Planets around Tau Ceti

Putative planet with period 300 days is ok but 600 days rather not especially that there is planet with 642 days

The Debris Disk of Solar Analogue τ Ceti: Herschel Observations and Dynamical Simulations of the Proposed Multiplanet System
http://arxiv.org/abs/1408.2791

Quote :

Abstract: τ Ceti is a nearby, mature G-type star very similar to our Sun, with a massive Kuiper Belt analogue (Greaves et al. 2004) and possible multiplanet system (Tuomi et al. 2013) that has been compared to our Solar System. We present Herschel Space Observatory images of the debris disk, finding the disk is resolved at 70 and 160 microns, and marginally resolved at 250 microns. The Herschel images and infrared photometry from the literature are best modelled using a wide dust annulus with an inner edge between 1-10 AU and an outer edge at ~55 AU, inclined from face-on by 35± 10 degrees, and with no significant azimuthal structure. We model the proposed tightly-packed planetary system of five super-Earths and find that the innermost dynamically stable disk orbits are consistent with the inner edge found by the observations. The photometric modelling, however, cannot rule out a disk inner edge as close to the star as 1 AU, though larger distances produce a better fit to the data. Dynamical modelling shows that the 5 planet system is stable with the addition of a Neptune or smaller mass planet on an orbit outside 5 AU, where the Tuomi et al. analysis would not have detected a planet of this mass.

Ah, good to see the, er, second? third? nearest debris disk (is the Kuiper belt a debris disk?) get some proper characterisation! But, as the authors do not hesitate to state, the inner regions of the disk still need proper observation.

E-K belt and scattered disc should be associated with dust, IIRC the closest known extrasolar analogues are zeta-2 Ret and q1 Eri (HD 10647).

Then you have the warmer dust in the inner system associated with Jupiter-family comets, the asteroid belt, etc.

ALMA observations of the Tau Ceti disc. Confirms that the disc is much wider than the Kuiper belt and potentially extends into the region of the (still unconfirmed?) planetary system.

MacGregor et al. "ALMA Observations of the Debris Disk of Solar Analogue Tau Ceti"
http://arxiv.org/abs/1607.02513

Color difference makes a difference: four planet candidates around tau Ceti
https://arxiv.org/abs/1708.02051

Quote :

The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial velocity method to detect Earth analogues. We analyze radial velocity data of tau Ceti and find robust evidence for wavelength dependent noise. We find this noise can be modeled by a combination of moving average models and "differential radial velocities". We apply this noise model to various radial velocity data sets for tau Ceti, and find four periodic signals at 20.0, 49.3, 160 and 642 d which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 d while the other two previously suspected signals around 160 and 600 d are quantified to a higher precision. The 20.0 d candidate is independently detected in KECK data. All planets detected in this work have minimum masses less than 4M⊕ with the two long period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the HARPS around 0.2 m/s. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m/s level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m/s, demonstrating the ability of the radial velocity technique to detect relatively weak signals.

Farewell to b,c and d, hello to g and h. I see the minimum masses for e and f have gone down, although under the assumption that they share the inclination of the disc (35±10°) then it still looks most likely that these are mini-Neptunes rather than terrestrials.

A request for you: has anyone got a paper from 2000 or 2002 (author(s) Butler or Marcy, I don't remember) inferring two (presently discarded) Neptune-size planets around Tau Ceti in close-in orbits? Once I had it but I cannot retrieved it.