The Transition from Super Earth to Mini Neptune

In the just-published April 2014 issue of Sky & Telescope is an article detailing the results announced at the January American Astronomical Society meeting on some of the latest analyses of the Kepler mission’s exoplanet transit data and follow up observations to show the relationship between planetary radii and mass for planets somewhat larger than our own known as “super-Earths” [1].   For rocky planets like the Earth, as the mass increases, it is expected that the planet’s average density would also increase as a result of gravitational compression (i.e. the gravity generated by an increasingly more massive planet squeezes the material inside the planet to make it more dense).   At some point, however, an increasingly massive planet would begin to retain ever larger amounts of less dense water and especially gases like hydrogen and helium to become a mini-Neptune instead of a super-Earth.  No such planets exist in our solar system in this mass range (i.e. ~1 to ~15 Earth masses) but such planets have been detected in other planetary systems.

Famed exoplanet hunter Geoff Marcy and his team have been following up Kepler planetary transit detections with their own Doppler velocity measurements to determine the masses of Kepler’s planet candidates (or upper limits for the smallest planets when they happen to fall below their detection threshold) in order to independently confirm Kepler’s discoveries.  By combining the planet radii data provided by Kepler data and masses determined by Marcy et al., it is possible to determine the densities of these planets – one of the key parameters needed to determine the nature of a planet.  In order to explore this transition from super-Earths to mini-Neptunes, Marcy at al. used four years worth of data on 42 planets in 22 systems.

Another team led by Yoram Lithwick of Northwestern University has taken a different approach to determining the mass of planets detected by Kepler called transit timing variation or TTV.  TTV has proven to be a useful technique in determining the masses of planets that are too small to be accurately measured using the well established Doppler velocity measurement technique.  Variations in the precise timings of planetary transits observed by Kepler can be the result of changes in a planet’s orbit brought about by perturbations caused by other planets in the system.  By analyzing TTV, it is possible to determine the masses of the planets in the system including those that were not observed creating transits in the Kepler data.  Using this approach, Lithwick et al. have determined the masses and densities of 58 planets in addition to the 42 in the work by Marcy et al.

Both groups came to similar conclusions: While there is a wide range of planetary densities (indicating a spectrum of planetary compositions and types as a function of mass), for planets around the size of the Earth and slightly larger the general trend is one of increasing density with increasing mass as one would expect with terrestrial type super-Earths.  Somewhere in the 1.5 to 2.0 Earth-radii size range, however, a transition occurs and larger planets start to become noticeably less dense as a result of retaining ever larger quantities of gas.  Based on a relation derived from Marcy’s work on the observed planetary density as a function of radius, this transition corresponds to planets with masses approximately 4.8 to 14 times that of the Earth.

For decades, the general “rule of thumb” has been that terrestrial planets would typically be less than about ten times the mass of the Earth [2].  Planetary embryos larger than this were calculated to be massive enough to accrete gas directly from the protoplanetary disk out of which the planet was forming to increase quickly in mass to become gas giants [3].  Using the relationship determined by Marcy et al. in their latest work, a ten-Earth-mass planet would have a radius ~1.8 times that of the Earth – almost exactly in the middle of the range determined by the two groups.   While this early work shows that there are a range of planet types for any given mass or radius (and therefore there is no definitive demarcation between the largest terrestrial planets and the smallest gas giants), it is reassuring that the old “rule of thumb” that has been used for so long has some validity.  Future observations are sure to help us better define the upper limits of the size of Earth-like terrestrial planets.


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Related Reading

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

“The Extremes of Habitability” (cover story), SETIQuest, Volume 4, Number 2, pp. 1-8, Second Quarter 1998 [Article]

“Habitable Moons”, Sky & Telescope, Volume 96, Number 6, pp. 50-56, December 1998 [On line version]

“Habitable Moons: A New Frontier for Exobiology”, SETIQuest, Volume 3, Number 1, pp. 8-16, First Quarter 1997 [Article]



1) Monica Young, “…and is ‘Super Earth’ a Misnomer?”, Sky & Telescope, Vol. 127, No. 4, p. 12, April 2014

Earlier on-line version: Monica Young, “Are Super-Earths Really Super?, News from Sky & Telescope (On line), January 7, 2014 [Article]

2) James F. Kasting, Daniel P. Whitmore and Ray T. Reynolds, ‘Habitable Zones Around Main Sequence Stars”, Icarus, Vol. 101, pp. 108-128, January 1993

3) P. Bodenheimer and J.P. Pollack, “Calculations of the Accretion and Evolution of Giant Planets – The Effect of Solid Cores”, Icarus, Vol. 53, pp. 391-408, September 1986