Gliese 892 - six planets (as least one planet in transit)

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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Lazarus on 5th July 2016, 4:40 pm

The mass value seems to have been revised somewhat, the Motalebi et al. paper gave 2.67 Earth masses, while Vogt et al. gave 0.11 Jupiter masses (=3.5 Earth masses)
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by tommi59 on 6th July 2016, 2:31 am

I think you mean 0.011. I noticed also much higher mass for c than previously announced (maybe revised stellar mass and radius) and I wonder whether mass of b is revised upward. Nevertheless chance for transits of outer planets increased slightly
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Led_Zep on 6th July 2016, 2:56 am

Two other slides :

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Re: Gliese 892 - six planets (as least one planet in transit)

Post by tommi59 on 6th July 2016, 6:23 am

2 planets only?
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Edasich on 6th July 2016, 1:12 pm

tommi59 wrote:2 planets only?

2 certain ones for sure. Wink
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Shellface on 6th July 2016, 1:36 pm

Though it's hard to read the connotations from the slide alone, that is far too pessimistic.

First, the periods used to refer to "e" and "h" (which, in the discovery papers, refer to the same signal) are different to the previous values. The period of "e" is now close to the proper period of "h", while "h" has moved to the actual period of the magnetic cycle. This doesn't actually change the planet count because they were the same object in the first place.

Unlike rotation, it is physically not possible for harmonics (integer divisions) of magnetic cycles to cause an RV signal because they do not represent surface variability, so "e" cannot be activity. The same conclusion is reached by Johnson et al., and I cannot imagine why the opposite is insinuated by this slide.

The rotational period of the star appears to be about the period of d (~42 d, which Motalebi et al. detect), so the first harmonic is about the period of f (~23 d, which is the signal Johnson et al. detect). However, Motalebi et al. find the RV signal to be more coherent than the activity signal and, more convincingly, the two variables are decorrelated (figure 8 ). Though their model for d is actually a combination of d and f, it remains that there would be a correlation with the activity indicators if the RV signal was activity, which is not observed.

Conclusions for f are more difficult because Motalebi et al. do not parse it, but Vogt et al. find that the signal is coherent:

In each case, the P =22.8-day periodicity is the location of the strongest peak in the power spectrum of the velocity residuals (between
P = 10 days and P = 30 days).  This suggests that the P = 22.8-day signal is not the  product  of  spot  modulation,  and  that  it  has been present and stable throughout the full time span of our observations.
Though the conclusion is weaker, the evidence favours a Keplerian origin for the signal than activity.

As for g, I don't even know. I guess because it's about twice the rotational period? But multiples of rotational periods don't mean anything - it is not possible for spots or plages to work like that, because then fluid dynamics would break.


Stellar activity, be it rotational or basal convection, possess three important properties. They are not coherent over long timescales, they are not perfectly periodic, and they leave signals in certain spectral lines (which is how activity indicators are derived). It is very unwise to dismiss planets because they happen to have periods sort of like activity without actually testing whether this is supported by activity indicators, especially when simple tools like correlations are enough to determine whether an RV signal is activity or not.

But then again, all of the papers I'm citing (and this slide cites…) reach the same conclusions as what I just wrote, so I don't actually know who I should be directing this to. And there may be some missing context.

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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Lazarus on 6th July 2016, 2:00 pm

Johnson et al. (2016) favour the interpretation that the 46.7-day and 2200-day signals are planetary, in which case the system consists of at least 4 planets (b, c, d, h). They don't seem to recover the signal for g from their data but they note (§4.1) that the uneven time sampling in the Keck data is problematic, they don't seem to detect c either (but transits give a fairly good indication that it does exist).

From the conclusion:
Conversely, however, our work provides evidence that the 46.7 day HD 219134 d and the 2200 day HD 219134 h are likely to be actual planets; M15 and V15, respectively, had expressed some concerns about whether these RV signals could be related to stellar rotation or activity.

It will be interesting to see what further data reveals about the system. The revised masses in those slides do suggest that there is a new analysis of the RVs.
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by tommi59 on 7th July 2016, 3:21 am

They don't seem to detect c either  (but transits give a fairly good indication that it does exist).

So if transit would not happen then planet would go to unconfirmed ? The 22.8 days signal is very strong and looks like genuine planet what a pity inclination will not allow it to transit (just only 2.4 % chance for transit).We can not rule out planet existence only because rotational period of the host is close to period of the planet
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Led_Zep on 2nd March 2017, 7:15 pm

http://www.nature.com/articles/s41550-017-0056?utm_content=buffer91f35&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Nature :

Two massive rocky planets transiting a K-dwarf 6.5 parsecs away

The transiting nature of both HD 219134 b and c increases the
probability that planets d and f also transit. Using the formalism of
previous work4, we compute posterior transit probabilities of 13.1%
and 8.1% for planets f and d
(…)
Although such a transit search is probably out of reach of ground-based
telescopes, it could be performed again by Spitzer, whose operations have
been extended to end-2018, or by the space missions TESS and
CHEOPS, which are both due to launch in 2018.

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Re: Gliese 892 - six planets (as least one planet in transit)

Post by tommi59 on 3rd March 2017, 4:59 am

So I hope for transit of 22.7 days planet although.At what inclination such transit would occur? 89.3 degrees?
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Lazarus on 3rd March 2017, 1:33 pm

From what I can see they do not seem to discuss whether or not the 22.7-day signal is activity-related, as previously suggested. Though Johnson et al. (2016) did suggest that detection in the HARPS-N data would favour the stellar origin interpretation:
The activity level of HD 219134 is currently increasing, with the next maximum expected around late 2018. If the 22.8-day RV signal begins to appear in continuing HARPS-N observations as the activity level increases, this would be strong evidence for the stellar origin of this signal. In order to test for this possibility V15 split their dataset into three portions, and did recover the 22.8-day periodicity in all three subsets; however, they did not quote the significance level of the recovery or if the other parameters are consistent between the different subsets, preventing us from making a more detailed analysis of this issue. Additionally, high-cadence observations near the cycle maximum could potentially probe whether the 22.8 day periodicity is a harmonic of a longer rotation period.

The model used in the Nature paper takes account of RHK but not sure how this relates to the SHK analysed in the Johnson et al. paper.
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Re: Gliese 892 - six planets (as least one planet in transit)

Post by Led_Zep on 7th March 2017, 2:53 pm

On arXiv :

https://arxiv.org/abs/1703.01430

Two massive rocky planets transiting a K-dwarf 6.5 parsecs away
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Re: Gliese 892 - six planets (as least one planet in transit)

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