6 ms·
There seems to be some confusion about the meaning of "within the habitable zone". From the paper (Introduction and Conclusions, respectively) [1]: The habita
by T-A 5y ago
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/eso2202/eso2202a.pdf https://www.eso.org/public/archives/releases/sciencepapers/e...
- avsteele 5y agoGood 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.
- ncmncm 5y agoIt is certainly tidally locked. So it has a hot side, a cold side, and an intermediate ring at the terminator. Probably not very comfortable.
- slim 5y agohmm seems like a good candidate for paradise/hell /s
- deleted 5y ago[deleted]
- ErikCorry 5y agoWhy is it certainly tidally locked when Mercury is not?
- ncmncm 5y agoMercury orbits in 88 days. This other place orbits in 5 days. And Mercury is almost tidally locked. Venus will be, before long.
- ErikCorry 5y agoThx
- jrumbut 5y agoFor 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?
- s1artibartfast 5y agoRotational 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.
- BeefWellington 5y agoThere'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.
- samatman 5y agoMy 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.
- JumpCrisscross 5y ago> IR tends to twist or translate bonds (warming) Is this unique to IR?
- samatman 5y agoIt'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.
- ncmncm 5y agoThere 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.
- ethbr0 5y agoHere's a good collection of details about the entire star system of Proxima Centauri, including its planets. https://www.star-facts.com/proxima-centauri/ 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/Nearest-stars-trajectory.png 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.
- rnk 5y agoThanks 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.
- irrational 5y agoI only counted 5 (including Proxima Centauri). What are the six that you see?
- LeifCarrotson 5y agoBarnards 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.
- ruffrey 5y agoFor reference, Saturn is about 9-10 AU from the Sun.