The first few exoplanets were discovered in the early 1990s. But it wasn’t until the early 2000s, when astronomers began conducting large-scale, long-term surveys of other stars, that we began to get the first hints that our solar system—with its neat arrangement of four rocky planets and then four gas giants—might be unique.
For decades, the High Accuracy Radial Velocity Planetary Searcher in Chile and the California Legacy Survey have tracked the clear orbital fluctuations that exoplanets might induce in other stars. Although these surveys didn’t discover as many exoplanets as later telescopes like Kepler and TESS, they found signs of just how unusual our solar system is.
Our Sun, for example, is larger than 90 percent of other stars. It is also alone, unlike other stars that have at least one or two close neighbors. Our planets are also rare: only about 1 in 10 stars have a Jupiter-sized planet, and when they do, those worlds are often on very different trajectories from Jupiter’s neat circular orbit. We lack the planets common to most other star systems—those known as super-Earths or sub-Neptunes, with masses of about 2 to 10 Earth masses. What’s more, while we’ve found thousands of exoplanets, we’ve yet to spot an Earth-like planet around a Sun-like star, let alone alien life.
“Weird things are both what we have and what we don’t have. When we put it together, we’re definitely weird,” he says Sean Raymond at the University of Bordeaux in France. “It’s not clear yet whether we’re weird at the 1 percent level, which is kind of 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 away, about 700 million kilometers from the Sun, rather than a fifth of that distance as we see Jupiter-sized planets in most other planetary systems. The strange orbits of certain exoplanets have caused astronomers to rethink the history of our system, such as the Nice model, first dramatically proposed in 2001, which posits that a rearrangement occurred not long after the Solar System originally formed, pushing Jupiter to the periphery and hurling many of the asteroids and moons we see today into new orbits.
“The idea that this could even happen came directly from exoplanets,” says Raymond. “Nine out of every 10 giant exoplanet systems have gone through instability, and what we’re seeing is the aftermath … People have seen it and put it together and said, ‘Well, if it happened out there, could it happen here?’
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