I discovered while trying to get to my copy of Stephen Dole’s Habitable Planets for Man that my office was so choked with stacks of books mixing with Christmas gifts about to be wrapped that I couldn’t reach the necessary shelf. Thus space studies end inevitably in office cleaning, the only benefit of which is that there is now a clear path to the most distant of the bookshelves and Dole’s book (this is the 1964 edition written with Isaac Asimov) now sits before me. I was feeling nostalgic and wanted the Dole volume to remind myself of my early enthusiasm for the nearby star Tau Ceti.
The news that five planet candidates have been identified around this star — one of them in the habitable zone — brings back the fascination that was piqued when Frank Drake made Tau Ceti one of his two targets in 1960’s Project Ozma, a search for extraterrestrial radio signals from Green Bank, WV. And in fact what I found in Dole’s book on the subject of Tau Ceti was mostly about Drake’s interest in this G-class star, along with the even closer Epsilon Eridani. Dole liked Tau Ceti’s similarity to the Sun but in those days we had no evidence of any extrasolar planets.
Image: Photograph of the constellation Cetus, with Tau Ceti identified. Credit: Westlake.
Pushing Data Modeling to the Limit
The latest news, from an international team of astronomers, causes us to look at Tau Ceti in a new and deeper way. Mikko Tuomi (University of Hertfordshire), lead author of the paper on this work, says that the team has found a way to detect radial velocity signals half again as small as any that have been worked with before. Says Tuomi:
“We pioneered new data modeling techniques by adding artificial signals to the data and testing our recovery of the signals with a variety of different approaches. This significantly improved our noise modeling techniques and increased our ability to find low mass planets.”
The big problem in radial velocity studies, according to the paper on this work, is stellar noise, which the researchers refer to as ‘stellar jitter.’ While its magnitude can be uncertain for given stars, the shape of its noise distribution and its variability over time, as well as its dependence on other properties of the star, have not been a major focus of investigation. The team added artificial signals to the radial velocity data of Tau Ceti to see which of its statistical noise models could best extract the artificial noise from the data. Its refined models then allowed the search for low-amplitude signals that had heretofore escaped detection.
As the paper explains further:
To verify the trustworthiness of our noise models in extracting weak signals from the data, we first test their performance by adding artificial signals to the HARPS data set for HD 10700. The models that enable the recovery of the artificial signals are then compared using the Bayesian model selection techniques to find the most accurate descriptions of these HARPS RVs. Finally, we search for periodic signatures of planetary companions in the HARPS velocities.
Just how hard is this paper pushing available radial velocity methods? For a take on that, I turned to planet hunter Greg Laughlin (UC-Santa Cruz), who noted the excitement of what he called ‘this deep dive into the three extremely extensive data sets on a household-name star.’ On the methods involved, Laughlin said this:
…it’s clear that the community is pushing up against the limits of the extant radial velocity data. It will take quite a while to get a substantial confirmation of these planets, since increases in signal-to-noise scale with the square root of the number of measurements, and there are a lot of measurements already.
It’s interesting to see that time-series techniques familiar from econometric analyses (e.g. ARMA(p,q) ) are being applied in this new context. I think that the methods they are using (which would be completely familiar to a Wall Street Quant) have a lot of promise in this particular area.
A Candidate System
Out of this work, conducted with spectroscopic data from HARPS on ESO’s 3.6m telescope at La Silla (Chile), UCLES on the Anglo-Australian Telescope in Siding Spring, Australia and HIRES on the 10m Keck telescope on Mauna Kea, Hawaii, we have five planets with masses between two and six times that of Earth. The candidate in the habitable zone — orbiting its star every 168 days — is about five Earth masses, which the researchers say would make it the smallest planet orbiting in the habitable zone of any Sun-like star. The periodicities of the five in order are 13.9, 35.4, 94, 168, and 640 days, a system telling us that the new noise modeling techniques may have paid off handsomely.
Laughlin’s take on the Tau Ceti candidates is that they are representative of what we have been finding elsewhere:
…the system that they’re proposing is completely _unsurprising_. Kepler and HARPS have demonstrated that the galaxy’s default mode of planet formation is to produce multiple super-Earth/sub-Neptune category planets on nearly circular orbits with orbital periods in the range from days to weeks. This system is certainly a standard-issue example of such a configuration.
Interestingly, the team picked Tau Ceti for the study because they thought it contained no signals. Indeed, Tau Ceti, at just under 12 light years from the Solar System, is a quiet and inactive star for which the HARPS spectrograph had not been able to generate any planetary signatures despite more than 4000 spectral observations. Nor had other searches revealed any signals until this new work. The star is known, however, to have a bright debris disk with a mass estimated to be an order of magnitude greater than the mass of the Kuiper Belt in our own Solar System, extending out to about 55 AU, leading some to assume that any planets there would likely undergo an extensive period of bombardment from objects entering the inner system.
Confirming the Tau Ceti planets will be a story worth watching, for this is a star close enough to the Sun to factor into our thinking for spectroscopic atmospheric analyses in future missions. Quoting the paper again:
With a distance of only 3.7 pc, HD 10700 [Tau Ceti] is the third closest star reported to be a host to a putative planetary system after Epsilon Eridani (Hatzes et al., 2000) with a distance of 3.2 pc and α Centauri B (Dumusque et al., 2012) with a distance of 1.3 pc, though both of these remain to be confirmed and Zechmeister et al. (2005) have cast considerable doubt on the existence of a planet around Epsilon Eridani. This makes HD 10700 an ideal target for future direct-imaging missions. The signals we find, which suggest the presence of low-mass planets, are consistent with both current theoretical models for low-mass planet formation and extant observational evidence for the presence of low-mass planets in the immediate Solar neighbourhood.
The paper is “Signals embedded in the radial velocity noise Periodic variations in the τ Ceti velocities,” accepted for publication in Astronomy & Astrophysics and available online.
Since it’s beginning to look like Super Terresrials are likely the
closest potentially benign worlds (I use that term loosely). I’d like
to see exensive modeling on atmospheric cycles, Geology, for differing
S-T sizes, to see what kind of microbial or pre-cambrian lifefoms are allowed in such a world. Surely a 5Me world with a similar density of
Earth would make life outside an ocean a great challenge. I would
expect that the route to land dwelling plants/animals would be much more
difficult and thus less likely. It is unfortunate that HARPS, is at the limit
of its detecting capability, as I assume that unless it’s a special sitution,
Earth sized planets cannot be detected by HARPS in the habitable zone of
Sun like stars.
Fun with microbes: how about using a centrifuge as a petri dish. Use a special centrifuge loaded with a pressure vessel inside of which is a steamy
atmosphere of 70% N 30% Water vapor, set gravity at 2g, temp at 150 C place suitable medium and anerobic bacteria, see if you get any survivors.
Better news even than ?Cen Bb, really, though I’m still holding out hope for further interesting stuff around the ?Cen stars.
The Tau Ceti news is intriguing, but I wonder if the proposed super Earth orbiting in the habitable zone (Tau Ceti e) is more likely a small gas giant rather than a rocky terrestrial world. There appears to be evidence that Tau Ceti’s rotational axis is pointed towards Earth, and hence the orbital plane of its planetary system would be oriented nearly perpendicular to our point of view. If so, then the observed radial velocity shifts of Tau Ceti e are a fraction of the total shift, and Tau Ceti e’s mass is likely far larger than the proposed minimum of 4.3 Earths.
Christmas came early this year!
Also Gliese 667 C is back: claims that the system hosts multiple planets in its habitable zone. So much for the claim that the narrow width of the M-dwarf habitable zone is a problem?
Gregory (2012) “Evidence for Multiple Planets in the Habitable Zone of Gliese 667C: A Bayesian Re-analysis of the HARPS data”
Maybe there are some stability issues with the proposed planetary system though…
Minor bibliographic note: the Asimov-enhanced version of the book in question was retitled simply Planets for Man (habitability assumed?). Great book in either form, though in rather different ways.
Onward to Tau Ceti!
I find the definition of habitable zone stretched inwards in this case.
For our solar system, more modern definitions put the inner boundary at 0.9 times the distance of Earth. This is ~135 M km. Older papers fond .725.
Now, Tau Ceti is reported by the paper to be .488 as luminous as the sun.
The equivalent distance is therefore 135*sqrt(.488)= 94 M Km.
Now P = 2 *pi *d/sqrt(G*M/d) where P is the period for a planet, G the gravitational constant, d the distance and M the star mass.
With a reported star mass of .783 times the one of the sun, and a period of 168 days, the resulting distance is 82 M Km. Well outside the HZ.
Not only that, but a planet with 4-5 (or more) earth masses is likely to have a deep atmosphere that, if anything, would push the HZ for that planet outward.
Planetary Hab Lab is claiming that possibility exists of the second planet in Tau Ceti being habitable(very broad definition).
http://phl.upr.edu/press-releases/twonearbyhabitableworlds
What leaps out of the data is the disproportionate gap between planets e and f. This is actually a good sign. Likely there are other planets in there which are not massive enough to be detected by RV. There could easily be a habitable Earth twin with a period of 250 days. We have pushed the RV technique right to the limit. Bring on direct imaging!
@Rob Flores ” I would expect that the route to land dwelling plants/animals would be much more difficult and thus less likely.”
I do not see why. It would simply reduce the maximum size of organisms. Small organisms would be fine. There would even be benefits as the higher atmospheric pressures could allow faster metabolisms. There might even be more [small] body plans that could fly.
Joy
That does not matter, because any planet orbiting Tau Ceti would face far more impact events than the Earth. So The chance id high that there will never be complex life.
Long time lurker here. I’m curious: if you applied the techniques used to tease out the Tau Ceti candidates to an equivalent number of measurements of our own sun from 12LY, would any of our worlds be detectable? i.e. are we able to detect a true solar system analogue at all?
We live in exciting times, recently Alpha Centauri B b and now this!
I am honestly presently surprised (contrary to Laughlin) about this abundance of planets around Tau Ceti, because of its very low metallicity.
I know it is known that low metallicity is mainly a show-stopper for giant planets, but TC’s metallicity is really low: I have seen measurements of (Log) -0.39 to -0.55, corresponding with 28 – 40% of solar metallicity.
And then there is this very thick debris disk of dust, planetoids and comets, which seemed to suggest a failed planetary system to me.
So planets can even exist under these very low metallicity conditions. I still would not expect to find any giant planets in this system though.
With regard to Habitable Zone planets, I tend to agree with Enzo: If we generously assume the HZ in our solar system to extend from 0.93 – 1.5 AU, this would correspond to 0.65 – 1.05 AU for Tau Ceti.
In other words, planet e at 0.55 AU is way on the inside of the HZ. And planet f is well on the outside of it.
So, I hope with Joy that there is still another, really earthlike small planet in between the two.
Paul,
that does not sound quite right to me. Should that have been square root instead of square, maybe?
Rob Flores: There is no reason to worry about gravity. Gravity is unsubstantial for microbes, they will thrive well and not even notice a difference at 100 g. This is because the effect of gravity scales linearly with size. So, the worst 2 gees of gravity can do is keep the Tau Ceti equivalent of Dinosaurs at half the size of ours. Still pretty impressive.
Atmospheric density is much more worrisome than gravity. A larger planet will tend to have a much denser atmosphere. If the atmosphere is too dense, light will not shine through to the surface, and photosynthesis may not evolve. Although, if the atmosphere is dense enough, organisms could perhaps live suspended in it away from the surface. The truth is, we really have no idea whether a bigger planet increases or decreases the chance of life.
Agreed with enzo here.
At distance 0.552 Au, Tau Ceti e will receive 1.6x amount of light that earth receive from earth if Tau Ceti’s luminosity is 0.488x of sun. Not venus level (2x amount of light earth receive), but it enough to make habitability claim questionable.
Eniac writes:
Let me ask Greg — I’ll post his response.
Eniac was right — this just in from Greg Laughlin:
“Thanks for the e-mail. Yes, the way I’ve phrased it, I should have written “square root” rather than “square”. Thanks to the reader for catching that!”
Good point. I am not really an expert, but according to what I do know the answer is a resounding NO.
In my opinion all claims that “our solar system is special” should be held back until we have the methods sensitive to systems like ours.
Really? Well yes there is a large debris disc in the outer system but that doesn’t necessarily mean there will be high impact rates on the inner planets. In fact having such a large debris disc in such an old system would suggest that there isn’t much going on to remove material from the reservoir of small bodies producing the dust, e.g. by flinging it into the inner system.
The disc also suggests that Tau Ceti never formed giant planets. Giant planets are good at removing debris: first in the initial epoch of planet-planet scattering after the protoplanetary disc disappears (an atypically nonviolent example of this may have triggered the Late Heavy Bombardment in our solar system), and also they are very good at dynamically stirring the disc.
Having lots of warm dust in the inner system on the other hand would probably be a bad sign, but that has not been detected at Tau Ceti.
In response to ALex Tolley:
Well, yes, perhaps Land animals in 2G worlds would just be thicker.
But how extra difficult would it be Achieveing Space Flight in such
world. Right Now I believe we can plut only about 7-10% of total
fueled launch mass into LEO. A 2G world would make vertical
lauches very inefficient. An equatorial sled launch system would probably
be more workable for such a world.
Why? To show that our solar system is in the minority does NOT require being able to detect systems like ours. All you need to do is to show is that planetary systems that have architectures unlike our own occur around more than 50% of the stars.
The occurrence rate of systems with close-in super-Earths around solar-type stars does indeed seem to be above 50% (see the introduction to this paper) according to the results of the RV surveys. If so, this allows you to state that the solar system is not in the majority even without being able to detect a solar system analogue around another star, by the simple fact that all the fractions must add up to 100%.
“That does not matter, because any planet orbiting Tau Ceti would face far more impact events than the Earth.”
It would increase the likelihood of exogenesis event though. More impacts-more material able to land with surviving primitive life on other planets in the system with favorable conditions for existence.
@Col
An astronomer in the Tau Ceti system observing the Sol system since 1995 with the RV tools we have would have detected Jupiter by now. Jupiter has a 12.4 m/s RV change on Sol and a full orbit would have been observed.
The Saturn RV effect is 2.7 m/s, which is detectable with second generation spectographs, but with an orbital period of over 29 years there would not have been enough observation time to claim detection yet.
None of the inner planets of Sol could be detected with current equipment, and might never be detectable with RV.
Can the gaps between e and f is asteroid belt? And the “debris disk” signal that we get is coming from it?
Andy,
Yes, this is of course correct. Given that we have in fact found planets closer than Mercury in more than 50% of ALL stars (is that really correct?), you can indeed state confidently that the solar system is in the minority on that count. But that is a very weak assertion compared to “our system is rare”. There are probably a handful of categories, and we happen to be in one that is not the largest. Or, none of them exceed 50%, which is also possible.
To me it starts looking like our system may be unusual (if at all) for having very few planets. With an absence of evidence for the absence of outer planets, we have to assume that undiscovered outer planets also exist together with the close inner planets that we know about.
Unless, that is, we have very good theoretical grounds to assume that inner and outer planets are mutually exclusive. I doubt this, but will accept expert opinion on it.
Andy, calling our system of planets special is a very interesting claim, but rather than the handwaving of all previous statements of that nature I would at least like to see such speculation put on an objective basis. Is the prediction…
1) It seems that our multiple planet candidate data, is just starting to point to there being a few discrete categories of such systems, and indications are that we are one of the smaller such categories.
2) There appear as yet to be no such categories, but the data that we do have could be interpreted as indicating that if all local planetary system were known, and a measure of the standard deviation from the norm taken of there parameters, then our system would be one of the most deviant.
It seems to me that without such a basis it is easy for you and Eniac to talk past each other.
Andy,
Right, Chiang and Laughlin indeed report that “Close-in super-Earths, with radii R = 2-5 R_Earth and orbital periods P < 100 days, orbit more than half, and perhaps nearly all Sun-like stars in the universe". Interesting. If it is the "nearly all" part that turns out to be true, that would make our system a bit of an oddity among sun-like stars, with no super-Earths at all.
Still, words like "special" or "rare" should probably be reserved for categories of 5% or less, for which there is certainly no evidence, at this point.
Eniac, there are 2 points that I would like to add to your dissuasion of aerobiota here.
1) SuperEarths would have far more difficulty in loosing hydrogen than Venus. As we previously noticed. The zone on Venus with Earth sea-level pressures is very temperate, so for floating or suspended life it is well within the life zone.
2) A dinosaur-killing sized impact on such a world would just fertilise the life zone. Thus an outer dense asteroid or Kuiper belt and an implied stream of small to moderate sized impacts would provide little danger, but might greatly boost life’s prospects there.
Andy, Eniac, I think you are both right in a way: the compact systems of super-earths and subgiants (Neptune class) may be by far the most common, however, in the universe rarity is a relative concept. Even if our type of planetary system constitutes a small minority category (a few percent?, 5, 10%?), it may still be rather abundant in abolute terms.
For instance, despite extensive RV searches (HARPS and others), no such compact systems has yet been found for nearby sunlike stars such as: Delta Pavonis, Beta Canum Venaticorum, Zeta Tucanae, Beta Comae Berenices, Alpha Mensae, Zeta 1 and 2 Reticuli, 58 Eridani, 18 Scorpii, just to name a few within 50 ly.
So a minority category can still be very relevant.
And remarkably, of 10 of the most sunlike solar twins (18 Scorpii, HD 44594, HD 195034, HD 138573, HD 142093, HD 98618, HD 143436, HD 129357, HD 133600, HD 101364), *none* has been detected with such a compact system.
Research by Melendez, Ramirez and Asplund on solar twins seems to indicate that about 15% of solar twins and analogs have (inner) systems comparable to ours.
@Joy & Eniac, thanks for the answers. Interesting that just Jupiter and possibly Saturn are detectable with something comparable to our methods.
I am just a layman when it comes to this stuff, but having been raised on a steady diet of classic Science Fiction (cut my teeth on Heinlein, Asimov, Clarke & others) I find the emerging reality of the local systems to be endlessly fascinating. I don’t think any of the old masters of SF ever envisaged worlds like Alpha Centauri Bb, or the tightly wound systems of super-earths that are turning up.
Even if only 5% of systems around G & K stars have an architecture similar to our own, that still implies a huge harvest of potentially habitable worlds (our definition of habitable, narrow as it is). Not to mention the unanswered question of what lies beyond those tight little super-earth systems.
What is clear is that we need far more investment in ground and space based observatories to tease out the full census of worlds within 50 to 100 LY of home. We need a map to guide those future (robotic) explorers.
It is suprising to me that Abel Mendez has listed BOTH habitable zone planets for Tau Ceti on his PHL catalog, BUT; NONE of the NEW habitable zone planets for Gliese 667C. He does list the Tau Ceti planets as “candidates”. Does this mean that he does not consider the new Gliese667C data good enough for even “candidate” status? This may be that Mikko Touni HAS done a long-term stability study for his Tau Ceti system, whereas Philip Gregory has NOT for the REVISERD Gliese 667C system(s). FINALLY: I am waiting with baited breath for Toumi’s INEVITABLE re-analysis of the HARPS Alpha Centauri data to see if Fischer and Laughlin’s 250 day (or any other interesting) signal(s) pop out for Alpha Centauri B!
Rob:
I am not sure I understand what your point is here. My interpretation/comment would be: 1) Losing less hydrogen, of course, means more water and thus greater opportunity for life, and 2) sea-level pressure is not at all special. Life can exist at any pressure sufficient to permit liquid water. There is no obvious upper limit at all.
Intuitively, I would say chances for life probably go up with increasing pressure, especially for aerobiota. There is of course the matter of optical depth: In a thick atmosphere you would not expect photosynthetic activity below a certain depth, but on Venus it is bright enough still at 100 bar (the surface), so this is not a very strict limit.
Good point that about the impacts.
Col:
Very sensible comments!
“FINALLY: I am waiting with baited breath for Toumi’s INEVITABLE re-analysis of the HARPS Alpha Centauri data to see if Fischer and Laughlin’s 250 day (or any other interesting) signal(s) pop out for Alpha Centauri B!”
Yes, so am I; do you have any further information on ‘Fischer and Laughlin’s 250 day signal(s) for Alpha Centauri B’? I mean, the discovered AC B b planet is a much shorter period, are there indications for another, longer-period planet?
Ronald:
This seems to me in direct contradiction to Chiang and Laughlin’s “Close-in super-Earths orbit more than half, and perhaps nearly all Sun-like stars”. If it is 50% or more in general, but zero amongst the closest 10 examples, what is that? Incredibly bad luck? Ten tails in a row? I must be missing something.
Andy, any idea?
It is interesting how the new solar system of Tau Ceti looks suspiciously like that of 82 Eridani (another low metallicity star). Perhaps one can conclude that:
(1) Planets like Jupiter and Saturn (or another steller partner like Alpha Centauri A) disperse much of the nebular mass that might otherwise fall over close orbit around the central star.
(2) When a giant planet or steller partner is absent, much of the nebular mass does fall into close orbit around the central star (unless it is dispersed by other factors such as photoevaporation from giant stars in the steller birthplace)
(3) When the nebular mass does fall into close orbit around the central star, it will accumulate into Super-Earth or Neptune mass bodies. It will also bring a great deal of volatiles into close orbit around the central star.
(4) Logically, Earth mass or smaller bodies would be formed also. In such a case, I do not see any reason why the presence of a Super-Earth of Neptune in very close orbit around the star would forbid the creation of an Earth in or near the habitable zone (unless the Super-Earth or Neptune is in a sufficiently eccentric orbit).
(5) Apparently, if Jupiter did indeed pass within 1.5 AUs of the Sun in its youth, Jupiter did not prevent the formation of Mars (although Mars became much smaller than it might have been) or the formation of Earth.
R Kelley:
And Nu2 Lupi, same story of low metallicity, rather old age, low-medium mass planets.
What you say under 1) (and maybe also 2) ) is indeed along the lines of Wyatt, M. C. et al. (2012). “Herschel imaging of 61 Vir: implications for the prevalence of debris in low-mass planetary systems”. Monthly Notices of the Royal Astronomical Society.
Quote: “systems that form low-mass planets are also able to retain bright debris disks.” I think what is really meant here is ‘systems that form *only* low-mass planets’.
Ronald, Google http://www.oklo.org, click Alpha Centauri, and read ALL of Gregory Laughlin’s posts!
UPDATE to Ronald: Click “Alpha Cen” and read ONLY the Alpha Centauri: Market Outperform post
FINAL update to Ronald: ALSO check out the Alpha Centauri Bb post on the “latest posts” section. At the BOTTOM you will get an even MORE detailed account!
Harry, thanks, I may have missed something but with regard to your ‘latest posts, Alpha Centauri Bb’ post I did not get much more news on any posible planet in the HZ (250 day), rather that stellar noise necessitates longer observation time. I did not find there how much longer.
What struck me is that ‘no news is good news’ with regard to large mass planets in the HZ.