When we started finding planetary systems around other stars we thought many of them would be like ours. We’ve now found hundreds – and it’s so far, so wrong
1 December 2021
Once upon a time, there was a solar system. In it lived four small rocky planets called Mercury, Venus, Earth and Mars. Four big gassy planets lived there too: Jupiter, Saturn, Uranus and Neptune. The four small planets lived close to the sun because it was very hot there, and everything else had been blasted away, leaving only rocks to make planets. But further out it was colder and there was lots of ice around, so the planets there grew into great big gassy giants.
This story wasn’t originally written as a fairy tale. Until relatively recently, it was our solidly sourced story of how our solar system formed – in fact, how any solar system would form. But in the past decade or so, it has started to look, well, just a little contrived.
As we find large numbers of solar systems elsewhere in our galaxy, none of them look like ours. There are gas giant planets close to their parent stars, rocky planets larger than Earth, compact systems with rocky worlds slotted in between gas giants – anything goes. At first, we could dismiss these exotic exoplanets as oddballs, but after thousands more discoveries, that is starting to look untenable.
Instead, a new picture is emerging of how solar systems form in a chaos of planet building with no certain outcome. That has made us revisit our own solar system’s history, and as we do, a nagging question is becoming louder: instead of being the archetypal solar system, are we actually the freak?
Things started to look weird with our system in the 1990s, when the first exoplanets were discovered …