spaceSpace and Physics

Distant Exoplanets Around Other Stars More Likely To Be Ice Giants Like Neptune


Jonathan O'Callaghan

Senior Staff Writer

Artist's impression of a Neptune-mass exoplanet. NASA/Goddard/Francis Reddy

Distant exoplanets around other stars are more likely to be ice giants like Neptune, claims a new study. And these worlds may be important in the development of habitable Earth-like worlds closer to the star.

The study was published in The Astrophysical Journal. To come to the conclusion, scientists from NASA looked at planets that had been discovered via a method called microlensing.


Microlensing involves a star passing in front of a more distant massive object, like another star, from our field of view. When this happens, the light from the more distant star is bent around the lensing star, sometimes letting us see planets orbiting the lensing star.

By its nature though, microlensing is limited. Each event occurs only once, so there’s no opportunity for a repeat viewing of a planet via this method. As such, only about 50 exoplanets have been discovered using it, compared to thousands found by other methods – mostly the transit method, when a telescope sees a planet crossing the face of a star.

But one major benefit of microlensing is that it can see planets that are much further from their star than other methods. And in this study, the authors found that many of these planets at such a location – up to about 10 times as distant as Earth from the Sun – tended to be the mass of Neptune or larger.

A typical planet-hosting star with about 60 percent the Sun’s mass was most likely to have a world between 10 and 40 times Earth’s mass (Neptune is 17 times Earth’s mass). This suggests Neptune-like worlds are most likely to form beyond the “snow line”, the region in a system beyond which water is most likely to remain frozen. Our Solar System’s snow line is about 2.7 times Earth’s distance from the Sun, in the middle of the asteroid belt today.


"We've found the apparent sweet spot in the sizes of cold planets,” lead scientist Daisuke Suzuki, a post-doctoral researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland Baltimore County, said in a statement.

“Contrary to some theoretical predictions, we infer from current detections that the most numerous have masses similar to Neptune.”

Upcoming telescopes like NASA’s Wide Field Infrared Survey Telescope (WFIRST) in the mid-2020s will look for many more planets via microlensing events, and help confirm if this prediction is true.

The implications of this are that Neptune-mass worlds may play a key role in delivering water to rocky planets. Worlds like Earth that form nearer their star can be fiery, hot worlds early in their formation. But these distant ice giants could fling water-rich asteroids into the inner part of the system, delivering water and playing a key role in habitability.



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