From Keck: “Unexpected Classification of Exoplanets Discovered -Two Distinct “Species” of Exoplanets Illuminated”

Keck Observatory

Keck Observatory.

Keck Observatory

Mari-Ela Chock, Communications Officer
W. M. Keck Observatory
(808) 554-0567



NASA/Kepler Telescope


Assembly Line of Planets: This diagram illustrates how planets are assembled and sorted into two distinct size classes. First, the rocky cores of planets are formed from smaller pieces. Then, the gravity of the planets attracts hydrogen and helium gas. Finally, the planets are “baked” by the starlight and lose some gas. At a certain mass threshold, planets retain the gas and become gaseous mini-Neptunes; below this threshold, the planets lose all their gas, becoming rocky super-Earths. CREDIT: NASA/KEPLER/CALTECH (R. HURT)





Most of these close-in planets were found to be roughly between the size of Earth and Neptune, which is about four times the size of Earth. But, until now it was not known they fall into two distinct size groups.



The Caltech team—together with colleagues from several institutions, including UC Berkeley, the University of Hawaii, Harvard University, Princeton University, and the University of Montreal—took a closer look at the Kepler planets’ sizes with the help of Keck Observatory’s High-Resolution Echelle Spectrometer (HIRES).


They spent years gathering HIRES spectral data on the stars hosting 2,000 Kepler planets, allowing them to obtain precise measurements of the sizes of the stars; these measurements, in turn, helped determine more accurate sizes for the planets orbiting those stars.

“Before, sorting the planets by size was like trying to sort grains of sand with your naked eye,” says Fulton. “Getting the spectra from Keck Observatory is like going out and grabbing a magnifying glass. We could see details that we couldn’t before.”

With Keck Observatory’s HIRES data, the researchers were able to measure the sizes of the 2,000 planets with four times more precision than what had been achieved previously.


Mind the Exoplanet Gap: Researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission have discovered a gap in the distribution of planet sizes, indicating that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earths and super-Earths (similar to Kepler-452b), and the mini-Neptunes (similar to Kepler-22b). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. CREDIT: NASA/AMES/CALTECH/UNIVERSITY OF HAWAII (B. J. FULTON).



The second possible reason that planets don’t land in the gap has to do with planets losing gas. If a planet does happen to acquire just a little bit of gas—the right amount to place it in the gap—that gas can be burned off when exposed to radiation from the host star.


New Branch in Exoplanet Family Tree: This sketch illustrates a family tree of exoplanets. Planets are born out of swirling disks of gas and dust called protoplanetary disks. The disks give rise to giant planets like Jupiter as well as smaller planets mostly between the size of Earth and Neptune. Researchers using data from the W.M. Keck Observatory and NASA’s Kepler mission discovered that these smaller planets can be cleanly divided into two size groups: the rocky Earth-like planets and super-Earths, and the gaseous mini-Neptunes. CREDIT: NASA/KEPLER/CALTECH (T. PYLE)


The study, titled The California-Kepler Survey. III. A Gap in the Radius of Distribution of Small Planets, was funded by NASA and the National Science Foundation.


The High-Resolution Echelle Spectrometer (HIRES) produces spectra of single objects at very high spectral resolution, yet covering a wide wavelength range. It does this by separating the light into many “stripes” of spectra stacked across a mosaic of three large CCD detectors. HIRES is famous for finding planets orbiting other stars. Astronomers also use HIRES to study distant galaxies and quasars, finding clues to the Big Bang.  Support for this project was provided by the William J. and Dorothy K. O’Neill Foundation, and Joseph and Deborah Schell.

See the full article here .

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