Up to only a couple of decades ago the perception of our Solar System was so simple. Every planet is unique: Saturn has its spectacular rings; Venus has a toxic atmosphere; and Uranus and Neptune are grand icy giants. All in a neat order, from small rocky planets near the Sun to large gassy and icy planets further out. It was a picture that astronomers assumed would be represented elsewhere in the Universe.
But these assumptions were shattered once we began to discover planets outside our Solar System in the early 1990s. When astronomers identified the first exoplanet to closely orbit a star like our Sun in 1995, they were in for a surprise. Big and gaseous, this planet looked more like Jupiter than any other example from our solar system, earning it the nickname “hot Jupiter” – a label that would apply to numerous similar discoveries to follow.
Hundreds more exoplanets have been discovered over the years, and their strangeness continues to confound previously accepted theories about planets. Some exoplanets appear to be made of water, with small rocky cores. Others orbit a set of binary stars, reminiscent of the planet that was home to Luke Skywalker in Star Wars. Occasionally, exoplanets appear to move in the opposite direction to their parent star’s rotation. And some hot Jupiters make it all the way around their parent star in just four days. Stranger still are the multi-planet systems, where as many as five planets can be packed so closely together that, were they in our solar system, they would squeeze their orbits inside the gap between Earth and Venus. “These planets’ bulk densities suggest that they could be made from anything ranging from styrofoam to lead,” says Dr David Sing, an exoplanet scientist at the University of Exeter, UK.
It also calls into question existing theories about how the solar system came to be. Over 450 years since Copernicus radically changed our view of the Universe by working out that the Earth is not at its centre, we are now learning that our solar system is not alone or particularly dramatic. In the words of the biologist JBS Haldane, “the universe is not only stranger than we imagine, it is stranger than we can imagine.”
Rewrite the books
In hindsight, the astronomy of twenty years ago seems quite conservative. “We lacked imagination – you get very blinded by what you know, and think that determines what is out there,” says Charles Beicham, Executive Director of Nasa’s Exoplanet Science Institute. “We have only seen the tip of the iceberg,” adds Ben Oppenheimer, Assistant Curator at the American Museum of Natural History. “There is huge diversity in [planets’] properties.”
The fact that scientists have been so wrong-footed suggests that we might live in a somewhat atypical neighbourhood. It also clearly makes the case that more imaginative astronomical theories are needed. “The oddballs help to tell us the most about any holes in our theories,” says Heather Knutson, an associate professor at the California Institute of Technology.
There are two main theories about how planets form. The first, core accretion, states that planets originate as dust in a disc of gas around a young star, and gradually coagulate by collisions into rocks and then rocky planets. Further from the star, beyond a "snow-line" in the disc, solid ices can form as well as rock, and this helps the planets grow faster (becoming the ice giants like Neptune). The biggest of these can then suck up gas from the disc by gravitational attraction and become gas giant planets like Jupiter.