Planets around Tau Ceti

http://www.lesechos.fr/entreprises-secteurs/innovation-competences/sciences/0202457162447-decouverte-d-une-planete-situee-en-zone-habitable-521829.php

(sorry it's in french)  

5 planets (2-6 Earth mass)
One in Habitable zone !!

Where are they getting this from? I'm hoping they aren't just getting this and GJ 667C confused.
Given the system inclination, I would guess they have rather high true masses.

It's under embargo until tomorrow 18:00 (french time)

Ugh. Embargo breaches. Wasn't ours this time though, and since there's no information that isn't publicly available here, I don't see a need to intervene. So we'll wait until tomorrow...

Led_Zep wrote:

http://www.lesechos.fr/entreprises-secteurs/innovation-competences/sciences/0202457162447-decouverte-d-une-planete-situee-en-zone-habitable-521829.php

(sorry it's in french)

5 planets (2-6 Earth mass)
One in Habitable zone !!

They can't have found my abode!

Ahem... Seriously. I'm waiting for the official announcement (possibly the arXiv paper) before any inference. It seems there's a A&A publication involved.

Two classic sci-fi systems in one year... didn't expect that

Lazarus wrote:

Two classic sci-fi systems in one year... didn't expect that

And we need more. Sigma Draconis is gotta be next.

Thank goodness for auto translate!!!

-M-

Also I guess it isn't going to be so good as an RV standard now...

Signals embedded in the radial velocity noise. Periodic variations in the tau Ceti velocities
http://arxiv.org/abs/1212.4277

Abstract wrote:

According to our analyses, moving average components with an exponential decay with a timescale from a few hours to few days, and Gaussian white noise explains the jitter the best for all three data sets. Fitting the corresponding noise parameters results in significant improvements of the statistical models and enables the detection of very weak signals with amplitudes below 1 ms−1 level in our numerical experiments. We detect significant periodicities that have no activity-induced counterparts in the combined radial velocities. Three of these signals can be seen in the HARPS data alone, and a further two can be inferred by utilising the AAPS and Keck data. These periodicities could be interpreted as corresponding to planets on dynamically stable close-circular orbits with periods of 13.9, 35.4, 94, 168, and 640 days and minimum masses of 2.0, 3.1, 3.6, 4.3, and 6.6 M respectively.

They seem to be taking a rather cautious tone in the paper. Reasonable I suppose. I'm curious as to how much we can consider these "confirmed."

Planet e (if it exists, note both e and f only were detected after the datasets were combined, not in the individual datasets) seems to have roughly 60% more irradiation than the Earth. Close to the runaway greenhouse limit for cloud-free planets.

Sirius_Alpha wrote:

Given the system inclination, I would guess they have rather high true masses.

There is something I'm missing. Do you mean there is somewhat an estimate of P_rot sin i from which roughly deducing putative orbital inclinations of Tau Ceti planets, following Simpson et al. (2010)'s assumption of coplanarity between stellar rotation plane and planet's orbital plane?

If so, what kind of inclination should we expect? More or less than, to say, 13-20°?

True masses can be as high as 15-20 earth mass because of high inclination and rather only 3 planets next 2 rather uncertain and 60% more irradiation than earth for one of putative planets is not really big success

tommi59 wrote:

True masses can be as high as 15-20 earth mass because of high inclination and rather only 3 planets next 2 rather uncertain and 60% more irradiation than earth for one of putative planets is not really big success

So 11 < i < 15° ? Not bad as system. Like HD 69830's one, though richer in worlds.

Edasich wrote:

Do you mean there is somewhat an estimate of Prot sin i from which roughly deducing putative orbital inclinations of Tau Ceti planets, following Simpson et al. (2010)'s assumption of coplanarity between stellar rotation plane and planet's orbital plane?

Not that I have seen. The debris disk at Tau Ceti is known to be nearly face-on, and I was assuming that the planetary system is well-aligned with the disk. If so, then they necessarily have low inclinations.

Some interviews quoting a few well-known scientists here.

Sara Seager wrote:

They're really digging deep into the noise here. The [astronomical] community is going to find it hard to accept planet discoveries from signals so deeply embedded in noise.

Greg Laughlin wrote:

They're pushing the envelope. Some or even many of these planets could go away. But I think that they've done absolutely the best job that you can do, given the data.

Quote :

Team member Chris Tinney, an astronomer at the University of New South Wales in Sydney, Australia, acknowledges the problem. "It's certainly very tantalizing evidence for potentially a very exciting planetary system," Tinney says, but he adds that verifying the discovery may take 10 years, and the scientists didn't want to wait that long. "We felt that the best thing to do was to put the result out there and see if somebody can either independently confirm it or shoot it down."

So what is the inclination of Tau Ceti?

The debris disc around τ Ceti: a massive analogue to the Kuiper Belt
http://adsabs.harvard.edu/abs/2004MNRAS.351L..54G

Quote :

These disc dimensions correspond to 130 by 70 au, or a maximum radius of ≈55 au once deconvolved from the beam. The minor axis is unresolved, but if the contour were up to 5 arcsec wider (from –1σ changes in flux density), then the minor axis radius could be up to 25 au. Assuming a geometrically thin disc, the inclination is then 60°–90°, close to edge-on. However, the star seems to be viewed more pole-on as the observed rotational velocity (sin i) is only 0.4 ± 0.4 km s^-1 (Saar & Osten 1997). Combined with the known stellar rotation period and radius (Saar & Osten 1997; Di Folco et al. 2004), the implied inclination of the stellar equator is ≈0°–40° from the sky plane. Thus the disc results are marginally inconsistent with the stellar equator and disc plane being aligned, but could be reconciled with a slightly larger error bar (v sin i above 1.05 km s^-1 implies i > 60°). It is also possible that the peaks are features within a more pole-on disc, as in the case of Vega (Holland et al. 1998), where there are circular contours surrounding two peaks. Because the τ Ceti emission is much fainter than that of Vega, such a disc component would be hard to detect above the background limits.

Tricky tricky, especially since low-mass systems seem to be typically well-aligned...

According to Greg Laughlin, one thing speaks for the existence of the planets: The system is very much run-of-the-mill what comes to its configuration.

Does the system configuration and orbital stability permit a planet between planets e and f, with an orbital period of 310 to 320 day, exactly in the middle of the habitable zone? Something not detectable yet, with a mass like 1 Earth mass?

There should be room for extra planets.

http://ciel.science-et-vie.com/2012/12/19/cinq-planetes-autour-de-tau-ceti/

Interesting point from an article by Serge Brunier (journalist specialized in astronomy)


« …The proximity of the star Tau Ceti, and great distance from their star of Tau Ceti e and f, augur future exciting observations. Indeed, it is likely that these planets, perhaps Rocky, and whose surfaces resemble, perhaps, that of Mars, Venus and the Earth will be, in the near future, directly observable to the telescope! Take the calculator: Tau Ceti lies 11.9 light years away, and its planets Tau Ceti e and f to 75 and 200 million kilometres. This means that, seen from Earth, the maximum deviation between the star and the planet Ceti e reached, all 84 days, 0.13 seconds of arc. For Ceti f, the angle is even greater: 0.36 seconds of arc. These angles, these differences are within the reach of the most powerful current telescopes: Hubble in space, and the most powerful ground-based telescopes are able to "separate" two stars located approximately 0.05 seconds of arc from the other. But here, of course, the difficulty, huge, again, is the gap of brightness between the star, blinding, and their tiny planets. It is unlikely that the current generation of telescopes able to photograph directly Tau Ceti (e) and (f) embedded in the brightness of their star, but the next generation of giant telescopes, with their power of the order of 0.01 arc second resolution will achieve this without doubt, in ten to twenty years. These instruments will be able, also, to determine the temperature of these planets, the presence of an atmosphere around them, and their chemical composition… »

(Serge Brunier)

Another delve into Underwood, Jones and Sleep (2003)

T_star = 5344 K
L_star = 0.488 solar

Giving:

Recent Venus = 0.533 AU
Runaway greenhouse = 0.608 AU
Water loss = 0.674 AU

First CO₂ condensation = 0.974 AU
Maximum greenhouse = 1.20 AU
Early Mars = 1.27 AU

In this set of estimates the candidate "e" looks like it could be rather toasty, while planet "f" may be too cold, unless you do things with greenhouse gases other than CO₂ perhaps.

Or alternatively Selsis et al. (2007):

Venus criterion: 0.510 AU
0% clouds (inner): 0.593 AU
50% clouds (inner): 0.482 AU
100% clouds (inner): 0.328 AU

Mars criterion: 1.27 AU
0% clouds (outer): 1.20 AU
50% clouds (outer): 1.40 AU
100% clouds (outer): 1.71 AU

Maybe more promising? Not particularly convinced about the actual viability of the cloudy inward extensions though.

http://www.newscientist.com/article/dn23021-nearby-tau-ceti-may-host-two-planets-suited-to-life.html



« …Abel Méndez at the University of Puerto Rico at Arecibo has independently analysed the data and says that the fifth planet, Tau Ceti f, may also be in the habitable zone. Jones is cautious, however. "It's not as strong a case, it's only just in the habitable zone," he says…. »
(…)
Xavier Dumusque of the Geneva Observatory in Switzerland, who led the discovery of Alpha Centauri B b, says Tau Ceti e seems promising, but adds that the new technique used by Jones's team has to be independently verified. "If the planet is really there, it would be the best candidate so far to harbour life," he says.

Greg Laughlin's take on it at Systemic