Mini-Neptunes Are Everywhere In Milky Way, According To New NASA Kepler Spacecraft Data

Kepler Candidate Planets
Kepler Candidate Planets as of Jan. 2014.

A review of planet candidates discovered by NASA’s Kepler spacecraft reveals just how common “Super-Earths” are in the universe. In addition to discussing potential planets, the space agency has announced five new rocky planets discovered by Kepler.

Super-Earths are planets with a mass larger than Earth but smaller than Neptune's or Uranus'. They're also rocky planets, whereas mini Neptunes have thick atmospheres of hydrogen and helium with layers of ice and possibly oceans of water or ammonia. These types of exoplanets are believed to be some of the most common types of planets found in the Milky Way, and the latest Kepler data provides even more evidence for this theory. Despite its prevalence throughout the Milky Way, these types of planets are not found in our solar system.

The review of the Kepler revealed “More than three-quarters of the planet candidates discovered by NASA's Kepler spacecraft have sizes ranging from that of Earth to that of Neptune, which is nearly four times as big as Earth,” notes NASA in a statement.

Astronomers also confirmed the discovery of five rocky planets. These planets are larger than Earth, ranging from 10 percent to 80 percent larger, and two planets, Kepler-99b and Kepler-406b, are 40 percent larger than Earth and are incredibly dense with their mass likened to that of lead. Kepler-99b orbits its host star in less than five days and Kepler-406b orbits its host star in just three days are too hot to sustain life, reports NASA.

To confirm these planets, researchers used Doppler measurements to confirm the existence of these planets and to learn more about their mass. Doppler measurements are based on the “wobble” of the host star, caused by the gravitational pull of a planet. The Dopler study was led by Geoff Marcy, from the University of California, Berkeley, and published in the Astrophysical Journal.

Researchers determined mass of 16 exoplanets from additional observations using Earth-based telescopes at the W.M. Keck Observatory, located in Haiwaii, and Kepler supplied the diameter of these planets. Based on these sets of data, the astronomers were able to determine the composition and density of these exoplanets, classifying these planets as “rocky or gaseous, or mixtures of the two.” These ground-based observations confirmed 41 planets discovered by Kepler.

Based on the density calculations, the researchers discovered the chemical composition of the 16 mini-Neptunes which consists of a rocky core surrounded by layers of hydrogen, helium and hydrogen. Some of these exoplanets do not have this gaseous layer surrounding the rocky core.

Marcy said in a statement, “This marvelous avalanche of information about the mini-Neptune planets is telling us about their core-envelope structure, not unlike a peach with its pit and fruit.”

Another researcher reviewed the Kepler data to verify 15 pairs of planets by measuring the transit timing variations, TTV, which analyzes changes in orbit speed of a planet based on the gravitational pull of a neighboring planet. The verified planets ranged in size, from Earth-like to slightly larger than Neptune, notes NASA. The TTV research was led by Ji-Wei Xie, from the University of Toronto, and published in the Astrophysical Journal.

The new analysis of Kepler data helps create a way to differentiate between a super-Earth and mini-Neptune as planets that have a radius of 1.5 times that of Earth, or smaller, tend to be rocky planets.

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