ESO's Paper on Prox B
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ESO's Paper on Prox. B - http://www.eso.org/public/archives/releases/sciencepapers/eso1629/eso1629a.pdf?utm_medium=social&utm_campaign=SocialSignIn&utm_source=Twitter
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zoo3hans
in response to johnfairweather's comment.
So the sensation is perfect. A planet around the nearest star, and in the habitable zone as well!
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Artman40
http://arxiv.org/pdf/1608.06291v1.pdf
Paper that came out in previous year showed that stellar activity and magnetic cycle don't match the 11.2day period of the planet, further solidifying the evidence of the planet.
Rotation period of the star - 82.5...84 days.
Magnetic cycle - evidence of 442-day period. Currently harder to detect due to quiet phase.
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Artman40
But wait a minute.
http://www.planetary.org/blogs/guest-blogs/2016/0824-proxima-centauri-b-have-we.html
One of the comments said:
"it is worth noting that previous radial velocity surveys of Proxima Centauri found a completely different periodicity of 83.5 days. This was attributed to the rotation of the star and not to any exoplanet. In the latest survey, a periodicity of 11.2 days was found, but this was attributed to an exoplanet instead.
This I find very curious. The new measurements appear to contradict the finding of a rotational period of the star of 83.5 days. Surely a stellar rotational component should show up in the radial velocity? If the rotational period of Proxima is NOT 83.5 days, then what is it?
The obvious answer, I'm afraid, is that the new (and we assume improved – due to much better instrumentation) radial velocity periodicity is down to the rotation of the star, and that period is 11.2 days.
All stars rotate, but not all stars have planets. Elementary reasoning (Occam's Razor) requires us therefore to assume that stellar rotation is the most parsimonious explanation for periodic radial velocity variations. We can only impute the existence of an exoplanet once the rotational effect has been otherwise accounted for which, in the present case, does NOT appear to have been done. Again, NO EVIDENCE of the 83.5 day rotation period has been found in the latest data.
It is also, I would suggest, suspicious that 11.2 days is a very typical rotation period for a star (the Sun rotates in about 25 days at the equator)."
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ajamyajax
in response to Artman40's comment.
Very observant, thanks for the post. I wish we each had more time to look at all the interesting datasets that are available. Alas we can only do so much. If you have the time though pls keep contributing your insights, because you never know who might come along and read them. 😃
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Shellface
in response to Artman40's comment.
No no, this is insane.
The logic used here seems to be something like the following:
- "previous radial velocity surveys" ** found an 83.5 d rotational period.
- the new periodicity is 11.2 days
- hocum pocum, the new periodicity is the rotational period
- [trash talk the authors]
I find this bizarre and entirely disagreeable, so I am compelled to deconstruct it.
- "The 11.2 day periodicity in the radial velocities is rotational"
- Radial velocities are, at best, not well-suited for detection of rotational variability. The correlation(s) between apparent velocity and magnetism are complex, but at current RV precision they are not detectable in inactive stars. Proxima is, by most metrics, fairly inactive. It is quite a strong flare star, but flaring is not greatly connected to magnetic variability for M-dwarfs.
- On the other hand, photometry is an excellent way of determining rotational periods - which should be indisputable, considering the type of data we are here to deal with. An 11.2 day rotational period has never previously been observed in photometry (or any other magnetic proxy); in addition, the Pale Red Dot campaign took photometric observations for precisely the reasons being considered, and did not detect an 11.2 day periodicity. How could the radial velocities detect a supposed rotational signal but all the activity indicators not? Well…
- The rotational period of Proxima is ~83 days. This can be found in various pieces of literature, and importantly it is also seen in this paper. So, the RVs detect a signal that the activity indicators do not because… it's not activity.
Even the above position, without considering contradictory evidence from the actual activity indicators, is untenable. Surface activity features on stars (starspots et al.) are transient, and usually disappear after 3-4 rotational periods They are also not perfectly periodic, and vary in apparent period by a few percent over time. This paper considers 16 Earth years of radial velocity data, which is ~130 cycles of the 11.2 day period. The signal remains consistent in phase, amplitude and period in that time, which is genuinely impossible to reproduce by activity.
- " Again, NO EVIDENCE of the 83.5 day rotation period has been found in the latest data." [sic]
All four photometric series show evidence of a long period signal compatible with a photometric cycle at 83-d (likely rotation) reported before.
As in the photometry, the rotation period and its first harmonic (~40 days) are clearly detected in the PRD campaign. […] The analysis of the HARPS pre-2016 also shows very strong evidence that m2 is tracing the photometric rotation period of 83 days.
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I'm fairly certain there is no way to construe this other than willing ignorance of what is actually written in the paper.
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"This paper doesn't account for activity"
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Section 6 meticulously details how the authors… account for activity. Specifically, they consider linear correlations between 8 activity indicators for three radial velocity datasets; only one, the differential of the photometry, has a statistically significant correlation with the RVs. The correlation(s) are then subtracted from the data, as in the activity is removed from the RVs.
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The lack of significant correlations between the RVs and any other activity indicator is extremely strong support of the low level of activity in the RVs of Proxima, which directly supports the not-activity interpretation of the 11.2 day signal.
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"11.2 days is a very typical rotation period for a star" [sic]
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The nature of rotation in stars varies considerably. However, one feature that unites stars between F7 and roughly the hydrogen fusion lower mass limit is that the rotational period decays over time (due to loss of angular momentum to stellar wind), with the function being roughly Prot α (Age)^0.5. Though the rotational periods of these stars are all fairly close to equal at the start of the main sequence, those of later-type stars decay faster than earlier-type stars; the ~12 Gyr old G8V star HD 20794 has a rotational period of ~40 days, but the ~10 Gyr old M3V star Gliese 581 has a rotational period of ~120 days. For an M5.5V star like Proxima, a rotational period of ~10 days can only be maintained at ages below ~1 Gyr; however, we know that Proxima is ~5 Gyr old from the ages of Alpha Centauri AB, to which Proxima is associated. That is, when accurately considering Proxima's nature, an 11.2 day rotational period is not possible, and conversely an ~83-day rotational period is normal.
In short, maintaining that the 11.2-day RV signal is rotational requires you to a.) ignore (all) useful evidence, b.) ignore what is written in the paper and poo-poo the authors and their methodology, and c.) ignore any and all scientific reasoning.
I have my reservations on the workings Anglada-Escudé and Tuomi, but here their results are ironclad. Thus, it boils down to this: Artman, who do you trust more - a huge team of RV professionals including some of the most innovative researchers in the field, or an armchair detractor in some comment section?
Proxima Centauri b is just as real as Kepler-186f, or any other planet deeply studied to exclude a false positive.
What is scientifically doubtful in the paper is using data to validate a result which has been specifically refuted BY YOUR OWN RESEARCH!
The appropriate response: "What is truly "scientifically doubtful" is ignoring all data and writing that has been presented which invalidates your untenable hypothesis, in the name of attempting to throw the discovery of the decade under a bus"
** He explicitly refers to a radial velocity detection of the rotational period, but then refers to a paper about photometry. The only real previous high precision RV observations were by UVES (which were reprocessed in the new paper), but neither Endl & Kuerster (2008) or Zechmeister et al. (2009) detect any signal in their analysis of the UVES RVs. Still, Proxima's rotational period is actually 83 days.
Make no mistake, this is one of the most important exoplanet discoveries so far. While there was the recent refutation of Alpha Centauri Bb - defeated by, of all things, sampling(!) - Proxima b has no visible avenues for contest, and the methods used in the discovery paper are truly airtight. The announcement has prompted at least five papers on the habitability of the planet, and they all conclude that appropriate climates are possible but highly sensitive to the volatile content of the planetary surface and atmosphere. It will be many years until it will even be possible to determine Proxima b's surface composition, so in the meantime let's cross out fingers for H2O!
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Orbital Period is given here, as 11.2 days - http://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=planets
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ajamyajax
in response to Shellface's comment.
... "Proxima b's surface composition, so in the meantime let's cross our fingers for H2O!"
"Planet in star system nearest our Sun 'may have oceans'"
http://phys.org/news/2016-10-planet-star-nearest-sun-oceans.htmlPosted
