The Composition of Super-Earths

One of the primary motivations of NASA’s Kepler mission was to find Earth-size planets in Earth-like orbits around Sun-like stars by observing the transits of these planets. As work continues to identify such worlds in the data from Kepler’s primary mission, on going analysis of this mission’s data is allowing scientists to address a host of issues about the nature of planets smaller than the gas giants typically found by surveys using other methods.  A large number of planets smaller than Neptune have already been identified by Kepler with more discoveries certainly yet to come. While the overwhelming majority of these planets have no prospect of being habitable, their study is shedding light on the nature of a new class of planets dubbed “super-Earths” and the limits of planetary habitability.

Diagram showing the major components of NASA Kepler spacecraft. (NASA/Kepler Mission/Ball Aerospace)

Before the Kepler mission, planets larger than the Earth but smaller than Neptune were expected to exist but their exact properties were unknown because none are present in our solar system. As a result, the position in parameter space of the transition from rocky to non-rocky planets and the characteristics of this transition were unknown. So when astronomers were developing size-related nomenclature to categorize the planets they expected to find using Kepler, they somewhat arbitrarily defined “super-Earth” to be any planet with a radius in the range of 1.25 to 2.0 times that of the Earth (or RE) regardless of its actual composition which could not be directly determined using Kepler data alone. Planets in the 2.0 to 4.0 RE range were dubbed “Neptune-size” (although many still refer to planets in the lower part of this range “super-Earths”). Now that Kepler has found planets in this size range, astronomers have started to examine the mass-radius relationship of super-Earths and determine their bulk compositions. The latest extrasolar planet in this size range to have its mass accurately measured is Kepler 93b.

 

Kepler 93b

Kepler 93b was detected in the first four months of Kepler data by Borucki et al.. This super-Earth is in a close 4.7-day orbit around a Sun-like star only a little smaller and less massive than the Sun with about 78% of its luminosity. As a result, there is no expectation that Kepler 93b, which has a mean stellar flux about 280 times that of the Earth, is a potentially habitable planet. Using data from Kepler and the Spitzer Space Telescope, Ballard et al. were able to refine the radius measurement of Kepler 93b to be 1.47±0.02 RE  – the most precise radius measurement for any extrasolar planet to date. About a year ago, Marcy et al. made the first rough estimate of the mass of Kepler 93b using 32 Keck HIRES radial velocity measurements acquired between July 2009 and September 2012. But with a mass measured to be 2.6±2.0 times that of the Earth (or ME), the measurement was too imprecise to constrain the potential composition of this world. Subsequent work by Ballard et al. using an additional 14 HIRES measurements from the 2013 observing season was able to refine the mass estimate of Kepler 93c to 3.8±1.5 ME but the measurement uncertainty was still too large.

In new work submitted for publication by Dressing et al., a more precise and useful mass determination for Kepler 93b was made by an international collaboration of astronomers. For this task, the original HIRES data for Kepler 93 were combined with an analysis of 86 spectra acquired during the 2013 and 2014 observing seasons using the HARPS-N spectrograph on the 3.57-meter Telescopio Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory in the Canary Islands. As had been observed earlier by Marcy et al., there was a pronounced trend in the radial velocity measurements indicating the presence of a second object, currently designated Kepler 93c, that is now estimated to be in an orbit with a period greater than a decade (corresponding to an orbital radius greater than about 4½ AU) and a mass greater than 8.5 times that of Jupiter. More data over a longer period of time will be needed to determine the orbit and nature of this object but most likely it is either a small star or a brown dwarf.

View of the 3.57-meter Telescopio Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory in the Canary Islands. Precision radial velocity measurements are made on this telescope using the HARPS-N spectrograph. (TNG)

Once the radial velocity trend for Kepler 93c is removed from the data, a clear signature for Kepler 93b was observed with a semiamplitude of 1.63±0.27 m/s. Combined with the Kepler observations, this yields a mass of 4.0±0.7 ME for Kepler 93b and a mean density of 6.9±1.2 g/cc. Comparing these parameters with models of planets of various compositions, Kepler 93b appears to have a bulk composition consistent with being a fully differentiated planet made of 83% magnesium silicates (MgSiO3) and 17% iron (Fe) – very similar to the bulk composition of Earth and Venus.

 

Comparison with Other Planets

Dressing et al. took their analysis one step further and compared the properties of Kepler 93b with those of nine other extrasolar planets whose radii are less than 2.7 RE and have had their masses determined to an accuracy of 20% or better. Earth and Venus were also added to their analysis. Interestingly, the seven planets in this group with masses less than 6 ME fell neatly along a line corresponding to an Earth-like composition of 83% MgSiO3/17% Fe. Even when the Earth and Venus are excluded from the analysis, the tight correlation remains. All the planets in this group with masses in excess of 6 ME have densities consistent with a composition that includes a large fraction of volatiles such as water, hydrogen and helium.

The mass-radius relationship for all known extrasolar planets smaller than 2.7 RE and masses known to a precision of 20% or better. The various curves indicate models for planets of various compositions. The dashed blue line represents and Earth-like composition with the shaded region surrounding this line indicating the observed variation is the compositions of the extrasolar planet host stars. Click on image to enlarge. (Dressing et al.)

Dressing et al. then assessed the effect on planet density of varying the ratios of iron to silicates and magnesium to silicates within the range observed for the photospheric abundances of these elements in nearby hosts of extrasolar planets. The assumption, based on a study of elemental abundances in our solar system, is that the ratios in the host stars reflect the ratios of the material out of which their planets formed. Dressing et al. found that varying these ratios in their models produced a 2% variation in the mean density of the planets which neatly matches the 1.9% variance they observed between the actual density of the seven extrasolar planets with masses less than 6 ME and the model corresponding to a composition of 83% MgSiO3/17% Fe. While much more data are required, this finding suggests that there is a single mass-radius relationship for all rocky planets with masses less than about 6 ME which corresponds to a radius less than about 1.6 RE. The data suggest that planets larger than this would not be rocky planets but volatile-rich mini-Neptunes instead.

This finding agrees very well with earlier work on the mass-radius relationship of extrasolar planets that had been published during 2014. Using planetary radii determined from Kepler data and masses found by precision radial velocity measurements and analysis of transit timing variations (TTVs), Marcy et al. as well as Weiss and Marcy had found that the density of super-Earths tended to rise with increasing radius as would be expected of rocky planets. But somewhere around the 1.5 to 2.0 RE range, a transition is passed where larger planets tended to become less dense instead. The interpretation of this result is that planets with radii greater than about 1.5 RE are increasingly likely to have substantial envelopes of various volatiles such as water (including high pressure forms of ice at high temperatures) and thick atmospheres rich in hydrogen and helium that decrease a planet’s bulk density. As a result, these planets can no longer be considered rocky planets like the Earth but would be classified as mini-Neptunes.

Plot of planet density versus radius for 33 extrasolar planets and the planets in our solar system included in an early analysis by Marcy et al.. The newer analysis by Dressing et al. is confined to planets with better characterized masses. (Marcy)

The new work by Dressing et al. agrees very well with a detailed statistical analysis of the mass-radius relationship for 47 extrasolar planets with well-determined masses and radii submitted for publication in July 2014 by Leslie Rogers (Hubble Fellow at the California Institute of Technology). Rogers’ analysis clearly showed that a transition took place between rocky and non-rocky planets at 1.5 RE with a sudden step-wise transition being mildly favored over more gradual ones. Taking into account the uncertainties in her analysis, Rogers found that the transition from rocky to non-rocky planets takes place at no greater than about 1.6 RE at a 95% confidence level. The mass of a 1.6 RE planet with an Earth-like composition would be 6 ME – exactly what was found in the new analysis by Dressing et al..

This new finding lends further support to the view that super-Earths with radii greater than 1.6 RE or masses in excess of 6 ME are in fact most likely mini-Neptunes in composition and therefore can not be considered potentially habitable as has been claimed in the media and even some peer-reviewed papers (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). This result implies that most of extrasolar planets that have been claimed to be “potentially habitable” are likely not even terrestrial planets never mind habitable. The only reasonable habitable planet candidates among the currently known extrasolar planets with masses less than 6 ME or radii less than 1.6 RE are Kepler 62f and Kepler 186f ( see “Habitable Planet Reality Check: Kepler 186f”).

A comparison of the size of Earth and Neptune along with an approximation of the size of a planet at the transition from rocky to non-rocky planet.

While there is evidence for a step-wise transition from rocky to non-rocky planets as a function of planetary radius around 1.5 RE to probably no greater than 1.6 RE, recent evidence suggests that the transition might be a little more gradual as a function of planetary mass. In November 2014 Schmitt et al. announced the discovery of an extrasolar planet orbiting Kepler 289 they dubbed PH3 c after the Planet Hunters citizen scientist program that found it in archived data from NASA’s Kepler mission. The Kepler data showed that the planet has a radius of 2.7 RE. Using the TTV technique, Schmitt et al. found that PH3 c has a mass of 4.0±0.9 ME (which was too uncertain to be included in the analysis by Dressing et al.). These radius and mass estimates yield a mean density of 1.2±0.3 g/cm3 or only about one-fifth that of the Earth. Models indicate that this density is consistent with PH3 c possessing a deep, hot atmosphere of hydrogen and helium making up about half of its radius but only about 2% of its total mass. Once again, much more data will be required to work out the details of the mass-radius relationship and the transition from rocky to non-rocky planets but the properties of PH3 c suggests that there might be a more gradual transition as a function of mass from predominantly rocky to predominantly non-rocky planets in the 4 to 6 ME mass range.

 

Conclusion

The latest work by Dressing et al. further supports the results from earlier analyses of Kepler data for sub-Neptune size extrasolar planets that strongly suggests that super-Earths with masses greater than 6 ME or radii greater than 1.6 RE are predominantly mini-Neptunes and not rocky planets. All the known extrasolar planets smaller than this (and which have tightly constrained masses) closely follow a mass-radius relationship consistent with an Earth-like bulk composition of 83% MgSiO3 and 17% Fe. While much more data are required to better characterize the mass-radius relationship of super-Earths and the nature of the transition from rocky to non-rocky compositions, it seems likely that the majority of the planets that have been claimed to be “potentially habitable” are not even terrestrial planets never mind habitable. The only reasonable candidates for being potentially habitable among the currently known extrasolar planets are Kepler 62f and Kepler 186f.

 

A French translation of this article by Alexandre Lomaev is also available: “La composition des super-terres”, Extrasolar.fr – Encylopédie des Mondes Extérieurs, August 28, 2015 (in French) [Post]

 

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Related Reading

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

“The Transition from Rocky to Non-Rocky Planets”, Centauri Dreams, November 14, 2014 [Post]

“Habitable Planet Reality Check: Kepler 186f”, Drew Ex Machina, April 20, 2014 [Post]

“The Transition from Super-Earth to Mini-Neptune”, Drew Ex Machina, March 29, 2014 [Post]

 

General References

Sarah Ballard et al., “Kepler-93b: A Terrestrial World Measured to within 120 km, and a Test Case for a New Spitzer Observing Mode”, The Astrophysical Journal, Vol. 790, No. 1, Article id. 12, July 2014

Borucki et al., “Characteristics of Planetary Candidates Observed by Kepler. II. Analysis of the First Four Months of Data”, The Astrophysical Journal, Vol. 736, No. 1, Article id. 19, July 20, 2011

Courtney D. Dressing et al., “The Mass of Kepler-93b and the Composition of Terrestrial Planets”, arVix 1412.8687 (accepted for publication in The Astrophysical Journal), December 30, 2014 [Preprint]

Geoffrey W. Marcy et al., “Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets”, The Astrophysical Journal Supplement, Vol. 210, No. 2, Article id. 20, February 2014

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

Schmitt et al., “Planet Hunters VII: Discovery of a New Low-Mass, Low Density Planet (PH3 c) Orbiting Kepler-289 with Mass Measurements of Two Additional Planets (PH3 b and d)”, The Astrophysical Journal, Vol. 795, No. 2, ID 167, November 10, 2014

Lauren M. Weiss and Geoffrey W. Marcy, “The Mass-Radius Relation for 65 Exoplanets Smaller than 4 Earth Radii”, The Astrophysical Journal Letters, Vol. 783, No. 1, Article id. L6, March 2014

 

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