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.
The second theory is known as the gravitational collapse model, where the gas disc around the young star fragments and collapses directly into giant planets through its own gravity. This should form only large gas planets, and probably far out from the star, although you might be able to form rocky planets by this method if the high energy radiation from the star can evaporate the gas from the planet and leave behind the heavier rocky material.
“Most people favour the core accretion method for the formation of most planets,” says Dr Pete Wheatley at the University of Warwick, UK. However, many scientists now agree that the gravitational collapse model could be very important “especially when explaining multiple large planets a long way out from the star”.
Thanks to exoplanets the field is now paying more attention to other theories that have traditionally lacked widespread acceptance. One such idea is migration, in which Jupiter, Uranus and Neptune are possible candidate solar system immigrants. “They probably formed at longer distance then moved in,” says Adam Burrows at Princeton University, New Jersey. This would explain why hot Jupiters that are close to their parent star seem so odd to astronomers used to surveying our Solar System.
Other possibilities involve an early violent phase in a solar system’s formation, where planets start as unstable objects far from the parent star, and then settle in more stable orbits. Another theory to explain how rocky planets formed involves X-rays from a planet’s parent star heating a gas planet’s upper atmosphere so much that it evaporates, leaving behind the rocky core. “There are one or two planets very close to stars where we know this is likely to have happened,” says Wheatley, suggesting the event can be thought of like an extreme version of an aurora.
Certainly, the list of ideas considered plausible is only expanding. “Discovering so many new exoplanets hasn’t eliminated any theory of planet formation”, admits Burrows.
Perhaps a more profound aspect of exoplanet hunting is the possibility of finding a planet with environmental conditions similar to our own: an “Earth twin”. The hunt has thrown up plenty of potential candidates, although definitions can be misleading. There is no reason why life would be more likely to exist on a planet that is similar to our own than one with different characteristics. “Prejudices about our solar system slip into our terminology,” notes Bruce Macintosh, an astronomer at the Lawrence Livermore National Laboratory in northern California. Of all the planets discovered so far, a quarter are known as “super Earths”, defined as those with a mass substantially bigger than our own planet, but much smaller than the gas giants of the solar system. It’s unlikely that they bear much of a resemblance to our Earth.
Picking clues from the available data about what these planets are made of is no simple matter. What astronomers know depends on what they can observe. Nasa’s main planet-finding mission, Kepler, which launched in March 2009 is now defunct, leaving vast amounts of data still to be trawled. Direct imaging of exoplanets has only recently become possible.
Several projects that should reveal the densities and the atmospheric characteristics of hundreds of exoplanets are on the horizon. Nasa has The American Gemini Direct Imager, to be based in Chile, and the Transiting Exoplanets Survey Satellite, due to launch in 2017. The UK has a Next Generation Transit Survey in the works, with a focus on discovering objects of Neptune’s size and smaller around bright stars.
It means we are entering an exciting time, according to Dr Mark Marley, a researcher who studies the atmospheres of other planets at Nasa Ames. “In the next few years people will start to be able to identify the true Earth twins,” he says.
But compared to stars, planets are very hard to decipher in general, points out Burrows. Simply put, many more things can happen to them. Earth, for example, has a moon because it was hit by another object in its early life; Venus, although similar in size to Earth, has been left with a greenhouse effect caused by a toxic and thick atmosphere; and Saturn has its famous rings, which could have once been a moon. “Each one seems to have its own unique story [of how it formed],” says Marley, “details matter a lot.”
Of course, theories will be adapted as more observations come in. While “super Earths” are currently the most common category of exoplanets, Beicham believes that the smaller rocky planets will soon overtake them in number, purely on the basis that it seems reasonable that smaller things may be more common in the Universe. Similarly, we will know more about whether our own solar system is atypical or commonplace. Just a quarter of 1% of the sky has been surveyed by planet hunters so far, which means there will almost certainly be many more outlandish discoveries to come.
Burrows is philosophical about the point of all this effort. “If you want to understand your origins, you have to understand them in the context of everything else,” he offers. By that metric, the human species seems a very long way from an objective appreciation of its place in the Universe. “You can’t really say you understand Earth until you understand others like Earth.”
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