HD 125612 - No c planet?

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HD 125612 - No c planet?

Post by Sirius_Alpha on 20th July 2014, 1:11 am

So I downloaded all the available data on HD 125612, which is reported to have three planets. The innermost planet is a hot Neptune at 4 days.

I can recover the b and d planets, but I'm having trouble seeing the c planet in the HARPS RV dataset. Anyone have any ideas?

Two-planet fit in Systemic.


Periodogram of residuals.



They used Keck+HARPS data. 58 HARPS measurements, whereas I had access to 76 HARPS measurements. The fact that it doesn't show up in the expanded HARPS-only dataset could mean it's just an artifact of merging the datasets.

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Re: HD 125612 - No c planet?

Post by Shellface on 20th July 2014, 8:57 am

That's a very large outlier. What happens if you remove it? (You can do this within systemic by pressing ctrl+d, uncheck the datatpoint, then press "apply")

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Re: HD 125612 - No c planet?

Post by Sirius_Alpha on 20th July 2014, 9:10 am

I found a two-planet solution with Chi-squared = 27.1853, with an rms of 8.6, and a jitter of 8.2.

Playing with it some more, I was able to recover the c planet finally, but only if I disallow the two-planet fit from reaching a minimum chi-squared (such as 27.1853). Systemic found a two-planet fit with a chi-squared of ~31, but then got it down to ~5 by fitting the c planet, though the jitter and rms are only marginally better at ~6.

So if you can force an n-planet fit into a local chi-square minimum, and by doing so eliminate the signal of additional planets in your periodogram, how do you go about evaluating whether or not an additional planet is necessary?

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Re: HD 125612 - No c planet?

Post by Lazarus on 20th July 2014, 2:17 pm

From what I can see the parameters of the best 2-planet fit are slightly different from the parameters of the same planets in the 3-planet fit. In particular, the period of planet b seems to be the most relevant change - in Systemic 2 it tends to end up at around 490 days in a highly eccentric (orbits crossing) 2-planet solution which does not show planet c. The 3-planet solution can be obtained only after forcing the second planet's period to 550 days, optimising that (which substantially decreases the eccentricity of the outer planet), once this is done the period of planet b can be left as a free rather than fixed parameter.

(Clarification: this based off fit to HARPS data only, excluding the Keck data)
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