Habitable Planet Reality Check: GJ 581d

Back in July 2014, the press was filled with stories about the apparent disappearance of two potentially habitable extrasolar planets thought to be orbiting the nearby star, GJ 581 (see “The Disappearing Habitable Planets of GJ 581”). Of course these planets did not literally disappear. Instead it was found that the subtle variations in the radial velocity of GJ 581 used to infer the presence of as many as six planets was instead probably due to only three planets combined with natural noise or “jitter” from the star itself.

As happens so often in science, this interpretation has been questioned raising the possibility that at least one of these planets, GJ 581d, may exist after all. And as happens so often in the media today, press reports about the situation are frequently incomplete and laced with hype claiming that GJ 581d is “Earth-like” and even “habitable”. In this essay, the history of the controversy surrounding GJ 581d since its discovery in 2007 is reviewed as are its prospects for being potentially habitable.

Background

GJ 581 is a type M3V dwarf star located 20.3 light years away in the constellation of Libra. This fairly ordinary red dwarf has an estimated mass of 0.31 times that of the Sun, a radius of 0.29 times and a luminosity of just 0.013 times. Like most nearby stars, GJ 581 has been part of various surveys searching for extrasolar planets using a range of techniques. In 2005, the European team using the HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph on ESO’s 3.6-meter telescope at the La Silla Observatory in Chile announced the discovery of a planet orbiting GJ 581 with a period of 5.4 days and a minimum mass of 17 times that of the Earth (or M_E). As it would turn out, this would be only the first of many discovery announcements to come.

Two years later, the HARPS team announced the detection of an additional pair of planets: GJ 581c in a 12.9-day orbit with a minimum mass of 5 M_E and GJ 581d in a 83-day orbit with a minimum mass of about 8 M_E. It was further claimed in the discovery paper by Udry et al. that GJ 581c orbited at the inner edge of the habitable zone of that system while GJ 581d orbited at the outer edge making each of these worlds potentially habitable planet. However, an analysis of this system by Selsis et al. published not long afterwards concluded that the habitability claim of the former was overstated. GJ 581c receives about 30% more energy than Venus does from our Sun and their modelling found that it is unlikely to be habitable. This planet is much more likely to be an especially large version of Venus.

In 2009, the HARPS team announced the discovery of yet another planet in this system: GJ 581e. This new work by Mayor et al. allowed them to further refine the properties of the other planets especially GJ 581d which now seemed to be in a moderately eccentric orbit with a period of 67 days and a slightly lower minimum mass of about 7 M_E. Normally, radial velocity measurements alone are incapable of determining the actual mass, M_P, of a planet since the inclination of the orbit to the plane of the sky, i, is unknown and must be found by other means. As a result only the minimum mass or M_Psini of the planet can be determined. By performing dynamical simulations of the stability of this four-planet system, Mayor et al. were able to constrain the actual masses of the planets as being no more than about 1.6 times the measured minimum mass assuming the orbits were approximately coplanar.

Diagram showing the 6-planet system of GJ 581 as known in 2010 and an optimistic view of the habitable zone compared to our Solar System. Click on image to enlarge. (ESO/Henrykus)

In September 2010, a team of astronomers led by Steven Vogt claimed to have discovered two additional planets, GJ 581f and g, by combining their data acquired using the HIRES spectrograph as part of the Lick-Carnegie Exoplanet Survey at the Keck Observatory with the published HARPS data. GJ 581g, with a minimum mass of 3.1 M_E, was discovered in a 36.6-day orbit comfortably inside this system’s habitable zone prompting much interest from habitable planet enthusiasts. However, almost immediately doubts were cast on the existence of these last two planets. The HARPS team, in a paper submitted for publication by Forveille et al., found no evidence for GJ 581f and g in the analysis of their new expanded data set.

More Doubts

While other teams performed their own analyses of the data which cast doubt on the existence of GJ 581f and g, one of them also called into question the detection of GJ 581d on the outer edge of the habitable zone. Roman Baluev (Pulkovo Observatory) performed an independent analysis of the HARPS and HIRES data sets which was published in March 2013. He found that the data contained an important correlated noise component which produced what he characterized as “misleading effects” in the earlier analyses. Using a different analysis technique that better accounted for the type of noise present in the data, he was able to definitively confirm the existence of GJ 581b, c and e. Unfortunately, he found that GJ 581f and g were likely just illusions caused by the noise he identified and that the reality of GJ 581d was questioned.

Serious doubt was cast on the existence of GJ 581d in an analysis by a team led by Paul Robertson (Penn State) which was originally published on-line by Science on July 3, 2014 (and subsequently published in their print-edition on July 25). Instead of looking at just radial velocity measurements alone as had been done in the previous analyses, they examined the 239 publicly available HARPS spectra of GJ 581 from which these measurements were originally derived. While the HARPS team had looked for the signature of star spots and other obvious signs of stellar activity that have been known for decades to generate signals that can mimic those of extrasolar planets, this new analysis was looking for much more subtle variations in magnetic surface activity by examining the hydrogen-α emissions of GJ 581.

An artist’s depiction of star spots on a red dwarf star orbited by a planet. (David A. Aguilar – CfA)

Robertson’s team found that the radial velocity signature for GJ 581d seemed to be strongly correlated with the star’s magnetic activity.  In other words, when magnetic activity was high, the apparent radial velocity signature for GJ 581d was strong,  When magnetic activity was low, the signature for GJ 581d was weak. When the team corrected the radial velocity measurements for the subtle effects of the observed magnetic surface activity in GJ 581, they found that the signal for GJ 581d had decreased to just 1.5 times the noise level in the data or half the value typically required to constitute a statistically significant detection.

Coupled with the fact that the new rotation period the Robertson et al. had derived for GJ 581 of 130±2 days was almost exactly twice that of the reported 66-day orbital period of GJ 581d, the radial velocity signal corresponding to the planet was most likely just a harmonic of the star’s rotationally modulated magnetic activity signal. The residual signal left after accounting for stellar activity was likely due to an imperfect correction for the magnetic activity they detected probably caused by shifts in the location of surface activity across the star’s surface over time which would slightly alter its Doppler signature. Another casualty of this correction process was GJ 581g whose radial velocity signature completely disappeared in the reprocessed data. No sign of GJ 581f was found in their analysis. Robertson et al. would go on to use the same technique to demonstrate that some of the planets of the red dwarf GJ 667C were also stellar activity masquerading as planets (see “Habitable Planet Reality Check: GJ 667C”).

Eight months after the publication of the analysis by Robertson et al., a formal comment by HARPS team members Guillem Anglada-Escudé and Mikko Tuomi was published in Science. Anglada-Escudé and Tuomi questioned the statistical rigor of the analysis used by Robertson et al. pointing out that their analysis method was “inappropriate and promotes inadequate tools”. Specifically, Anglada-Escudé and Tuomi took issue with the use of periodogram analysis (which looks for periodic signals in data such as radial velocity variations caused by an orbiting planet) on residuals left after Robertson et al. removed the effects of stellar activity from the radial velocity data – a process known as “detrending”. Anglada-Escudé and Tuomi rightfully point out that detrending can lead to erroneous results that can affect detection thresholds and skew false alarm probabilities. The appropriate method of analysis would be to include the properties of the “noise” (in this case, stellar activity) along with the effects of orbiting planets in the model of the data rather than removing the noise first by detrending. These mathematical subtleties become all the more important when trying to extract small signals from comparatively noisy data as occurs when trying to detect small planets using precision radial velocity measurements.

Robertson et al. responded to this criticism in the very same issue of Science. They agreed with Anglada-Escudé and Tuomi that the ideal means of analyzing the radial velocity data in the presence of noise caused by stellar activity is by multiparametric modeling where the noise and orbits are analyzed simultaneously. Even though their analysis method was admittedly statistically and conceptually simple, Robertson et al. emphasized that Anglada-Escudé and Tuomi focused on the statistical analysis issues while failing to address the underlying physical problem with interpreting the 66-day periodicity in the data as being the result of a planet, GJ 581d. The 66-day orbital period of GJ 581d is almost exactly one-half of the 130-day rotational period Robertson et al. derived for the red dwarf, GJ 581. As a result, the observed periodicity is likely a harmonic of rotationally modulated stellar activity and can not be taken as reliable evidence of an orbiting extrasolar planet regardless of how the data are analyzed.

Robertson et al. remind us that it is standard practice to reject potential planetary detections with orbital periods that are whole number fractions of the stellar rotation period because a non-planetary explanation is more likely. They cite the recent example of Astudillo-Defru et al. where these HARPS team members rejected the planetary interpretation of a signal in the radial velocity measurements of GJ 3543 because its apparent orbital period was half of the star’s known period of rotation. Robertson et al. contend that if GJ 581d was found today knowing that its apparent orbital period was half of the rotational period of GJ 581 (a fact not realized until seven years after the announcement of the discovery of GJ 581d), the planetary interpretation of the signal would be rejected as well just as it should be now.

Potential Habitability

Pushing aside the on going controversy and assuming for the moment that GJ 581d exists, we can begin to assess its potential habitability. Based on the best formally published results by Mayor et al., GJ 581d has a M_Psini value of 7.1 M_E. It has a period of 66.8 days, an eccentricity of 0.38 and a semimajor axis estimated to be 0.22 AU. Combined with the luminosity of GJ 581, this orbit implies an effective stellar flux, S_eff, of 0.25 times that of the Earth. Assuming a surface temperature of 3480° K for GJ 581, the conservative outer limit of the habitable zone corresponding to the maximum greenhouse limit as defined by Kopparapu et al. has an S_eff of 0.25. This would place GJ 581d right on the edge of the habitable zone. Factor in the inevitable uncertainties in the properties of the orbit of GJ 581d and its sun, and there is about an even chance that this planet orbits just beyond the outer edge of the habitable zone.

Wordsworth et al. published an analysis on the potential habitability of GJ 581d in 2011 where they used a detailed three-dimensional climate simulation with a range of atmospheric compositions, spin states and other properties. Since GJ 581d orbits so closely to its sun, it is probably a synchronous or, at very least, a slow rotator which increases the likelihood of atmospheric freeze out at low values of S_eff. Wordsworth et al. found that the atmosphere of GJ 581d would freeze out on the dark side of the planet as well as its poles for partial pressure of CO₂ less than about 10 bars – the typical value for the maximum greenhouse limit. Only marginal habitability was possible with higher concentrations of atmospheric CO₂ near the equator of the day lit side. They found that the presence of a “super greenhouse” effect involving gases in addition to water vapor and CO₂ was necessary to raise global temperatures to the freezing point of water.

While GJ 581d appears to be right on the cusp of habitability, this assumes that it is a terrestrial planet with a primarily rocky composition. Unfortunately, work by Rogers analyzing the mass-radius relationship of planets found by NASA’s Kepler mission indicates that planets transition from being mainly rocky planets like the Earth to volatile-rich planets like Neptune at radii no greater than 1.6 times that of the Earth. Assuming an Earth-like composition, this optimistically corresponds to a transition at a mass of no greater than 6 M_E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). Since GJ 581d has a minimum mass of 7.1 M_E and, if the results of the dynamical studies by Mayor et al. are correct, a maximum mass about 1.6 times greater still, it is highly probable that GJ 581d is a mini-Neptune with no prospect of being habitable – IF GJ 581d exists at all!

Summary

The prospects for the potential habitability of GJ 581d are not good. Detailed modeling of the climate of this world shows that it is right on the cusp of being habitable in an Earth-like sense. Only by the presence of a “super greenhouse” effect that involves gases other than CO₂ and water vapor would the surface temperatures be high enough to support liquid water globally. Unfortunately this analysis assumes that GJ 581d is a rocky planet. With a minimum mass of 7.1 M_E and an actual mass that it is likely higher, it is much more probable that GJ 581d is a mini-Neptune based on studies of the mass-radius relationship of extrasolar planets. As a result, it is highly unlikely that GJ 581d is either “Earth-like” or “habitable” as has been stated in media reports.

Of course, this analysis assumes that GJ 581d even exists in the first place. While the debate continues about how the radial velocity data for GJ 581 should be analyzed, the simple fact of the matter is that the apparent 66-day periodicity in the radial velocity measurements of GJ 581 is almost precisely one half of the star’s rotational period. This fact alone strongly favors a non-planetary interpretation of the signal as has been the done with similar detections for other stars including by the HARPS team. Unless a convincing case can be made that this signal has nothing to do with rotationally modulated stellar noise, there is simply no compelling evidence for the existence of GJ 581d and the discussion about its questionable potential for being habitable is moot. Taken together, the evidence suggests that it is highly unlikely that the 66-day periodicity in the radial velocity of GJ 581 represents a potentially habitable planet.

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Related Reading

“The Disappearing Habitable Planets of GJ 581”, Drew Ex Machina, July 7, 2014 [Post]

“Habitable Planet Reality Check: Terrestrial Planet Size Limit”, Drew Ex Machina, July 24, 2014 [Post]

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

“Habitable Planet Reality Check: GJ 667C”, Drew Ex Machina, September 7, 2014 [Post]

“A Review of the Best Habitable Planet Candidates”, Centauri Dreams, January 30, 2015 [Post]

General References

Guillem Anglada-Escudé and Mikko Tuomi, “Comment on ‘Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581’”, Science, Vol. 347, No. 6226, p. 1080, March 6, 2015

Astudillo-Defru et al., “The HARPS search for southern extra-solar planets XXXV. Planetary systems and stellar activity of the M dwarfs GJ 3293, GJ 3341, and GJ 3543”, arXiv 1411.7048 (submitted for publication in Astronomy & Astrophysics), November 25, 2014 [Preprint]

Roman V. Baluev, “The impact of red noise in radial velocity planet searches: only three planets orbiting GJ 581?”, Monthly Notices of the Royal Astronomical Society, Vol. 429, No. 3, pp. 2052-2068, March 2013

T. Forveille et al., “The HARPS search for southern extra-solar planets XXXII. Only 4 planets in the Gl 581 system”, arXiv:1109.2505, submitted September 12, 2011

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

M. Mayor et al., “The HARPS search for southern extra-solar planets. XVIII. An Earth-mass planet in the GJ 581 planetary system”, Astronomy and Astrophysics, Vol. 507, No. 1, pp. 487-49, November 2009

Paul Robertson, Suvrath Mahadevan, Michael Endl and Arpita Roy, “Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581”, Science, Vol. 345, No. 6195, pp. 440-444, July 25, 2014

Paul Robertson, Suvrath Mahadevan, Michael Endl and Arpita Roy, “Response to Comment on ‘Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581’”, Science, Vol. 347, No. 6226, p. 1080, March 6, 2015

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, arXiv 1407.4457 (submitted to The Astrophysical Journal), July 16, 2014 [Preprint]

F. Selsis et al. “Habitable planets around the star Gliese 581?”, Astronomy and Astrophysics, Vol. 476, No. 3, pp. 1373-1387, December 2007

S. Udry et al., “The HARPS search for southern extra-solar planets. XI. Super-Earths (5 and 8 M_⊕) in a 3-planet system”, Astronomy and Astrophysics, Vol. 469, No. 3, pp. L43-L47, July III 2007

Robin D. Wordsworth et al., “Gliese 581d is the First Discovered Terrestrial-mass Exoplanet in the Habitable Zone”, The Astrophysical Journal Letters, Vol. 733, No. 2, article id. L48, June 2011