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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