Habitable Planet Reality Check: Proxima Centauri b

The nearby star system of α Centauri, with its pair of Sun-like stars 4.37 light years away and a distant red dwarf companion, for decades has been a target for people with a range of interests and motivations: astronomers hoping to understand stars like the Sun, visionaries considering plans to travel to and even colonize any worlds that orbit them, not to mention the writers and fans of science fiction stories in print as well as on film set in this system. Even though it is the closest known star system to us, our efforts to find planets here have been frustrated by the difficulty of the task and the limits of our technology. With the recent revelation that the Earth-size planet “found” in a close orbit around α Centauri B in 2013 was just an artifact of noise in the data, we have been left waiting as the search for planets continues (see “The Discovery of Alpha Centauri Bb: Three Years Later”).

Dr. Guillem Anglada-Escudé is shown during the ESO press conference in Munich, Germany during the announcement of the discovery of Proxima Centauri b on August 24, 2016. (ESO/M. Zamani)

But now the wait finally seems to be over. During the first quarter of 2016, an international team of astronomers started an intensive and very public campaign to observe the red dwarf of the α Centauri system known as Proxima Centauri. Called the Pale Red Dot, the purpose of this project was to follow up on earlier radial velocity measurements which suggested that Proxima Centauri was orbited by a planet in a short-period orbit. After almost two weeks of unconfirmed reports and rumors, a team led by Guillem Anglada-Escudé (Queen Mary University of London) announced on August 24, 2016 that they had discovered a planet now called Proxima Centauri b. Not only did they confirm the existence of the previously suspected extrasolar planet, they found that it was an approximately Earth-mass planet orbiting comfortably inside the habitable zone (HZ) of its star. While this important discovery has been at the eye of a storm of media hype, what do we actually know about Proxima Centauri b and how Earth-like is it really?

 

Background

Proxima Centauri is a very small type M5.5V red dwarf star that displays all the characteristics of a magnetically active star including occasional intense flares. According to the data used by Anglada-Escudé et al., Proxima Centauri has an estimated mass of only 0.120±0.015 times that of the Sun, a radius of 0.140±0.012 times that of the Sun and an effective temperature of 3050±100 K. It is among the smallest main sequence stars known with a mass only about a third again more than the least massive normal star theoretically possible. Because of its low luminosity of just 1.55±0.06X10-3 times that of the Sun, Proxima Centauri has a V magnitude of only 11.0 and requires a small telescope to spot visually even though it is the closest known star at a distance of 4.24 light years – just a touch closer to us than the pair of Sun-like stars at the heart of the α Centauri system hence its name “Proxima”. Despite its dimness, it still has a higher apparent brightness than most known red dwarfs and, as a result, it has been a target of study by those with an interest in this most numerous class of star.

A view of Proxima Centauri acquired by the Hubble Space Telescope. (ESA/Hubble/NASA)

Because of its close proximity to α Centauri AB and its shared apparent motion, it has been generally believed since its discovery in 1915 by Scottish astronomer Robert Innes that Proxima Centauri is bound to the pair of larger, Sun-like stars at the center of this system. But studies over the past two decades on the relative motions of these stars tends to suggest that Proxima Centauri might not be gravitationally bound to α Centauri AB about 15,000 AU distant. Instead these stars might be independent members of a stellar moving group that happen to be close to each other at the present time.

Diagram showing the relative sizes of the Sun and the three stars of the α Centauri system. (PHL@UPR Arecibo)

A study by Jeremy Wertheimer and Gregory Laughlin (University of California Observatories/Lick Observatory) published in 2006 using the best available information on the positions and motions of these stars shows that Proxima Centauri, with a velocity relative to α Centauri AB of 530±140 meters per second, is right on the cusp of being bound to α Centauri AB. The best measurements suggest that Proxima Centauri is near the apastron or most distant point in a highly eccentric orbit around α Centauri AB with an orbital period measured in hundreds of thousands to maybe even millions of years. However, given the measurement uncertainties, the possibility can not be excluded that Proxima Centauri might be unbound and simply passing by α Centauri AB. Only future improvements in the accuracy of the absolute radial velocity of Proxima Centauri will definitively resolve the issue.

 

The Search for Planets

Because of its small mass and closeness, Proxima Centauri has been the target of searches for substellar and planetary companions for decades. During the last quarter of a century, increasingly sensitive detection methods have been employed in this search capable of finding ever smaller companions and ultimately planets (for a detailed summary of these earlier searches, see “The Search for Planets Around Proxima Centauri”).

One of Hubble’s Fine Guidance Sensors being exchanged by astronaut Gregory J. Harbaugh during Servicing Mission 2 in February 1997. (NASA/ESA)

During the 1990s, a team led by G. Fritz Benedict (McDonald Observatory) used the Hubble Space Telescope (HST) to make precision astrometric measurements looking for the telltale wobble caused by the reflex motion of an orbiting companion. But instead of using one of Hubble’s imaging instruments, Benedict et al. employed the Fine Guidance Sensors (FGS) used to keep the telescope locked in position while other instruments collect data. Hubble’s FGS #3 was fitted with a broadband visible light filter specifically designed to make astrometric measurements like this. Benedict et al. failed to detect any evidence for planets orbiting Proxima Centauri based on measurements acquired between March 1992 and October 1997. This null result from the work of Benedict et al. excluded the presence of unseen companions more massive than Jupiter (or 1 MJ) with orbital periods between 50 and 1000 days to a 95% certainty level or better. For orbital periods greater than 400 days, the detection limit is less than the mass of Saturn or 0.3 MJ.

Plot showing the detection limits (at a 95% confidence level) as a function of companion period for Proxima Centauri based on the astrometric work by Benedict et al. using the Hubble Space Telescope. Click on image to enlarge. (Adapted from Benedict et al.)

More recently, a team led by John C. Lurie (University of Washington – Seattle) made astrometric observations of Proxima Centauri as part of the RECONS (Research Consortium On Nearby Stars) long-term astrometric and photometric survey program using the CTIO/SMARTS (Cerro Tololo Inter-American Observatory/Small and Moderate Aperture Research Telescope System) 0.9-meter telescope in Chile. For their analysis, they used data from 205 observations acquired over 14 seasons from mid-2000 to early-2013. While the accuracy of their astrometric measurements was lower than was possible with  HST, Lurie et al. were able to search for planets in orbits with longer periods owing to the longer time span of their observations. To a 90% confidence level, Lurie et al. found that there were no planets present with masses greater than about 1.0 MJ and 0.6 MJ with periods of 5 and 12.6 years, respectively. The combination of astrometric results from Benedict et al. and Lurie et al. effectively eliminated the possibility that Proxima Centauri has any companions with a mass comparable to or greater than Jupiter in orbits with periods ranging from 0.14 to 12.6 years (corresponding to orbital radii of about 0.13 and 2.67 AU, respectively).

The detection thresholds at a 90% confidence level for five of the stars studied by Lurie et al.. Proxima Centauri is the bottom curve in the group. Click on image to enlarge. (Lurie et al.)

The other means of measuring the reflex motion of a star orbited by an unseen companion is precision radial velocity measurements. In fact, before NASA’s highly successful Kepler mission, the majority of extrasolar planets were discovered using this technique. Since this method is more sensitive to planets in smaller orbits, it nicely complements the astrometric method which is more sensitive to planets in larger orbits. One of the drawbacks of the radial velocity method not shared by astrometry is that only the minimum mass of a planet or MPsini (where i is the inclination of the exoplanet’s orbit to the plane of the sky) can be calculated. Since the inclination can not be determined from radial velocity measurements alone, it must be found by other means or, failing that, only the probability that the actual mass of a planet falls within some range of interest can be calculated.

While published results from radial velocity searches for unseen companions of Proxima Centauri go back over a decade and a half, among the most accurate radial velocity results up until recently were based on the work of Michael Endl (McDonald Observatory) and Martin Kürster (Max Planck Institute) published in 2008. Endl and Kürster were involved in an ongoing program to survey 40 M-dwarf stars using the UVES spectrograph on the 8.2-meter telescope designated UT2 – one of four such telescopes that make up the VLT or Very Large Telescope at the European Southern Observatory on Cerro Paranal in Chile. For their analysis of Proxima Centauri, they used 229 individual spectra obtained over the course of 76 nights between March 2000 and March 2007. The differential radial velocity measurements they obtained had an RMS scatter of 3.11 meters/second – a factor of 17 better than earlier published radial velocity results.

View of the European Southern Observatory’s Very Large Telescope array at Cerro Paranal in Chile. One its telescopes, UT2, has been used to measure the radial velocity of Proxima Centauri in search of planets using the UVES spectrometer. (J.L. Dauvergne & G. Hüdepohl/ESO)

As with the astrometric results, Endl and Kürster found no signal which would indicate the presence of an orbiting extrasolar planet. Based on an analysis of the data and the ability of their analysis software to detect simulated planet signals, it was found that there were no planets detected orbiting Proxima Centauri with MPsini values of two times that of the Earth (or 2 ME) with an orbital period of 2 days and up to 20 ME for planets with orbital periods of nearly 2000 days (corresponding to orbital radii of 0.015 to 1.53 AU, respectively). Planets with MPsini values in the 2 to 3 ME range were excluded from the conservatively defined habitable zone of Proxima Centauri which, according the the definition adopted by Lurie et al., spans orbital periods of 3.6 to 13.8 days and distances of 0.023 to 0.056 AU.

Minimum mass upper limits as a function of period for planets in circular orbits around Proxima Centauri based on work by Endl and Kürster. The shaded area labelled “HZ” displays the approximate location of the conservatively defined habitable zone. Click on image to enlarge. (Endl and Kürster).

Taking into account the probabilities of various sini values, these radial velocity survey results effectively eliminated the possible presence any Saturn-size planets (95 ME or 0.3 MJ) with orbital periods less than about 2000 days or Neptune-size planets (17 ME or 0.05 MJ) with orbital periods less than about 40 days to a 95% confidence level or better. Inside of the habitable zone, there are no planets with masses greater than the 6 to 10 ME at the same confidence level. While the existence of planets about as small as mini-Neptunes had been effectively excluded, still smaller planets could be present and have escaped detection – smaller planets that have a much higher likelihood of having a rocky composition and therefore being potentially habitable.

 

The Pale Red Dot Campaign

The UVES observation program described by Endl and Kürster was not the only campaign to make precision radial velocity measurements of Proxima Centauri looking for orbiting planets. As part of their systematic survey of nearby M-dwarf stars conducted between 2003 and 2009, the European-based HARPS (High Accuracy Radial velocity Planet Search) team made a handful of precision radial velocity measurements of Proxima Centauri looking for signs of variations in this and other nearby red dwarfs. Any significant variations in the radial velocity of stars in their survey could indicate the presence of extrasolar planets that would then be followed up by a more thorough observation campaign to characterize the system.

The ESO 3.6m Telescope equipped with HARPS. (ESO/H.H.Heyer)

With the HARPS spectrograph attached to the European Southern Observatory’s 3.6-meter telescope in La Silla, Chile, an initial group of 27 radial velocity measurements were made between May 2004 and February 2009. In 2012, Guillem Anglada-Escudé (then with the Carnegie Institution of Washington and now with Queen Mary University of London) and famed exoplanet hunter, R. Paul Butler (Carnegie Institution of Washington), published the results of their analysis of these data using their new HARPS-TERRA analysis software. When searching for periodicities in the radial velocity measurements of Proxima Centauri which had an RMS variation of 2.05 meters per second, Anglada-Escudé and Butler found what they described as “a very marginal peak at 5.6 days”. In an analysis of all the M-dwarf stars in the HARPS survey published the following year with team leader Xavier Bonfils (Institut de Planétologie et d’Astrophysique de Grenoble) as the first author, it was noted that there was convincing evidence of some sort of periodic signal in the expanded data set of 32 radial velocity measurements. While Bonfils et al. found a velocity dispersion of 2.1 meters per second in their observations with a measurement uncertainty of 1.6 meters per second, no single or multi-planet solution generated a statistically convincing match for the available data.

Plot of the radial velocity data acquired by HARPS and analyzed in 2012 by Anglada-Escudé and Butler using their old analysis method, CCF, and their new one, TERRA. Click on image to enlarge. (Anglada-Escudé and Butler)

With more data needed, two high-cadence data collections were made with HARPS in 2013. The first had total of 143 spectra obtained on three consecutive nights between May 4 and 7 with another 25 spectra acquired by May 16. A second data collection of 23 spectra from December 30, 2013 to January 10, 2014 followed. The data clearly showed a periodicity of about 11.2 days – twice the period originally found by Anglada-Escudé and Butler which, in retrospect, was probably a harmonic of this newly identified signal. Unfortunately, the false alarm probability even with the greatly expanded data set was too high to be considered a reliable detection. In addition, there were alternate periods of 13.6 and 18.3 days possible which were caused by a “nontrivial window function” resulting from the uneven sampling of the data over long periods. Combining these data with the UVES data by Endl and Kürster produced a false alarm probability of about 1% but there were still many issues associated data sampling, stellar activity and the assumptions made in processing the data. What was needed was much more high-cadence data over an extended period of time.

The upper panel shows the HARPS radial velocity measurements of Proxima Centauri from May 2013. The periodogram in the bottom panel shows a broad peak at 11 days. (Anglada-Escudé )

Over the next two years, Guillem Anglada-Escudé organized a dedicated, high-cadence observation campaign to look for this possible exoplanet (or exoplanets) orbiting Proxima Centauri which would eventually be known as the Pale Red Dot – a play on Carl Sagan’s reference to the habitable Earth as a pale blue dot. But more than just precision radial velocity measurements would be required to confirm the possible signature of an orbiting planet with any confidence. At this time, there was growing concern in the exoplanet science community about the effects of natural stellar noise or jitter on the analysis of the data. While the readily noticeable effects of starspots rotating into and out of view over time were already appreciated, more subtle forms of chromospheric activity modulated by the rotation of the star might also mimic a planetary signature. The existence of potentially habitable planets orbiting GJ 581, GJ 667C and Kapteyn’s Star has been called into question because of such natural noise (see “Habitable Planet Reality Check: GJ 581”, Habitable Planet Reality Check: GJ 667C” and “Kapteyn b: Has Another Habitable Planet ‘Disappeared’?”). The putative Earth-size exoplanet α Centauri Bb orbiting one of Proxima’s Sun-like neighbors, is now generally regarded to be a window function artifact caused by the irregular sampling of various sources of noise including jitter (see “The Discovery of Alpha Centauri Bb: Three Years Later”). These issues are especially important with red dwarf stars which are prone to bouts of enhanced activity and flaring.

Pale Red Dot was an international search for an Earth-like exoplanet around the closest star to us, Proxima Centauri, which employed HARP on ESO 3.6-meter telescope in La Silla, Chile (shown here) and other southern hemisphere observatories. (ESO/Pale Red Dot)

To address these concerns, Anglada-Escudé and his team enlisted the aid of additional astronomers to monitor Proxima Centauri while they were making their radial velocity measurements using HARPS. The 0.4-meter ASH2 (Astrograph for the South Hemisphere II) telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile was used for photometric observations at the narrowband wavelengths corresponding the SII and Hα which are frequently employed by astronomers to monitor stellar activity. The one-meter telescopes of LCOGT.net (Las Cumbres Observatory Telescope network) made observations over the same time interval in the classic Johnson B and V bands. One of the BOOTES (Burst Observer and Optical Transient Exploring System) stations was originally suppose to gather photometric data as well but apparently suffered technical difficulties which prevented its participation. Analysis of these photometric data taken during the same time period as the HARPS high-cadence radial velocity measurements would be able to help differentiate the signal of an orbiting planet from stellar activity. With partners in place, a block of observing time was secured for HARPS during the first quarter of 2016.

Telescopes like this LCOGT.net one-meter telescope were used during the Pale Red Dot campaign to acquire multi-band photometery of Proxima Centauri to monitor this star’s activity as HARPS took its radial velocity measurements. (LCOGT)

As part of the Pale Red Dot campaign, one spectrum of Proxima Centauri was obtained near the end of almost every night between January 19 and March 31, 2016. Out of 60 scheduled nights, 54 nights yielded a total of 56 spectra (with two nights generating two spectra each). These new measurements from 54 “epochs” were combined with 90 earlier observations from HARPS and 72 measurements obtained between 2000 and 2008 by UVES. The combined data set clearly showed a peak at 11.2 days with a false alarm probability of 7X10-7. The somewhat marginal signal observed in the pre-Pale Red Dot data had the same phase as that which was present in the new data with a much superior false alarm probability confirming that the periodic signals observed in both sets of data were in fact the same. Detailed analysis of the near-simultaneous photometric data showed a periodicity of about 80 days which is close to the previously determined 83-day rotation period of Proxima Centauri. No periodicities were noted in the photometric data near 11.2 days. With stellar activity eliminated as the possible cause of the observed radial velocity variations, the only other explanation was the presence of a planet with an orbital period of 11.2 days.

This plot shows how the radial velocity of Proxima Centauri and how it varied during the Pale Red Dot campaign of the first quarter of 2016. (ESO/G. Anglada-Escudé)

Anglada-Escudé et al. published their results in the August 25, 2016 issue of the peer-reviewed science journal Nature. Fitting a Keplerian orbit to their radial velocity data, Anglada-Escudé et al. found a planet with an orbital period of 11.186 +0.001/-0.002 days and a Doppler semiamplitude of 1.38±0.21 meters per second. Combined with the presumed properties of its host star, the new exoplanet, dubbed Proxima Centauri b, has a minimum mass or MPsini of 1.27 +0.19/-0.17 ME and mean orbital radius of 0.0485 +0.0041/-0.0051 AU – a mere 7 million kilometers from its host and squarely inside most definitions of the habitable zone (HZ). The residuals from this fit suggests that maybe there is a second, possibly super-Earth-size planet with an orbital period in the 60 to 500 days range and a Doppler semiamplitude of less than three meters per second. The true nature of this signal is unclear at this time because of stellar activity and inadequate sampling. As before, more data will be needed to sort this out.

Based on the findings of Anglada-Escudé et al., it seems that the detection of Proxima Centauri b is robust. While it needs independent verification, Proxima Centauri b appears to be a roughly Earth-mass exoplanet orbiting inside the HZ of our nearest known stellar neighbor. But is this world really potentially habitable?

 

Potential Habitability

While a full assessment of the habitability of any exoplanet would require very detailed information about all of its properties, obtaining such information about even this nearest known exoplanet is simply beyond the reach of our current technology. At this early stage in our search for other Earth-like worlds, the best we can do is compare what properties we can derive to our current expectations of the range of properties for habitable worlds to determine if a new find is potentially habitable. And by “habitable”, I mean habitable in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface. While there may be other worlds that might possess environments that could support life (e.g. Mars or the tidally heated oceans on the moons Europa and Enceladus), these would not be Earth-like habitable worlds of the sort being considered here.

The first piece of information that gives us a clue about the nature of Proxima Centauri b is its size: is it a small dense rocky planet like the Earth or a large volatile-rich mini-Neptune possessing a deep, thick atmosphere with little prospect of being habitable in the conventional sense? At this time, we only know the MPsini or minimum mass value of Proxima Centauri b. There is a 1.5% chance that the orbit of Proxima Centauri b is oriented to produce transits visible to us here on Earth. If such transits actually occur, the actual mass and the radius of this new exoplanet could be determined in the near future. Although Davenport et al. find that flaring complicates the search for such transits, efforts are being made to search for them including using the MOST (Microvariability and Oscillations of Stars Telescope) microsatellite. Without knowledge of the planet’s actual mass and radius in the mean time, there is no way to constrain its bulk composition. However, we can at least estimate the probability that Proxima Centauri b is a rocky planet, or not, by comparing this new find to the growing population of other exoplanets whose properties are better known.

Based on the measured minimum mass of Proxima Centauri b, it seems likely that it is a rocky planet like that shown in this artist’s depiction. (ESO/M. Kornmesser)

Based on statistical analysis of Kepler results performed by Leslie Rogers (Hubble Fellow at Caltech) and others, it is known that exoplanets seem to transition from being predominantly rocky to predominantly volatile-rich probably at a radius of about 1.5 RE and certainly no greater than 1.6 RE (see “Habitable Planet Reality Check: Terrestrial Planet Size Limits”). A planet with this radius corresponds to a mass of about 6 ME, assuming an Earth-like composition. With an unconstrained orbital inclination, there is about a 98% chance that Proxima Centauri b with a MPsini of 1.27 ME has an actual mass below this threshold. More recent work by Jingjing Chen and David Kipping (Columbia University) suggests that the gradual transition from rocky to volatile-rich exoplanets starts at about 1.2 RE or 2 ME with the probability that a planet is rocky decreasing with increasing radius or mass. There is still a 77% probability that the actual mass of Proxima Centauri b is below this lower threshold. While we will have to wait to measure the actual mass and radius of this new find for a definitive determination, the odds seem to heavily favor Proxima Centauri b being a rocky planet.

The next issue affecting the potential habitability of this world is the amount of energy it receives from its sun or the effective stellar flux, Seff. According to the work by Kopparapu et al. (2013, 2014) on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the outer limit of the HZ is conservatively defined as corresponding to the maximum greenhouse limit of a CO2-rich atmosphere where the addition of any more CO2 would not increase a planet’s surface temperature any further. For a star like Proxima Centauri with a surface temperature of 3050 K, this conservative outer limit for the HZ has an Seff of 0.23 times that of the Earth corresponding to a mean orbital radius of 0.081 AU.

This diagram compares the orbit of Proxima Centauri b with the same region of our Solar System. Also indicated is the position of the habitable zone (HZ) around Proxima Centauri. Click on image to enlarge. (ESO/M. Kornmesser/G. Coleman)

The inner limit of the HZ is conservatively defined by Kopparapu et al. (2013, 2014) by the runaway greenhouse limit where a planet’s temperature would soar and its atmosphere would lose all of its water in the process. For an Earth-size planet orbiting Proxima Centauri, this corresponds to an Seff of 0.92 or a distance of 0.041 AU. According to Ribas et al., the rotation of Proxima Centauri b has been tidally slowed so that it is either a synchronous rotator or, depending on its shape (which is unknown) and orbit eccentricity (which is poorly constrained by the current radial velocity measurements), in a 3:2 spin-orbit resonance where, like Mercury in our solar system, it rotates three times for every two orbits. Over the past two decades, climate modelling for slow rotators like this has shown that the onset of a moist runaway greenhouse effect takes place at much higher Seff values than for fast rotators like the Earth because of feedback from enhanced cloud formation on the planet’s sun-facing hemisphere.

According to a recent model by Yang et al., the Seff for the inner limit of the HZ for a slow rotator is 1.55 corresponding to a distance of 0.032 AU from Proxima Centauri. But since Proxima Centauri b probably has a comparatively short period of rotation of either 7.5 or 11.2 days, the enhanced Coriolis effect changes the global circulation patterns enough to affect cloud formation on the dayside. A more recent model by Kopparapu et al. (2016) which takes this effect into account suggests the inner edge of the HZ for Proxima Centauri is closer to 0.042 AU where the Seff is 0.87 that of the Earth. By whichever definition of the HZ that is used, Proxima Centauri b with its mean orbital radius of 0.0485 AU with an Seff of 0.65 as derived by Anglada-Escudé et al. seems to orbit comfortably inside the HZ.

Among the many papers submitted for publication with the discovery announcement by Anglada-Escudé et al. was a one with Martin Turbet (Laboratoire de Météorologie Dynamique, Sorbonne Universités) as the lead author which describes the results of climate models specifically for Proxima Centauri b. Turbet et al. used a 3D global climate model to simulate the new find’s atmosphere and water cycle for its two likely spin states while varying the planet’s currently unconstrained surface water inventory and the amount of CO2 greenhouse gas in its presumed N2-CO2 atmosphere (the most common atmospheric composition among the larger rocky planets of our solar system including the early Earth).

These diagrams show the various possible climate regimes for Proxima Centauri b as a function of water inventory and atmospheric CO2 content. The upper panel shows the results for a synchronous rotator with a period of 11.2 days while the lower panel shows the results for non-synchronous rotation with a period of 7.5 days. Click on image to enlarge. (Turbet et al.)

Turbet et al. identified a number of possible climate regimes and found that Proxima Centauri b could support liquid water on its surface under a wide range of conditions. Which climate regime Proxima Centauri b actually possesses would depend on its inventory of water and the amount of CO2 in its atmosphere. The feedback from mechanisms like the carbonate-silicate cycle, which would regulate atmospheric CO2 level and was not included in the 3D global climate model explored by Turbet et al., would tend to maintain above-freezing temperatures. These 3D global climate model results seem to support the view that Proxima Centauri b is at least potentially habitable.

 

Other Issues

In addition to the basic properties of an exoplanet and its orbit affecting its potential habitability, a number of issues peculiar to red dwarfs may also have important consequences as well. One of the classic issues which still exists in the minds of some is synchronous rotation caused by strong tidal effects of the close-in HZ typical of red dwarfs. As we have already seen in the discussion above, over the last couple of decades this has been shown not to be a showstopper for habitability in general and the work by Turbet et al. supports this in the case of Proxima Centauri b in particular.

Flares and other types of stellar activity could complicate the habitability of exoplanets orbiting small red dwarfs like that shown in this artist’s depiction. (David A. Aguilar – CfA)

Another issue of genuine concern is strong flares as well as other forms of episodic stellar activity for which Proxima Centauri and similar red dwarfs are known. This and other issues are addressed in another paper with Ignasi Ribas (Institut de Ciències de l’Espai) as the lead author. In addition to determining the current rotation state of Proxima Centauri b, Ribas et al. attempted to model the evolution of the planet’s water inventory. Despite the high level of activity and the enhanced luminosity of Proxima Centauri during the first 100 to 200 million years of its life, they find that the newly discovered exoplanet should have lost less than the equivalent of an Earth-ocean’s worth of water even if it formed where it is found today. While there is a large uncertainty about the exoplanet’s initial water budget, Ribas et al. conclude that Proxima Centauri b “is a viable candidate habitable planet”.

In yet another paper with Rory Barnes (University of Washington – Seattle) as the lead author, various evolutionary scenarios for Proxima Centauri b are examined. Barnes et al. found that for a range of potential compositions, radiogenic abundances and tidal heating scenarios, it seems likely that Proxima Centauri b should possess a magnetic field which should help shield its atmosphere from the effects of the host star’s more extreme activity. Barnes et al., however, paint a less rosy picture about the potential impact of water loss than in Ribas et al.. Barnes et al. find that if Proxima Centauri b formed in place, the enhanced luminosity of its sun as it evolved onto the main sequence would have subjected the planet to a runaway greenhouse effect for the first 160 million years of its history which may have permanently desiccated the planet and possibly even left it with abiotic O2 in its atmosphere. However, Barnes et al. note that if Proxima Centauri b started with a thin H2 atmosphere with a mass <1% of the planet, it might have been enough to staunch this water loss.

This plot shows how the position of the habitable zone (HZ) of Proxima Centauri changed as it settled onto the main sequence. The blue shaded area is the conservative HZ of Kopparapu et al. (2013) while the dashed lines indicate the inner limit of the HZ for a desiccated planet with the indicated albedo. (Barnes et al.)

Of course these scenarios assume that Proxima Centauri b formed in place. Barnes et al. found that it is likely that Proxima Centauri has had close encounters with α Centauri AB during the past possibly disrupting the red dwarf’s planetary system. This could have moved Proxima Centauri b from its original location where it formed and caused interactions with other planets in the system including the possible second planet whose presence is suggested by the current radial velocity data. Possible formation scenarios examined in a paper with Gavin Coleman (Queen Mary University of London) as the lead author (and including exoplanet codiscoverer, Guillem Anglada-Escudé) include some in which Proxima Centauri b migrated inwards from beyond the snowline. This opens the possibility that Proxima Centauri b avoided desiccation during the earliest period of its sun’s evolution and might even be an ocean planet.

The simple fact of the matter is that we do not currently know the details about these important aspects of the evolution of Proxima Centauri b as well as many of its current properties that can affect its habitability. What can be said is that based on what we know makes the Earth habitable and given our currently limited knowledge about this new exoplanet, it appears that Proxima Centauri b is potentially habitable, although with its slow rotation rate, short year and the effect this would have on global climate, it could hardly be considered Earth-like. We will have to wait to see what future observations reveal about this world. But given the close proximity of this newly discovered exoplanet and its prime location comfortably inside the HZ, Proxima Centauri b will be an excellent target to explore the potential habitability of all red dwarf stars which are the most common star type in our galaxy.

 

Summary

Based on the limited data we currently have about Proxima Centauri b, it seems to be potentially habitable, although not very Earth-like. Its MPsini of 1.27 ME suggests that it is a rocky planet instead of a mini-Neptune with little prospect of being habitable in the conventional sense. It is also located inside the habitable zone (HZ) of its host star and detailed 3D global climate models suggest that it could be habitable over a wide range of realistic conditions. Comparing it to other potentially habitable exoplanets currently known (a short list dominated by planets of other red dwarfs), Proxima Centauri b is probably one of the most promising on par with Kepler 186f (see “Habitable Planet Reality Check: Kepler 186f Revisited“). Although this is a hopeful start, we need to avoid getting too wrapped up in the media hype surrounding this important discovery since there is still much that remains to be learned about this new world in the months and years ahead which could affect this assessment.

While Proxima Centauri b, shown in this artist’s depiction next to the Earth, has good prospects for being potentially habitable, it will not be a very “Earth-like” world. (PHL@UPR Arecibo)

There have been a number of issues raised over the years about the ability of red dwarfs in general and Proxima Centauri in particular to support habitable planets. But at the same time there have been other studies which suggest otherwise, as can be seen among the number of papers submitted for publication at the same time the discovery of Proxima Centauri b was announced. Being so close to the Earth, Proxima Centauri b is perfectly situated to test these various ideas about the habitability of planets orbiting red dwarfs – potentially the most numerous class of habitable planet in our galaxy (see “Occurrence of Potentially Habitable Planets Around Red Dwarfs”). Along with the continued study of other potentially habitable worlds (and even the “near misses”) which have been discovered over the last few years, we stand to learn much no matter how the issue is resolved and get a better handle on where to look for life beyond our solar system.

 

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Related Video

Here is a video of the ESO press conference in Munich, Germany where the discovery of Proxima Centauri b was announced on August 24, 2016.

 

 

Related Reading

“The Search for Planets Around Proxima Centauri”, Drew Ex Machina, February 23, 2015 [Post]

“The Hubble Space Telescope and the Search for Faint Extrasolar Companions”,  SETIQuest, Volume 3, Number 2, pp. 1-9, Second Quarter 1997 [Article]

“Occurrence of Potentially Habitable Planets Around Red Dwarfs”, Drew Ex Machina, January 12, 2015 [Post]

 

General References

Guillem Anglada-Escudé and R. Paul Butler, “The HARPS-TERRA Project. I: Description of the Algorithms, Performance and New Measurements on a Few Remarkable Stars Observed by HARPS”, The Astrophysical Journal Supplement, Vol. 200, No. 15, June 2012

Guillem Anglada-Escudé et al., “A terrestrial planet candidate in a temperate orbit around Proxima Centauri”, Nature, Vol. 536, No. 7617, pp. 437-440, 25 August, 2016 [Preprint]

Sarah Ballard and John Asher Johnson, “The Kepler Dichotomy among the M Dwarfs: Half of Systems Contain Five or More Coplanar Planets”, The Astrophysical Journal, Vol. 816, No. 2, Article id. 66, January 8, 2016

Rory Barnes et al., “The Habitability of Proxima Centauri b I. Evolutionary Scenarios”, arXiv 1608.06919 (submitted to Astrobiology), August 24, 2016 [Preprint]

G. Fritz Benedict et al., “Interferometric Astrometry of Proxima Centauri and Barnard’s Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions”, The Astronomical Journal, Vol. 118, No. 2, pp. 1086-1100, August 1999

X. Bonfils et al., “The HARPS search for southern extra-solar planets XXXI. The M-dwarf sample”, Astronomy & Astrophysics, Vol. 549, ID A8, January 2013

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, arXiv 1603.08614, March 29, 2016 [Preprint]

G.A.L. Coleman et al., “Exploring Plausible Formation Scenarios for the Planet Candidate Orbiting Proxima Centauri”, arXiv 1608.06908 (submitted to Monthly Notices of the Royal Astronomical Society), August 24, 2016 [Preprint]

James R.A. Davenport et al., “MOST Observation of our Nearest Neighbor: Flares on Proxima Centauri”, arXiv 1608.06672 (submitted to Astrophysical Journal Letters), August 24, 2016 [Preprint]

Courtney D. Dressing and David Charbonneau, “The Occurrence of Potentially Habitable Planets Orbiting M Dwarfs Estimated from the Full Kepler Dataset and an Empirical Measurement of the Detection Sensitivity”, The Astrophysical Journal, Vol. 807, No. 1, Article id. 45, June 30, 2015

M. Endl and M. Kürster, “Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri”, Astronomy and Astrophysics, Vol. 488, No. 3, pp.1149-1153, September 2008

R.K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID. 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., “The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models”, The Astrophysical Journal, Vol. 819, No. 1, Article ID. 84, March 2016

John C. Lurie et al., “The Solar Neighborhood. XXXIV. A Search for Planets Orbiting Nearby M Dwarfs Using Astrometry”, The Astronomical Journal, Vol. 148, No. 5, Article id. 91, November 2014

Ignasi Ribas et al., “The Habitability of Proxima Centauri b I. Irradiation, Rotation and Volatile Inventory from Formation to the Present”, arXiv 1608.06813 (submitted to Astronomy & Astrophysics), August 24, 2016 [Preprint]

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

Martin Turbet et al., “The Habitability of Proxima Centauri b II. Possible Climate and Observability”, arXiv 1608.06827 (submitted to Astronomy & Astrophysics), August 24, 2016 [Preprint]

Jeremy G. Wertheimer and Gregory Laughlin, “Are Proxima and α Centauri Gravitationally Bound?”, The Astronomical Journal, Vol. 132, No. 5, pp. 1995-1997, November 2006

“Planet Found in the Habitable Zone Around Nearest Star”, ESO Science Release 1629, August 24, 2016 [Press Release]