The year 2017 is certainly proving to be a fertile one for the discovery of potentially habitable exoplanets. Just a year ago there were maybe five exoplanets identified as having genuinely good prospects of being potentially habitable (see “Top Five Known Potentially Habitable Planets”) and only one of them, Proxima Centauri b discovered in 2016, was relatively nearby (see “Proxima Centauri b: The Search for More Exoplanets Continue”). Over the last several months, that list has expanded significantly as a result of a number of ongoing surveys taking place around the globe and in space. New additions to the list of nearby potentially exoplanets from 2017 include possibly three out of the seven exoplanets found orbiting TRAPPIST-1 along with individual exoplanets found orbiting GJ 273 and LHS-1140 (see “Habitable Planet Reality Check: The Seven Planets of TRAPPIST-1”, “Habitable Planet Reality Check: The Nearby GJ 273 or Luyten’s Star” and “Habitable Planet Reality Check: A Super-Earth Orbiting the Nearby LHS 1140”). While still far too distant to reach with today’s technology, these nearby exoplanets would be potential targets of exploration if interstellar travel proves to be practical in the future.

Now the European team of astronomers operating the HARPS (High Accuracy Radial velocity Planet Search) spectrograph attached to the European Southern Observatory’s (ESO’s) 3.6-meter telescope in La Silla, Chile have announced the discovery of yet another potentially habitable exoplanet as a result of their long-term survey of nearby stars. In a paper to be published in the peer-reviewed European astronomical journal, Astronomy & Astrophysics, with Xavier Bonfils (currently at Université Grenoble Alpes) as the lead author, the HARPS team describes their latest find – a temperate, roughly Earth-mass object found orbiting the nearby star commonly known as Ross 128. So what are the prospects for the potential habitability of this new exoplanet given what we now know about it?

Background

The star Ross 128 is an V magnitude 11.1 star located in the constellation of Virgo – The Virgin. Its common name is derived from being the 128th star cataloged by American astronomer Frank E. Ross (1874-1960) during his early work at the Yerkes Observatory while searching for and characterizing dim variable stars. First appearing as part of the fourth installment of Ross’ catalog published in 1926, subsequent work showed it to be an intrinsically dim, nearby red dwarf star with a distance currently pegged at 11.02±0.02 light years based on the initial astrometric measurements from the ESA Gaia mission. This makes Ross 128 the 12th closest known star system to the Sun. Because of its closeness, Ross 128 was included in the first edition of the Gliese Catalogue of Nearby Stars in 1957 earning it the designation of GJ 477 after the creator of the catalog, German astronomer Wilhelm Gliese (1915-1993), and his long time collaborator on later editions, Hartmut Jahreiß.

According to the best data available on Ross 128 compiled by Bonfils et al., this spectral type M4V red dwarf has a radius of 0.197±0.008 times that of the Sun and a surface temperature 3192±60 K. The luminosity is calculated to be 0.0036±0004 times that of the Sun and the mass is estimated to be 0.17±0.02 times. Occasional flares have been noted for this star over the decades which can dramatically increase its brightness for periods of several minutes earning it the variable star designation of FI Virginis. But based on a detailed analysis of its activity compared to other red dwarf stars, Ross 128 seems to be among the least magnetically active red dwarfs currently known suggesting that it is a fairly evolved object. This low level of activity combined with its Sun-like metallicity, its orbit around the galaxy and its long rotation period estimated to be about 121 days all suggest an age well in excess of five billion years.

Like many nearby stars, Ross 128 has been the target of exoplanet searches for decades. The most sensitive search results previously published were from an analysis of HARPS radial velocity (RV) measurements published in 2013 again with Xavier Bonfils (then with Observatoire de Genève) as the lead author. With only a half dozen measurements available at the time, the star’s RV seemed to vary on the order of a meter per second suggesting that the reflex motion of an exoplanet orbiting Ross 128 was being observed although it was impossible to claim a definitive exoplanet detection or characterize its properties with so little data. With this promising start, additional precision RV measurements were made by the HARPS team over the following years.

For their most recent work, Bonfils et al. started with a total of 157 precision RV measurements made of Ross 128 using HARPS between July 2, 2005 and April 26, 2016. Because of an upgrade to the HARPS spectrograph’s fiber optic feed introduced in May 2015 which significantly altered the line spread function (as well as improve the instrument measurement stability), the two parts of the RV data were processed separately using the team’s proven set of data reduction tools to find a clear signal with a period of about 9.9 days. Detailed modelling of the data assuming various sources of natural and instrument noise found a well-sampled periodic signal in the RV measurements with a semiamplitude of 1.7 meters per second consistent with the presence of an exoplanet in a circular orbit with a period of 9.86 days. Plugging in the properties of the star itself yields a mean orbital radius of 0.049 AU (just 7.3 million kilometers) and a M P sini of 1.35 times that of the Earth (or M E ) for the exoplanet. Since the inclination of its orbit to the plane of the sky, i, is not currently known, the actual mass, M P , of what is now designated Ross 128b will almost surely be larger. The amount of energy Ross 128b receives from its host star, the effective stellar flux of S eff , is about 1.38 times that of the Earth based on these derived properties.

In order to help eliminate the possibility that the observed variations in RV were not some form of stellar activity mimicking a planetary signature, Bonfils et al. also analyzed two sets of photometric data for Ross 128. They secured over nine years of ground-based V-band brightness measurements made by the All Sky Automated Survey (ASAS) and 82 days of almost continuous precision photometry obtained by NASA’s Kepler spacecraft as part of Campaign 1 of its extended K2 mission running from June to August 2014 (see “The First Year of Kepler’s K2 Mission”). There was no hint of any photometric variability corresponding to the ten-day period of Ross 128b strengthening the case that this is a planet.

After taking into account the effects of Ross 128b on the precision RV measurements, the only other significant signals were found to have periods of about 123 and 52 days. The ASAS photometry of Ross 128 clearly displays regular variations with a period of 121 days corresponding to the rotation of the star (the K2 photometry could not used in this assessment since it did not cover a complete rotation). The 123-day periodicity in the RV data is clearly associated with stellar activity modulated by the rotation of the star with the slight variance with 121-day period found in the photometry probably being due to differential rotation. The 52-day periodicity is explained as aliasing of half the 123-day signal caused by the annual gaps in the HARPS data set. While no other convincing exoplanet candidates can be identified in the current set of RV measurements, there could easily be additional members of this system awaiting discovery.

Potential Habitability

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 – 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 Ross 128b 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 possessing a deep hot atmosphere dominated by hydrogen overlaying layers of exotic high temperature ices with little prospect of being habitable in an Earth-like sense? Unfortunately, the only information currently available about this new exoplanet that could help make such an assessment is its M p sini or minimum mass value. The actual mass and the radius of Ross 128b are needed to calculate this exoplanet’s density and help constrain its bulk composition.

With such a tight orbit around its sun, Ross 128b has about a 2% probability that the plane of its orbit is oriented by random chance to produce transits observable from Earth. Such transits could be used to pin down the orbit inclination, i, allowing the actual planet mass to be determined as well as measure the exoplanet’s radius. Unfortunately, there were no hints of such transits found in Kepler’s K2 photometry from 2014. Any transiting exoplanet passing directly in front of Ross 128 as viewed from our solar system with a radius larger than 0.19 times that of the Earth would have been detected to a 99% confidence level. Grazing transits of exoplanets closer to Earth in size have also been excluded to high confidence.

Bonfils et al. openly discuss the possibility of using the 39-meter European Extremely Large Telescope (E-ELT) currently under construction for ESO on top of Cerro Armazones in northern Chile to observe Ross 128b after it is commissioned in 2024. The large size of this telescope combined with the latest adaptive optics technology (no to mention the superb seeing in Chile’s Atacama Desert) should allow Ross 128b to be resolved at its maximum elongation distance of just 15 milliarc seconds from its host star. Combined with an appropriately designed high spectral dispersion instrument which allows selection of bands to improve the contrast ratio (i.e. the ratio of the apparent brightness of the host star and its orbiting planet) and ease detection through the glare of the host star, Bonfils et al. believe that Ross 128b could be directly detected almost as easily as Proxima Centauri b (an exoplanet that has already been studied in detail as a prospective future E-ELT target).

In addition to providing vital spectral information about the world, the orbit inclination could be determined using E-ELT observations. If E-ELT proves incapable of making the required observations, 10+ meter-class space-based telescope built with features specifically to support exoplanet detection should be able to make the required observations in the next decade. While these sort of observations can be used to help constrain the radius of Ross 128b, direct measurements of its size will likely require a space-based interferometer of the sort that will probably not be available until mid-century.

While we wait until measurements such as these come from E-ELT or other proposed instruments in the next decade, 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 R E but more likely at 1.5 R E (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 R E and an Earth-like composition would have a mass of about 6 M E . With its currently unconstrained orbit inclination, there is about a 3% chance that Ross 128b, with a M P sini of 1.35 M E , exceeds this 6 M E threshold.

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 M E . There is about a 26% chance that Ross 128b exceeds this threshold suggesting that it has a small but non-zero chance of being a mini-Neptune. Until a more quantitative estimate can be made based on an analysis of the available data, it seems likely (but not certain) that Ross 128b is a rocky planet with a bulk composition probably not too dissimilar from Earth’s.

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 S eff . 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 CO 2 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 Ross 128, this happens at an S eff value of 0.93 which corresponds to a mean orbital distance of 0.063 AU. With a S eff calculated to be 1.38, Ross 128b orbits too close to its sun to be considered habitable by this definition. However, there are other definitions for the HZ worth considering for this case.

Because of the tight orbit of Ross 128b and its age, 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 S eff 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 Ross 128 would have an S eff of 1.58 corresponding to an orbital distance of just 0.048 AU. This places Ross 128b comfortably inside the HZ for synchronous rotators.

But before we invest too much into this result, a more recent paper by Kopparapu et al. (2016) which takes into account the effects of short orbital periods on atmospheric circulation also suggests that the feedback mechanism that maintains the reflective cloud layer on the daylight side starts to breakdown for synchronously rotating exoplanets in tight orbits. This is due to the Coriolis effect which breaks up this dayside cloud layer resulting in a moist runaway greenhouse effect at lower S eff values than found by Yang et al.. For a cool star like Ross 128, the S eff for the inner edge of the HZ would be about 1.23. An even more recent paper by Kopparapu et al. (2017) which incorporate the latest data of how key greenhouse gases transmit and absorb infrared radiation suggests that changes in the atmospheric structure can lead to rapid and permanent water loss for an Earth-size exoplanet at an S eff of around 1.20 even before a runaway greenhouse effect sets in. Although Ross 128 is a relatively quiescent red dwarf today, various forms of elevated activity it would have surely experienced earlier in its life would be yet another loss mechanism that would exacerbate the situation. With the permanent loss of water, the carbonate-silicate cycle which helps act as a global thermostat breaks down allowing CO 2 to build up in the atmosphere resulting in a dry runaway greenhouse much as Venus experiences today in our own solar system.

Given the uncertainties in the various red dwarf HZ models currently available, it seems that it is a toss up as to whether or not Ross 128b orbits inside the HZ although the situation does not appear especially promising at the moment. For this reason, Bonfils et al. characterize Ross 128b as a “temperate planet” instead of a “habitable planet”. There are possible “temperate” scenarios where an exoplanet could be stripped of most of its volatiles (including excess amounts of greenhouse gases like water and CO 2 ) early in its evolution leaving an arid desert world which might have some environments which could support life – essentially hot versions of Mars sharing its thin atmosphere and hyperarid surface conditions. But such scenarios, if they prove to be physically plausible, would probably start deviating too much from Earth-like habitability being considered in this assessment. More detailed models specifically for Ross 128b will surely become available in the near future to address this exoplanet’s possible evolutionary paths and shed more light on its potential habitability.

Summary

The discovery of Ross 128b is especially exciting since it provides astronomers with yet another opportunity to study a nearby exoplanet in detail with the next generation of astronomical instruments. Based on what little we know at this time about this newly discovered exoplanet, the odds seem to favor it being a rocky world like the inner planets of our own solar system. But given the high effective stellar flux and the current uncertainties in the various HZ models for red dwarfs, the best we can hope to claim at this stage is that this newly discovered exoplanet orbits somewhere near the inner edge of the HZ – maybe just inside the HZ as a warm yet habitable exoplanet but maybe more likely just outside the HZ to become a non-habitable Venus-like world.

Based on this assessment, it seems that Ross 128b has only moderate chances of being potentially habitable in an Earth-like sense – certainly not as good as the prospects for red dwarf exoplanets like Proxima Centauri b, GJ 273b or LHS-1140b but definitely better than those for the recently discovered GJ 625 (see “Habitable Planet Reality Check: Is GJ 625b a Super-Earth or a Super-Venus”). Fortunately, future observations of Ross 128b will help provide vital data for its current state allowing HZ models to be validated. And there still remains the possibility of additional exoplanets in this system including in more distant orbits comfortably inside of the habitable zone. Regardless of whether or not Ross 128b is potentially habitable, it is most definitely an exoplanet worthy of further detailed study.

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Related Video

This brief ESO video provides an artist’s impression of what Ross 128b might look like.

Related Reading

For a complete collection of articles about our other neighboring star systems and the searches for exoplanets orbiting them, see Drew Ex Machina’s page on Nearby Stars.

General References

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

Xavier Bonfils et al., “A Temperate exo-Earth around a quiet M dwarf at 3.4 parsecs”, arXiv 1711.06177 (accepted by Astronomy & Astrophysics), November 16, 2017 [Preprint]

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

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

Ravi Kumar Kopparapu et al., ” Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs”, The Astrophysical Journal, Vol. 845, No. 1, Article ID. 5, August 2017

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

Jun Yang et al., “Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate”, The Astrophysical Journal Letters, Vol. 787, No. 1, Article id. L2, May 2014

“Closest Temperate World Orbiting Quiet Star Found”, ESO Press Release 1736, November 15, 2017 [Press Release]