Talk:Alpha Centauri Bb/Archive 1

Archive 1

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	To all the people who made this article possible so quickly and at such a level of quality. Amazing work! U5K0'sTalkMake WikiLove not WikiWar 18:40, 17 October 2012 (UTC)

Size?

Somebody should add the size/mass of the planet. Nergaal (talk) 15:00, 17 October 2012 (UTC)

 Done by another user. StringTheory11 (t • c) 04:35, 18 October 2012 (UTC)

Temperature

Both the sources cited for the temperature give an estimate of 1,200°C. Twice editors have changed this to "at least 1,200°C", which I believe is incorrect as the sources aren't giving a lower bound. I've changed this back now. If there is a more trustworthy source for the planet's estimated temperature I'd love to see it. LukeSurl ^{t c} 10:04, 18 October 2012 (UTC)

Contested deletion

This article should not be speedy deleted as lacking sufficient context to identify its subject, because... the contents is way too important (closest exoplanet) — Preceding unsigned comment added by 69.77.164.46 (talk) 20:24, 18 October 2012 (UTC)

This article should not be speedy deleted as lacking sufficient context to identify its subject, because... (your reason here) --64.131.193.65 (talk) 20:26, 18 October 2012 (UTC)

This is an important scientific discovery!

No need to worry. It was tagged briefly, but it isn't going to be deleted. --Bongwarrior (talk) 20:27, 18 October 2012 (UTC)

Temperature on the lee side

The article mentions one temperature. But for a tidally locked planet one would expect a very high temperature on the side facing the sun, and a lower temperature on the other. Could there then be a "horizon" with a temperature similar to ours? The article on "Tidal locking" does not provide this information either. If relevant information were available in wiki a link could be added to this article. — Preceding unsigned comment added by 213.252.171.254 (talk) 06:47, 18 October 2012 (UTC)

Most likely, heat is transmitted from the star-facing side to the 'lee' side either through currents of molten rock or through conduction. If there is any atmosphere (perhaps of material that would be solid on Earth or likely some mixture of nitrogen and carbon dioxide) it probably whips around the planet quickly. Pbrower2a (talk) 14:53, 18 October 2012 (UTC)

Those speculations sound like original research.Eregli bob (talk) 16:24, 19 October 2012 (UTC)

Whether there is a temperature difference will strongly depend on the (unknown) density of the atmosphere. Looking at our own system, Mercury and Venus both have a months-long day-and-night cycle not so far from complete tidal locking. But on Mercury, temperatures between day and night vary by 600 K (700 K vs. 100 K) because it has practically no atmosphere, while Venus' dense atmosphere leads to constant temperatures at day and night. --Roentgenium111 (talk) 15:54, 18 October 2012 (UTC)

Just about every molten material has some vapor pressure, and I would expect such to be true with molten silicate rocks. There would almost certainly be some atmosphere. Water vapor would not be one of those gases. Of course anyone who knows about the physical properties of molten silica and silicates at such temperatures can say far more than I can. This very hot planet would have a truly strange atmosphere. Pbrower2a (talk) 00:04, 20 October 2012 (UTC)

"The bright star Alpha Centauri"

The second picture labelled "The bright star Alpha Centauri" is not useful. What is this picture actually of ? Alpha Centauri is a binary star system. There should be two of them. There is only one big blurry star in this photo. Is this photo just one of the stars ? Which star is it ? Is the planet visible in this photo ? I don't see any point in having this photo here, it contributes nothing and is just confusing.Eregli bob (talk) 16:22, 19 October 2012 (UTC)

The two component stars are so close that they (plus the planet) appear to be a single "star" to the naked eye, and presumably on this photograph as well. (That's why the term "binary star" (singular) is often used, because it's a "star" that upon closer inspection turns out to be two.) The planet cannot be seen/photographed directly with current technology. I suppose the point of the image is to show the "parent star" of Bb, but a photograph resolving the binary would indeed be better, if one exists. --Roentgenium111 (talk) 17:34, 19 October 2012 (UTC)

I've clarified the caption a bit. --Mirokado (talk) 00:44, 20 October 2012 (UTC)

liquid water?

"At such extreme temperatures water is extremely unlikely to exist in liquid form on the planet's surface" - let's just change this to impossible!HammerFilmFan (talk) 03:25, 18 October 2012 (UTC)

If the atmospheric pressure is high enough, liquid water is possible. However, it is unlikely that the atmosphere pressure is very high, given the planet's mass and proximity to solar wind. GBC (talk) 04:24, 18 October 2012 (UTC)

Not at the temps reported (so far) for this planet - just too hot.HammerFilmFan (talk) 19:18, 18 October 2012 (UTC)

and if the temperature is very low on the side facing away from the sun, anything could be possible? (I would like that clarified.) — Preceding unsigned comment added by 213.252.171.254 (talk) 06:50, 18 October 2012 (UTC)

It is certainly not impossible for water to exist on this planet. We just do not know enough to rule it out completely. Even if it exists in small amounts as a supercritical fluid for short periods of time, it is possible for it to be present on the surface given an unlikely combination of factors (and planets have been known to possess such varying and unusual properties). For example if it has a partially molten and sufficiently metallic core and generates a strong enough magnetosphere then it could well sufficiently deflect the stellar wind enough to maintain an atmosphere slightly thicker than Mars, especially if combined with Earth-like gravity and density. If it has migrated inward (the system is slightly older than our own) then it may have originally been partially composed of ices. A cloud layer, high albedo and certain atmospheric properties can create an anti-greenhouse effect. Tidal locking has not been proven to rule out plate tectonics which can contribute to high elevations and carbon cycles. Potential heat distribution to the far side creates an environment in which the temperature can reach a stable point anywhere from the terminator to equator or the poles. It gets cold enough in some pockets of Venus' atmosphere that it "snows" material to the surface, similarly water ice is present at the poles on Mercury, assuming it had an atmosphere that is not fully overheated, then these pockets could instead be small pools of liquid water ... only spectroscopy would rule it out completely and this is not going to be possible, probably ever - unless it is found to transit. Having said all this, its is not even a remote candidate for life, but the existence of a terrestrial planet so close certainly makes it possible that there could be other Earth-like planets Centauri system. --EvenGreenerFish (talk) 10:38, 18 October 2012 (UTC)

I'd agree with HammerFilmFan here - liquid water (in the usual sense, i.e. excluding supercritical fluids) is indeed impossible at temperatures above the critical point, which is "only" 374°C. A sentence "At such extreme temperatures water cannot exist in liquid form on the planet's surface" is therefore absolutely correct, since any potential cold spots are not covered by the qualifier "at such extreme temperatures". (Nor would it be misleading, since the combination of a substantial atmosphere AND spots cold enough for liquid water seems extremely improbable to me, given that an atmosphere distributes heat around the surface. Mercury and Venus both have only one of the two, and therefore no surface liquid water at all.) --Roentgenium111 (talk) 13:11, 18 October 2012 (UTC)

Exactly. 1200 degrees centigrade (over 2,000 F) - that a 'cold spot' could exist somewhere amidst that sort of mean temperature boggles the mind.HammerFilmFan (talk) 19:22, 18 October 2012 (UTC)

This sentence seems to be gone but without commenting on the above discussion we definitely shouldn't be randomly changing things like that. If the source say 'extremely unlikely' or something similar, then this is what we should say. If people dispute a well sourced statement, they need to find better references. (If there's no source or the source is poor and people are disputing the accuracy of the statement then rather then changing it to something they regard as more accurate the best bet is generally to just tag or remove the statement until a {better} source if found.) Nil Einne (talk) 11:35, 22 October 2012 (UTC)

Mmm, WP:NOR comes into mind here. LukeSurl ^{t c} 11:55, 22 October 2012 (UTC)

The disputed claim was unreferenced to begin with, AFAIK. The suggested correction follows immediately from the phase diagram of water and so needs no further source IMO - but I agree that it is no loss to just remove it. --Roentgenium111 (talk) 16:13, 23 October 2012 (UTC)

It is highly unlikely that the planet's gravitation could hold water vapor. At 305K, roughly the temperature of the hottest surface of the Pacific Ocean (the highest consistent temperature on Earth -- hot deserts can have chilly nights) the Earth has gravitation strong enough to hold gases with atomic weights above about 6 (which includes water vapor) but cannot hold either helium or hydrogen. Venus and Mars are on the margin of holding water vapor due to either high temperature or low gravity, respectively. A planet with Earth-like gravitation but an absolute temperature more than three times that of the warmest ocean surfaces on Earth could not hold water vapor in any form. Water vapor would be at most a transient item in the atmosphere of Alpha Centauri Bb much as helium is a transient item in the Earth's atmosphere. Pbrower2a (talk) 17:32, 22 October 2012 (UTC)

Orbital elements, semi-amplitude

This should be the radial velocity of the planet in its orbit (compare for example the value in Jupiter), not that of the central star along our line of sight resulting from this interaction. I will replace the current value by a comment for now. Like the planet's mass, we will not know this accurately until we know the orbital inclination. --Mirokado (talk) 11:55, 20 October 2012 (UTC)

This is incorrect. The semi-amplitude is the component of the reflex velocity of the star along the line of sight. It is the observable that you get from the radial velocity method, and differs from the orbital speed value we have for Jupiter (which is correctly listed as "average orbital speed", not semi-amplitude). 46.126.76.193 (talk) 18:20, 20 October 2012 (UTC)

Ah, yes, I now see you are quite correct, thanks for restoring the original. Clearly "a little knowledge is a dangerous thing"! I think we need a better wikilink than "amplitude" for that entry, I will try to sort something out. --Mirokado (talk) 20:29, 20 October 2012 (UTC)

Please see Wikipedia talk:WikiProject Astronomical objects#Template:Planetbox orbit, comments welcome. --Mirokado (talk) 22:25, 20 October 2012 (UTC)

A matter that will also need to be determined is if Bb's orbit is precisely circular, or is elliptical. If it is elliptical, then the sun (A.C. B) will move in the planet's sky as it orbits, from east to west and back again. Areas near the terminator will see the sun rise then set on the same horizon, every three and a quarter days. The night side will get a day and a half of light from Alpha Centauri A for every one of the planet's 3 1/4 day long years, and really be the only "normal" sunrise and sunset the planet gets. The light may, at periastron, be bright enough to read a book by. GBC (talk) 18:48, 20 October 2012 (UTC)

AFAIK, a 1:1 tidally locked planet is always expected to have its orbit circularised, due to the same tidal effect. An elliptical planet should not have fixed day and night sides: e.g. Mercury's orbit is highly elliptical, and it is in a 3:2 tidal locking. --Roentgenium111 (talk) 21:02, 23 October 2012 (UTC)

Is the age of the star system necessary?

Information about the age of the Alpha Centauri binary system is highly technical, and doesn't further any knowledge of the planet. I think it should either be removed, or made more relevant. — Preceding unsigned comment added by 64.131.193.65 (talk) 18:40, 18 October 2012 (UTC)

The age of the system essentially also gives the age of the planet, since planets form "immediately" after their stars. So I would consider it somewhat relevant. But this info actually isn't in the article (as of now), AFAICS...--Roentgenium111 (talk) 11:45, 19 October 2012 (UTC)

The IP went ahead and removed it. Njardarlogar (talk) 14:25, 19 October 2012 (UTC)

I put it back. The technical details are not important - anyone can appreciate the value of the age datum. If there are companion planets in the habitable zone, this gives the reader some idea of how long biological evolution could potentially have been going on. WolfmanSF (talk) 19:17, 19 October 2012 (UTC)

OK, if this is relevant for hypothetical habitable zone companion planets, I'm moving it there. — Preceding unsigned comment added by 64.131.193.65 (talk) 18:43, 20 October 2012 (UTC)

It's also relevant for Bb itself, as I said. I would rather move it back. --Roentgenium111 (talk) 21:06, 23 October 2012 (UTC)

Habitable Zones of Binary Star Systems

The paper Circumstellar Habitable Zones of Binary Star Systems in the Solar Neighborhood is relevant here. Typesometext (talk) 02:02, 22 October 2012 (UTC)

The habitable zone given for Alpha Centauri in this article is wrong. It's not 0.5 to 0.9 AU it's more like 0.7 to 1.0 AU. It is very easy to calculate the distance of the habitable zone from the paper 'Kasting et al - 1993'

${d}={\left({\frac {L/L_{\odot }}{S_{\rm {eff}}}}\right)}^{0.5}$

${RunawayGreenhouseAlphaCentauriB}={\left({\frac {0.5/1}{1.41}}\right)}^{0.5}$ = 0.6 au

${WaterLossAlphaCentauriB}={\left({\frac {0.5/1}{1.10}}\right)}^{0.5}$ = 0.7 au

${FirstCarbonDioxideCondensationlimitAlphaCentauriB}={\left({\frac {0.5/1}{0.53}}\right)}^{0.5}$ = 1.0 au

And the figures above assume that Alpha Centauri B gives out the same proportion of infra-red light as our Sun does. In fact Alpha Centauri B gives out more infra-red light, as a proportion of its luminosity, so the actual habitable zone is probably a bit further out from the Alpha Centauri B, more like 0.75 AU to 1.05 AU! — Preceding unsigned comment added by 86.171.30.202 (talk) 03:29, 22 October 2012 (UTC)

• • There's more than one definition of the habitable zone you can find in different papers. It's probably better we are qualitative rather than quantitative in the article. LukeSurl ^{t c} 10:29, 22 October 2012 (UTC)

• • It seems the 0.5 - 0.9 figure comes from a paper directly relating to Alpha Centauri B [1]. While we could qualify it as "one estimation of the habitable zone", this article is probably not the space to expound the entire scientific debate. LukeSurl ^{t c} 10:34, 22 October 2012 (UTC)

No! Just glancing for a few minutes at that paper[2] you can see that it does not try to discover the habitable zone around Alpha Centauri B. It is just going on about the limits of where planets can form around Alpha Centauri B, in relation to a habitable zone already established, by some other paper. The 0.5-0.9 au comes from 'Kasting et al - 1993'. I think they get the 0.5-0.9 au indirectly from Kasting et al, through other papers quoted in that paper that used Kasting et al 1993 definition of a habitable zone. 0.5-0.9 au is obviously a mistake, isn't it!?

Come on, 0.54 au is where in the Alpha Centauri system you'd get a 'recent Venus'! That's assuming a solar flux of 1.76, and it's probably lower for a K0.5V star and so it's more like 0.6 au for a recent Venus around Alpha Centauri B.

${RecentVenus}={\left({\frac {0.5/1}{1.76}}\right)}^{0.5}$ = 0.55 au

Anyway, for Alpha Centauri Bb to form with the mass, it supposedly has, of 1.13-2.3 Earth mass, it would need to have formed quite a bit beyond 0.5 au. So perhaps, the paper 'Planet formation in the habitable zone of Alpha Centauri B' is now out of date. I don't know anything about astrophysics but could Alpha Centauri Bb really have acquired that much mass by slowly spirally in from just 0.5 au to 0.04 au? Seems unlikely, doesn't it? I think past papers that tried to run simulations on planet formation in the Alpha Centauri B system sometimes did find planets forming close in, though not as close as 0.04 au, I think it was more like 0.2 au. But, they gave the close in planets mass as much less than 1.13 Earth mass because there simply wouldn't be enough dust, planet forming material, that close in to Alpha Centauri B to make a planet that massive. I am sure there was some recent paper that claimed that the lowest metallicity a star must have for an Earth mass planet to form within its habitable zone was no lower than 10% metallicity. Following that paper's logic then Alpha Centauri Bb would have to have formed around 0.8 au, at a minimum, roughly. So, Alpha Centauri A and B were further apart in their early history than they are now.

No, the 0.5-0.9 au habitable zone was calculated incorrectly from Kasting et al. 1993, it's just a mistake that nobody has bothered correcting and nothing more than that, probably. Surely scientists don't just make new defintions for habitable zones, I would imagine their calculations are based on past research, like Kasting et al. 1993. — Preceding unsigned comment added by 86.171.30.202 (talk) 16:16, 23 October 2012 (UTC)

For a topic such as this there isn't one range that is "correct" and all others are false. It's continually debated in the scientific literature. As there's only one planet that's currently known to support life, there's no dataset to extrapolate from, assessments have to be based on theory. If you see Habitable_zone#Circumstellar_habitable_zone you'll see there's quite a large margin on the estimates of the habitable zone of our own solar system, let alone that around other stars. LukeSurl ^{t c} 16:30, 23 October 2012 (UTC)

Using the interpolations of the Kasting (1993) formulae from [3] (attempting to take the different spectrum for different stellar temperatures into account), a stellar temperature of 5214 K and a luminosity (bolometric) of 0.500 solar luminosities:

Limit	Relative flux (Earth = 1)	Distance (AU)
Recent Venus	1.70	0.542
Runaway greenhouse	1.29	0.622
Water loss	1.06	0.685
First CO2 condensation	0.509	0.991
Maximum greenhouse	0.333	1.22
Early Mars	0.296	1.30

This neglects the influence of star A which should cause small oscillations in the habitable zone extent [4]. Nevertheless this is getting quite deep into WP:OR and in any case is utterly irrelevant for Alpha Centauri Bb which is clearly far too close to the star. Such discussions may be better located at the Alpha Centauri article itself. 46.126.76.193 (talk) 21:52, 24 October 2012 (UTC)

Least massive planet?

The lead calls this "the least massive planet known to orbit a star similar to the Sun". Kepler-20 calls that star "Sun-like" and gives masses for Kepler-20e and f that are comparable to α Cen Bb. Could this apparent contradiction be resolved? --ἀνυπόδητος (talk) 21:05, 19 October 2012 (UTC)

I resolved it by adding the word 'nearby' in intro about the least massive planet known around a star like our Sun. Also in the same talk in the image caption, I added 'one of'. Kepler-20e is less massive than Alf Cen Bb at 0.72 Earth masses. Kepler-20f has mass identical to minimum mass of this planet. Both of these planets were discovered 10 months prior to this topic planet. So Alf Cen Bb is not the least massive planet known around a sun-like star, but it is tied for the second least massive. PlanetStar 22:50, 19 October 2012 (UTC)

It's not so clear-cut: Kepler-20e and f have upper mass limits higher than Alpha Centauri Bb's minimal mass (see Kepler-20 or [5]), so it is possible (though apparently not probable) that Bb is lighter than them. --Roentgenium111 (talk) 23:56, 19 October 2012 (UTC)

The discovery paper says "Here we report the detection of the smallest minimum mass planet detected so far around a solar-type star." so is referring to the minimum mass because it was detected with the radial velocity method. Unlike the Kepler planets that were detected by the transit method. And this is true is the smallest minimum mass planet detected so far around a solar-type star. Link to paper http://www.eso.org/public/archives/releases/sciencepapers/eso1241/eso1241a.pdf Quantanew (talk) 05:34, 20 October 2012 (UTC)

• This phrase, excluing the word "minimum", came from the ESO website. For some reason someone had commented out the ref. LukeSurl ^{t c} 11:47, 20 October 2012 (UTC)

Yes, ESO calls it the smallest unconditionally. I think the point of the OP was to point out that this claim was dubious nevertheless. Adding the "minimum mass" qualifier makes it uncontroversial, but since that term can be ambiguous to laypeople, I'd prefer calling it the "smallest planet discovered by radial velocity" (if we decide not to trust the ESO press release, that is). --Roentgenium111 (talk) 16:01, 23 October 2012 (UTC)

It was left out of the press release. The discovery paper refers this discovery as the smallest minimum mass planet detected so far around a solar-type star, that removes the dubiousness of the assertion. I agreed that it can be ambiguous to laypeople, but I feel we could take a key property off the article if we rephrase it like the way you propose. Quantanew (talk)

Well, I see little difference in content between the current wording and my suggestion, since the minimum mass of exoplanets (as in the context of the paper) can only be determined by radial velocity. But if you prefer, we could also write something like "smallest minimum mass planet (as measured by radial velocity)"; this does not change the content of the assertion at all, and clarifies the definition of minimum mass used. (I replaced "discovered" by "measured" to account for the few cases of planets discovered by other methods but later measured by RV.)

Even following the wikilink to minimum mass does not remove the ambiguity in the current wording, by the way (it is "defined" as any "lower bound calculated mass" in the lede paragraph), so we should certainly avoid it IMO. (At least in the lede; I wouldn't mind stating it this way in the main, together with the definition of minimum mass.) --Roentgenium111 (talk) 23:20, 30 October 2012 (UTC)

View of Proxima?

In the article, the magnitudes of Alpha B (parent star), Alpha A, and Sol are given, but not Alpha C (Proxima). How bright would the red dwarf be from the planet (ignoring unknown atmospheric effects, of course)? CFLeon (talk) 23:08, 1 November 2012 (UTC)

I think it would about mag 4.5, but I don't have a reliable source which we could use to cite it. --JorisvS (talk) 12:29, 2 November 2012 (UTC)

I believe Proxima would be barely visible, if at all. Our Sun would be one of the brightest stars in Cassiopeia, near the five stars that form an M or W shape. It is possible an atmosphere might be so unstable due to convection from the day side that stars of such magnitude would not be visible. GBC (talk) 04:19, 5 November 2012 (UTC)

Temperature

The infobox says the temperature is over 5000 K while the text cites just over 1200. What is correct?

Also it seems that mean temperature has no meaning on a tidally locked planet if it has no atmosphere (very likely). The far side can be of any temperature, from very high to very low.

At the same time the presence of the other star at 11 AU makes the far side well illuminated with a light-day period of 3.2 Earth days.--Anixx1 (talk) 16:24, 10 November 2012 (UTC)

Where does it say that? The infobox gives 5214 ± 33 K as the parent star's temperature and ~1500 K as that of the planet, which is agreement with the 1200°C. Temperature means surface temperature. --JorisvS (talk) 16:49, 10 November 2012 (UTC)

The article doesn't actually state "mean temperature", just "temperature". It's not clear to me from the sources if they refer to the average surface temperature (of day and night side) or the surface temperature on just the day side. If Bb is tidally locked and doesn't have a dense atmosphere, the average temperature would indeed be of little value. But we can only report what the sources say. --Roentgenium111 (talk) 19:15, 10 November 2012 (UTC)