麻豆传媒

ONE sure bet for 2017 is that astronomers will find more of the alien worlds called super-Earths.

The existence of these planets, with masses greater than Earth鈥檚 and less than Neptune鈥檚, was largely unanticipated by science. The first found around a star similar to ours was Gliese 876 d, in 2005. They are now thought to be the most common planet type, making our solar system a bit of an oddity.

This may explain why science didn鈥檛 really see them coming. However, science fiction did.

Jack Vance鈥檚 Big Planet (1952) is probably the most famous example. His world is Earth-like, with one Earth gravity but about 鈥渢en times the volume鈥� and the average density correspondingly smaller, 鈥渙nly a third of Earth鈥檚鈥�, because it is low on metals. The appeal is the romance of huge landscapes, coupled with the basic plausibility of the planet鈥檚 defining numbers: 鈥淭he face of Big Planet dropped below, spreading wider and wider, and where an Earthly eye might expect a horizon, with a division into land and sky, there was only land, and then still more land鈥︹��

The earliest fictional super-Earths are older still. The psychic 鈥渃osmical explorers鈥� of Olaf Stapledon鈥檚 Star Maker (1937) encounter many super-Earths. Stapledon argued that big worlds might need to be aqueous to be habitable by complex creatures, with the water鈥檚 buoyancy countering the effects of higher gravity. He imagined, for example, an oceanic world inhabited by 鈥渓iving ships鈥�. Now such worlds are seen as plausible, one candidate being Gliese 1214 b, spied in 2009.

Many imagined super-Earths are, retrospectively, beyond the bounds of scientific plausibility. The parameters of Robert Silverberg鈥檚 Majipoor (Lord Valentine鈥檚 Castle, 1980), some 10 times Earth鈥檚 size, are more like gas giant Saturn鈥檚. But the legacy of this subgenre is more impressionistic. The writers intuited the strangeness of these worlds; huge, varied, perhaps with surface conditions unlike anything we have experienced.

Of course, other types of exotic exoplanet have been anticipated. Tatooine in Star Wars (1977), like Brian Aldiss鈥檚 Helliconia (1982), is a planet of a double star 鈥� and in 2011, a 鈥渞eal鈥� Tatooine was found: Kepler 16 b. Planets around red dwarf stars, like the 鈥淓arth-like鈥� Proxima b glimpsed this year, were also anticipated by Stapledon.

So far only a tiny portion of the sky has been checked for a limited range of exoplanets. NASA鈥檚 Transiting Exoplanet Survey Satellite, to launch next year, will mount the first all-sky search.

Expect more super-Earths to turn up. Many are likely to be exotic, and astronomers could do worse than dip into the catalogue of imagined versions for a glimpse of what may be to come.

This article appeared in print under the headline 鈥淏est of both worlds鈥�