The year 2019 is proving to be a fruitful one for the discovery of exoplanets orbiting nearby stars especially our smallest neighbors, red dwarfs. On August 13, 2019 a preprint of a paper submitted for publication in the Monthly Notices of the Royal Astronomical Society with Stefan Dreizler (University of Göttingen) as the lead author announced the discovery of a trio of exoplanets orbiting the nearby red dwarf GJ 1061 just a dozen light years away. Most exciting of all, one of these three newly discovered worlds seems to orbit comfortably inside the habitable zone (HZ) of this star prompting much interest from astronomers as well as groups who are looking for targets for future interstellar missions, not to mention exoplanet enthusiasts around the world. So, what are the prospects for the potential habitability of these new exoplanets given what we now know about them and planetary habitability?

Background

GJ 1061 is a V magnitude 13.1 red dwarf located in the southern constellation of Horologium – the Pendulum Clock. The star was first cataloged by the Dutch-American astronomer Willem Jacob Luyten (1899-1994) and appeared with the number L 372-58 in his Proper Motion Survey with the forty-eight inch Schmidt Telescope published in 1941. The star caught Luyten’s attention because of its 0.83 arc second per year proper motion and received the designation LHS 1565 in the Luyten Half-Second Catalogue published in 1979. Because of a distance estimate of about 25 light years based on photometry obtained in 1974, the star was included in the second edition of the Gliese Catalogue of Nearby Stars published in 1979 by Wilhelm Gliese (1915-1993) and his collaborator, Hartmut Jahreiß, earning it the designation GJ 1061.

The first trigonometric parallax measurement for GJ 1061 was not published until 1997 as part of the ongoing RECONS (Research Consortium on Nearby Stars) effort. It was found to be much closer than previously estimated making it the 20th closest known star system. The current distance measurement based on Gaia data places the system at a distance of 11.980±0.003 light years. A summary of the best current values for this star’s key properties are listed in the table below.

Properties of GJ 1061

Spectral Type M5.5V Surface Temperature 2953±98 K Mass (Sun=1) 0.12±0.01 Radius (Sun=1) 0.156±0.005 Luminosity (Sun=1) 0.0017±0.0001 Rotation Period (days) ~130 Age (Gyr) >7.0±0.5 Distance (LY) 11.98

Overall, the properties of GJ 1061 are similar to those of Proxima Centauri (see the Proxima Centauri page). The major difference between the two is the much lower level of stellar activity observed on GJ 1061. Combined with the slow rotation, GJ 1061 is estimated to be in excess of about seven billion years old. While substantially older than the Sun, it is still only a small fraction of the main sequence lifetime of this small star which would be measured in trillions of years.

The Search for Planets

Like other nearby red dwarfs, GJ 1061 has been considered a good target for exoplanets. Direct imaging searches and astrometric measurements performed up to this decade had effectively eliminated the possibility that GJ 1061 had any super-Jovian or larger companions with orbital periods greater than about a year. But that still left plenty of room for planets which were smaller and in tighter orbits.

Because of its low level of stellar activity, GJ 1061 is considered a good target for searches using precision radial velocity (RV) measurements to detect the reflex motion of orbiting planets especially those with short periods. As part of a campaign to survey nearby red dwarfs for RV variations indicative of orbiting planets, GJ 1061 was observed using 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. The results based on four sample RV measurements published in 2013 with Xavier Bonfils (then with Observatoire de Genève) as the lead author showed an RMS variation of just 4.2 meters per second – consistent with the calculated internal measurement errors. The HARPS team continued to make RV measurements of GJ 1061 after the results of Bonfils et al. were published as part of their long term survey effort with the precision of those measurements improving after an optical fiber upgrade was made in May 2015 which improved instrument stability.

The results of another RV survey were published in 2014 with J.R. Barnes (University of Hertfordshire – UK) as the lead author. The Red Optical Planet Survey (ROPS) made five precision RV measurements of GJ 1061 made in late July 2012 using the UVES (Ultraviolet and Visual Echelle Spectrograph) on the 8.2-meter Very Large Telescope (VLT) UT2 in Cerro Paranal, Chile. These more sensitive results showed an RMS variation in the RV of just 2.4 meters per second. The result confirmed that GJ 1061 was exceptionally quiet and did not possess any large exoplanets in short-period orbits.

In 2018, GJ 1061 became a target of the ongoing Red Dots collaboration along with the nearby red dwarfs, Lacaille 9352 (Gl 887) and YZ Ceti (Gl 54.1). Red Dots is an outgrowth of the Pale Red Dot campaign of 2016 which discovered Proxima Centauri b (see “Habitable Planet Reality Check: Proxima Centauri b”). Starting in 2017, the new and expanded Red Dots campaign selected nearby red dwarfs for a season’s worth of intensive observations which are capable of spotting Earth-mass exoplanets in short period orbits. Unlike other exoplanet surveys which take data over long periods of time, Red Dots makes nearly daily RV measurements using HARPS over a season at the same time partner observatories are performing photometry to help differentiate stellar activity from an actual planet. During the 2017 observation season, Red Dots continued observing Proxima Centauri but also added Barnard’s Star and Ross 154 (see “Red Dots: The Search for Nearby Earth-Size Exoplanets”). Although no new planets were detected with certainty, the 2017 campaign did confirm the presence of Proxima Centauri b and provided fresh hints of a more distantly orbiting exoplanet (see “Proxima Centauri b One Year Later: The Search for More Exoplanets Continues”).

The results of the Red Dots observation campaign of GJ 1061 are described in a paper submitted for publication in the Monthly Notices of the Royal Astronomical Society with Stefan Dreizler (University of Göttingen) as the lead author. Dreizler et al. started with a total of 114 publicly archived HARPS RV measurements 107 of which were obtained after the HARPS upgrade in 2015. An additional 54 nights of HARPS measurements made between July and September 2018 as part of the Red Dots campaign significantly enlarging the available data set with homogeneous, high-cadence measurements. Contemporaneous photometry in support of Red Dots was obtained from three sources: the 0.4-meter ASH2 (Astrograph for the South Hemisphere II) telescope at the San Pedro de Atacama Celestial Explorations Observatory in Chile, the 1.2-meter MONET-South (MOnitoring NEtwork of Telescopes) telescope located in Sutherland, South Africa and the AAVSO (American Association of Variable Star Observers) network of amateur astronomers. Also available was about 53 days of high cadence photometry from NASA’s ongoing TESS (Transiting Exoplanet Survey Satellite) mission acquired during the Red Dots campaign as well as archived data from MEarth transiting exoplanet search program during 2016 and 2017 and ASAS-SN (All-Sky Automated Survey for Supernovae) made from 2014 to 2017.

Analysis of the RV data set by Dreizler et al. clearly showed three periodic signals which did not correspond to any observed in the photometry or activity indices derived from the HARPS spectra. The first two signals had periods of 3.204±0.001 and 6.689±0.005 days with semiamplitudes of 2.43±0.24 and 2.48 +0.28/-0.29 meters per second, respectively. A third signal with a period of 13.031 +0.025/-0.032 days and a semiamplitude of 1.86±0.25 meters per second was also found. However, because of aliasing caused by sampling of the data, this third signal is currently indistinguishable from one with a period of 12.434 +0.031/-0.023 days and a semiamplitude of 1.76±0.28 meters per second. The longer period 13.03-day period is favored since it seems these three exoplanets form a resonance chain to a ratio of 1:2:4 (and twice the orbital period of the second planet is 13.38 days). More data will be required to resolve this issue. The observed RV variations and the coherence of the signals across the Red Dots and earlier HARPS data sets are consistent with the presence of three exoplanets moving in low-eccentricity orbits.

Using the properties of GJ 1061, it is possible to derive the properties of these new exoplanets designated GJ 1061b, c and d. These properties are summarized along with their uncertainties in the table below. The properties of GJ 1061c are based on the assumption of the longer period of 13.03 days (which, given the uncertainties in the parameters, are almost indistinguishable from those with a 12.43 day period). Also included in the table is the effective stellar flux, S eff , which provides a measure of how much energy the planet receives from its sun.

GJ 1061 Exoplanet Properties

Planet b c d Orbit Period (days) 3.204 ±0.001 6.689 ±0.005 13.031 +0.025/-0.032 Orbit Semimajor Axis (AU) 0.021 ±0.001 0.035 ±0.001 0.052 ±0.001 Orbit Eccentricity <0.31 <0.29 <0.54 M P sini (Earth=1) 1.38 +0.16/-0.15 1.75 ±0.23 1.68 +0.25/-0.24 S eff (Earth=1) 3.8±0.7 1.4±0.2 0.6±0.1

Since the inclination, i, of the orbits of these exoplanets to the plane of the sky cannot be derived directly from RV measurements, only the minimum mass or M P sini values can be derived with the actual mass, M P , likely being larger.

In addition to the three periodic signals, a long term trend in the RV amounting to 1.8±0.4 meters per second per year was also observed hinting at the presence of a more distant exoplanet in a long-period orbit. More data over a longer observational baseline will be required to confirm its presence. A fourth periodic signal with a possible period of about 53 days was also detected – about a factor of four longer than the orbital period of GJ 1061d and in keeping with the apparent 1:2 resonance chain of the system.

However, based on the analysis of the photometry and activity indices which show a series of periodic signals in the 50 to 130 day range, it cannot be ruled out that this signal is a result of stellar activity modulated by the star’s rotation. More high-cadence data over a five month baseline will be needed to determine the origin of the signal. Dreizler et al. state that sub-Earth-mass exoplanets with periods in excess of about 20 days would be undetectable in the current data set but could be revealed with further precision RV measurements.

Potential Habitability

So, what are the habitability prospects for these new exoplanets? 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 the newly discovered exoplanets orbiting GJ 1061 is to determine what sort of worlds they are: are they rocky planets like the Earth or volatile-rich mini-Neptunes possessing deep hot atmospheres 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 the planets of GJ 1061 that could help make such an assessment are the M p sini or minimum mass values. The actual masses and the radii of these worlds are needed to calculate their density and help constrain their bulk compositions.

With such tight orbits around their sun, GJ 1061b, c and d have a probability of 3.5%, 2.1% and 1.3%, respectively, that the planes of their orbits are oriented by random chance to produce transits observable from Earth. Such transits could be used to pin down not only the orbit inclination, i, allowing the actual planet mass to be determined, but also the exoplanet’s radius. Dreizler et al. were able to search the 27,602 brightness measurements made of GJ 1061 by TESS looking for transits with the same periods as the exoplanets they found. With depths expected to be in the 3 to 40 millimagnitude range (depending on the actual mass and composition of these planets), such transits should be easily spotted. Unfortunately, no transits corresponding to periods of about 3, 7 and 13 days were observed.

Until we get the data needed to determine the bulk densities of these new finds, statistical arguments can be made about the probability they have 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 . Even with their currently unconstrained orbit inclinations, the chances that the actual masses of GJ 1061b, c and exceed this 6-M E threshold are only around 2.6%, 4.3% and 4.0%, respectively. However, Dreizler et al. performed an analysis of the stability of this three-planet system and found that their actual masses, M P , cannot be greater than three to four times their M P sini values otherwise the system would be unstable. In this case, none of these three exoplanets exceed the 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 . The chances that GJ 1061b, c and d exceed this threshold are somewhere around 25%, 45% and 40%, respectively, keeping in mind the maximum mass limits found by Dreizler et al.. This suggests that these newly discovered exoplanets still have a small chance of being mini-Neptunes. Until a more quantitative estimate can be made based on an analysis of the available data, it seems likely (but not certain) that all three of these exoplanets are rocky planets with a bulk composition more similar to Earth’s than Neptune’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 outer limit of the HZ is conservatively defined by the maximum greenhouse limit beyond which a CO 2 -dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO 2 beyond this point makes the atmosphere more opaque causing the surface temperatures to drop instead of increase. Kopparapu et al. (2013, 2014) suggests an S eff value of about 0.22 for the outer limit of the HZ of GJ 1061 corresponding to a mean orbital distance of 0.087 AU. The semimajor axes of the orbits of all three newly discovered exoplanets orbiting GJ 1061 are comfortably less than this outer limit.

Kopparapu et al. (2013, 2014) conservatively define the inner edge of the HZ 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 its water in a geologically brief time in the process. For an Earth-size planet orbiting GJ 1061, this happens at an S eff value of 0.86 which corresponds to a mean orbital distance of 0.044 AU. With a S eff calculated to be about 0.6, GJ 1061d orbits well beyond this limit and comfortably within this system’s HZ. GJ 1061c, with an S eff of 1.4, and especially GJ 1061b, with an S eff of 3.8, orbit too closely to their sun to be considered habitable by this definition.

Because of the tight orbits of the GJ 1061’s exoplanets and their age, they would be expected to be synchronous rotators which keep the same side pointing towards their sun. Detailed climate modeling over the last two decades has shown 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. A study by Kopparapu et al. (2017) which incorporates not only the latest data of how key greenhouse gases transmit and absorb infrared radiation but the effects of short-period synchronous rotation on cloud formation as well suggests that the inner limit of the HZ for Earth-sized synchronous rotators orbiting GJ 1061 would be at an S eff of around 1.16 corresponding to a mean orbital distance of 0.038 AU. GJ 1061b definitely orbits well outside of this definition of the HZ and is likely to be a hotter, super-sized version of Venus (assuming that it has a more Earth/Venus-like composition). Unfortunately, GJ 1061c seems to orbit just beyond the edge of the HZ. While there is a chance that, after getting a better grip on the properties of this exoplanet and its sun, we could find GJ 1061c orbits the inside inner part of the HZ, it seems more likely it is a larger, albeit cooler, version of Venus.

This leaves us with GJ 1061d as being the best bet for a potentially habitable exoplanet in this system. And just as GJ 1061 is similar to Proxima Centauri, their potentially habitable exoplanets are also similar as well – they have about the same mass and S eff values. While GJ 1061, unlike Proxima Centauri, is a relatively quiescent red dwarf today, various forms of elevated activity it would have surely experienced earlier in its life would have had a major impact on the evolution of its planets’ volatile inventories including water. Detailed modelling of the volatile inventories of these worlds validated by observations of these or similar exoplanets will be needed to determine just how “habitable” GJ 1061d is. Given the uncertainties in the various red dwarf HZ models currently available, Dreizler et al. characterize their new find as a “temperate planet” instead of a “habitable planet”.

Summary

The discovery of exoplanets orbiting the GJ 1061 just a dozen light years away offers astronomers yet another opportunity to study a nearby planetary system – the tenth closest currently known. In addition, GJ 1061d appears to meet the basic criteria for being potentially habitable – the second closest such world currently known after Proxima Centauri b whose overall properties are similar. Further examination of this new worlds should contribute to our understanding of planetary habitability in red dwarf systems. Future study of this system should not only help refine our knowledge of these three exoplanets’ properties but also promises to uncover another one or two (or even more) exoplanets in this system. As Red Dots and other ongoing exoplanet search programs continue, we should find even more such systems among our neighbors.

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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

J R. Barnes et al., “Precision radial velocities of 15 M5-M9 dwarfs”, Monthly Notices of the Royal Astronomical Society, Vol. 439, pp. 3094-3133, February 20, 2014

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”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

S Dreizler et al., “Red Dots: A Temperate 1.5 Earth-mass planet in a compact multi-terrestrial planet system around GJ 1061?”, arXiv 1908.0o4717 (submitted to Monthly Notices of the Royal Astronomical Society), August 13, 2019 [Preprint]

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., “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

“Red Dots #2 starts July 8th, contributions welcome”, Red Dots website, July 8, 2018 [Announcement]