The first few exoplanets were discovered in the early 1990s. But it wasn’t until the early 2000s, when astronomers began carrying out large-scale, long-term surveys of other stars, that we started to get the first hints that our solar system – with its neat arrangement of four rocky planets, then four gassy giants – might be unique.
For decades, the High Accuracy Radial Velocity Planetary Searcher in Chile and the California Legacy Survey watched for telltale orbital wobbles that exoplanets might induce in other stars. Though these surveys didn’t discover as many exoplanets as later telescopes like Kepler and TESS, they did find signs of just how unusual our solar system is.
Our sun, for instance, is larger than 90 per cent of other stars. It is also alone, unlike other stars that have at least one or two close neighbours. Our planets, too, are rare: only around 1 in 10 stars have a Jupiter-sized planet, and when they do, these worlds are often on very different trajectories to Jupiter’s neat, round orbit. We are missing planets common to most other star systems – those known as super-Earths or sub-Neptunes, of between about 2 and 10 Earth masses. What’s more, even after finding thousands of exoplanets, we have yet to spot an Earth-like planet around a sun-like star, not to mention alien life.
“The weird things are both what we have and what we don’t have. Putting those together, we’re definitely weird,” says Sean Raymond at the University of Bordeaux in France. “It’s not clear yet whether we’re weird at the 1 per cent level, which is a little bit weird, or whether it’s really at the 1 in a million level.”
These discoveries also raised questions about how our solar system formed, such as why Jupiter is so far out, at around 700 million kilometres from the sun, rather than a fifth of that distance as we see for Jupter-sized planets in most other planetary systems. The strange orbits of certain exoplanets made astronomers rethink our system’s history, such as with the Nice model, first suggested in 2001, which posits that a dramatic rearrangement occurred not long after the solar system initially formed, kicking Jupiter out to the periphery and flinging many of the asteroids and moons we see today into new orbits.
“The idea that that could have happened at all came straight from exoplanets,” says Raymond. “Nine out of every 10 giant exoplanet systems underwent an instability, and what we see is the aftermath… People saw that and connected the dots and said, ‘Well, if it happened out there, could that have happened here?’”
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