The Kepler Mission has discovered over 4000 exoplanets, the vast majority of which are smaller than Neptune, and many of which are as small as Earth.  In our solar system, Earth and the other terrestrial planets Venus, Mercury, and Mars have rocky surfaces with very thin or no atmospheres*.  By contrast, Neptune and Uranus are about four times the radius of Earth, and most of their volume is a mixture of hydrogen and helium gas.

So are Kepler’s small exoplanets rocky like Earth, with the potential for hosting life, or are they more like Neptune, smothered in hydrogen without a place to stand?  Or are they something else entirely?

One way to address this question is to measure the masses of a bunch of small exoplanets.  We can divide each planet’s mass by its volume to calculate a bulk density within the planet. 

Armed with this plan and a wealth of telescope time at the W. M. Keck Observatory, my PhD advisor and I measured the masses and densities of 42 small exoplanets.  The planets were selected for their small sizes (smaller than Neptune) and for their sun-like stars.  We supplemented this sample with a few small exoplanets with already measured densities.  All in all, we examined the densities of 65 exoplanets smaller than Neptune to look for patterns that might lead to clues about the planets’ compositions.  The densities of the planets and their sizes are shown in the graph below, along with the patterns we found.

density-radius relation
This plot shows planet bulk density vs. planet radius. The exoplanets (shown as circles, with blue squares identifying average values at a given radius) and solar system planets (shown as diamonds) reveal that on average, planets achieve their peak density at a size of 1.5 Earth radii.  The smaller planets (fit by red line) are likely rocky.  Rocky planets do not all have the same density because rock is slightly compressible.  The larger planets (fit by black line) must have thick gaseous envelopes to explain why their densities are lower than the peak value.

We discovered that rocky planets can be as large as 1.5 times the size of Earth, but not much larger.  Planets that were 1.5 times the size of Earth were the densest planets in our sample.  Their average density was 7.6 grams per cubic centimeter, which is slightly denser than Earth (5.5 grams per cubic centimeter).  This makes sense because even rock is slightly compressible when about an Earth-mass of material is squished together into a rocky planet.  Rocky exoplanets 1.5 times the size of Earth are about 5 times the mass of Earth.

What happens when planets are larger than 1.5 times the size of Earth?  In our sample, we found that their densities were much lower than the peak density, meaning they must have thick layers of gas.  Since gas has a much lower density than rock, adding just one percent of a planet’s mass in a gaseous form increases the planet radius and lowers the bulk density.

We don’t know the exact mechanism yet, but for some reason, planets larger than about 1.5 times the size of Earth accumulated (or retained) a thick envelope of gas that most of the smaller planets in our sample do not have.  The gas is likely mostly hydrogen and helium, like in the sun.  It is possible that some of the gas envelopes could also contain superionic water or other heavier-than-hydrogen materials, but the very low densities of many exoplanets indicate that hydrogen is a major component.  For example, exoplanets that are approximately the size of Neptune have much lower masses and densities than Uranus and Neptune, suggesting that the Neptune-sized exoplanets might have different interior compositions from our solar system “ice giants*.”

What materials make up the low-density envelopes surround that small exoplanets?  How, when, and where did the gas envelopes form?  Why is it that most planets smaller than 1.5 times the size of Earth do not have gas envelopes?  How do other properties of the planet, such as its distance from the star, affect the observed relationship between planet density and radius?  These questions will continue to shape exoplanet research in the decades to come.

*Earth’s atmosphere comprises only one millionth of the mass of Earth.

** The exact interior composition of Uranus and Neptune is unknown.  If they do contain water, the water is at too high a pressure to be in the ice phase and is likely a superionic water.

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