Planets larger than Neptune have elevated eccentricities

The shape of a planet’s orbit is one of its fundamental properties, along with its size and distance from its host star. Earth and the other planets in the solar system have nearly circular orbit, but some extrasolar planets have very elliptical or "eccentric" orbits. Greg Gilbert, Paige Entrican, and I just published a paper in the Proceedings of the National Academy of Science characterizing the eccentricities of planets from the size of Jupiter down to the size of Mars. We found that planets smaller than Neptune have nearly circular orbits, while larger planets have orbits about four times more elliptical.

We used data collected by NASA’s Kepler telescope, which monitored 150,000 stars and measured dips in their brightness caused by transiting planets to discover thousands of exoplanets. The measurements of stellar brightness over time are called light curves. We performed a detailed analysis of the light curve dips to extract information about the shape of the planets’ orbits.

The speed of a planet on an elliptical orbit changes over the course of the orbit. We measured the speed of the planet orbits from their light curves and used that to measure their eccentricities.

The eccentricity split coincides with several other iconic features in the exoplanet population, such as the high abundance of small planets over large planets and a tendency for giant planets to form only around stars enriched in heavy elements such as oxygen, carbon and iron. Astronomers call these heavy elements metals.

Small planets are common; large planets are rare. Large planets need metal-rich stars in order to form; small planets do not. Small planets have low eccentricities, and large planets have large eccentricities

The coincidence of trends in abundance, metallicity and eccentricity points to two distinct pathways for forming planets larger/smaller than Neptune. Exactly how all these observables are connected is not obvious but one possibility is that metal-rich stars are more likely to form more than one super-Neptune-size planets and that these large planets stir each other up, exciting their eccentricities.

Zooming out, I think it's remarkable what we’ve been able to learn about the orbits of planets around other stars using the Kepler Space Telescope. The telescope was named after Johannes Kepler, who, four centuries ago, was the first scientist to appreciate that the planets in our solar system move on slightly elliptical rather than circular orbits. His discovery was an important moment in human history because it showed that the sun, rather than the Earth, was at the center of the solar system. I’m sure Kepler, the man, would be delighted to learn that a telescope named in his honor measured the subtle shapes of orbits of Earth-size planets around other stars

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Additional resources

• Published paper in Proceedings of the National Academy of Science 

• UCLA press release