Exoplanets: an introduction for worldbuilders

With the recent discovery of Kepler-452b, planets beyond our solar system—called extrasolar planets or exoplanets—have come into the news again. This has already happened a few times: the Gliese 581 system in 2007 (and again a couple of years ago); the early discoveries of 51 Pegasi b and 70 Virginis b in the mid 1990s; and Alpha Centauri, our nearest known celestial neighbor, in 2012.

For an author of science fiction, it’s a great time to be alive, reminiscent of the forties and fifties, when the whole solar system was all but unknown and writers were only limited by their imaginations. Planets, we now know, are just about everywhere you look. We haven’t found an identical match to Earth (yet), and there’s still no conclusive evidence of habitation on any of these newfound worlds, but we can say for certain that other planets are out there. So, as we continue the interminable wait for the new planet-hunters like TESS, the James Webb Space Telescope, Gaia, and all those that have yet to leave the drawing board, let’s take a quick look at what we know, and how we got here.

Before it all began: the 1980s

I was born in 1983, so I’ve lived in four different decades now, and I’ve been able to witness the birth and maturity of the study of exoplanets. But younger people, those who have lived their whole lives knowing that other solar systems exist beyond our own, don’t realize how little we actually knew not that long ago.

Thirty years ago, there were nine known planets. (I’ll completely sidestep the Pluto argument in this post.) Obviously, we know Earth quite well. Mars was a frontier, and there was still talk about near-term manned missions to go there. Venus had been uncovered as the pressure cooker that it is. Jupiter was on the radar, but largely unknown. Mercury was the target of flybys, but no orbiter—it was just too hard, too expensive. The Voyager mission gave us our first up-close looks at Saturn and Uranus, and Neptune would join them by the end of the decade.

Every star besides the Sun, though, was a blank slate. Peter van de Kamp claimed he had detected planets around Barnard’s Star in the 1960s, but his results weren’t repeatable. In any case, the instruments of three decades past simply weren’t precise enough or powerful enough to give us data we could trust.

What this meant, though, was that the field was fertile ground for science fiction. Want to put an Earthlike planet around Vega or Arcturus? Nobody could prove it didn’t exist, so nobody could say you were wrong. Solar systems were assumed to be there, if below our detection threshold, and they were assumed to be like ours: terrestrial planets on the inside, gas giant in the outer reaches, with one or more asteroid belts here or there.

The discoveries: the 1990s

As the 80s gave way to the 90s, technology progressed. Computers got faster, instruments better. Telescopes got bigger or got put into space. And this opened the door for a new find: the extrasolar planet. The first one, a huge gas giant (or small brown dwarf, in which case it doesn’t count), was detected in 1989 around the star HD 114762, but it took two years to be confirmed.

And then it gets weird. In 1992, Aleksander Wolszczan and Dale Frail discovered irregularities in the emissions of a pulsar designated PSR B1257+12. There’s not much out there that can mess up a pulsar’s, well, pulsing, but planets could do it, and that is indeed what they found. Two of them, in fact, with a third following a couple of years later, and the innermost is still the smallest exoplanet known. (I hope that will be changed in the not-too-distant future.) Of course, the creation of a pulsar is a wild, crazy, and deadly event, and the pulsar planets brought about a ton of questions, but that need not concern us here. The important point is that they were found, and this was concrete proof that other planets existed beyond our solar system.

Then, in the middle of the decade, the floodgates opened a crack. Planets began to be discovered around stars on the main sequence, stars like our sun. These were all gas giants, most of them far larger than Jupiter, and many of them were in odd orbits, either highly eccentric or much too close to their star. Either way, our solar system clearly wasn’t a model for those.

As these “hot Jupiters” became more and more numerous, the old model had to be updated. Sure, our solar system’s progression of terrestrial, gaseous, and icy (with occasional asteroids thrown in) could still work. Maybe other stars had familiar systems. After all, the hot Jupiters were an artifact of selection bias: the best method we had to detect planets—radial velocity, which relies on the Doppler effect—was most sensitive to large planets orbiting close to a star. But the fact that we had so many of them, with almost no evidence of anything resembling our own, meant that they had to be accounted for in fiction. Thus, the idea of a gas giant having habitable moons begins to grow in popularity. Again, there’s no way to disprove it.

Acceptance: the 2000s

With the turn of the millennium, extrasolar planets—soon to be shortened to the “exoplanet” moniker in popular use today—continued to come in. Advances in technology, along with the longer observation times, brought the “floor” of size further and further down. Jupiter analogues became fairly common, then Saturn-alikes. Soon, Uranus and Neptune had their clones in distant systems.

And Earth 2 was in sight, as the major space agencies had a plan. NASA had a series of three instruments, all space-based, each increasingly larger, that would usher in a new era of planetary research. Kepler would be launched around 2005-2007, and it would give us hard statistics on the population of planets in our galaxy. The Space Interferometry Mission (SIM) would follow a few years later, and it would find the first true Earthlike planets. Later, in the early to mid 2010s, the Terrestrial Planet Finder (TPF) would locate and characterize planets like Earth, showing us their atmospheres and maybe even ocean coverage. In Europe, ESA had a similar path, with CoRoT, Gaia, and Darwin.

And we know how that turned out. Kepler was delayed until 2009, and it stopped working a couple of years ago. SIM was defunded, then canceled. TPF never got out of the planning stages. Across the ocean, CoRoT launched, but it was nowhere near as precise as they thought; it’s given us a steady stream of gas giants, but not much else. Gaia is currently working, but also at a reduced capacity. Darwin met the same sad fate as TPF.

But after all that doom and gloom had passed, something incredible happened. The smallest of the new discoveries were smaller than Neptune, but still larger than Earth. That gap in mass (a factor of about 17) is an area with no known representatives in our solar system. Logically, this new category of planet quickly got the name “super-Earth”. And some of these super-Earths turned up in interesting places: Gliese 581 c was possibly within its star’s habitable zone, as was its sister planet, Gliese 581 d. Sure, Gliese 581 itself was a red dwarf, and “c” has a year that lasts less than one of our months, but it was a rocky planet in an orbit where liquid water was possible. And that’s huge.

By the end of 2009, super-Earths were starting to come into their own, and Kepler finally launched, promising to give us even more of them. Hot Jupiters suddenly became oddballs again. And science fiction has adapted. Now there were inhabited red dwarf planets, some five to ten times Earth’s mass, with double the gravity. New theories gave rise to imagined “carbon planets”— bigger, warmer versions of Titan, with lakes of oil and mountains of diamond—or “ocean worlds” of superheated water, atmospheric hydrogen and helium, and the occasional bit of rocky land.

Worldbuilding became an art of imagining something as different from the known as possible, as all evidence now pointed to Earth, and indeed the whole solar system, as being an outlier. For starters, it’s a yellow dwarf, a curious part of the main sequence. Just long-lived enough for planets to form and life to evolve, yet rare enough that they probably shouldn’t. Red dwarfs, by contrast, are everywhere, they live effectively forever, and we know a lot of them have planets.

Here and now: the 2010s

Through the first half of this decade, that’s pretty much the status quo. Super-Earths seem to be ubiquitous, “gas dwarfs” like Neptune numerous, and hot Jupiters comparatively rare. There’s still a lot of Kepler data to sift through, however.

But now we’ve almost come full circle. At the start of my lifetime, planets could be anything. They could be anywhere. And planetary systems probably looked a lot like ours.

Then, we started finding them, and that began to constrain our vision. The solar system was now rare, statistically improbable or even impossible. Super-Earths, though, were ascendant, and they offered a new inspiration.

And, finally, we come to Kepler-452b. It’s still a super-Earth. There’s no doubt about that, as even the smallest estimate puts it at 1.6 Earth masses. But it’s orbiting a star like ours, in a spot like ours, and it joins a very select group by doing that. In the coming years, that group should expand, hopefully by leaps and bounds. But it’s what 452b states that’s important: Earthlike planets are out there, in Earthlike orbits around Sunlike stars.

For worldbuilders, that means we can go back to the good old days. We can make our fictional worlds match our own, and nobody can tell us that they’re unlikely to occur. Thirty years ago, we could write whatever we wanted because there was no way to disprove it. Now, we can write what we want because it just might be proven.

What a time to build a world.