While the flood of exoplanetary discoveries from NASA’s Kepler program has dominated headlines in recent years, there are a host of lesser known programs which have been searching for exoplanets around most of the nearby stars much closer to home. One of the larger and longer running exoplanet survey programs which uses precision radial velocity (RV) measurements to detect the subtle reflex motion of orbiting exoplanets has been HARPS (High Accuracy Radial velocity Planet Search). Sponsored by Observatoire de Genève, HARPS uses a spectrograph attached European Southern Observatory’s 3.6-meter telescope in La Silla, Chile to measure the RV of target stars down to the meter per second level. Among the more recent discoveries of the European HARPS team are potentially habitable exoplanets orbiting nearby stars like Proxima Centuari (see “Habitable Planet Reality Check: Proxima Centauri b”) , Wolf 1061 (see “Habitable Planet Reality Check: Wolf 1061 Revisited”), Luyten’s Star (see “Habitable Planet Reality Check: The Nearby GJ 273 or Luyten’s Star”) and LHS 1140 (see “Habitable Planet Reality Check: A Super-Earth Orbiting LHS 1140”).
In addition to the ESO facility in Chile, the HARPS team has collaborated with other institutions to create a northern hemisphere equivalent called HARPS-N. In 2012, installation of a HARPS spectrograph was completed on the Italian 3.58-meter Telescopio Nazionale Galileo (TNG) located at the Roque de los Muchachos Observatory on the island of La Palma in Spain’s Canary Islands. Since its activation, this facility has been used for the HADES RV program (HARPS-N Red Dwarf Exoplanet Survey Radial Velocity) as part of a collaboration between the Italian Global Architecture of Planetary Systems (GAPS) consortium, the Institut de Ciències de l’Espai de Catalunya (ICE), and the Instituto de Astrofísica de Canarias (IAC). The radial velocities of over six dozen late K and early M-type stars are being monitored looking for the presence of variations indicating the presence of orbiting exoplanets.
In 2016, HARPS-N scored its first discoveries – a pair of super-Earth size exoplanets orbiting the nearby red dwarf GJ 3998. On May 18, 2017, the HADES RV program announced its next find described by the title of its discovery paper with Alejandro Suárez Mascareño (Observatoire de Genève) as the lead author: “A super-Earth on the inner edge of the habitable zone of the nearby M-dwarf GJ 625”. So, how does this claim for this new nearby exoplanet bear up under closer scrutiny?
GJ 625 is a V-magnitude 10.17 star located in the northern constellation of Draco – The Dragon. The star was first charted by the Catania Observatory in Sicily during the early 20th century as part of the Astrographic Catalog: a decades-long international astronomical project organized in 1887 where 20 observatories around the globe measured the brightness and positions of millions of stars down to the 11th or 12th magnitude using photographic telescopes of a standardized design. In recent decades, GJ 625 has frequently gone by the designation of AC +54° 1646-56 found in that catalog.
It was Russian-American astronomer Alexander Vyssotsky (1888-1973) of the University of Virginia’s McCormick Observatory who, in 1956, published the first measurement of this star’s proper motion and determined that it was a spectral type M2 red dwarf star. Based on spectrophotometric measurements, it was also realized that this star was relatively nearby at a roughly estimated distance of around 47 light years earning it a place in the first edition of the Gliese Catalogue of Nearby Stars in 1957 and its designation of GJ 625 after the creator of the catalog, German astronomer Wilhelm Gliese (1915-1993), and his collaborator on later editions, Hartmut Jahreiß. The initial trigonometric parallax measurement from the ESA Gaia mission published in 2016 much more accurately places GJ 625 at a distance of 21.2±0.1 light years firmly establishing it among the closest stars known.
As part of the HADES RV campaign, astronomers have also made an effort to characterize the stars in their survey more accurately. In a paper with Jesus Moldanado (INAF or Instituto Nazionale di Astrofisica) as the lead author (as well as one of the coauthors of Suárez Mascareño et al.), the properties of 71 M and K-type dwarf stars the program is observing were presented including those of GJ 625. Using data from TNG, they confirmed the M2 spectral classification found six decades earlier by Vyssotsky and found an effective surface temperature of 3499±68 K. They estimate that GJ 625 has a mass of 0.30±0.07 times that of the Sun, a radius of 0.31±0.06 times and a luminosity of 0.013±0.005 times. With a period of rotation in excess of 70 days and a low level of activity (which suggest that it is a mature star billions of years old), GJ 625 is an ideal candidate for a sensitive exoplanet search.
The Search for Planets
For their analysis, Suárez Mascareño et al. started with 151 HARPS-N spectra taken over a 3½ year period. The radial velocities were determined from these spectra using two different methods. The standard algorithm used to find radial velocities is a cross correlation function (CCF) of a HARPS spectrum with a synthetic stellar template to find the wavelength offset. With this offset, the Doppler velocity is determined which, after correcting for the motions of the Earth, yields a barycentric RV. Since the accuracy of this method suffers because of the large number of blended lines in the spectra of cool M-dwarfs, Suárez Mascareño et al. also used the TERRA data processing pipeline which employs a more sophisticated template matching algorithm to find the position of spectral features used to determine the RV.
These raw RV measurements on their own are insufficient to unambiguously detect an orbiting exoplanet. While the readily noticeable effects of starspots rotating into and out of view over time have been appreciated for decades, in more recent years it has been found that 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’?”).
In order to track the activity level of GJ 625, Suárez Mascareño et al. used the HARPS-N spectra to measure the star’s Ca II and Hα lines which are used in the calculation of standard stellar activity indices. Based on these indicators, 11 RV measurements acquired during apparent flare events were excluded. This left 140 HARPS-N measurements acquired between May 26, 2013 to September 14, 2016 for analysis. As an independent measure of any activity GJ 625 might display, photometric measurements in the standard B, V, R and I bands were acquired by the INAF-Catania Astrophysical Observatory (the modern counterpart of the institution which originally charted GJ 625 a century ago) using the EXORAP (EXOplanets RoboEc APT Photometry) program’s 80-centimeter APT2 robotic telescope located at Serra la Nave on Mount Etna. Over 100 sets of photometric measurements were secured during a 2.6 year period from May 5, 2014 to December 23, 2016.
In order to eliminate the possibility that a nearby companion (real or apparent) could be affecting the observations made of GJ 625, the star was observed on June 6, 2016 using the FastCam instrument on the 1.5-meter Carlos Sánchez Telescope in the Observatorio del Teide on Tenerife in the Canary Islands. Out of a total of 50,000 30-millisecond exposures acquired in the near infrared I-band, about 30% of the raw images were used to create a diffraction-limited image of GJ 625 with an equivalent integration time of 450 seconds. With a minimum detectable I-band magnitude difference of 4.5 to 5.0 at one arc second from GJ 625 (the equivalent of a projected distance of ~6.5 AU), no dim companions were detected in the image.
Based on a periodogram analysis of the RV and other data, Suárez Mascareño et al. found six different periodic signals. The longest, with a period of 3.4 years, was found in the activity indices and appears to be linked to the star’s magnetic activity cycle. Although significantly shorter than the 12-year cycle of the Sun, it is comparable in length to that found in similar size red dwarfs which are on the verge of being totally convective. The next longest 1.7-year signal is either a harmonic of the 3.4-year signal or the result of a “flip-flop” cycle of the spatial arrangement of active regions across GJ 625. The next group of signals found in the various data sets with periods of 74, 82 and 85 days appear to be related to the rotation of GJ 625. The spread of values could indicate differential rotation with regions closer to the poles moving more slowly. These rotation related effects create a periodic fluctuation in the RV with a semi amplitude of about 1.8 meters per second.
This leaves only one last periodic signal which is observed only in the RV data. This signal was found to have a period of 14.629±0.069 days and 14.629±0.077 days in the CCF and TERRA series, respectively, with semi amplitudes of 1.85±0.13 meters per second and 1.65±0.18 meters per second, repectively. Since there is no hint of this periodicity in the photometric data or activity indicators, the best explanation with a false alarm probability of <0.1% is an orbiting exoplanet now designated GJ 625b.
A Markov Chain Monte Carlo (MCMC) Bayesian analysis performed by Suárez Mascareño et al. of the CCF and TERRA RV results found that the best fit for the data is an exoplanet in an orbit with a semimajor axis of 0.0784 AU and a period of 14.628±0.013 days. The analysis finds that GJ 625b has a minimum mass or Mpsini of 2.82±0.51 times that of the Earth (or ME). Since the inclination of the exoplanet’s orbit with respect to the plane of the sky, i, is not known, the actual mass of this exoplanet is almost definitely larger. But given the most probable actual mass values, Suárez Mascareño et al. have labelled this new find as a “super-Earth” – a term currently with no formal scientific definition but which is typically used to describe planets larger than the Earth (but not necessarily otherwise Earth-like) yet much smaller than Neptune which has a mass of 17 ME. So while the size of GJ 625b is consistent with it being a super-Earth, what about the claim made by Suárez Mascareño et al. that their new find orbits at the inner edge of the habitable zone (HZ)? Is GJ 625b even potentially habitable?
A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state, the evolution of its volatile content and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not habitable today), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water on the planet’s surface – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.
The first step in assessing the potential habitability of the GJ 625b is to determine what sort of world it is: is it a rocky planet like the Earth or is it a volatile-rich mini-Neptune with little prospect of being habitable in a conventional sense. Unfortunately, the only information currently available about this new planet is its MPsini or minimum mass of 2.82 ME. The actual mass and the radius are needed to calculate its average density which would allow us to constrain the bulk composition of this exoplanet.
There is the possibility that GJ 625b has its orbit aligned by random chance to produce transits observable from the Earth. Such transits would allow the planet’s radius and orbit inclination, i, to be determined. Suárez Mascareño et al. calculate that there is a 1.5 to 2.5% probability that GJ 625b produces transits visible from the Earth. The detection of up to a couple of tenths of a percent decrease in brightness caused by any transits of this roughly Earth-size planet could be possible using existing ground-based instruments. Failing this, NASA’s TESS (Transiting Exoplanet Survey Satellite) spacecraft scheduled for launch in March 2018 should be able to spot any transits from GJ 625b or even other currently undetected exoplanets that might accompany it. If the orbit geometry proves favorable, at least one and likely two transits of GJ 625b could be observable during a standard 27.4-day TESS observing cycle. Suárez Mascareño et al. rightfully point out that if GJ 625b does produce transits, it also offers scientists an excellent opportunity to probe its atmosphere using transmission spectroscopy in addition to providing another data point to characterize the mass-radius relationship of super-Earths.
In lieu of this vital information, statistical arguments can be made about the probability this new exoplanet has a rocky composition. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Rogers strongly suggests that the population of known exoplanets transitions from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 times that of the Earth or RE and more likely at 1.5 RE (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While rocky planets larger than this are possible, they become more uncommon with increasing radius. A planet with a radius of 1.6 RE and an Earth-like composition would have a mass of about 6 ME. With its currently unconstrained orbit inclination, there is about a 12% chance that GJ 625b, with a minimum mass of 2.82 ME, exceeds this 6 ME threshold.
However, before we read too much into this assessment, more recent work by Chen and Kipping with a larger sample of exoplanets suggests that the gradual transition of the exoplanetary population from predominantly rocky planets to volatile-rich worlds starts at about 2 ME. This much lower threshold for the start of the transition significantly increasing the chances that GJ 625b, with only its Mpsini value known, is a mini-Neptune. Until a more quantitative estimate can be made based on an analysis of the available data, we can only state that it would seem likely that this exoplanet is rocky but there is still a fairly substantial, non-zero probability that it is a mini-Neptune instead.
Another important criterion which can be used to determine if a planet is potentially habitable is the amount of energy it receives from its parent star known as the effective stellar flux or 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 inner limit of the HZ is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO2 present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For an Earth-size planet orbiting GJ 625, this happens at an Seff value of 0.93 which corresponds to a mean orbital distance of 0.117 AU. For a more massive 5 ME exoplanet, the Seff for the inner edge of the HZ is a slightly higher 1.00 or a distance of about 0.113 AU due to the compression of the atmosphere caused by the higher surface gravity.
Because of the tight orbit of this exoplanet around GJ 625, it would be expected to be a synchronous rotator which keeps the same side pointing towards its sun. Detailed climate modeling over the last two decades shows that synchronous rotation is not the impediment to global habitability as it was once thought. In fact, it has been shown that slow or synchronous rotation can actually result in an increase of the Seff for the inner edge of the HZ owing to feedback mechanisms which result in the formation of a reflective cloud layer on the daylight side. According to the recent work by Yang et al., the inner edge of the HZ for a slow rotator orbiting a star like GJ 625 would have an Seff of 1.65 corresponding to an orbital distance of just 0.088 AU. A more recent paper by Kopparapu et al. (2016) which takes into account the effects of short orbital periods on atmospheric circulation also suggests a similar Seff value for the inner edge of the HZ for a synchronous rotators. Unfortunately, GJ 625b has an Seff of 2.1 – not only higher than these definitions for the inner edge of the HZ for synchronous rotators but it is also about 10% higher than that of Venus which is most certainly not a habitable planet.
So what is the basis of the claim made by Suárez Mascareño et al. that GJ 625b is “a super-Earth on the inner edge of the habitable zone” as stated in the title of the discovery paper? Suárez Mascareño et al. rely on surface temperature estimates they calculated based on ad hoc adjustments of the combination of planetary albedo and total atmospheric greenhouse effect using a simple model by Selsis et al. to show that the planet’s surface temperature can be kept to a 350 K temperature level. In a 2014 review of planetary habitability models by James Kasting (the “father” of our modern understanding of Earth-like habitability), Ravi Kopparapu (a former student of Kasting’s at Penn State who has continued work on habitability) and others, the approach used by Selsis et al. was criticized because of the unrealistic and arbitrary way in which the effects of clouds were included in the model. While one could easily contrive of some combination of planetary albedo and the magnitude of the greenhouse effect which could produce habitable surface temperatures, such combinations would be physically implausible if not impossible.
For example, in order to minimize the greenhouse effect, one could imagine a hyper-arid planet with an atmosphere containing no carbon dioxide and a low concentration of water vapor (which is a potent greenhouse gas) to minimize the greenhouse effect. But at the same time, global cloud coverage would be required to maintain habitability by raising the planet’s albedo to reflect excess energy away. Unfortunately, the water to produce the required clouds is not available (due to the low atmospheric water vapor content) and the atmospheric conditions at high altitudes can not support the formation of clouds even if the water vapor were available (because the temperatures never get low enough for water to condense at these high Seff values).
While there is little doubt that in terms of size, GJ 625b is certainly a super-Earth, the simple fact of the matter is that it appears that Suárez Mascareño et al. cherrypicked the most optimistic model from the peer reviewed literature to support the claim that their new discovery orbits at the inner edge of the HZ even though that model is known to involve conditions that are physically implausible. Looking at more conservative and physically realistic models of planetary climate and the definition of the HZ by Kopparapu et al., Yang et al. and others, there is no support for the claim being made by Suárez Mascareño et al. even when the admitted uncertainties of these better models are taken into account. I am afraid that it is much more likely that GJ 625b is a sweltering super-Venus than it is to be a habitable super-Earth. And when the very real possibility that GJ 625b is not a rocky world but is instead a mini-Neptune with a deep hot atmosphere of hydrogen and steam overlying exotic phases of ice which form only under high pressures and temperatures, the already poor prospects for habitability dim even further.
It appears that the claim made by Suárez Mascareño et al. in their discovery paper, “a super-Earth on the inner edge of the habitable zone of the nearby M-dwarf GJ 625”, is not entirely true. While the HADES RV team’s new discovery is most likely a “super-Earth” in terms of its size, in my opinion it does not orbit on the inner edge of the habitable zone (HZ). This latter claim made by Suárez Mascareño et al. is based on an overly optimistic definition of the inner edge of the HZ which relies on physically implausible circumstances. With an effective stellar flux 10% greater than that of Venus in our own solar system, it is much more likely that GJ 625b is a “super-Venus” orbiting well outside of the HZ. Even though this claim made by Suárez Mascareño et al. seems to have no merit, GJ 625b is nonetheless an important find. Just like the super-Venus recently found by the Lick-Carnegie Exoplanet Survey orbiting the nearby Lalande 21185 (see “Our New Neighbor Orbiting Lalande 21185”) and similar worlds turning up in other surveys, the study of GJ 625b will add much to our knowledge about this important class of exoplanet as well as help astronomers probe the limits of planetary habitability.
While it seems improbable that GJ 625b is habitable, there is still the possibility that there are potentially habitable planets orbiting this star that have gone undetected in the current work by Suárez Mascareño et al. A study of red dwarf planetary systems observed by NASA’s Kepler mission strongly suggests that half of such systems contain several planets in coplanar orbits (see “Architecture of M-Dwarf Planetary Systems”). Suárez Mascareño et al. claim that other planets producing RV variations less than ~1.5 meters per second in orbits with periods less than ~3 years would have escaped detection with the data they currently have available. This would include exoplanets with MPsini values less than ~3 to ~4 ME orbiting inside of the HZ as defined by Kopparapu et al. (2013, 2014, 2016). However, Suárez Mascareño et al. also note that the observed effects of stellar rotation will make the detection of low amplitude RV signals from exoplanets with orbital periods in the 35 to 120 days difficult – a range which covers the outer part of the HZ and beyond. Despite the difficulties, future observations using the RV method, photometric observations looking for transits or other exoplanet detection techniques may still reveal more exoplanets orbiting with GJ 625b including potentially habitable Earth-size worlds.
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“Our New Neighbor Orbiting Lalande 21185”, Drew Ex Machina, February 18, 2017 [Post]
“Abundance of Venus Analogs”, Drew Ex Machina, September 15, 2014 [Post]
“GJ 832c: Habitable Super-Earth or Super-Venus”, Drew Ex Machina, June 27, 2014 [Post]
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