Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 252

Nicely near-circular orbits are not part of a universal planetary
groundplan. Many of these planets have freakishly looping orbits.
'hot Jupiters' are not uncommon—although perhaps they are not as
relatively abundant as they seemed in the early days of exoplanet-
finding. Being the biggest type of exoplanet, and the closest to their
sun, they are, after all, also the easiest to detect.
There is another category that is looming large. These are the
'super-Earths', planets that are bigger than Earth but smaller than
Neptune. At the moment, they seem to make up somewhere between
a third and half of all planets found—and they are mostly in close and
tight orbits around their parent star. This has perplexed the planetary
theorists a little (a state that has become depressingly familiar to theo-
rists since the first exoplanet was found). It had been thought that
there was just not enough rocky material close to a star to grow any-
thing much bigger than an Earth, while farther out, beyond the snow
line, planets should quickly grow into gas giants.
To explain the hot Jupiters, theory was amended to allow them to
form in distant regions and migrate in towards its star. As for the
super-Earth-sized planets, the close-orbit zone, it was thought, should
have been swept clear of such bodies. In fact, it seems to be crowded
with them. When the mass of those super-Earths was measured (by
means of the wobble technique) and combined with the size as meas-
ured by the transit technique, it was clear that many of those closely
orbiting super-Earths have low densities—that is, they have small
rocky cores surrounded by gas. Did they form in the outer regions as
'failed Jupiters' before migrating inwards—or were they assembled in
place, close to the star? It is one of many open questions 157 at the
beginning of this, our golden age of planetary science.
The diversity of this menagerie is astounding, and, indeed, akin to
science fiction. A planet has been found that orbits a binary star sys-
tem, like Tatooine in Star Wars. 158 A planet larger than Jupiter has been