Missing Planets

The types of planets you have as neighbors may depend on your neighborhood. The planets discovered in our region of the Milky Way Galaxy come in just about every variety: hot Jupiters, super-Earths, mini-Neptunes, lava planets, ocean planets, and others.

But that may not be the case for other parts of the galaxy. As an example, there appear to be almost no super-Earths or mini-Neptunes outside the Milky Way’s disk.

The disk is about a hundred thousand light-years wide, but only a few thousand light-years thick. The Sun moves up and down within the disk, but never leaves it. But the orbits of many other stars are much more tilted than the Sun’s. These systems move above and below the disk like horses bobbing on a merry-go-round.

A recent study found that these stars have almost no super-Earths or mini-Neptunes, which are common in our region of the galaxy. Super-Earths are dense and rocky, like Earth, but a few times more massive. Mini-Neptunes are a bit heavier still — slightly less massive than Neptune, the smallest of the Sun’s giant planets.

Scientists aren’t sure why these types of planets are missing in the Milky Way’s outer precincts. Perhaps their star systems didn’t have enough raw materials for making such planets. Or maybe their systems have kicked out such planets. Or it could simply be that those kinds of worlds are harder to find — perhaps still awaiting discovery in their far-away neighborhoods.

Script by Damond Benningfield