Figure 1. Cartoon published in
2007 on the announcement of the alleged exoplanet Gliese 581 d, which has now been
declared an illusion induced by correlated noise in the stellar spectra
---------------
Exoplanetary astronomy is a brand-new field of inquiry,
originating only in 1992 with the detection of two barren planets orbiting a distant
neutron star (Wolszczan & Frail 1992). As an infant science, it is
especially prone to controversy
and revision.
No extrasolar
hullabaloo has generated so much wordage or persisted so long as the debate
on the nature of the planetary system around Gliese 581 (GJ 581), a red dwarf
star located just 20 light years away. For the past seven years, this system
has been repeatedly proposed as the home of one or more habitable
Super Earths, although the precise number and characteristics of these
marvelous worlds have remained in flux.
Just yesterday came a study that claims to settle, once and
for all, the question of habitable planets around GJ 581. According to Paul
Robertson and colleagues, the candidate
planet most favored to support life – GJ 581 d – simply does not exist
(Robertson et al. 2014). This news means that our Sun’s
back yard lacks any plausible contenders for habitability, with the
possible exception of GJ
667C.
Whether the study by Robinson’s team will actually close the
book on this controversy remains to be seen. Members of the original discovery
team (Stephane Udry, Xavier Bonfils, Xavier Delfosse, Thierry Forveille, Michel
Mayor) have generally been quite protective of their analyses of the radial velocity
data on GJ 581, and have publicly repudiated previous attempts to revise their
picture of the system.
Here is an abbreviated rundown of the controversy, which has
been mentioned more than once in this blog. Relevant publications appear in the
reference list at the end.
origin of the
controversy
In 2005, Bonfils and colleagues announced that radial
velocity data collected by the HARPS spectrograph indicated the existence of a
Neptune-mass exoplanet in a tight orbit around the nearby star GJ 581. The star
has a spectral type of M3, a mass 30% Solar (0.3 Msol), and a luminosity only
1.3% Solar. Following standard naming protocols, the new planet was designated GJ 581 b. It is at least 16 times as
massive as Earth (16 Mea), yet its orbital period is only about 5 days. Thus it
merits the nickname of Hot Neptune, as it is similar in mass to the real
Neptune (17.2 Mea) but even hotter than Mercury and Venus. At the time, this
discovery was notable because GJ 581 was only the third M dwarf to be
identified as a planet host. Nevertheless, astrobiologists took no interest,
since a planet with the characteristics of GJ 581 b has no hope of nurturing
life.
In 2007, things became much more interesting when the same HARPS
team, now led by Udry, announced two new Super Earths orbiting in or near the
proposed habitable zone of GJ 581. Planet c had a minimum mass of about 5 Mea,
a semimajor axis of only 0.07 astronomical units (AU), and an orbital period of
13 days. Planet d, which was detected with less confidence, had a minimum mass
of 7.7 Mea, a semimajor axis of 0.25 AU, and a period of 83 days. The discovery
team described the orbital environment of GJ 581 c as “the warm edge of the
habitable zone” and characterized the planet itself as “the closest Earth twin
to date.” The more massive outer candidate, GJ 581 d, was positioned “close to
the cold edge of the habitable zone.” The team called both objects “promising
targets” for future studies. One member of the team, Xavier Forveille,
delighted the press (CNN, Time Magazine, Agence France-Presse)
with an especially mediagenic remark about GJ 581 c: “On the treasure map of the universe, one would be
tempted to mark this planet with an X.”
In those pre-Kepler days, many researchers were inclined to
believe that a planet 5 to 10 times the mass of Earth would just be a bigger
version of Earth, rather than a smaller version of Uranus. I
never bought that argument myself, so I was surprised by the flood of articles
on the habitability of planet c that followed Dr. Forveille’s enthusiastic speculations.
However, the exoplanet community recovered from this irrational exuberance
pretty quickly. Before the year was out, at least two studies conceded that
even for a dim red dwarf, a semimajor axis of 0.07 AU (one-sixth of Mercury’s)
is just too hot for water and life (Selsis et al. 2007, Von Bloh et al. 2007). GJ 581 c might be a Super Venus, but it can't be a big sister to Earth.
Two years later, nevertheless, the HARPS team successfully
refreshed its picture of GJ 581 with new data and analyses. Now led by Michel
Mayor, the discoverer of the very first Hot Jupiter, they announced
yet another planet, GJ 581 e,
orbiting interior to planet b with a period of only three days. Its minimum
mass – a quantity known as m sin (i) in the astronomical literature – was only 1.9
Mea, at that time the lowest value ever reported for an extrasolar planet.
While any object with those characteristics must be a partially molten
Hellworld, the identification of planet e led to a revision in the parameters
of GJ 581 d and a welcome constraint on the orbital inclinations of the whole
system. The orbital period, semimajor axis, and m sin (i) of planet d were all
revised downward to 67 days, 0.22 AU, and 7 Mea, respectively. These reductions
placed GJ 581 d more securely in the system habitable zone. Meanwhile, a
stability analysis of the full ensemble of planets found that the maximum
inclination of their orbits (presumed co-planar) to the line of sight was only
30%, enabling upper as well as lower limits on planet masses.
With this publication, planet d definitively replaced planet
c as the most likely “habitable Super Earth.” Today, if you do a Google Image
search for “Gliese 581 d,” you can still find an abundance of imaginary views
of this putative Earth-like planet; here’s a pretty one
by DarinK on deviantART.
In 2010, a completely different team led by Steven Vogt used
radial velocity data from the HIRES program to confirm two new planets around
GJ 581. One member of the pair, GJ 581 g,
was described as a Super Earth of about 3 Mea with a period of 37 days, placing
it squarely in the system habitable zone (Vogt
et al. 2010). The other candidate, GJ
581 f, seemed to be a more straightforward Mini Neptune of about 7 Mea,
orbiting at an astrocentric distance of 0.76 AU, implying chilly temperatures
at its cloud tops. The smaller and warmer of these two worlds received instant
media acclaim as the latest truly habitable exoplanet, inspiring still prettier pictures.
Figure 2. Gliese 581, Then and Now
In
the upper panel, planets proposed by
the HARPS team are labeled in red; planets proposed by the HIRES team are
labeled in blue; planet b is assumed to have a hydrogen/helium envelope;
planets e, c, g, and d are rocky; and planet f is rocky with an ice veneer. In
the lower panel, planet e is assumed
to be rocky, and planets b and c have hydrogen/helium envelopes. Colors are arbitrary.
------------------------
Alas, even before the official publication of the discovery
paper, astronomers with the HARPS survey sharply challenged the reality of GJ
581 g. 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.”
The consensus of the astronomical community, as expressed
over the next few years (Gregory 2011, Tuomi 2011, Forveille et al. 2011, Tuomi
& Jenkins 2012), coalesced around the four-planet model favored by the
HARPS team. This model was limited to the four objects designated by red letters in the upper panel of Figure 2.
Yet the rumblings continued. Roman Baluev, a Russian
astronomer with no association with either HARPS or HIRES, published a new
analysis of both datasets (Baluev 2013). His results not only refuted the
existence of GJ 581 f and g but challenged the reality of GJ 581 d. Baluev
argued that both planet-hunting teams, while recognizing the need to filter random
white noise from their analyses, failed to realize that their data were also contaminated
by “red noise” – a non-random signal whose origin was indeterminate. Once Baluev
removed the red noise from his model, planet g disappeared and planet d became
dubious; yet the signals for planets b, c, and e became stronger.
Figure 3. Red
noise (red) and white noise (gray) in simulated radial velocity data; based on
Figure 1 of Baluev 2013.
Baluev proposed two potential sources for this vexing red
noise: either instrumental effects or some feature of the host star itself.
Since he was working with two different datasets collected by two different
instruments, he could easily rule out the first option. That left GJ 581 itself
as the source of the variation, perhaps through magnetic activity resulting in
starspots. Baluev also noted a bit ominously that “red RV noise might be a
rather common phenomenon,” possibly relegating many other widely accepted
detections to the freak tent of “unconfirmed, controversial.”
Despite so many naysayers, the ghost of GJ 581 g has found
no rest. It appeared as recently as 2012 in a list of habitable planets published
by the University of Puerto Rico Arecibo, and
it was still discussed as a potentially real object in a study published only last
month (Joiner et al. 2014).
And now, with the work of Robertson and colleagues, GJ 581 d
has also been consigned to the extrasolar dustbin. Their approach resembles
that of Baluev, insofar as they filtered correlated (non-random) noise from the
radial velocity data on the host star. However, they analyzed only the HARPS
dataset, since it covered a longer time span. Their conclusions are even more
robust than those of Baluev: they confirm planets b, c, and e while completely ruling
out planet d as well as planet g.
Robertson’s study foregrounds a key weakness of all previous
analyses: until this work, no one had ever managed to define the rotation
period of the host star. This was a crucial limitation, since one standard
approach to avoiding spurious exoplanet detections is to test whether the
proposed orbital period is related to the stellar rotation period. For the
first time, Robertson et al. determined the rotation of GJ 581, which turns out
to be a leisurely 130 days. As they argue, the signal identified as the phantom
planet d was simply a harmonic of this activity, corresponding to approximately
half of the rotation period.
where
does all this leave us?
I have never been attached to the notion that either GJ 581
c or GJ 581 d might be habitable, because even the low end of their proposed mass
ranges always exceeded 3 Mea (not to mention the hellish temperatures likely
for the warmer of the two). Therefore, losing planet d as a possibly
potentially habitable Super Earth brings no tears.
But I do find it troubling that a star system so nearby, to
which so much attention has been devoted for so many years, was so profoundly
misunderstood. If GJ 581 d isn’t real, then what about all the other
controversial objects in the Sun’s
back yard? Does Epsilon Eridani really harbor a cool gas giant? Does Alpha
Centauri B really host a Hot Super Earth? Do HD 40307 and Tau Ceti really have
extensive low-mass planetary systems? Or do all our exoplanetary dreams amount to a handful of dust?
Things probably aren’t that bad. Still, the GJ 581 affair
once again reminds us that most data on extrasolar planets are provisional and
subject to enormous changes at any moment. And so, on the assumption of
innocence rather than guilt, here is a revision of a
chart I published just last week, summarizing all systems discovered by
radial velocity searches with three or more low-mass planets:
Figure 4.
All twelve
radial velocity systems with at least three low-mass planets
Semimajor axes are measured in astronomical units (AU), where 1 AU
is the Earth-Sun separation; planet masses are indicated in Earth units (ME); star
masses appear in Solar units at right.
------------------------
REFERENCES
Anglada-Escudé G. (2010) Aliases of the first eccentric
harmonic: Is GJ 581g a genuine planet candidate? (First version, unpublished; a
second unpublished draft with the collaboration of Rebekah Dawson is available
at http://arxiv.org/abs/1011.0186.)
Baluev R. (2013) The impact of red noise in radial velocity planet
searches: Only three planets orbiting GJ 581? Monthly Notices of the Royal Astronomical Society 429, 2052-2068. Abstract:
http://adsabs.harvard.edu/abs/2013MNRAS.429.2052B
Bonfils X, Forveille T, Delfosse X, et al. (2005) The HARPS
search for southern extra-solar planets VI. A Neptune-mass planet around the
nearby M dwarf Gl 581. Astronomy &
Astrophysics, 443: L15-L18. Abstract.
Cameron AC. (2012) Astrophysical false positives. Nature 6 December 2012 (492:48-50).
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HARPS search for southern extra-solar planets XXII. Only 4 planets in the Gl
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Gregory PC. (2011) Bayesian re-analysis of the Gliese 581
exoplanetary system. Monthly Notices of
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Joiner DA,
Sul C, Dragomir D, Kane SR, Kress ME. (2014) A consistent orbital stability
analysis for the GJ 581 system. Astrophysical
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October 2010. Linked
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