Centauri Dreams

As the number of confirmed planets and planet candidates has grown, we’ve gone through a variety of techniques for exoplanet hunting, as Michael Lemonick’s new book Mirror Earth: The Search for Our Planet’s Twin (Walker & Co., 2012) makes clear. I’m only a third of the way into the book but I bring it up because it’s germane to today’s discussion in two ways. The first is purely administrative. Readers of Centauri Dreams are used to seeing information about the book I’m reading on the front page, but as many emails have reminded me, lately it’s been absent.

What’s happening is this: The software I use to display the book cover and progress bar is no longer being maintained by its creator, and the program has become flaky. I’ve discovered more and more that certain books will not display properly, so that although I can enter them in the configuration file, nothing shows up in the sidebar on the home page. As a result, I’m searching for alternatives that will display titles like Mirror Earth and anything else I’m currently reading.

But even though I’m early on in my progress through Lemonick’s book, I can already recommend it for the overview it provides. Lemonick writes for TIME and has written skillfully about astronomy’s history (William and Caroline Herschel were the topic of a recent book) and cosmology. He was an early entrant into exoplanet books with 1998’s Other Worlds. In short, Lemonick knows what he’s up to, and he offers the general reader a concise set of discussions with major exoplanet figures and a cogent history of how the field developed.

Astrometry was an early entrant in the exoplanet hunt as astronomers hoped to identify the side-to-side motion of a star being tugged by its planet, but early ‘discoveries’ made with the method proved bogus, and radial velocity emerged as the technique of choice, focusing on the movement of the star to and away from the Earth as measured by Doppler shift. We soon learned the effectiveness of transits (Kepler uses this method) and investigated gravitational lensing and, in rare cases, succeeded at direct imaging. What other methods will emerge?

As I proceed with the book, I’m also looking at recent news from the planet hunting HiCIAO ( High Contrast Instrument for the Subaru Next Generation Optics), used by an international team with the Subaru Telescope on Mauna Kea. HiCIAO has ways of blocking out the central star to help astronomers detect faint objects like planets or dust disks around it. The new work presents studies of the disk around the young star (about 9 million years old) HD 135344B, some 460 light years from Earth in the constellation Lupus. The disk under measurement is 20 billion kilometers in radius, which is about five times greater than Neptune’s distance from the Sun.

This is intriguing stuff because the team is looking at a spiral disk structure with two discernible arms. Applying density wave theory to the data, the researchers conclude that planets embedded within the disk may account for the spiral shape, and in doing so, they may be revealing another way of detecting exoplanets. The idea here is that regions of enhanced density develop inside a rotating disk because of differential rotation. A planet within the disk could produce the kind of density wave that results in the formation of just this kind of spiral structure. From the paper: “While we cannot uniquely identify the origin of these spirals, planets embedded in the disk may be capable of exciting the observed morphology. Assuming that this is the case, we can make predictions on the locations and, possibly, the masses of the unseen planets.”

Image: A comparison of the fit between the theoretical model and observational data. The red dashed line represents the shape of the disk based on modeling from density wave theory. The image shows that the data conform to the predictions of the theory and supports an explanation for the development of the structure in terms of this theory’s model. (Credit: NAOJ).

So far no planet candidate has emerged — we are simply considering the possibility that a planet is the cause of the density wave. But the use of density wave theory in measuring a protoplanetary disk helps us to understand how spiral disks form, and it may develop into a more precise way to detect exoplanets. The paper on this work suggests that HiCIAO may be capable of detecting the indirect signatures of planets down to 0.05 Jupiter mass, but it will take future observations at other wavelengths to see whether such predictions are borne out in fact.

The paper is Muto et al., “Discovery of Small-Scale Spiral Structures in the Disk of SAO 206462 (HD 135344B): Implications for the Physical State of the Disk from Spiral Density Wave Theory,” in Astrophysical Journal Letters 748 (April 2012), L22, 2012 (abstract).

In his new article on Alpha Centauri in Astronomy & Geophysics, Martin Beech (Campion College, University of Regina) noted that the Alpha Centauri stars seem to go through waves of scientific interest. Beech used Google’s Ngram Viewer to look for references to the system in both the scientific literature as well as general magazines and newspapers, finding that there is a 30-year interval between peaks of interest. The figure is suspiciously generational, and Beech wonders whether it reflects an awakening of interest in this nearby system as each generation of scientists and publishers arises.

I mentioned on Christmas Eve that the Beech paper was a real gift for the holidays, and for those of us who try to track developments about Alpha Centauri, it certainly is, drawing together recent work and commenting with care on the findings. The big issue for now is the existence of planets around these stars, a question Centauri B b will begin to answer if it can be confirmed. Everyone from astrophysicists to science fiction authors has noted at one time or another that we may have planets around all three of these stars, no doubt fueling that thirty-year spike Beech identified.

Image: Despite the overexposure of Centauri A and B that fuses the two stars into one at top left, I like it because it reveals Proxima Centauri (shown by the arrow at bottom right), an indication of how distant this nearest star to the Sun is from the two larger stars. Credit: 1-Meter Schmidt Telescope, ESO.

I remember a long-ago sixth grade afternoon when I asked the teacher, after a presentation about astronomy, whether the nearest star had planets like ours around it. After class, she slipped me a book to look at whose title is long lost to memory, but I recall it being stated with confidence that planets were all but impossible around binary stars. Now we know better, for we have planets around binaries elsewhere. As for Alpha Centauri, it was in 1997 that Paul Wiegert and Matt Holman showed that planetary orbits were viable here out to about 4 AU around Centauri A and B.

That would seem to rule out gas giants, which presumably would have formed beyond the snow line at roughly the same 4 AU and beyond, but terrestrial planets in closer orbits are still allowed. Beech runs through the recent scholarship for Centauri A and B, most of which shows the likelihood of planet formation, though in at least one case with a serious restriction:

Guedes et al.(2008) and Thébault et al. (2009) additionally find that planets should have formed around a Cen B. Assuming an initial co-planar distribution of planetesimals, Guedes et al. argue that multiple 1-2 _MEarth planets might reside within the region 0.5-1.5 au from a Cen B. Taking into account the details of planetesimal encounters and impact velocities, however, Thébault and co-workers argue that planet formation is only favoured within a restricted zone of 0.5 au about a Cen B. At this stage it will be the discovery of additional planets, with orbits well beyond that of a Cen Bb, that will guide the development of next-generation planetesimal accretion models.

For that matter, can we assume that the current orbital characteristics of Centauri A and B are those of the initial system? In the paper by Philippen Thébault and team referenced above, the authors point out that the odds of Alpha Centauri having gone through a close encounter with a nearby star system that could have affected stellar orbits may be as high as 1 in 2. It’s regrettable, then, that we have no knowledge of the cluster that spawned Alpha Centauri or the effects such encounters might have had. We do know that this system formed from a different cluster than the Sun — our stars are close now but the condition is temporary, and Alpha Centauri was likely 1 to 2 billion years old when our Solar System was beginning to form.

You’ll find most of the papers Beech refers to, the recent ones anyway, discussed in earlier Centauri Dreams posts, and you can use the search function to track them down by author. The same is true of work on Proxima, the M-dwarf that accompanies Centauri A and B, or at least is located at roughly the same distance and shares the same proper motion. It’s hard to believe that the three don’t form a single system, especially given their common chemical composition, but it has always been a tough call going back to the work of Joan Gijsbertus Voûte, who announced the first parallax findings about Proxima in 1917. Voûte pondered whether the three stars were physically connected or ‘members of the same drift,’ and Beech notes that despite the following century of observations, we are still working on the same question.

It’s also been amply demonstrated that both gas giants and rocky worlds can form around M-dwarfs, so while we’re gradually ruling out larger planets around Proxima, we’re far from being able to declare there are no terrestrial planets orbiting it. Here again we’ve run through the numbers in Centauri Dreams in recent memory, but Beech sifts through the findings, which increasingly exclude planets of Jupiter-mass, and he notes in particular the work of Michael Endl and team, which argues that any planet greater than two Earth masses within Proxima’s habitable zone should have been detected by now. That leaves room for smaller worlds in interesting places.

We can also exclude various kinds of planets around Centauri A and B:

With respect to a Cen AB, the five-year radial velocity survey starting in 1992 conducted by Endl et al. (2001) revealed the following constraints: for a Cen A there are no planets more massive than 1 M_Jupiter within 2 au, and no 2 M_Jupiter or larger mass planets within 4 au; for a Cen B there are no planets more massive than 1.5 _Jupiter within 2 au, and no planets more massive than 2.5 M_Jupiter within 4 au. On the larger scale, a deep CCD imaging survey of the region immediately surrounding a Cen AB revealed no co-moving companions with masses greater than 15 M_Jupiter at distances 100-300 au (Kervella and Thévenin 2007).

Thus the search goes on, and Beech believes the current surveys will be able to dampen the noise in the radial velocity signal of an Earth mass planet in a 1 AU orbit around Centauri A or B with about five more years of data gathering. Although the HARPS team at La Silla was first with an Alpha Centauri planet candidate, the hunt for an Earth-class world in a habitable orbit has only intensified. Dedicated Alpha Centauri search programs are in progress not just via HARPS but also through a Yale University program at Cerro Tololo (this one backed by the Planetary Society as well) and through the HERCULES spectrograph at Mt. John Observatory in New Zealand.

I’ve only touched on the highlights of this rich paper, whose bibliography alone makes it worth seeking out. The reference is Beech, “A journey through time and space: Alpha Centauri,” Astronomy & Geophysics, Volume 53, Issue 6, pp. 6.10-6.16 (abstract).

We saw yesterday how a newly refined radial velocity technique allowed researchers to identify five planet candidates around the nearby star Tau Ceti. The latter has long held fascination for the exoplanet minded because it’s a G-class star not all that different from the Sun, and one of the planets around it — if confirmed — appears to be in its habitable zone. But smaller stars remain much in the news as well, as witness Gl 667C, a red dwarf (M-class) star in a triple system that also contains two closely spaced K-class stars with a semimajor axis of 1.82 AU.

M-class stars offer a lot to planet hunters, as new work using the HARPS spectrograph at La Silla is making clear. For one thing, a planet of a given size induces more radial velocity variation around a low-mass star than around a larger one, making the planet easier to spot. For another, red dwarfs are dimmer than G and K-class stars, with a habitable zone much closer to the star. Here again we get a larger radial velocity wobble than we would find with a larger star.

Philip Gregory (University of British Columbia) has performed a re-analysis of the HARPS data on Gl 667C that is getting play in the press because it identifies not one but three planets in the habitable zone. The star has about a third of the mass of the Sun, so according to Gregory’s figures, a habitable zone planet there produces seven times the radial velocity signature that a similar planet around a G-class star would generate. In 2011 Gl 667C was already known to be orbited by at least one planet, Gl 667C b, with a 7.2 day orbit, and there was evidence for other worlds. Later work confirmed the planet Gl 667C c in a 28-day orbit in the habitable zone.

All of this, says Gregory in the paper on his new work, has driven a serious effort to improve the statistical tools used to analyze radial velocity data, and we saw yesterday how Mikko Tuomi (University of Hertfordshire) and team put their own methods to work on Tau Ceti. Using his own algorithms, Gregory has already been able to confirm the existence of a second planet around 47 Ursae Majoris, but Gl 667C will draw more attention because of the dramatic placement of the planet candidates here. In this case, the astronomer included stellar jitter and orbital drift factors to re-analyze the HARPS data using Bayesian methods to refine the probability estimates.

The result: The detected signals include the already established planets in 7.2 and 28.1-day orbits, but also show possible planets in 30.8 (d), 38.8 (e), 53.2 and 91.3-day orbits (f). Gregory discounts the 53.2-day signal because it seems to be the result of surface activity on the star. All these candidates are more massive than Earth, but e is only 2.4 Earth masses. The signals at 30.8 and 38.8 days, if confirmed, would join Gl 667C c as planets in the habitable zone. Gregory finds that the 91.3 day orbit would take that planet inside the outermost edge of the habitable zone, although its eccentric orbit would keep it outside the HZ for the majority of time.

Image: The darker area is the orbital region that remains continuously habitable during at least 5 Gyr as a function of the stellar mass (Selsis et al. 2007). The light grey region gives the theoretical inner (runaway greenhouse) and outer limits with 50% cloudiness, with H2O and CO2 clouds, respectively. The dotted boundaries correspond to the extreme theoretical limits, found with a 100% cloud cover. The dashed line indicates the distance at which a 1 M? planet on a circular orbit becomes tidally locked in less than 1 Gyr. Credit: Philip Gregory.

The scientist is careful to note that new simulations will be needed to determine which of the planetary signals are consistent with a stable planetary system. Look particularly at the closeness of the 28.1 and 30.8 day orbits, where the semi-major axis differs by a mere 0.007 AU. This sets up a closest approach, as Gregory notes, every 323 days, doubtless a fascinating astronomical spectacle from the surfaces of these possible worlds. And the author cites the Kepler mission’s own findings of systems with close planetary separations, including Kepler 36 b and c (0.014 AU), Kepler 42 b and d (0.0038 AU), and KOI 55b and c (0.0016 AU).

We may have to start getting used to solar systems with close planetary separations, unlike the relatively spacious inner system we see around the Sun. I’m reminded of something Steve Vogt (UC-Santa Cruz) said in the news release on the Tau Ceti story: “We are now beginning to understand that Nature seems to overwhelmingly prefer systems that have multiple planets with orbits of less than one hundred days. This is quite unlike our own solar system where there is nothing with an orbit inside that of Mercury. So our solar system is, in some sense, a bit of a freak and not the most typical kind of system that Nature cooks up.” Vogt was not speaking of M-dwarfs, of course, but the statement has no better illustration than Gl 667C’s possible planets.

The paper is Gregory, “Additional Keplerian Signals in the HARPS data for Gliese 667C from a Bayesian Re-analysis” (preprint).

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.