Anyone that has watched Star Wars always remembers the iconic scene of Luke Skywalker staring into the Tatooine dual sunset. Science fiction has imagined fantastic alien planets, like Tatooine with its two suns, and other planets with more suns (like in Futurama) and even planets around black holes (think Gargantua in Interstellar). And astronomers have discovered some of these planets!
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Extrasolar planets (or exoplanets) are planets that go around stars other than our own sun. Exoplanets had long been a thing of science fiction until their confirmation in the 1990s.
The first known exoplanet was discovered around a very dense, rapidly rotating dead star called a pulsar. The only planets that we knew of prior to this discovery were in our own solar system. We built models that explained how the planets might have formed around our sun, and why we have rocky planets closer to the sun and gas giants further away. Our model didn't even consider dead stars. And another thing - this pulsar must have exploded in a supernova. So how did the planets get there? Did they survive one of the most violent explosions in the universe? Or did they form after?
The next exoplanets that we discovered are called "hot Jupiters". These are Jupiter-sized - or larger - planets that are closer to their star than Mercury is to our sun. These planets also zoom around their stars every few days, sometimes even every few hours. Our models didn't predict these planets, because gas giants should be sitting further away from their stars, like in our own solar system. So how did they get there?
We have since continued to discover other weird planets, including free-floating planets (planets that don't have a star) and planets that are so dense we think they could be made of diamond. But we have also found planets like the ones imagined in science fiction - particularly planets with two suns.
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To date, we know of about 5000 exoplanets, and around 700 of those are planets in binary star systems (a system where two stars orbit each other). Most of these systems have planets orbiting one star, and the other star orbiting further away. Think of our solar system - if we also had another star orbiting further out than Pluto. We have also discovered about 20 circumbinary planets, which go around two suns.
These circumbinary planets are the planets of science fiction, like Tatooine, or my personal favourite, Magrathea from The Hitchhiker's Guide to the Galaxy. Douglas Adams describes Magrathea as "the most improbable planet that ever existed", but is it really?
Being a scientist, I wanted to determine the likelihood of planets like Magrathea or Tatooine forming. Less than 1 per cent of the exoplanets that we know of are circumbinary planets, but the problem is that most searches for planets have focused on planets going around one star because they are the easiest to find.
We are still learning about the true diversity of planets in the universe, and have a lot more searching to do. Who knows - we might even find planets around black holes, like in Interstellar. While science fiction hasn't predicted all the exotic worlds we have found, it continues to inspire us and our search for exoplanets.
- Rajika Kuruwita is an astrophysicist who is currently a postdoctoral researcher at the University of Copenhagen. She completed her PhD at the Australian National University.
