# Talk:K2-18b

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## Distance of 111ly is overly precise

The Wikipedia article says the distance is 111 light years away, referencing a BBC article.

The original paper in Nature Astronomy says it is "34 pc" away, or 34 parsecs. 34 pc = 110.8932 light years. With only 2 digits of precision, the range 33.5-34.5 pc is 109.2624 to 112.523963 light years. It seems more appropriate therefore to use "about 110 light years" as the distance, which also has 2 significant digits.

This is what TechCrunch does at https://techcrunch.com/2019/09/11/hubble-spots-liquid-water-on-a-super-earth-110-light-years-away/ and NBC News at https://www.nbcnews.com/mach/science/strange-alien-world-found-have-water-vapor-possibly-rain-clouds-ncna1052486 . — Preceding unsigned comment added by 83.255.22.84 (talk) 19:38, 11 September 2019 (UTC)

How about "about 111" which is closer to the estimates cited (have added 'about' to the number). Randy Kryn (talk) 19:49, 11 September 2019 (UTC)
The star K2-18 has a Gaia DR2 parallax of 26.2686±0.0546 mas. That's pretty precise. The {{starbox astrometry}} template would render this as 124.2±0.3 ly. Cloutier et al. (2019) state 38.025±0.079 pc for the Gaia DR2 distance. Lithopsian (talk) 20:02, 11 September 2019 (UTC)
I also see the NASA exoplanet database has 38.068 pc [1] but I can't see where they are getting that number. --Masem (t) 02:24, 12 September 2019 (UTC)
That comes from the parallax given in the same database, which is the Gaia DR2 parallax (with a rather pointless number of decimal places). A more interesting question might be why the published paper gives a slightly different value. 38.025±0.079 pc is very very close to the value given in Bailer-Jones et al. (2018), but not exactly. The most likely distance for a star based on a parallax with a finite margin of error is not the inverse of the parallax, as shown at the exoplanet database (and used in the starbox calculation). Either way, the distance of ~111 light years (34 pc) quoted in popular articles is woefully obsolete, somewhat sad that it the Nature paper using it. Lithopsian (talk) 13:49, 12 September 2019 (UTC)
As related, I am assuming it is more common that these papers all publish that distance in parsec so that when other sources (like mainstream) take rounded numbers and covert that to light-years, that's where the "error" is being introduced, right? So we should start from the best peer-reviewed published number in parsecs and then convert to ly for reader convenience. --Masem (t) 13:57, 12 September 2019 (UTC)
Also, the "Cloutier2019" article does go on to explain this all so I've included that (including the original 34 pc) so that we can account for older data. --Masem (t) 14:15, 12 September 2019 (UTC)

## An unpublished arXiv preprint as a source

An unpublished arXiv preprint [2] does not satisfy WP:RS and should not be used as a reference, especially for such an important scientific discovery. Nsk92 (talk) 02:11, 12 September 2019 (UTC)

I replaced the reference to this preprint with a published reference[3] in ScienceNews discussing both studies, and moved the preprint ref to external links. Nsk92 (talk) 02:43, 12 September 2019 (UTC)
As long as have at least one source that notes it, this solution should be fine. Probably worth the check in 6 months or so to see where its publishing status is at. --Masem (t) 04:08, 12 September 2019 (UTC)
We should not use an unpublished primary source as a reference, especially for such an important discovery, when multiple published reliable secondary sources are aviailable. Nsk92 (talk) 10:12, 12 September 2019 (UTC)
Probably shouldn't get excited about a preprint, and then relying on a secondary source based on the preprint (and possibly the Nature paper, but who can tell). Given that there is a peer-reviewed paper as well as the preprint, both making essentially the same claims, quoting both shouldn't be a problem. Then use the secondary sources to establish notability, and rely on the primary sources for "facts". No point in copying a copy, doubling the risk of misinterpretation or a simple transcribing mistake. Lithopsian (talk) 14:01, 12 September 2019 (UTC)

## "Habitable Zone" + water does not mean habitable

This article is sort of clickbait in that a) about 8x the mass of earth also means 8x the gravity. No human could survive for long on the surface. b) There are a lot of other things that define "habitable" besides proximity to a star. Water is probably present on any planet in a "habitable" zone whether detected or not. Obviously there is more to learn about this planet, but I have issue with the term "habitable" when not put in proper context - this planet is NOT habitable (at least to humans) based on mass/gravity alone. It also may be much too hot (much like Venus) with an orbital period of only 33 days. Liquid water is not indicated - only vapor. 2600:6C48:7006:200:B056:6066:1296:EF0B (talk) 00:33, 13 September 2019 (UTC)

All uses of the word "habitable" is with "habitable zone", exception for one place where it is explained that this does not likely mean the planet is habitable (that's wehre a lot of press is click-baity). Also, the star is a red dwarf, so actually no the surface temperature is not believed to be that hot. --Masem (t) 01:03, 13 September 2019 (UTC)
"8x the mass of earth also means 8x the gravity". True, at the same distance. However, what we're really interested in is not gravity at some arbitrary fixed distance, but surface gravity, i.e. the g-force on the surface of K2-18b, which is only ${\displaystyle M_{factor}/R_{factor}^{2}=8.63/2.71^{2}=1.18}$  times that of Earth. --46.242.12.78 (talk) 02:03, 13 September 2019 (UTC)
Ok, I accept your equation that perhaps the gravity is not an issue (for humans), assuming it has a solid surface close to its measured diameter. Or an atmosphere that is not crushing. "Assuming" a lot in this here article though. "Habitable" term is my hangup. I'd bet my life savings that nobody will prove it to be habitable to any life form in my lifetime... 2600:6C48:7006:200:B056:6066:1296:EF0B (talk) 01:50, 17 September 2019 (UTC)
It is an unfortunately situation today that astronomers chose the term "habitable zone" back, as non-astronomers are probably looking at that term in the press and reading it as "a zone that is habitable" rather than the astronomic definition which is very specific. We here on WP are definitely not trying to mislead to claim this planet is habitable. --Masem (t) 04:08, 17 September 2019 (UTC)
No human would want to be anywhere near that close to a red dwarf. You'd be alternately fried and irradiated, or frozen. "Habitable zone" and being habitable to some life form are two different things. Theories about the life at the border of day and night on such planets are just speculation. 104.169.37.72 (talk) 04:13, 13 September 2019 (UTC)
We can do a better job highlighting that "habitable zone" only refers to the potential to have liquid water at the surface, but in this ice/gas planet, apparently there is no potential for that, just vapor. Also, "habitable" in this context refers to microbial life, often extremophiles. Cheers, Rowan Forest (talk) 13:54, 13 September 2019 (UTC)
Whatever happened to "Goldilocks Zone"? "Habitable" zone carries too much baggage. 2600:6C48:7006:200:B056:6066:1296:EF0B (talk) 00:56, 17 September 2019 (UTC)
Goldilocks Zone redirects to Circumstellar habitable zone. Rowan Forest (talk) 00:58, 17 September 2019 (UTC)

## Composition?

The article currently says "This classifies the planet as a super-Earth, though its size makes it unlikely to be composed of rocky iron and silicates, and more apt to be formed of hydrogen, helium, and astronomical ice", and cites this to Rogers 2015. I have just taken a look at Rogers. Not being a scientist in the field, I could not properly follow the article, but my best reading of it was that a better wording would be "unlikely to be formed solely of rocky iron and silicates, and more likely to include hydrogen, helium, and ice". that is, I could not get from Rogers that K2-18b was likely to be ONLY H/He and volatiles. Could someone more knowledgeable in the field take a look and see what they think? hamiltonstone (talk) 12:43, 13 September 2019 (UTC)

Rogers has an analysis of exoplanets baesd on their sizes (relative to earth's size) and what is known about their compositions, and generally states that of the catalog of super-Earth (any planet from 1 to 10 times the mass of earth) that is larger than 1.5 the mass of earth is unlikely to have a rocky (iron/silicate core - what we know from our solar system) core, and instead are more likely to be H2/He with other gases. Papers on K2-18b refer to Rogers to re-assert this, since K2-18b is ~8 x the mass of Earth. --Masem (t) 14:10, 13 September 2019 (UTC)
And to be specific, Montet 2015 is that paper I was thinking of. --Masem (t)
Yes, but I am seeing a difference between the core and the composition of the whole planet. Just checking the article language is right. hamiltonstone (talk) 00:26, 14 September 2019 (UTC)
The way I understand the terms: the data is separating the atmosphere from the core (I may be wrong). They're looking at what light passes through the planet for the composition of the atmosphere, as light will not pass through the core - which could be a rocky core, or an extremely dense (by pressure) H2/He/ice core. The whole composition of atmosphere + core would be something different. --Masem (t) 00:52, 14 September 2019 (UTC)

## Mini-Neptune

It may qualify as a super Earth solely by mass, but by composition (ice/gas mix) it is better described as a mini-Neptune. It has been described as a mini-Neptune by reliable sources, some are:

• "Laura Kreidberg, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said that K2-18b might be better described as a “mini-Neptune” than a “super Earth.” Extensive research with computer models suggests that, at 1.6 to 1.8 times the mass of Earth, planets tend to become huge and gaseous, rather than rocky." [4]
• Ars Technica: "The planet, K2-18b, is certainly not habitable by us, as it's a mini-Neptune that may not have any solid surface and is likely to have a hydrogen/helium-rich atmosphere." [5]
• NASA: "[…] resembling a small version of Neptune." [6]
• The Verge: "[…] a type of world often referred to as a mini-Neptune". [7]

Cheers, Rowan Forest (talk) 13:47, 13 September 2019 (UTC)

I could have also sworn one of the journal reports we have also compared it to Neptune (due to being H2, He, and ices). I know another paper discussed the possibility of it having a more rocky core and did their models based on both assumptions, with the mini-Neptune having the more statistically significant result. The thing is that "Super-Earth" is the technical qualification based on it size, but its clear that in the field "super-Earth" does not mean it has a rocky core. It would be fair to say, after discussing its likely composition, it has been compared to a "mini-Neptune". --Masem (t) 14:07, 13 September 2019 (UTC)
It can be classified according to mass and according to composition. Lets mention both. Cheers, Rowan Forest (talk) 14:11, 13 September 2019 (UTC)
Checked the scholarly sources, and its discovery called it a mini-Neptune, but all papers since (likely with the improved numbers from Spitzer) use super-Earth. I did add mini-Neptune language though (since we have that article). --Masem (t) 14:26, 13 September 2019 (UTC)
Thanks. Just as it was done at K2-288Bb time ago. Cheers, Rowan Forest (talk) 14:28, 13 September 2019 (UTC)

## xkcd

Today's xkcd is apt: https://xkcd.com/2202/ Fdfexoex (talk) 18:57, 13 September 2019 (UTC)

## 20 to 50% water vapor?

In the article there is a line about the atmosphere being 20 to 50% water vapor that I cannot find confirmation of in the sources listed after the sentence. It might be contained in the nature article (which I can't access). I find this claim unlikely. Did I miss it in the sources after the sentence, or is it in another source? Rockphed (talk) 19:08, 13 September 2019 (UTC)

It's the Nature article, and that's why the ArXiv link is provided, the preprint is available. --Masem (t) 19:19, 13 September 2019 (UTC)
The nature article is also available on the arxiv at https://arxiv.org/abs/1909.05218 and on page 6 it mentions the 20 to 50% as one possible model with another model having as low as 0.01% so basically the wikipedia article is misrepresenting the source. Fdfexoex (talk) 19:22, 13 September 2019 (UTC)
I added the additional range as it depends what atmospherical model they use. --Masem (t) 19:30, 13 September 2019 (UTC)

## Does anyone know something that can be . . . habited?

There is no such thing as a "habitable zone". Nothing "habits" a space; rather, it inhabits it. What astronomers and astrophysicists mean and should say is: "inhabitable zone".

Merriam-Webster lists only one verbal meaning for "habit": "to clothe or dress". Merriam-Webster Dictionary, s. v. "Habit" It lists multiple verbal synonyms as well: Synonyms: Verb apparel, array, attire, bedeck, caparison, clothe, costume, deck (out), do up, dress, dress up, enrobe, garb, garment, get up, gown, invest, rig (out), robe, suit, tog (up or out), toilet, vesture

Does any one of the above fit? I fail to see how . . . unless, you mean it's the area in which planets put on their clothes.Mwidunn (talk) 13:24, 17 September 2019 (UTC)mwidunn

Understand that "habitable zone" is a term of art from the astronomical side of things, WP is not making this up. (Also, I would not be using a dictionary to try to counter scientific arguments. "Habitation" is the root word you want, which is where "Habitable" extends from, not from "habit". --Masem (t) 14:01, 17 September 2019 (UTC)
Merriam-Webster seems pretty lacking, then; Samuel Johnson, in 1755, offered "HABITABLE. adj. [habitable, Fr. habitabilis, Lat.] Capable of being dwelt in; capable of sustaining human creatures" with cites from Francis Bacon, John Donne and John Dryden. It's been in use by the best authors of Modern English for most of its 500 year existence.--Prosfilaes (talk) 07:48, 18 September 2019 (UTC)