Figure 1. Guillem
Anglada-Escude and colleagues have announced a planet candidate with a minimum
mass of 1.27 Earth units on a temperate orbit around Proxima Centauri, a tiny
red dwarf that happens to be the nearest star to our Sun. This artist’s view
shows the planet alongside its red host star, with the binary system of Alpha
Centauri visible in the distance. Image credit: Ricardo Ramirez.
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By now you’ve probably heard the news. Last week, a team
led by Guillem Anglada-Escude reported radial velocity data from the HARPS
spectrograph supporting the presence of a terrestrial planet orbiting Proxima Centauri, our Sun’s nearest
neighbor. Proxima is an especially tiny red star of spectral type M5.5. Its
mass is only 12% Solar and its luminosity is less than 1% Solar. Proxima b, as the new object is known,
is also quite small for an exoplanet: its minimum mass is estimated at only
1.27 Earth units (1.27 Mea). That finding has inspired journalistic catchphrases
such as “second Earth” and “Earth twin” (McKiernan 2016).
Just as newsworthy is the planet’s likely temperature.
Even though Proxima b has an orbital period of only 11.2 days and a semimajor
axis of only 0.049 astronomical units (AU), its host star is so dim that the
planet receives just 65% of the irradiance that bathes Earth. This results in a
blackbody equilibrium temperature (Teq) of 235 Kelvin (K) – which is actually
cooler than Earth’s Teq of 255 K, and a bit warmer than that of Mars, at 210 K.
Proxima b is located squarely in its system’s habitable zone.
Anglada-Escude and colleagues were unable to determine
the eccentricity of the new planet’s orbit, offering only an upper limit of
0.35. This is a notable gap in our understanding of the overall system
architecture. Nevertheless, the discovery team collected data suggesting a
possible second planet orbiting Proxima Centauri with an orbital period longer
than 100 days. This result meets expectations, given abundant evidence that
small planets like Earth and Proxima b often have companions of similar mass.
Another limitation in our knowledge stems from the fact
that Proxima b was detected by radial velocity measurements instead of transit
observations. Radial velocity data can provide only a minimum mass, not a true
mass, and in the absence of a transit, we have no idea of the radius of Proxima
b. Thus, we cannot calculate the planet’s bulk composition, a fundamental
determinant of surface conditions.
Nevertheless, despite the sparseness of the available
data, and despite the excessive hype that often surrounds announcements of
small planets, this is truly big news. Unlike the case of the phantom
planet formerly claimed for Alpha
Centauri B (Proxima’s
next-door neighbor), all commentators seem satisfied with the reality of
Proxima b. And if its host star were a G dwarf like our Sun or a K dwarf like HD
219134, I wouldn’t hesitate
to identify Proxima b as an extrasolar Holy
Grail: a potentially
habitable Earth-like planet.
But Proxima Centauri is an M dwarf, and a very puny one at
that. It lies near the bottom of the mass range for this spectral type, right
above the cut-off for brown dwarfs (which aren’t stars). Given the findings of Luger
& Barnes (2015) and Owen & Mohanty (2016) on the evolutionary and
energetic characteristics of M dwarfs, my instinct is to discount the
possibility that Proxima b could support oceans or complex life.
Then again, I’m not a professional astronomer whose research
depends on funding from government agencies and whose career benefits from
media attention. Governments and media evidently determined long ago that
taxpayers/consumers have no interest in exoplanets unless they resemble Earth, Pandora, or Tatooine. Accordingly, the publication of
Proxima’s detection last week in Nature
was accompanied by a posse of preprints on its potential formation history and
present habitability. All tried very hard to find scenarios that would yield a habitable
Earth-like planet, despite the unfavorable conditions predicted for the Proxima
Centauri system and others like it.
dim
stars are risky bets for life
Before looking at a selection of those preprints, let’s
briefly review the unfavorables, which are straightforward and stubborn:
First, an evolving M dwarf spends hundreds of millions of
years with temperatures far higher and stellar activity more energetic than it
will experience when it finally enters maturity on the main sequence (the phase
in stellar evolution when hydrogen fusion occurs). This developmental history
means that any planet orbiting in a red star’s mature habitable zone will likely
experience runaway greenhouse conditions for hundreds of millions of years. Loss
of atmosphere and water is probable, forestalling the emergence of life (Luger
& Barnes 2015).
Second, during much of their long lifetimes, M dwarfs are
subject to frequent flaring events and coronal mass ejections (CMEs), and they
emit high levels of extreme ultraviolet radiation. This behavior subsides very
slowly with age. Flares and CMEs are likely to erode the atmospheres and
volatile contents of any planets orbiting in the inner systems of M dwarfs,
where their habitable zones are found. Thus, even if an Earth-size planet
survived an intense greenhouse in its infancy, it would still be vulnerable to evacuation
of volatiles in its maturity (Luger & Barnes 2015). Again, a sterile desert
is more likely than a garden.
Most of the brand-new studies on Proxima b acknowledge
these two challenges.
Figure 2. Desert Sunset
Landscapes on red dwarf planets are more likely to resemble this photograph of sunset over the Sahara than the watery locales pictures by many optimistic space artists. Source: Wikimedia, with a red filter over the Sun.
proxima
centauri sub specie aeternitatis
A team of scientists led by James Davenport, none of them
associated with the Proxima discovery team, reported recent observations by the
MOST satellite that underscore the harsh conditions sketched above. They found
that Proxima Centauri emits flares at a tempo of at least 63 per Earth day,
with superflares occurring about 8 times per Earth year (approximately once every
4 orbits of Proxima b). As they remark:
“If these flares regularly impacted Proxima b, the
atmosphere would never fully recover. While this is not known to be a ‘show-stopper’
for habitability, it clearly necessitates a more detailed investigation of
atmospheric response […] and photoevaporation […] for Proxima b” (Davenport et
al. 2016).
Another group led by Gavin Coleman, consisting of a
subset of the discovery team, conducted a set of numerical simulations to
investigate four potential formation scenarios for the new planet. Each was
intended to produce an analog of Proxima b within a specified range of planet
masses and orbital periods. One problem with their approach, from my
perspective, is that all their simulations assume an implausibly large mass for
the host star’s protoplanetary disk, amounting to 4.5% of the stellar mass. By
contrast, a large and growing body of observations indicates that a typical protoplanetary
disk contains about 1% or less of the mass of its parent star (Williams &
Cieza 2011, Andrews et al. 2013).
The first scenario explored by Coleman & colleagues
was in situ accretion from a swarm of
embryos and planetesimals after the dissipation of the gaseous component of the
protoplanetary disk. This is similar to the process invoked to produce the four
inner planets of the Solar System. Numerical simulations tended to produce compact
multiplanet systems with one
or more Earth-mass planets like Proxima or Venus, often with a few lower-mass
planets alongside them, smaller than Venus but bigger than Mars. Across
simulations, their compositions ranged from water-rich to dry, and their orbits were
somewhat eccentric.
The second scenario followed the migration of several embryos with initial masses ranging
from 0.05 to 0.2 Earth masses (Mea) – roughly similar to the mass of Mars –
within icy and dusty regions of a gaseous protoplanetary disk extending outward
to 9 AU – equivalent to the orbit of Saturn in our system. This scenario started
earlier in system history than the first one, so the simulations modeled
interactions between the growing embryos and the gas disk. Both inward and
outward migration were enabled, and forming planets could accrete hydrogen from the
disk. This scenario tended to yield one or two Earth-mass planets accompanied by
a few smaller planets, as in the first scenario. However, all these planets were
rich in volatiles, prompting the designation of “Ocean Planets” (Coleman et al.
2016). Their orbital eccentricities were generally smaller than those in the
first scenario.
The third scenario was similar to the second, except that
it featured a single migrating embryo that formed at a distance of several AU.
The object was accompanied on its inward journey by a swarm of planetesimals. Each
simulation in this set produced just one volatile-rich planet, whose mass varied
from one to several Earth masses. Many simulation runs failed to produce
planets with orbits as tight as Proxima b.
The fourth scenario also featured a single embryo growing
in a gas disk, except that it was accompanied by pebbles instead of
planetesimals. This scenario had the most difficulty forming an analog of
Proxima b, even though the embryo tended to migrate over long distances. The
authors observed that the formation of “a true Proxima analog” in this scenario
would require a substantially larger disk mass than they assumed (even though
they were already using an unrealistically massive disk).
Coleman & colleagues conclude by suggesting
observational tests for each of their four scenarios. These tests hinge on
obtaining a precise estimate of the orbital eccentricity of Proxima b;
establishing the presence or absence of additional planets in the system; and
determining Proxima’s bulk composition (dry versus watery, hydrogen versus
heavier atmospheric gases). Apart from pointing out the difficulties involved
in the single-embryo scenarios, the authors do not attempt to rank their models
according to likelihood.
Although I admire Gavin Coleman’s work, I see two limitations
in this study. First, as noted earlier, the investigators used an
unrealistically large disk mass, and second, they did not consider potential
planet masses in excess of 2 Mea, even though Proxima b’s true mass could
easily be 4 or 5 Mea instead of 1.3 Mea. How would their results change if they
broadened their simulation parameters to include these possibilities?
Two other new studies focused on the habitability of
Proxima b. One included members of the discovery team joined by several other
distinguished researchers, mostly affiliated with European institutions (Ribas
et al. 2016). The other was conducted by astronomers who were not involved with
the discovery; all are affiliated with U.S. institutions (Barnes et al. 2016).
Notably, the latter group includes Rodrigo Luger and Rory Barnes, who wrote
that widely cited study on the likelihood of extreme water loss for M dwarf planets
(2015).
proxima centauri as a habitable planet: barnes & colleagues
Barnes & colleagues (2016) begin with an expansive
review of our knowledge of Proxima Centauri. The first question they consider
is whether Proxima is the third member of a triple star system centered on
Alpha Centauri A and B, which are both Sun-like stars chemically enriched in metals.
In the present epoch of our Galaxy, Proxima is separated from our Sun by 1.3
parsecs, but its distance from Alpha Centauri AB is only 15,000 AU. The chances
are one in a million that Proxima could be found so close to the binary unless all
three stars are physically associated. Indeed, observations over the past 100
years show that Proxima shares a common motion through space with its brighter
neighbors. Yet not even the latest measurements, using the most sophisticated
instruments and analytic methods, have been able to establish any curvature in
Proxima’s trajectory.
Thus, this ruddy little twinkler might simply be part of
a moving group that includes the
Alpha Centauri binary as well as other star systems. As such, Proxima might or
might not have formed in the same molecular cloud as Alpha Centauri AB. If it
did, then we can assume that its age and metallicity are similar to those of
the dazzling binary: age about 3 to 6 billion years (preferred value 4.8
billion years) and [Fe/H] about +0.25. These numbers imply a star that is
slightly older and far richer in heavy elements than our Sun. Barnes &
colleagues argue that stellar enrichment in metals is evidence that Alpha
Centauri formed substantially closer to the Galactic Core than did our Sun,
since the local concentration of metals increases with proximity to the Core.
Given the possibility that Proxima is bound to Alpha
Centauri AB as a third member of the star system, the authors point to the
recent work of Kaib & colleagues (2013) on the consequences of wide stellar
orbits. Barnes & colleagues consider it likely that Proxima’s planetary
system has been disrupted by a close encounter with the bright binary at some
point during its history, especially if the hypothetical Alpha Centauri trinary
migrated to its present Galactic orbit from the inner Milky Way. If it did,
Proxima b might once have followed a wider orbit around Proxima Centauri, but
was driven closer to the star by perturbations induced by Alpha Centauri AB.
Such perturbations might have excited the planet’s orbital eccentricity and
inclination.
Therefore, Barnes & colleagues find that Proxima b
could exist in two different orbital regimes: either 1) it is tidally
locked, with one hemisphere
perpetually assaulted by flares and CMEs and the other in endless night, or 2) it
is engaged in a 3:2 spin-orbit resonance like the planet Mercury, spinning 3
times for every 2 orbits around the host star. The former regime would be
consistent with a circular orbit, the latter with a moderately eccentric orbit.
Having
reviewed the planet’s motion through space, the authors explore several
additional factors, including atmospheric escape, tidal evolution, and
radiogenic heating, before they get down to business. As they tell us, their
aim is to investigate “plausible evolutionary scenarios, focusing on cases that
allow the planet to be habitable.” Their bias in favor of habitable outcomes is
explicit.
Their
approach is analytic, based on an original software package called VPLANET. They
consider two different formation histories for Proxima b. In one, it is a
water-rich planet with a hydrogen envelope amounting to 1% or less of its total
mass. In the other, it is a water-rich planet without any gaseous hydrogen. Their
primary analysis assumes a default planetary mass of 1.27 Mea, but they also
consider more massive cases.
Barnes
& colleagues conclude that, if Proxima b achieved its present orbit at the
time of its formation, it would have to support at least 10 Earth oceans in
order to retain 1 Earth ocean today. If the planet’s water content were any
smaller, given the intense stellar flux, “it is likely desiccated today.” They
also find that a hydrogen envelope any smaller than 1% would readily dissipate,
but that larger concentrations would linger, with negative consequences for
water and life. Larger planet masses would be an additional factor preventing
the escape of a primordial hydrogen envelope.
They
discuss seven possible atmospheric states for Proxima b in the present epoch.
In the “habitable but dry” case, the
planet avoids a hydrogen greenhouse and retains a small quantity of water, but
it is much dryer than Earth – more like Dune or Barsoom than a Cryogenian snowball or a Carboniferous jungle. In the “Venus-like” case, it retains a thick CO2 atmosphere but is
completely desiccated and uninhabitable. In the “Neptune-like” case, it is similarly desiccated and hellishly hot,
but the culprit is an extensive hydrogen envelope that failed to dissipate,
either because the planet is more massive than 1.27 Mea or because the original
envelope exceeded 1% of the bulk composition. In the “abiotic oxygen” case, photolysis of the planet’s original water
content led to a complete loss of hydrogen, while the liberated oxygen accumulated
in the atmosphere and saturated global geochemistry. In this case, they argue,
the intensely oxidized environment would prevent the emergence of life, even if
quantities of water escaped photolysis (with that possibility defining the “water and oxygen, but uninhabitable”
scenario).
They
also discuss an outcome they consider especially unlikely: the “no atmosphere” case. While they
concede that the host star’s intense flaring activity is capable of stripping
an Earth-like atmosphere from Proxima b, they argue that such a catastrophe
would be followed by outgassing from the planet’s interior, which could
re-establish an atmosphere. Only if the planet’s core has solidified, quenching
planetary magnetism, or if the star is a few billion years older than their preferred
estimate of 4.8 billion years, providing enough time for the mantle to
completely devolatilize, would its atmosphere be permanently destroyed.
The
authors devote the most time and space to the “Earth-like” case, consistent with their stated aims. Within this
case they find three pathways to a happy ending. In one, Proxima b originally
formed as an Earth-like planet on an orbit well outside the system’s mature
habitable zone, neatly avoiding ablation of volatiles. After the star settled
down on the main sequence, a close encounter between Proxima and Alpha Centauri
AB scattered the planet into its present orbit, where it managed to maintain
its atmosphere and water despite continuing flares and CMEs. In the second
pathway, Proxima b achieved its present orbit in primordial times and suffered
desiccation as in the “Venus-like” case, but then a close passage involving
Proxima and the bright binary launched icy asteroids and comets on a collision
course with the desert planet. This bombardment re-hydrated the environment and
enabled oceans and life. In the third pathway, Proxima b started its existence
as a gas dwarf with a hydrogen envelope comprising about 0.1% of its bulk
composition, plus a water inventory amounting to 4.5 Earth oceans. The host
star then blew off the envelope and evaporated most of the water, and thus
unveiled, the planet brought forth life. This is the “habitable evaporated core” scenario, which Barnes & colleagues
consider the likeliest of the three pathways to Eden.
In their
closing remarks, nonetheless, they concede that life-friendly outcomes
represent a small subset of the possible scenarios for our new neighbor. Having
reviewed the many mechanisms by which the planet can end up a blasted, lifeless
desert, they “identify the retention of water as the biggest obstacle for
Proxima b to support life.”
proxima centauri as a habitable planet: ribas & colleagues
Ignasi Ribas led another group including Reiners, Morin, and Anglada-Escude from the discovery team, as well as Sean Raymond, Jeremy Leconte, Franck Selsis, Emeline Bolmont, and others. Their study is substantially briefer and less expansive than that of Barnes & colleagues, but they cover much of the same ground, while reaching somewhat rosier conclusions.
They begin with formation scenarios, considering various possibilities
also discussed by Coleman & colleagues and Barnes & colleagues:
formation in situ, formation in situ with late water delivery by
bombardment, and formation by accretion with long-distance migration. They also
consider a subset of the evolutionary outcomes explored by those studies,
including Ocean Planets, completely desiccated planets, and of course
Earth-like planets. Among likely orbital states, they agree with Barnes &
colleagues regarding the possibility of two regimes: a “synchronous” or tidally
locked case and a 3:2 spin-orbit resonance.
In their exploration of stellar irradiation and the
potential for erosion of atmosphere and water, Ribas & colleagues offer a
broad range of outcomes. Although they recognize that the host star’s
troublesome behavior could desiccate a temperate planet even if it originally
supported 21 Earth oceans, they also find cases where the planet could lose
less than a single Earth ocean. It’s no surprise that this special case assumes
special prominence in their overall findings. As they put it, the “general
conclusion from our study is that Proxima b could have liquid water on its
surface today and thus can be considered a viable candidate habitable planet.”
I’d say the fix is in.
wrapping
it up
After typing these thousands of characters,
I’ve only scratched the top layers of the new literature on Proxima b. Members
of Ribas’ group have also produced a study of the planet’s potential climate (Turbet
et al. 2016), members of Barnes’ group just circulated a lengthy treatment of its
potential environments and their observational signatures (Meadows et al.
2016), while a solo author offers a study of heat distribution (Goldblatt
2016). So many experts contributing so much brain power to that pale red dot!
Here’s what I think. Given the limited available data,
it’s premature to speculate about the possibility of Earth-like conditions
on Proxima b. Given my understanding of human behavior under the regime of
desiring-production enforced by terminal commodity capitalism, however, such
speculations are inevitable. We want excitement, and we want it now!
Before Proxima b was announced, we already knew that M
dwarfs readily supported Earth-size planets in their classical habitable zones,
and we already knew that the likelihood of life on such planets was far lower
than on their counterparts orbiting Sun-like stars. The announcement of Proxima
b hasn’t changed any of that. But it has inspired a lot of cogitation and
calculation, and fortunately, that’s not likely to stop.
REFERENCES
Andrews SM, Rosenfeld KA, Kraus AL, Wilner DJ. (2013)
The mass dependence between protoplanetary disks and their stellar hosts. Astrophysical Journal 771, 129.
Anglada-Escude G, Amado PJ, Barnes J,
Berdinas ZM, Butler RP, Coleman GAL, de la Cueva I, Dreizler S, Michael Endl M,
Giesers B, and 21 others. (2016) A terrestrial planet candidate in a temperate
orbit around Proxima Centauri. Nature
536, 437-440.Barnes R, Deitrick R, Luger R, Driscoll PE, Quinn TR, Fleming DP, Guyer B, McDonald DV, Meadows VS, Arney G, Crisp D, Domagal-Goldman SD, Lincowski A, Lustig-Yaeger J, Schwieterman E. (2016) The habitability of Proxima Centauri b I: Evolutionary scenarios. In press. Abstract: 2016arXiv160806919B
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Meadows VS, Arney GN, Schwieterman E, Lustig-Yaeger J, Lincowski AP, Robinson T, Domagal-Goldman SD, Barnes RK, Fleming DP, Deitrick R, Luger R, Driscoll PE, Quinn TR, Crisp D. (2016) The habitability of Proxima Centauri b: II: Environmental states and observational discriminants. In press.
Owen JE, Mohanty S. (2016) Habitability of terrestrial-mass planets in the HZ of M Dwarfs. I. H/He-dominated atmospheres. Monthly Notices of the Royal Astronomical Society 459, 4088-4108.
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