Comment by T-A | Hacker News Reader

Comment by T-A | Hacker News Reader

There seems to be some confusion about the meaning of "within the habitable zone".

From the paper (Introduction and Conclusions, respectively) [1]:

The habitability conditions of Proxima b, which orbits within the HZ of the star, have been extensively studied (e.g. Barnes et al. 2017; Ribas et al. 2016; Turbet et al. 2016; Meadows & Barnes 2018). On the other hand, the candidate Proxima d orbits much closer to the star and outside the HZ range.

In particular, the discovery of an Earth-mass planet orbiting Proxima Centauri (Anglada-Escudé et al. 2016), our closest stellar neighbour, was one of the most significant results in the field, in part because the planet orbits inside the habitable zone (HZ) of the star (e.g. Kopparapu et al. 2013).

So, Proxima d is too close to the star to be inside the HZ; it's within the HZ in the sense that the HZ is further out from the star.

The author of the Nature commentary, which claims that "it could have oceans of liquid water that can potentially harbour life", apparently didn't read the paper.

[1] https://www.eso.org/public/archives/releases/sciencepapers/e...

Here's a good collection of details about the entire star system of Proxima Centauri, including its planets.

https://www.star-facts.com/proxima-centauri/

Also fascinating, a relative distance vs time graph of close stars. It turns out we'll have closer distances for 6 in the next 10,000 - 50,000 years.

https://www.star-facts.com/wp-content/uploads/2020/08/Neares...

Somehow I feel like Elon Musk has a print out of that graph on his office wall.

I only counted 5 (including Proxima Centauri). What are the six that you see?

Barnards Star, Lalande 21185, then Alpha Centauri, then Proxima Centauri, then Ross 248, then Gliese 445 makes 6.

There are 6 if you count Proxima Centauri and Alpha Centauri separately; 7 if you count both stars in the Alpha Centauri pair separately, and 5 if you count Alpha and Proxima Centauri separately.

Alpha Centauri is the binary system of Alpha Centauri A and Alpha Centauri B, which are separated by about 10-30 AU. It's very, very close in terms of interstellar distances to Proxima Centauri, which is a red dwarf that orbits Alpha Centauri A and B at a distance of 13000 AU, or 0.2 LY - a remote enough orbit that it makes sense to keep track of whether it's on the near side or far side of the triple-star system.

For reference, Saturn is about 9-10 AU from the Sun.

Making an assumption here about Proxima Centauri's own Oort Cloud, but seems like if it's the same size as ours, they'd overlap? Seems like a potentially interesting way for "stuff" to get transferred from one solar system to another.

I believe we are discovering that space isn’t as empty as we once imagined. Rock-hopping could be a viable method of (slow) interstellar travel.

Eh, we're talking about a section of an arm on a galaxy that we're in. I'm pretty sure there's nothing in between most galaxies.

https://en.wikipedia.org/wiki/Intergalactic_star#Mass

Isn't that how you end up in an endless Solar System war?

Belta-lowda! Belta-lowda! Belta-lowda!

If you take long enough at it, you get comfortable out there and don't need stars anymore.

"Unlike the Sun, which will evolve into a red giant in 6.5 billion years and reach the end of its life at an age of about 10 billion years, Proxima will stay on the main sequence for another four trillion years due to its low mass and low energy production."

They laughed at me when I bought investment property on Proxima Centauri b, but who'll be the one laughing in 10.000000001 billion years?

“Stars with a mass of less than 0.25 solar masses do not evolve into red giants.… Toward the end of the blue dwarf phase, Proxima will become considerably more luminous, reaching up to 2.5% of the Sun’s luminosity. Once it runs out of hydrogen fuel, it will evolve into a white dwarf, skipping the red giant phase, and gradually lose all its heat energy.”

So we just need to tackle climate change and learn to live sustainably on Earth, then we’ll have a few billion years to figure out how to get to Proxima Centauri.

A good place to learn how to reverse entropy.

Just read Asimov's "The Last Question" yesterday and then this discussion pops up today on HN :-)

Fantastic read isn’t it!

Loving the Foundation series sofar as well on AppleTV, but the books are great ofcourse.

I thought that was interesting. Especially when the next news article I read called it a "dying star" (presumably their literary license on red dwarf). Everything's relative!

Thanks for that chart, that site is really worth checking out! There is a ton of cool stuff there, like the nearest starts, when we computed their distances, and other thing.

Good catch

"With the small stellar radius, Proxima d ... equilibrium temperature may reach 360K,"

NB: Earth's is 255K. Venus 260K, Mars 215K. So this doesn't tell the whole story, but is probably quite hot.

It is certainly tidally locked.

So it has a hot side, a cold side, and an intermediate ring at the terminator.

Probably not very comfortable.

hmm seems like a good candidate for paradise/hell /s

Why is it certainly tidally locked when Mercury is not?

Mercury orbits in 88 days. This other place orbits in 5 days.

And Mercury is almost tidally locked. Venus will be, before long.

Thx

For us kids at the back of the class, how are those two facts related?

Why couldn't something orbit the star fast without being tidally locked?

Rotational drag depends on distance between the star and planet.

Planets with fast orbits are closer to the star and get more drag- hence more likely to be locked.

There's also the consideration of the host star to take into account.

I dislike the term "Earth-like" in these publications because it conjures images of a strange alien world able to support (human) life. Even assuming there were life capable of withstanding the constant flares from the host star, I suspect it would have to look very different in color and likely composition to the plant life here due to needing to capture a different part of the spectrum.

My education was in chemistry, I've wondered about this topic before. Visible light is actually special after removing the happenstance of DNA-based visual systems from the picture: IR tends to twist or translate bonds (warming) and UV tends to ionize.

Visible light being right in the middle is the trick which gives us photosynthesis, and sight, because in both cases a photon can be captured by a photosensitive bond. It's harder to come up with plausible photosensitive bonds for IR light, and most anything is photosensitive once you get far enough into UV.

> IR tends to twist or translate bonds (warming)

Is this unique to IR?

It's almost definitional of IR in fact, but not quite.

Waves which are relaxed enough to pass through ordinary matter, but interact in interesting ways with metals and other electron clouds, we call radio. Microwaves start out doing this: usefully, they spin water molecules around, hence microwave ovens. What we call microwaves 'bottom out' on not doing so, it's a useful but somewhat artificial category which I would split between radio and IR if you gave me the gavel.

Unlike visible, which is 'visible' because it has distinct properties.

There is a strong selection bias at work: molecules made and operated on by living systems are chosen according to available energy.

A world evolved under lower energies and temperatures would select chemicals with weaker bonds. Other worlds evolved under higher energies and temperatures would select chemicals with stronger bonds.

Visible light is visible to us because it is important for molecules we use.

This is not to say that all points on the energy continuum are equally favorable to potential metabolic processes, but life adapts to conditions, even where they are not optimal. I don't know of any reason to assume the regime where Earth life has landed is optimal, but it has proven more than adequate.

Yes, "Earth-like" caught my attention too because red dwarfs are flare stars. I wanted to check my facts before commenting which led me down the rabbit hole of the history of Proximas flare activity, which is quite interesting.

Fun fact: Go outside on a clear night, and you can see 5-10,000 stars. There are 8 times that many red dwarfs in your field of view, and none of them are visible to the naked eye. But in 2016 Proxima was for a few minutes.

In March 2016 we saw a superflare: Proxima briefly became nearly a factor of 100 brighter, reaching a brightness just visible to the naked eye from dark sites.

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