Tuesday, January 1, 2013

Year of the Signal

Figure 1. “Raw” (top) versus “non-noisy” (bottom) periodograms for radial velocity data from Alpha Centauri B. Analyses by Xavier Dumusque and colleagues led to the announcement of Alpha Centauri Bb, a hot, rocky exoplanet just 4 light years away. (Based on Figure S10 from Dumusque et al. 2012.)

Compared to the excitement of 2011, the offerings of the exoplanetary year just ended seem a bit sedate. Granted, the Kepler Mission continues to report new worlds, and its vast and growing dataset will surely feed astronomical research for years to come. But the pace of breakthroughs has slackened. Kepler candidates in 2012 were typically more of the same (a few more circumbinaries, many more hot, low-mass planets), while the mission itself lost a bit of luster through a combination of hardware failure, higher-than-anticipated rate of false positives (Cameron 2012), and serious scarcity of Earth-size planets on Earth-like orbits. Ground-based programs continued to yield notable results, but overall, exoplanets bagged fewer headlines than phenomena in adjacent disciplines.

In particle physics, the Higgs boson underwent a media apotheosis as the God Particle, with articles about its discovery leading most science news for several weeks over the summer. In planetary astronomy, space robots found ice at Mercury’s north pole (Paige et al. 2012) and a dry riverbed on Mars, making both places feel ever so slightly more homey. The best that the exoplanetary headlines could muster were more of those deeply dubious “habitable Super Earths” (which generally turn out to be Mini Neptunes, when they turn out at all) and a few days of sci-fi speculation on rogue planets hurtling sunless across the Galaxy.

Figure 2. Artist’s view of Alpha Centauri Bb. Credit: University of Puerto Rico Arecibo

Nevertheless, this past year saw provocative and likely historic developments in exoplanetary science:
  • A team of astronomers using the HARPS spectrograph announced an Earth-mass planet orbiting Alpha Centauri B (the K-type component of the very nearest star system) in a period of just 3.24 days (Dumusque et al. 2012). Detecting such a faint radial velocity signal pushed instrumental sensitivity to the limit.
  • The Kepler team reported the first system of multiple planets in a circumbinary configuration – Kepler-47, where a Sun-like star and an M dwarf share a circular orbit of 7.5 days, and two Uranus-like planets (gas dwarfs) orbit them on periods of 50 and 303 days (Orosz et al. 2012).
  • An international team using the Herschel Space Observatory reported an extensive debris disk – much larger than our own Kuiper Belt – around the nearby M dwarf GJ 581, better known as the host of a multiplanet system with a Super Earth or Mini Neptune orbiting near the habitable zone (Lestrade et al. 2012). This remarkable discovery, which has gone largely unnoticed by the science journalists, adds to our exiguous knowledge of debris disks around M dwarf stars. Only one other red dwarf system (AU Microscopii) supports a dust disk that has been spatially resolved. Just as important, the newly revealed parameters of the GJ 581 disk bring us closer to understanding the overall architecture of a complex and perennially fascinating exoplanetary system. 
  • Several candidate low-mass planets, some orbiting in their systems’ habitable zones, were proposed by astronomers who applied sophisticated statistical methods to extract faint signals from publicly available radial velocity data on nearby stars, all but one of which are already known to host planets. The list of revisited systems includes GJ 667C, GJ 676A, HD 10180, HD 40307, and – most notably – Tau Ceti, the nearest Solar analog (i.e., single G-type star). Most of these candidate exoplanets remain controversial.
  • The orbiting CoRoT spacecraft, launched in late 2006 to search for transiting planets, suffered an apparently fatal computer error. I haven’t yet seen an official post mortem, but from my back alley perspective, this five-year mission has not met expectations. Despite much pre-launch optimism (Leger et al. 2004, Garrido & Deeg 2006), the CoRoT team so far has reported a single transiting Super Earth (CoRoT-7b, a prototypical Hellworld with a “year” that lasts 20 hours), a single transiting Warm Jupiter (CoRoT-9b, with a period of 95 days), and dozens of transiting Hot Jupiters. Early CoRoT predictions, by contrast, envisioned the detection of “between 10 and 40 terrestrial planets . . . some of them in the ‘habitable zone’ ” (Boisnard & Auvergne 2006).
  • The homepage of the indispensable Extrasolar Planets Encyclopaedia underwent its first major redesign since its launch in 1995. Despite persistent bugs, the site now looks at home in the 21st century.
I’ve already blogged about the two top discoveries, and I plan to discuss the GJ 581 system in an upcoming post about the immediate Solar neighborhood. So for now, I’ll just take a look at the fourth phenomenon – planet candidates at the very limit of detection, and the potential reality thereof – and then conclude with a few remarks about the notion of confirmed vs. controversial discoveries.

planetary conflict in Ursa Major and Libra

Controversy is an inevitable outcome of scientific endeavor. A classic example in exoplanetary astronomy involves 47 Ursae Majoris (47 UMa), a nearby G-type star that became one of the first confirmed exoplanetary hosts. In 1996, Butler and Marcy announced a single gas giant, 47 UMa b, orbiting at a semimajor axis of 2 AU (Butler & Marcy 1996). The planet was included in the census of the Extrasolar Planets Encyclopaedia (EPE), and within a few years its orbital parameters were independently confirmed by other studies (Naef et al. 2004, Wittenmyer et al. 2007). In 2002 the original discovery team, with three new collaborators, analyzed a longer data series that indicated the presence of a second gas giant on a wider orbit (Fischer et al. 2002). The new planet, christened 47 UMa c, was estimated to have a period of at least 7 years, corresponding to the largest semimajor axis then known (3.73 AU).

Unfortunately, other search programs could not detect planet c (Naef et al. 2004, Rivera & Haghighipour 2007, Wittenmyer et al. 2007, 2009). EPE continued to list it as a confirmed exoplanet, despite the controversy, and some years later, a single member of the 2002 team collaborated with Philip Gregory on a new study (Gregory & Fischer 2010). By then the radial velocity observations of 47 UMa accumulated by the Lick and McDonald Observatories covered 22 years. After conducting “a Bayesian adaptive hybrid Markov chain Monte Carlo analysis” of the data series, Gregory & Fischer felt confident in confirming planet c, now with a slightly shorter period of 6.55 years and a slightly smaller semimajor axis of 3.6 AU. Their analysis also detected an uncertain signal potentially corresponding to a third gas giant on a still wider orbit, with a semimajor axis of at least 11.6 AU (compare Saturn at 9.5 AU, with a period just under 30 years), They cautioned, however, that “the longest period orbital parameters are still not well defined.” Nevertheless, all three planets continue to be listed by EPE, and no further challenges have emerged. For the time being, then, 47 UMa has three gas giants.

Another controversy has not subsided so quietly. In 2010, the same year in which Gregory & Fischer published their results on 47 UMa, a team led by Steven Vogt announced the discovery of a new Super Earth in the habitable zone of GJ 581 (Vogt et al. 2010). This nearby M dwarf was already known to host four low-mass planets – i.e., Mini Neptunes or Super Earths – which were originally reported by the HARPS team. The new planet candidate, GJ 581 g, was nicknamed “Zarmina’s World” after the lead astronomer’s wife, and received instant media acclaim as the first habitable exoplanet. Alas, even before the official publication of the discovery paper, astronomers with the HARPS survey sharply challenged the reality of Zarmina’s World. As Francesco Pepe argued, “The signal amplitude of this potential fifth planet is very low and basically at the level of the measurement noise” (Mullen 2010). He considered it likely that the purported “signal” was “just produced ‘by chance’ out of the noise.”

This signal vs. noise controversy continues into the present. Because it involves a “habitable Super Earth” instead of a potential Saturn analog (without the rings), it has attracted much more attention than the challenges to 47 UMa c. Although Guillem Anglada-Escude quickly defended the findings of Vogt and colleagues in an unpublished study (Anglada-Escude 2010), prevailing opinion has gone against them (Gregory 2011, Tuomi 2011, Tuomi & Jenkins 2012). In fact, a recent study challenges the existence not only of GJ 581 g but also of GJ 581 d (Baluev 2012), which has otherwise retained its crown as the most likely “habitable Super Earth.” In the case of Zarmina, I’ll go on the record in support of the skeptics. For me the most persuasive evidence is an as-yet unpublished study by the HARPS team featuring new radial velocity data that, in their analysis, contain no evidence for more than four planets around GJ 581 (Forveille et al. 2011). Yet despite so many naysayers, the ghost of planet g has found no rest, as evidenced by its prominent appearance in a poster just published by the University of Puerto Rico:

Figure 3. Recent poster by the University of Puerto Rico Arecibo

phantom planets in the reality continuum

The chronicles of 47 UMa and GJ 581 set the stage for the still more remarkable detections proposed in 2012. We have already met the astronomers – Philip Gregory, Guillem Anglada-Escude, Mikko Tuomi, and their colleagues – as well as their methods, among which Bayesian inference looms large. A handful of systems are involved, all but one quite nearby. I’ll consider the most representative case first.

GJ 667C  A red dwarf with spectral type M1.5 and mass 0.31 Solar, GJ 667C is the third member of a triple star system located just 7 parsecs (23 light years) away (Anglada-Escude et al. 2012 say 6.8 pc, Gregory 2012 says 7.23 pc). Members of the HARPS team have announced two low-mass planets: GJ 667C b with a period of 7 days and a minimum mass of 5.46 Mea, and GJ 667C c with a period of 28 days and a minimum mass of 4.25 Mea (Bonfils et al. 2011, Delfosse et al. 2012). Those numbers make GJ 667C c the best candidate yet for the media status of “habitable Super Earth,” although in my judgment the minimum mass is probably high enough to exclude it from consideration.

Two additional studies by astronomers outside the HARPS team followed soon afterward, one by Gregory and the other led by Anglada-Escude and including 16 additional collaborators. Gregory reanalyzed the data used by Delfosse et al.; Anglada-Escude reanalyzed the data of Bonfils et al., adding observations by members of his own group. Both studies agreed on the reality of the 7-day and 28-day planets, with Gregory suggesting a mass of 4.8 Mea for planet c and Anglada-Escude suggesting 4.54 Mea. Thereafter their conclusions diverged quite dramatically. Anglada-Escude’ group noted a potential signal with a period of 75 days, corresponding to an object similar in mass to planets b and c; nevertheless, they recommended “due caution” in considering a planetary interpretation. Gregory, on the other hand, found evidence for three more planets, two of them also in the habitable zone, and all in the mass range of Super Earths. Gregory’s five-planet solution includes planets b (7.2 days, 5.4 Mea), c (28.1 days, 4.8 Mea), d (30.8 days, 3.1 Mea), e (38.9 days, 2.4 Mea), and f (91.3 days, 5.4 Mea). This is a remarkably crowded system of Super Earths, but nowhere do we see an object with a period near 75 days.

It’s impossible for anyone without the requisite education and experience to assess the likelihood of either reanalysis. If Gregory’s findings are accurate, we’d best launch that interstellar probe to GJ 667 C without delay. Yet EPE currently lists only two planets in this system; candidate d is tagged “controversial” and candidates e and f are not acknowledged. To me that suggests a skeptical position. Even Gregory does not claim that his numerical model corresponds to a working dynamical architecture. As he puts it, “N-body simulations are required to determine which of these signals are consistent with a stable planetary system.” That leaves me wondering whether we are contemplating pretty planets or pretty numbers.

GJ 676A  Although classified M0, this star is as robust as many K dwarfs (0.71 Solar masses). It has a smaller companion star of type M3 at a separation of at least 800 AU. In 2011, Thierry Forveille and colleagues reported a surprisingly massive gas giant (4.9 Mjup) orbiting star A in a period of almost three years (Forveille et al. 2011). While noting that the object might actually have a longer period and a higher mass, making it a brown dwarf or a very dim red dwarf, they were more confident of a planetary origin for the radial velocity signals. EPE duly added this object to the catalog of confirmed exoplanets, where it appeared as recently as May 27, 2012.

Over the next few months, Anglada-Escude & Tuomi collaborated on successive drafts of a study in which they applied Bayesian methods to the published HARPS radial velocities of GJ 676A. Not only did they confirm the giant orbiting at about 1.8 AU; they also detected two low-mass planets on short-period orbits – a potential Super Earth with a minimum mass of 4.4 Mea orbiting in 3.6 days, and a Warm Uranus with a minimum mass of 11.5 Mea orbiting in 35.4 days (Anglada-Escude & Tuomi 2012). This architecture, if real, would make GJ 676A the only known system beside HD 10180 and our own to host at least two low-mass planets inside the ice line and at least one gas giant outside it.

But how likely is it to be real? The Exoplanet Orbit Database, produced by Jason Wright and Geoff Marcy, lists only planet b for GJ 676A, while EPE, for reasons unstated, has completely expunged this star from its catalogs! Evidently the purge happened in June or July, and it was as total as anything envisioned (or unenvisioned) by George Orwell. Not even the freak tent of “unconfirmed, controversial, or retracted planets” has room for this oversized dwarf. GJ 676 Ab is, at least for now, an unplanet. [UPDATE: As of January 2, 2013, planet b is back in the freak tent, tagged by EPE as a "controversial" object. The other candidates remain unplanets.] 

HD 10180  This yellow star, about 39 parsecs (128 light years) away, was identified in 2010 as the host of at least five low-mass planets orbiting within 1.42 AU, with a small gas giant at about 3.4 AU and a seventh suspected planet with a mass similar to Earth’s and an orbital period shorter than two days – i.e., a Hellworld somewhat like CoRoT-7b (Lovis et al. 2011). Until the announcement of the Kepler multiplanet systems, this was the most packed system architecture known. Yet even amid such an abundance of companions, not a single one is both cool enough and lightweight enough to qualify as a potentially habitable rocky planet.

Enter Mikko Tuomi, who as before used Bayesian methods to reanalyze the published HARPS radial velocities for HD 10180. His reanalysis confirms all seven planets suggested by Lovis and colleagues and adds two more low-mass objects: planet i, which orbits between planets c and d in a period of about 10 days, and planet j, which orbits between planets d and e in a period of about 68 days. Both candidates have minimum masses below 5.5 Mea. Neither has even a remote chance of habitability.

As before, Tuomi’s candidates remain candidates; EPE continues to list only planets c through g in the main catalog, while planets b, i, and j are “unconfirmed, controversial.”

Figure 4. Highly imaginative artist’s view of the hypothetical planet HD 40307 g, along with two better constrained siblings. Credit: J.T. Pinfield

HD 40307 Back in 2008, Michel Mayor and colleagues reported a system of three Super Earths orbiting this amber star of spectral type K2, which is located 12.8 parsecs (42 light years) away. Confined within a semimajor axis of just 0.13 AU, all three must support environments the likes of which even Dante never imagined. In 2012, Tuomi and Anglada-Escude led a team of eight astronomers (including such noted planet hunters as R. Paul Butler, Eugenio Rivera, and Steven Vogt) in yet another Bayesian reanalysis of published HARPS data. They recovered three additional signals: planet e, with a period of 35 days and a minimum mass of 3.5 Mea; planet f, with a period of 52 days and a minimum mass of 5.2 Mea; and planet g, with a period of 198 days and a minimum mass of 7.1 Mea. Since HD 40307 is substantially less luminous than our Sun, the orbit of planet g occupies the system habitable zone, a feature that guaranteed a bumper crop of headlines in the weeks before Thanksgiving.

Remarkably, EPE accepted these results and added all three Bayesian planets to the official catalog. We can only wonder why this reanalysis seemed more persuasive than others by members of the same team. HD 40307 now officially qualifies as a system of six planets, all with minimum masses under 10 Mea and orbital periods shorter than a Venusian year. And as we saw in Figure 3 above, HD 40307 g has joined the happy club of possibly potentially hypothetically habitable Super Earths.

Ah, fluffy white clouds, deep blue seas, rugged green continents! Figure 4, an artist’s view of this new world, gives us all those delightfully familiar options. Nevertheless, as I’ve taken pains to establish in earlier posts, a cool planet with a minimum mass as high as 7 Mea is extremely unlikely to resemble Earth. Such a massive object is likely to retain a deep hydrogen atmosphere, and it is doubtful that it could sustain either plate tectonics or oceans (which according to some theorists are mutually constitutive phenomena – you can’t have one without the other).

But I say let the dreamers go on dreaming if they won’t wake up.

Figure 5. Artist's view of the proposed Tau Ceti system. Credit: J.T. Pinfield

Tau Ceti  Now we reach the biggest dream on our end-of-the-year agenda. Just in time for Christmas, Tuomi and crew (including many of his collaborators from the study of HD 40307) reported results on a star with no previously suspected planets: Tau Ceti, the nearest Solar analog. Located just 3.65 parsecs (12 light years) away in the constellation of Cetus the Whale, this star was always high on those lists of “nearby stars that might have solar systems” that circulated during the remote era before radial velocity and transit searches (i.e., my childhood).

Along with Alpha Centauri and Epsilon Eridani, Tau Ceti was targeted by all the earliest search programs. Null results continued even as other nearby systems – Gamma Cephei, Epsilon Eridani, 82 Eridani, 61 Virginis – began to yield up their secrets. Eventually, infrared observations revealed the presence of a two debris belts around Tau Ceti: one cool and massive and the other warmer and more tenuous (Greaves et al. 2004, Di Folco et al. 2007). Since debris is widely regarded as a signpost of planet formation, it seemed unlikely that Tau Ceti could harbor asteroids and comets but no planets. Nevertheless, the Great Whale refused to sing.

By the time Tuomi and colleagues began their Bayesian reanalysis, they were able to use extensive, high-quality datasets from three different search programs, including HARPS, with solid coverage over 13 years. Using sophisticated and highly complex modeling, they recovered signals corresponding to a system of five planets (b-f) that range in minimum mass from 2 to 6.6 Mea, and in period from 14 to 642 days.

Most notable is candidate planet e, with a mass of 4.3 Mea and a period of 168 days. Given the host star’s spectral type and effective temperature (G8, 5344 K), this orbit occupies the circumstellar habitable zone. Thus, provided it exists, Tau Ceti e meets most definitions of a potentially habitable Super Earth. If the results of Tuomi and colleagues had been firm enough to count as a detection, Tau Ceti would have topped my list of Notable Exoplanet News of 2012, hands down.

I’m hardly alone in my eagerness to find planets around Tau Ceti. Astronomer Greg Laughlin, never one to suffer fools, commented favorably on Tuomi and colleagues’ study in a New Year’s Eve blog post, while EPE included their findings as one of only two notable “News” items for all of 2012 (the other one was neither Alpha Centauri Bb nor Kepler-47, but “Hint for a transiting extended atmosphere on 55 Cnc b”).

Yet no one, not even Tuomi and his collaborators, is pressing this claim too strongly. EPE lists the “Tau Ceti Five” as “unconfirmed, controversial,” and Tuomi’s study concludes, “these issues remain merely speculative until the planetary origin of the signals can be verified by an independent detection.” Once again we contemplate pretty numbers instead of pretty planets.

where is the referee?

The International Astronomical Union (IAU) is the only organization officially vested with the authority to decide whether an object is a planet, and what to call it if it is (or isn't). In 2006, the IAU memorably intervened in the debate over the nature of Pluto. As soon as it classified that distant icebox as a dwarf planet and decreed that our Solar System has only eight full-size companions, textbooks were revised, museum exhibits were redesigned, and a surprising number of ordinary people complained.  Nevertheless, the decision stands, and virtually all astronomers are satisfied.

So far the IAU has made no decisions about exoplanets. That's part of the reason why we have no universally accepted definition of a Super Earth and no consensus on which planetary candidates are real and which are imaginary: there's no exoplanetary referee in deep space. EPE has therefore become the de facto decider, given its frequent citation in exoplanet studies and the wide acceptance of its published extrasolar census.

As we have seen, however, EPE isn't consistent in its decisions, and still worse, the site typically provides no alerts when it deletes or reclassifies an object, nor any explanation for such changes. In the ensuing vacuum, so to speak, we get publications like the poster shown in Figure 3. The fact that the authors have added asterisks to two of the names (GJ 581 g and HD 40307 g) inspires lots of lulz at my coordinates. On the one hand, study after study has rejected the existence of GJ 581 g, while on the other, HD 40307 g appears as a bona fide planet in EPE. So why do both get the same asterisk?

Still worse, we see this funny object designated Gliese 163 c, with no asterisk or scare quotes to tip us off to the fact that it has yet to be published, even in a preprint. Oh wait, it's listed in EPE anyway -- but as a small gas giant with a minimum mass of 0.226 Mjup (72 Mea). That's not much less than the mass of Saturn. One can only shrug.

Meanwhile, I'm waiting for that extrasolar umpire to turn up with an unequivocal call. I wonder how many more light years it/they/she/he still has to traverse.


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