head-scratch
地球に似た惑星を発見したという記事です。地球から約560光年先に地球よりも密度が高いKepler-10cと呼ばれる惑星です。地球の密度が5.5g/cm3なのに対し，Kepler-10cは7.5g/cm3あります。密度が高いと重力も地球よりも大きいため，木星のように水素をたくさん蓄えております。しかし，木星がガス型の惑星であるのに対して，Kepler-10cは地球と同じ岩石型の惑星です。
記事の名で気になった単語は"head-scratcher"です。"scratch"は「（痒いところなどを）かく」といういみですが"er"が付くと，「かゆみを和らげるために引っ掻く人」という意味になります（Weblio）。つまり"head-scratch"「頭をかく」ということです。転じて「難問、難題」という意味になります（SPACEALC）。難しい問題を解くときに頭をポリポリとかくような動作を思い浮かべました。（Ume）
'Godzilla of Earths' identified
There is a new class of planet out there that astronomers are calling the "mega-Earth".
It is an object with a hard surface like our own world but much, much bigger.
The necessity for the new designation follows the discovery of a planet which has a mass some 17 times that of Earth.
Known as Kepler-10c, it orbits a star about 560 light-years away. Scientists described its properties at an American Astronomical Society meeting in Boston.
They confess it is something of a head-scratcher.
Theorists had always thought that any planet that large would pull so much hydrogen on to itself that it would look more like a Neptune or a Jupiter.
"The proper way to call it is something bigger than a 'super-Earth, so how about 'mega-Earth," Prof Dimitar Sasselov, of the Harvard-Smithsonian Center for Astrophysics (CfA), told reporters. He also used the phrase, "the Godzilla of Earths!".
Kepler-10c, as the name suggests, was detected by the US space agency's Kepler telescope.
This finds new worlds by looking for the tiny dip in light as they pass in front of their parent stars.
