Cold Neptunes: An Exoplanet Sweet Spot? NASA Goddard video

A new statistical study of planets found by a technique called gravitational microlensing suggests that Neptune-mass worlds are likely the most common type of planet to form in the icy outer realms of planetary systems. The study provides the first indication of the types of planets waiting to be found far from a host star, where scientists suspect planets form most efficiently. Contrary to some theoretical predictions, the most numerous cold exoplanets have masses similar to Neptune, and there doesn’t seem to be the expected increase in number at lower masses.

Lead scientist Daisuke Suzuki, a post-doctoral researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, infer this from current detections. The team concludes that Neptune-mass planets in outer orbits are about 10 times more common than Jupiter-mass planets in Jupiter-like orbits. Gravitational microlensing takes advantage of the light-bending effects of massive objects predicted by Einsteins general theory of relativity. It occurs when a foreground star, the lens, randomly aligns with a distant background star, the source, as seen from Earth.

As the lensing star drifts along in its orbit around the galaxy, the alignment shifts over days to weeks, changing the apparent brightness of the source. The precise pattern of these changes provides astronomers with clues about the nature of the lensing star, including any planets it may host. Typically, the technique provides the mass ratio of the planet to the host star and their separation. Microlensing holds great potential. It can detect planets hundreds of times more distant than most other methods, allowing astronomers to investigate a broad swath of our Milky Way galaxy. The technique can locate exoplanets at smaller masses and greater distances from their host stars, and its sensitive enough to find planets floating through the galaxy on their own, unbound to stars.

Microlensing surveys complement other methods best suited to find planets closer to their stars. Researchers determined the frequency of planets compared to the mass ratio of the planet and star as well as the distances between them. For a typical planet-hosting star with about 60 percent the suns mass, the typical microlensing planet is a world between 10 and 40 times Earths mass. For comparison, Neptune in our own solar system has the equivalent mass of 17 Earths.

The results imply that cold Neptune-mass worlds are the most common types of planets beyond the so-called snow line, the point where water remained frozen during planetary formation. In the solar system, the snow line is thought to have been located at about 2.7 times Earths mean distance from the sun, placing it in the middle of the main asteroid belt today.

NASA’s Wide Field Infrared Survey Telescope (WFIRST), slated to launch in the mid-2020s, will conduct an extensive microlensing survey. Astronomers expect it will deliver mass and distance determinations of thousands of planets, completing the work begun by NASA’s Kepler mission and providing the first galactic census of planetary properties. A new statistical study of planets found by the gravitational microlensing technique suggests that Neptune-mass planets may be the most common worlds in the outer reaches of planetary systems.

Credit: NASA’s Goddard Space Flight Center/Scott Wiessinger

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