View Full Version : Tau Ceti, the story so far

Roger E. Moore
2018-Jul-26, 06:13 PM

Tau Ceti loomed large in my science-fiction drenched childhood, particularly in Larry Niven's A Gift from Earth. As I got older I thought less about tau Ceti, figuring lack of mention of it in scientific works meant there were no planets around it.

I was joyously wrong.


Signals embedded in the radial velocity noise: Periodic variations in the τ Ceti velocities
M. Tuomi, H. R. A. Jones, J. S. Jenkins, C. G. Tinney, R. P. Butler, S. S. Vogt, J. R. Barnes, R. A. Wittenmyer, S. O’Toole, J. Horner, J. Bailey, B. D. Carter, D. J. Wright, G. S. Salter, and D. Pinfield
Context. The abilities of radial velocity exoplanet surveys to detect the lowest-mass extra-solar planets are currently limited by a combination of instrument precision, lack of data, and “jitter”. Jitter is a general term for any unknown features in the noise, and reflects a lack of detailed knowledge of stellar physics (asteroseismology, starspots, magnetic cycles, granulation, and other stellar surface phenomena), as well as the possible underestimation of instrument noise.
Aims. We study an extensive set of radial velocities for the star HD 10700 (τ Ceti) to determine the properties of the jitter arising from stellar surface inhomogeneities, activity, and telescope-instrument systems, and perform a comprehensive search for planetary signals in the radial velocities.
Methods. We perform Bayesian comparisons of statistical models describing the radial velocity data to quantify the number of significant signals and the magnitude and properties of the excess noise in the data. We reach our goal by adding artificial signals
to the “flat” radial velocity data of HD 10700 and by seeing which one of our statistical noise models receives the greatest posterior probabilities while still being able to extract the artificial signals correctly from the data. We utilise various noise components to assess
properties of the noise in the data and analyse the HARPS, AAPS, and HIRES data for HD 10700 to quantify these properties and search for previously unknown low-amplitude Keplerian signals.
Results. 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.


The Disk and Planets of Solar-Analogue τ Ceti
S.M. Lawler, J. Di Francesco, G. Kennedy, B. Sibthorpe, M. Booth, B. Vandenbussche, B. Matthews, M. Tuomi
Abstract. τ Ceti is a nearby, mature 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 infrared and submillimeter observations of the debris disk from the Herschel Space Observatory and the James Clerk Maxwell Telescope (JCMT).We find the best model of the disk is a wide annulus ranging from 5-55 AU, inclined from face-on by 30◦. Tuomi et al. (2013) report five possible super-Earths tightly nestled inside 1.4 AU, and we model this planetary system and place dynamical constraints on the inner edge of the disk. We find that due to the low masses and fairly circular orbits of the planets, the disk could reach as close to the star as 1.5 AU, with some stable orbits even possible between the two outermost planets. The photometric modelling cannot rule out a disk inner edge as close to the star as 1 AU, though 5-10 AU produces a better fit to the data. Dynamical modelling shows that the 5 planet system is stable with the addition of a Saturn-mass planet on an orbit outside 5 AU, where the Tuomi et al. analysis would not have detected a planet of this mass.


The Chemical Composition of τ Ceti and Possible Effects on Terrestrial Planets
Michael Pagano, Amanda Truitt, Patrick A. Young, Sang-Heon Shim
τ Ceti (HD10700), a G8 dwarf with solar mass of 0.78, is a close (3.65 pc) sun-like star where 5 possibly terrestrial planet candidates (minimum masses of 2, 3.1, 3.5, 4.3, and 6.7 Earth masses) have recently been discovered. We report abundances of 23 elements using spectra from the MIKE spectrograph on Magellan. Using stellar models with the abundances determined here, we calculate the position of the classical habitable zone with time. At the current best fit age, 7.63 Gy, up to two planets (e and f) may be in the habitable zone, depending on atmospheric properties. The Mg/Si ratio of the star is found to be 1.78, which is much greater than for Earth (about 1.2). With a system that has such an excess of Mg to Si ratio it is possible that the mineralogical make-up of planets around τ Ceti could be significantly different from that of Earth, with possible oversaturation of MgO, resulting in an increase in the content of olivine and ferropericlase compared with Earth. The increase in MgO would have a drastic impact on the rheology of the mantles of the planets around τ Ceti.


ALMA Observations of the Debris Disk of Solar Analogue τ Ceti
Meredith A. MacGregor, Samantha M. Lawler, David J. Wilner, Brenda C. Matthews, Grant M. Kennedy, Mark Booth, James Di Francesco
We present 1.3 mm observations of the Sun-like star τ Ceti with the Atacama Large Millimeter/submillimeter Array (ALMA) that probe angular scales of ~1" (4 AU). This first interferometric image of the τ Ceti system, which hosts both a debris disk and possible multiplanet system, shows emission from a nearly face-on belt of cold dust with a position angle of 90◦ surrounding an unresolved central source at the stellar position. To characterize this emission structure, we fit parametric models to the millimeter visibilities. The resulting best-fit model yields an inner belt edge of 6.2 +9.8 −4.6 AU, consistent with inferences from lower resolution, far-infrared Herschel observations. While the limited data at sufficiently short baselines preclude us from placing stronger constraints on the belt properties and its relation to the proposed five planet system, the observations do provide a strong lower limit on the fractional width of the belt, R/R > 0.75 with 99% confidence. This fractional width is more similar to broad disks such as HD 107146 than narrow belts such as the Kuiper Belt and Fomalhaut. The unresolved central source has a higher flux density than the predicted flux of the stellar photosphere at 1.3 mm. Given previous measurements of an excess by a factor of ~2 at 8.7 mm, this emission is likely due to a hot stellar chromosphere.


F. Feng, M. Tuomi, H. R. A. Jones, J. Barnes, G. Anglada-Escude, S. S. Vogt, R. P. Butler
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 τ 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 τ 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 4 M⊕ 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 0.3m/s, demonstrating the ability of the radial velocity technique to detect relatively weak signals.


We know there's something there, but we don't know what. Can't wait for the story to continue, no matter how it comes out.

Roger E. Moore
2018-Oct-02, 02:44 PM
If the stellar wind from t Ceti is quite low, then it bodes well for survivability on habitable planets in the system (if any), or for human space operations in general. Note than many sunlike stars nearby are included in this study (delta Pavonis, epsilon Indi, etc.).

Implications of Recent Stellar Wind Measurements

Brian E. Wood (Submitted on 4 Sep 2018)

Very recent measurements of stellar winds are used to update relations between winds and coronal activity. New wind constraints include an upper limit of Mdot < 0.1 Mdot_sun for Tau Ceti (G8 V), derived from a nondetection of astrospheric H I Lyman-alpha absorption. This upper limit is reported here for the first time, and represents the weakest wind constrained using the astrospheric absorption technique. A high mass loss rate measurement of Mdot= 10 Mdot_sun for Delta Pavonis (G8 IV) from astrospheric Lyman-alpha absorption suggests stronger winds for subgiants than for main sequence stars of equivalent activity. A very low mass-loss rate of Mdot ~ 0.06 Mdot_sun recently estimated for GJ 436 (M3 V) from Lyman-alpha absorption from an evaporating exoplanetary atmosphere implies inactive M dwarfs may have weak winds compared with GK dwarfs of similar activity.

QUOTES: Wind effects on exoplanets are another motivating factor for improving our understanding of coronal winds. Most of the exoplanets that have been discovered orbit very close to their stars, where they will likely see very high wind fluxes due to their close proximity.

This leaves only τ Ceti, which has very advantageous values of VISM = 56 km s−1 and θ = 59◦, and a low ISM column density of logNH = 18.01. Furthermore, τ Ceti is particularly close, with d = 3.65 pc, making it even more likely to be surrounded by partially neutral ISM material like that around the Sun. Thus, an ˙M upper limit for τ Ceti is inferred here for the first time.

Roger E. Moore
2018-Nov-26, 01:55 AM
Tau Ceti has a gas giant! Many good items in this paper, enjoy the read!


Stellar and substellar companions of nearby stars from Gaia DR2 - Binarity from proper motion anomaly of stars within 50 pc

Pierre Kervella, Frédéric Arenou, François Mignard, Frédéric Thévenin (Submitted on 21 Nov 2018)

The census of stellar and substellar companions of nearby stars is largely incomplete, in particular towards the low mass brown dwarf and long-period exoplanets. It is however of fundamental importance for stellar and planetary formation and evolution mechanisms. We aim at characterizing the presence of physical companions of stellar and substellar mass orbiting nearby stars. Orbiting secondary bodies influence the proper motion (PM) of their parent star. Using the Hipparcos (Hip) and Gaia DR2 (GDR2) catalogs, we determine the long-term PM of each star. We then search for a proper motion anomaly (PMa) between the long-term PM and the GDR2 (or Hip) measurements, indicative of the presence of a secondary object. We present a catalog of the PMa of 6741 nearby stars located within 50 pc. A fraction of ~40% of these objects presents a PMa at a level of more than 2σ, and ~30% at more than 3σ. We present a few illustrations of the PMa analysis. We set upper limits of 0.1 - 0.2 MJup to potential planets of Proxima between 1 and 10 au and 2.5 MJup on any stable orbit. We confirm that Proxima is gravitationally bound to alpha Cen. We recover the masses of the known companions of eps Eri, eps Ind, Ross 614, GJ 229, tau Boo and beta Pic. We also detect a possible long-period planet of a few jovian masses orbiting tau Ceti. The combination of the GDR2 with Hipparcos and the very high accuracy of the derived PMa already enables to set valuable constraints on the binarity of nearby objects. The detection of tangential velocity anomalies at a level of σ(dVtan) = 1.1 m/s per parsec of distance is already possible with the GDR2. This opens the possibility to identify long period orbital companions otherwise inaccessible. The complementarity of Gaia, radial velocity and transit techniques already appears as remarkably powerful.

Other discoveries:

1. Proxima Centauri does indeed orbit Alpha Centauri AB.
2. Barnard's star might have a Jovian-mass planet 1-20 AU out.
3. Epsilon Eridani's Jovian-type planet confirmed.
4. Ross 128 might have a Jovian-mass planet within 30 AU.
5. Tau Ceti might have a Jovian-mass planet within 30 AU.
6. Epsilon Indi's Jovian-type planet confirmed.
7. van Maanen's star (wd) might have a companion.
8. Fomalhaut is a very complicated triple system with a substellar object (confirmed).
9. 51 Pegasi might have a Jovian or brown-dwarf secondary companion other than the Jovian body it is known to have.

2018-Nov-27, 01:27 PM
Thanks Roger this is interesting. This kind of study has the capability of detecting long-period planets, something that has been a bit lacking up to now.

Hence we know of a lot of compact systems but few solar system analogues. Hopefully this can be redressed slightly.

The other thing is metallicity: Tau Ceti has about 1/3rd the metallicity of the sun, but if we believe it, has at least four super-Earths, an asteroid belt at least 10 times ours, and possibly a Jupiter according to this paper.

Roger E. Moore
2018-Nov-27, 03:18 PM
kzb, I was thinking that the more we learn about other planetary systems, the less we know about them. Nothing is coming out at all like science once expected. I remember discussing 51 Pegasi's discovery with a friend, and we could not believe you could stick a Jovian planet so close to a star, but it kept happening. Plus, I had expected all the big long-period planets would be found first (Barnard's star B, 61 Cygni C, etc.), but they were a bust. Now it's unpredictable.

The weirdest thing that I believe will come out of this is that only narrow categories of planets can be discovered by each style of searching, whether by transiting or direct imaging or star-wobble or whatever, even by different spacecraft. It will be decades before we get a true picture of what planets are out there, and meanwhile we'll be saying things like, "Barnard's Star has six ice giants, two asteroid belts, a brown dwarf, and five super-Earths? What the heck is going on?" And next month we find out something else completely unexpected. It's chaotic discovery in weird little batches. Every week in the reports, you get piles of short-period super-Earths from one space mission, then a batch of ringed gas giants from ground observatories, then who knows.

I admit I kind of like it, but keeping up with it is impossible.

LATER NOTE: Kepler, for instance. Kepler in reality had a very narrow window on the sorts of planets that are out there. They had to be within its field of vision, they had to be transiting planets, they had to be Earthlike in size, they had to be close-in, etc. TESS has a different set of capture rules, and ground observatory programs each have their own capture requirements, and so on. We aren't getting anywhere NEAR a complete and accurate "big picture" of what's out there. It's like a Jackson Pollack painting, big splashes and slashes of color that don't cover the entire canvas, just a chaotic mass of data which we struggle to view as logical and sensible.