunately, we can't even build enough housing for humans on our current planet.

Adept maybe but that doesn't mean that living in space wouldn't involve compromises that people would rather not make / have to make on a nice big habitable planet.

Perhaps I'm wondering away from the initial proposal but think there would be some compromises that make life on a planet more desirable.

It's an and, not an or. People get good at living in space. Some of them move next to a new planet because it has resources. Some of those people have to go to the planet to exploit those resources. The rest stay in space. Now you've got a populated planet. Maybe they'll do some terraforming. I'm not sure what will be practical, but it's not hard to imagine that at least some basic terraforming is probably feasible just by constructing life forms that can live on the planet and further move it in a direction intelligence likes, even ignoring massive, energy-intensive interventions.

Not all the space people have to move to the planet, just like when North America was populated it didn't depopulate where they came from.

Apart from what's already been mentioned...

Consider that the population of England during the colonization of America was around 3 million. Today, nearly 3 times that live in London alone. It's like asking why would anyone have bothered to risk traveling across the Atlantic for months to get to untamed wilderness when England could've just built more housing.

You can step outside without being in a vacuum.

Have you read the Expanse books? They kind of touch on this subject.

Maybe they meant more of an O'Neill Cylinder, in which case you can step 'outside' onto the inner surface of the cylinder and not be in vacuum. Space habitats also have advantages over planets in that they're easier to move, they can be designed to not have adverse conditions like freezing cold, extreme heat and dangerous weather and you can build more of them.

If you step outside of Earth's atmosphere you're in the same vacuum, it's just a bigger step.

That's exactly it

> It would be so much easier just to create the ideal environment in a large, rotating cylinder instead of having adjust a planet's environment to our needs.

Yea but people want to go outside.

Artificial outside? If the ship, for lack of a better term, were big enough its conceivable that you could have a larger open area with grass and trees which would feel much like a park.

Could even paint the roof blue.

I’m open to it, but I suspect it would be difficult to figure out all the things we’d miss even on a biological level. You don’t think people would read about planets and realize what they’re missing? I think they would curse their ancestors for ever leaving. This feeling of leaving some paradise and forgetting what we’ve even lost is a core theme of genesis, it’s as human as anything in civilization, and it’s only going to get stronger if we explicitly abandon where we came from (I mean this metaphorically, I imagine we’d be happy on earthlike planets, even if we do miss earth’s sun or whatever at first).

I think the reason earth-like exoplanets generate so much interest is not because of the terraforming / colonization potential but rather because they are more likely places where we might find alien life.

Totally agreed

We have to wonder if they're thinking the same about us.

"That 3rd planet looks easy to colonize. I wonder if there are any upright, talking primates there?"

Semi-related to this discussion is the novel _Seveneves_ by Neal Stephenson.

Essentially, the premise is that the Moon gets hit by a Very Large Thing, and humanity has to prepare for Earth being hit by some very large bit of the moon. Part of their planning is building a generation-spaceship, designed to last until Earth's hot atmosphere cools to a habitable level, so there is some discussion of "Why live on a spaceship when you can live on a planet?" However, it's not a deep focus.

The book has good reviews [1] and I'd strongly recommend it for those who like hard Sci-Fi. However, I would consider only reading the first two parts. The third part is rather extravagant and, imo, lost the thread of the first two parts. It certainly lost my interest.

[1]: https://www.goodreads.com/book/show/22816087-seveneves

I wouldn't be surprised if we develop brain uploads sometime in the next 400 years, at which point the focus on trying to recreate earth-like environments anywhere will look silly.

There's probably a lot of other advancements over the next few centuries that we can't imagine. Our current view of space colonization is probably going to have as much of a connection to the future as the Tower of Babel had to the Apollo program.

The concept of identity might change as well.

If your biological brain is you (to you only), then a digital copy is not you, it's a clone of you.

Also then, the only immortality we can hope to achieve is through those head jars so famous in Futurama.

According to this [1] it's probably tidally locked, which means that despite being in the goldilocks zone, the environment is likely problematic for life.

[1] https://www.sciencealert.com/nasa-found-a-tatooine-like-plan...

I imagine that there is a small area where life would be possible?

These types of planets are known as 'Eyeball planets'. [1] They are hypothesized to have a hot side and a cold side, and potentially a ring around the periphery that is 'just right' where liquid water could form and sustain life.

Not sure if this particular planet would meet those criteria.

[1] https://en.wikipedia.org/wiki/Eyeball_planet

Even if the planet had a strong magnetosphere, wouldn't atmosphere on the hot side be blown away by heat or solar wind? Thus atmosphere from the whole planet would be depleted.

Astronomers & cosmologists like to play games where they try to sketch out how a tidally-locked world might, just might, be inhabitable. I tend to think it's balancing on a pretty high-dimensional knife's edge on cosmological time scales; there's all sorts of directions that various resources can fall out of the habitable zone and not be able to come back.

For a terrestrial example of the sort of thing I mean, see things like https://www.astrobio.net/biosphere/earths-breathable-atmosph...; it is thought that a lot of resources vital to life on Earth would have left the biosphere before life could even get going if Earth, or before the biosphere could progress very far, were not tectonically active. Trying to put together a habitable eyeball-planet seems like it has that problem to the tenth power to me.

Seems that it be more likely to produce habitable eyeball planets than habitable Earth-like rotating planets - an Earth that was 50 degrees warmer might only be habitable at the poles, while an Earth that was on average 50 degrees colder might be a frozen wasteland all the way the equator - not to mention that seasonal swings in climate for temperate latitudes further decrease your options.

With an eyeball planet the acceptable distance from the star, heat flux, and other variables could produce a ring of acceptable climate even if the average global temperature was +/- 100C relative to Earth.

The problem is that around the margins where it's "just right", you'll probably have mega-winds and stuff, too.

The snow storms are pretty bad I imagine.

People keep saying that for extreme climates on Earth and are then constantly surprised by the variety and abundance of life - admittedly sometimes just microbiology - found in those locations. So I wouldn’t write off a planet just for tidal lock.

Probably best to say "unlikely to contain multicellular life" or maybe even complex life. As we know from here on Earth, microorganisms exist in almost every extreme location on Earth.

Given how fast the human race evolved (compared to how old the earth and life is,) I wouldn't make any assumptions about multicellular life.

However compared to how fast single-celled life formed it took a really long time for multicellular life to occur.

The problem is that the atmosphere and weather is global. Meaning that snow that falls on the cold side will never melt or evaporate. Therefore there's probably no liquid water.

Liquid oxygen is dense enough for icebergs to float over to the warm side, but there won't be any oxygen before life can start photosynthesizing, and it would boil off long before ice could float close enough to the hot side to melt or evaporate. Liquid nitrogen is less dense than ice, so that doesn't help. Most other plausible liquids (methane, ammonia) are also less dense than ice.

It's difficult to say life would be impossible on a tidally locked planet, but it's certainly very unlikely. Remember that it took Earth something like two billion years to develope life, and this place is downright cozy.

Last year there was a discovery where some single cell organisms were extracting electrons directly from solid rock. So I don’t think ice is a barrier.

I do agree that it’s unlikely that life would exist on that planet. But I think the chances of life being slim are more down to the difficulty in the bootstrapping stage rather than the environment being too inhospitable to host life. By that I mean DNA forming (for example) might be 90% of the problem and survival might be 10%. Figures obviously made up but I hope you understand my (crudely expressed) point.

There’s also the point that extraterrestrial life might not mimic the typical examples we see on Earth. In fact in plausible for silicon and/or arsenic to be building blocks for DNA and that’s only narrowly widening our expectations of life.

> But I think the chances of life being slim are more down to the difficulty in the bootstrapping stage rather than the environment being too inhospitable to host life.

I'm 100% in agreement on this.

My argument is that with no oceans, abiogenesis is basically impossible. There's not really a useful starting point if everything is rock and ice.

I have no doubt that in 200 years a prepared group of humans could be dropped there and thrive. (transportation issues not withstanding) But life couldn't start there from nothing.

I don't think silicon based life is plausible. It can't form double silicon silicon bonds. Overall its chemistry is utterly boring compared to carbon's, and carbon is far more common than silicon is. Again, same argument. If silicon based life were intelligently created, sure, it would probably thrive. But there's no way it would spontaneously create itself. And honestly, it would probably evolve to being carbon fairly quickly.

I’ve read plenty of material which would disagree with your skeptisim there eg (top link in DDG)

https://www.astrobio.net/news-exclusive/possibility-silicon-...

Regarding life, I don’t just think human life could thrive in such environments, I think even single cell organisms could...if implanted there that is. As I said earlier, we have life on Earth in seemingly less hospitable environments like naked rock faces, sulphur-rich volcanic lakes, etc. We even have microbial life on Earth that uses arsenic as one of the chemical elements for DNA. So while 99.99% of life here does fall into a narrow spectrum, even on Earth we are constantly surprised by the ability for life to thrive and adapt in environments it classically shouldn’t. Therefore I think we should be open minded to the possibility of surprises outside our own ecosystems. Albeit bootstrapping aside.

Not sure on the social norms here, I commented on the post of this news that didn’t rise to the front :

‘completes one orbit every 10 Earth days' Excellent news for fans of the French Revolutionary Calendar. 'The French Revolutionary Calendar, created at the same time as the metric system, was an attempt to create a metric calendar and time system.' https://www.cooksinfo.com/french-revolutionary-calendar https://en.wikipedia.org/wiki/French_Republican_calendar https://twitter.com/sansculotides

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